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Synthesis and Microstructural Characterization of Electrodeposited Nanocrystalline
Soft Magnets
by Cedric K.S. Cheung
A thesis submitted to the
Department of Materials and Metaiiurgical Engineering in codormity with the requirement for the degree of
Doctor of PhiIosophy
Queen's University
Kingston, Ontario, Canada August, 2001
copyright O Cednc KS. Cheung
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Abstract
Two of the most important properties of a soft magnet are the saturation
rnagnetization (M,) and the coercivity (K). An ideal soft magnet wouId have a very high
saturation magnetization and a very low coercivity. in recent years, there has been
considerable interest in the development of nanocrystailine materials, including magnetic
materiais. This is because at grain sizes Iess than lOOnm, they exhibit many enhanced
properties, for exampIe, increased hardness and strength. With respect to magnetic
applications, nano-processed materiais have the additional benefit of having an increased
eIecûicaI resistivitv. It is because of these factors that considerable efforts have been put
into the research of nanoqstalline mapets-
However, earlv attempts to produce such materiah by several synthesis techniques
have faiied to achieve the expected values for the saturation magnetization- This work is
concerned with the Jevelopment of synthesis methods for a variety of soft magnetic
materials that could surpass the saturation magnetization limitations obsented in previous
work. in partimlar, nickel-iron aiioys, pure cobalt and cobait-iron aiioys, in order of
increasing saturation magnetization, were produced by electrodeposition methods.
By aiioying iron with nickel, binary nickel-iron d o y deposits have been successfully
produced within a composition range of up to approximately 28wt.% uon. This
-.. Synthesk and LIimstructural Characîerization of Electrodeposited Nanoqstaihe Soft Magnets III
compositional region covers that of Pexmallo@ (Ni-20Fe), an extremely important nickel-
iron aiioy for magnetic applications. The grain size of these d o v s was found to be between
1Onm to Xnm, and was essentiaiiy a function of the iron content in the aiioy. With a
maximum hardness of over 6OOVHN, the microhardness of these materials was
sigruficantiy irnproved (a 6 to 7-fold inaease) as compared to conventional materials with
the same compositions.
in order to obtain a higher saturation magneüzation, operating windows for the
production of nanocrystalline pure cobalt were found using the puised current
e~ectrodeposition technique. These electrodeposits were found to have grain sizes between
8nm to 15nm, with microhardness values up to 650VHN, a factor of 2 to 3 increase hom
conventionaiiy produced pure cobalt.
To further inaease the saturation magnetization, iron was alloved with cobalt.
Cobalt-iron aiioys were successfulIy electrodeposited with iron contents up to about
Lwt-% iron. These binary aUoy deposits exhibited the same k e e distinct phases reported
using pyrometaiiurgical production methods. However, the phase fields were shifted,
along with a clifference in the compositional range each one covered. The grain size of
these deposits ranged from 2ûnm up to micrometers, with the mallest-grained aiioy
deposits in the middle of the composition range produced. The maximum hardness of
these binary alloy deposits was about 400VHN.
The results obtained in tfüs work constituted the first adUevement in the production
of nanocrystalline soft magnets thatcodd eventudy be tailored towards the ideal magnet.
Synthesis and Mimstnictural Characterization of Electrodepsiîed NanocrystaiIine SDft Magnets iv
Acknowledgements
First and foremost, 1 would like to acknowledge my farnily for their support and
encouragement in whatever endeavours I choose ta undertake, and for their genuine
interest both in my life and my studies. 1 ottiy wish i would make them proud one day.
Financial support from the Natural Sciences and Engineering Research Council of
Canada (NSERC) is gratefuiiv acknowledged for this work,
Personaiiy, 1 wouId iike to express my utmost gratitude towards my supervisor, Dr.
Uwe Erb, for his kinciness and patience throughout my tenure as his student. But most
importantiy, 1 deepIy appreciate his confidence in my abilities ancl his wiIlingness to afford
me the Iatitude necessary for me to accompiish what 1 have accomplished today.
Everyone in the nano group, both at Queen's University, and most recentIy at the
University of Toronto deserves mv most sincere thanks, for without them, iüe would not be
as interesthg, and research would not have been as fun. The support staff in Nicot HaU
have been invaIuabIe throughout the years for the numerous students that have crossed
their paths, and 1 am no exception, 1 just want to express my apprecïation for their support
in more ways than one.
in the past few years, I have been through some ciifficuit times and I wouId N e to
take this opportunity to acknowledge some of the speciai people in my Me that have
hdped me tremendously during those limes. 1 am honoured to have them as my friends
and 1 am privileged to have received the help and support 1 probably did not deserve.
Chid arnong them Martin Aus, Iain Brooks, Juiie Chan, Uwe Erb, Joseph Hui, Cindy Kwok,
Carmen Lai, Shig Saimoto, Howard Wu and Ling Yu- I owe them a whoie lot and I know I
could not possibly repay them
Synthesis and biicrosmcturaI Characterization of Eiecmdeposited Nanocrystabe Soft Magnets v
Synthesis and bIicmsentctutal Characterïzation of Electrodeposited NanocrystalIine Soft Magneh vi
Table of Contents
... Abstract ............................................................................................... LU
................................................................................... Acknowledgements v
.......................... ..........*.......*.....*.***.**....**...*.......... Table of Contents ..... vii
List of Figures ............................................................................................ x
List of Tables ............................................................................................. mi
Chap ter 1 introduction
1.1 Introducto~ Background .......................................................... 1
............................................................... 1.1.1 MagneticMateriaIs 3
..................................... 1.1.2 Characterization of Magnetic Materiais 4
1.2 MotivationforthePresentWork ............................................ 6
1.3 Objectives of the Present Work ...................................................... 11
1.4 Nanocrystaiiine Materiais ............................................................ 11 ..................................... . 1.5 Synthesis of Nanoqstaiiine Materials ... 13
............................ 1.5.1 inert Gas Condensation and Other Techniques 13
.................................................................. 1.5.2 Electrodeposition 15
............................ 1.6 Theoretical Understandmg of Electrocrystallization 16
........................................................................ 1.7 Research Stra tegy 10
................................................................................. 1.8 References 13
Synthesis and Microstructural Characterization of EIectrodeposited N a n ~ s h h e Soft Magnets vii
Chapter 5 Cobalt-iron Binary System
....................................................................... 5.1 Literature Review 127 ................................................................... 5.2 Experimental Details 129 ................................................................ 5.3 Resu1t.s and Discussion 131 ................................................................. 5.3.1 May Composition 131
...................................................... 5.3.2 X-Ray Diffraction haiysis 133
5.3.3 bficrostructures .................................................................... 143
..................................................................... 5.3.4 blicroharhess 149
Chap ter 6
.................................................................................. Summary 152
................................................................................. References 153
Conclusions
.................................................................... General Condusions 156 ............................................................ Nickel-bon B h r y System 156
..................................................................... PureCobaltSystem 157 ............................................................. CobaIt-bon Binary System 158
................. Outlook for Electrodeposited Nanoqstalline Soft Magnets 159 ............................................................. Contributions to the Field 161
References .............................................................................. 163
Chapter 7 Recornmendations for Future Work
........................................................... 7.1 General Recornmendations 164
î h a p ter 8 Appendices
8.1 Appendix A -Weil's Classification of the Stntcture of Electro- ........................................................................ deposited Metals 167
8.2. References ................................................................................. 169
.............................................................. Qiapter 9 C d d u m Vitae 170
Synthesis and iviicrostnictural Chatacteiuation of Etectrodeposibed Nanocrysbüine Saft bfagnek
List of Figures
Figure 1-1 A Spical hysteresis loop showing intemal domain alignments [Parker
(1990)].
Figure 1-2 Schematic diagram of hysteresis loops for a hard and soft magnet [Askeland
(1996)l.
Figure 1-3 Coercivity as a hinction of grain size for vanous soft magnetic rnaterials;
(legend: A Fe-Nb-Si-8; Fe-Cu-WSi-B; Fe-Cu-V-Si-B; M Fe-Zr-B; V Fe- Co-Zr; A+O Ni-Fe aiioys; O Fe-Si) [Herzer (1990)l.
Figure 14 Saturation magnetization as a function of grain size obtained by Aus rt r d
[19921 and Gong &n/. [19911.
Figure 1-5 Schematic diagram showing the saturation magnetization and coercivitv
combination of various soft magnetic rnaterials [adopted hom Hattendorf
(1995)l.
Figure 1-6 A schematic diagram showing an apparabs used in the inert gas condensation technique to produce nanocrystalline materials [Siegel and
Eastman (1989)I.
Figure 2-1 Schematic diagram showing the invariant nature of direct current densitv as
a function of tirne.
Synthesis and Lfiaostructural Charactgrization of EIectrodeposited Nanocqstailine Soft kiagnets x
Figure 2-2 Schematic diagram showing a hrpical D.C. electrodeposition experiment
setup.
Figure - 3 A schematic diagram detailing the cathode design used in this work [EI- Sherik (1993)l.
Figure 2 4 Schematic diagram showing the tirne-variant nature of a pulsed current
experiment.
Figure 2-5 A schematic showing a typical setup of a P.C. electrodeposition experiment.
Figure 2-6 Grain size as a Function of measured X-rav diffraction peak width (FWHM)
as calculated using the Scherrer formula.
Figure 3-1 Iron content (wtY6) of the electrociepositeci nickel-iron aUoys as a function of
iron chloride salt concentration (g/L) in the sotution; individual points
represent the average of at Ieast 10 EDS readings with the error bars
representing one standard deviatian.
Figure 3-2 Average Iron content (wt.76) of the electrodeposited nickel-üon aiioys as a
function of the nickel to iron cation ratio in the solution.
Figure 3-3 A scanning electron miaograph of an electrodeposited nickel-iron ailov with
a composition of Ni-20wt.X Fe.
Figure 3 4 X-Ray ciiffraction patterns of a series of eiectrodeposited nickel and nickel-
iron aitoys with compositions ranging up to 2.2wt.l'oFe.
Figure 3-5 JCPDS powder diffraction file for pure nickel (PDF No.4850) [JCPDÇ (1990)l; 20 values iisted are for copper K, radiation.
Figure 3-4 Grain size of the nickel-iron deys as a h c t i o n of the iron content (wt%).
Synthesü and ~crostructurai Characterizaiion of Electrodeposited Nanocrystalhe Soft Magnets xi
Figure 3-7 (a) Brightfidd transmission electron miaograph, (b) darkfield transmission
eIectron micrograph, (c) electron diffraction pattern and (d) grain size
distribution of the pure nanocrystalline nickel electrodeposit (the uncertainty
represents one standard deviation).
Figure 3-8 (a) Brightfield transmission electron micrograph, (b) darkfield transmission
electron micrograph, (c) electron diffraction pattern and (d) grain size
distribution of the nanocrystaüine electrodeposit with a composition of Ni-
20wt.%Fe (the uncertainty represents one standard deviation).
Figure 3-9 Equilibrium phase diagram for the binarv nickel-iron system [~Mt>tnb
Hnmibook (l99Ob) 1.
Figure3-10 Microhardness of the various electrodeposited nickel-iron alioys as a
function of the iron content; error bars indicate one standard deviation.
Figure3-11 A Hall-Petch plot for the etectrodeposited nickel-uon aiioys; error bars
represent one standard deviation.
Figure 41 Graphical representation of the three variables in the 23 factorial design
experiments.
Figure 4 2 Scanning electron miaographs showing examples of different morphologies
observed on electrodeposited pure cobalt produced from the saccharin-free
bath; (a) TON = amsec, Ton = lOmsec and I P ~ & = 0.2A/cmz, (b) TON = 8msec,
TOIT = lOmsec and 1peJk = 0.2A/cmz, and (cl TON = grnec, Ton = 40msec and
Ipmk = 0*2A/mz.
Figure 4-3 Graphical representation of the various response variables for the saccharin-
free bath as functions of the pulse plating conditions; (a) texture response, (b)
morphdogy response and (c) grain size response.
Synthesis and Mic~ostructural Characterization of Eiectrodeposïted Nanocrystahe Soft Magrtek
Figure 4-4 The two typical X-ray diffraction patterns obtained from cobalt
electrodeposits produced from the saccharin-free bath and its associated
surface structure; (a) pyramid type (LA) has a (1 120) preferred texture
while (b) needle surface morphology (LA) has a (10i0) texture.
Figure 45 Scanning electron micrograph of a typicai cobalt dectrodepasit obtained
from the saccharin-containing bath.
Figure 16 Graphical representation of the various response variables for the saccharin-
containing bath as Eunctions of the pulse pIating conditions; (a) texture
response, (b) morphology response and (c) grain size response.
Figure47 X-rav diffraction patterns from a typicai eIectrodeposited n a n o q s t a b e
cobalt as weU as a cobalt powder standard.
Figure44 JCPDÇ powder diffraction file for H.C-P. cobalt (PDF No, 5727) [JCPDS (1990)l; 20 values listed are for copper K, radiation.
Figure49 JCPDÇ powder diffractbn Me for F.C.C. cobalt (PDF No. 25-806) DCPDS (1990)]; 20 values listed are for copper K, radiation,
Figure 4-10 (a) Brightfield, (b) daridierd transmission eIectron micrograph and (c)
electron ditfraction pattern (order of Migr in increasing radii: (1010).
(0002) , (10Ü) , (10Î2) , (1 120), (103) ù)) and (d) grain s i x distribution (based
on 200 grains) of a nanocrystahe cobait electrodeposit with an average
grain size of 10 nm; the error represent one standard deviation.
F i e 1 A Hail-Petch plot for the electrodeposited nanoqstaiüne cobait obtained
from the saccharin-containing bath.
Figure 422 Scanning dectron micrographs of sampIes obtained under conditions A, B and C in TabIe 44.
Figue413 X-ray diffraction patterns of cobalt deposits produced for the studv of the
combined effect of saccharin and puise plating; (a) Iow current without
saccharin, (b) high current without saccharin and (c) low current with
saccharin addition.
Figure 414 Schematic diagrams (top view) showing setup of the experiments carried out
in the presence of an externaiiy applied magnetic field (3500G); (a) field
parailel to the deposit surface and @) field perpendicular to the deposit
surface.
Figure 4-15 TypicaI X-rav diffraction patterns of in-field plated cobaIt samples with the
applied field (a) paraiiel to the deposit surface, and (b) perpendicular to the
deposit surface.
Figure 5-1
Figure 5-2
Figure 5-3
Figure 5 4
Figure 5-5
Iron content of the electrodeposited cobalt-iron alloys as a hnction of the
iron sulfate sait concentration in the solution: (a) current densitv i =
50mA/crn2, (b) i =200mA/m~ and (c) a composite plot for both current
densities shown in (a) and (b); error bars represent one standard deviation.
X-ray Diffraction patterns of two cobalt-üon alloy electrodeposits from a
bath containing 20g/L of iron saIt obtained using current densities of (a)
50mA/m2 and (b) lOOmA/cm~.
Equilibrium phase diagram of the üon-cobalt system [Bozorth (197811.
X-ray diffraction patterns for cobait-üon eIectrodeposits produced at
100mi\/cm~; (a) Og/L Fe504 (pure cobalt), @) 5g/L Fe504 (Co-3.Ge), (c)
IOg/L FeSO4 (Co-5.83Fe), (d) 15g/L FeS0.t (Co-7.45Fe), (e) 20g/L FeSQ (Co-
10.44Fe), (f) 25g/L Fe504 (C+l266Fe), (g) 30g/L FeSO4 (Co-Ié%e), (h)
35g/L FeSû4 (Co46.43Fe), (i) 40g/L Fe934 (Co-17.4Fe), (j) Gg/L (Co- 19.69Fe) and &) 50g/L Fes0.1 (Co-21.08Fe).
Cornparison between the equilibrium phase iieids for cobalt-iron aiioys from
the cobalt-iron phase diagram and resdts obtained in this study, showing
changes in the width and region of occurrence for these phases.
Synthesu and b[icrosûucturaI CharaEterization of Eiectrodeposited NanocrystaIline Soft Magnets xiv
Figure 5-6 High magnification scanning electron micrograph showing an example of
the noduiar structure for some of the binary cobalt-üon electrodeposits for
which grain size couid not be resoived in the SEM. in this example, a deposit
with 10.5wt.% üon is shown.
Figure 5-7 Representative scanning electron miaographs showing the various
morphologies observed on the b i n q cobalt-iron electrodeposits: (a) low ùon
contents - type II-A (needles), (b) intermediate iron contents - type 1-B
(clusters) and (c) high iron contents - type LA (pyramids).
Figure 5-8 (a) Brightfield transmission electron micrograph, (b) darkfield transmission
electron micrograph, (c) electron ciiffraction pattern and (d) grain size distribution of a binary cobalt-iron deposit having nodular morphology (Co-
14.5Fe); uncertainty represents one standard deviation.
Figure 5-9 Miaohardness of the electrodeposited cobalt-ùon aiioys as a function of the
iron content in the doy: (a) 5 0 ~ / c m Z , (b) lOOmA/cm~, (c) composite of (a)
and (b); enor bars indicate one standard deviation.
List of Tables
Table 3-1 Composition of the eiectroplating solution developed for the nickel-iron binary
system.
Table 4-1
Table 4-2
Table 4 3
Table 4-4
Table 4 5
Table 4-6
Table 4-7
Table 4-8
Composition of the electroplating solution used in the pure cobalt system.
Low (-) and high (+) vaiues for pulse on time (TON), puise off time (Ton), peak
current density ( I P ~ ~ ) and saccharin concentration in the plating solution.
TON, TOR and IP,A settings for electrodeposition experiments 1 to 8 (saccharin-
free bath) and 1' to 8' (saccharin-containing bath).
Response variables for experiments 1 to 8 as a function of the TON, TOFF and ip,k
settings conducted in the saccharui-free bath.
Resuits of miaohardness measurements for cobalt samples obtained hom
experiments 1 to 8 (the saccharin-free bath); uncertainty represent one standard
deviation,
Response variables for experiments 1' to 8' as a function of the TON, TOIT and Ipe&
settings conducted in the sacchanniontaining bath.
ResuIts of microhardness for cobaIt sampIes Erom experirnents 1' to 8' (the
saccharin-containing bath); uncertainty indicates one standard deviation.
Parameters for experimentç to separate effects of pulsed current and saccharin.
Synthesis and Microstructurai Characterization of Wectrodepasïted Nancaystalline Soft Magnets
Table 4 9 Plating conditions used for the shidy of eIectrodeposiaon of cobalt in a magnetic
field.
Table 4-10 Microhardness of cobalt deposits in the absence of applied magnetic fieId, error represents one standard deviation.
Table 4-11 Results of the hardness measurements carried out on the study of magnetic field
on the eIectrodeposition of nanoaystalline pure cobalt, error represents one
standard deviation.
Table 5-1 Composition of ekctroplating solution used in the cobalt-üon systern.
Table 5-2 Equilibriurn wstai stnictures of cobalt-iron alloys.
Table 5-3 X-ray diffraction peak locations for H.C.P. and F.C.C, cobalt using copper K,
radiation.
Table 5 4 X-ray diffraction peak locations for B.C.C. iron under copper K, radiation.
Table55 Morphology and grain size (as per SEM) as a function of iron sulfate in the
solution (i = 50 mi\/&).
TabIe 5-6 MorphoIogy and grain size (as per S M ) as a function of iron sulfate in the
sohtion (i = 100 mA/cni').
Table57 CorreIation between surface morphology and structure of electrodeposited
cobalt-üon alioys produced at 100 di&.
TabIe6-1 Magnetic properties meanired for the various nanocrystalline soft magnetic
materiais produced in this work by M.J. Aus [Am (1999)] dong with pubhhed & values for their conventional counterparts.
Synthesis and biicrostnrctural Characterization of Uectrodeposited Nanoqstalluie Soft Magnets xvii
Table8-1 Summary of the classification scheme for as-plated surface structures of
electrodeposited metals developed by Weil [1951].
Synthesis and Microstructural Characterization of EIectrodeposited Nanocrystahe Soft Magnets xvui
Chapter 1
Introduction
1.1 In&oductory Background
The connection bebveen electri~ltv and magnctism was discovtxeri bv Hans
Christian 0rsted who, in 1820, observed that the neeJlc of a magnetic compass coulci bc
manipulated by a current-carrying conductor [White (1999)l. As a result, the origin of
magnetic behaviour of a material can be appreciated microscopicallv in the context of
moving charges.
Magnetic behaviour is determineci prectcirninantlv - bv * the tilectronic structure of CI
material- The principal cause of magnetic moment in an atom cornes from t.Iectrrin motion
(in contrast, thc nuclei only contributes about O.I% tu the magnetic prcprties ruheti
compareci with the electron). The electron mn contribute to the magnctic momcnt in hvci
ways: the eIectron spin and the orbital momentum. The magnctic field resultant from
electron spin is dependent on the quantum numbor, m,, whereas orhiting electrons ueate
magnetic fields around the atom. [n general, the net magnetic field from orbital
momentum of the eiectrons is zero. Consequentiy, the net magnetic fieId (rom an atom
cornes from the eIectronic spin.
Synthesis and bIicrostructur-1 Characteriution of Elwtrodeposited Nanocrytiillinc Soft hlagnets
Each disneet energy Iwel in an atom coulci have bvo eiectrons with opposite spin
because of Pauli's exclusion principle. In other ruords, for a complctelv fiiled energ! Icvel,
the net magnetic moment is zero. From this ntionale, any atoms witli unpaired electrons
rvill have a net magnetic moment. However, this hvpothesis does not holri. This is Pecause
for most of these elements, the unpaireri electrons are in the valence sheil. anci clectrons
from different atoms intenct with each other, resulting in a cancellation of magnetic
moments [Askcland (2996) 1, hence, exhibiting no net magnetic moment.
in vacuum, a magnetic field, H, inriuces lines of flux, aiici thc ilus ilensih., or
inductance, 6, is relatcd to the appliecf field bv the folIowing:
B = u , H
where po is a constant known as the magnetic permeabiliv in vacuum.
When a material is in thc presence of a magnetic fieId, the permanent magnctic
dipotes mav interact with the field, either contributing or reducing the fielcl witiiin the
material, causing a change in the overd1 inductance, which can be denoted bv:
B =,tif-! Er!. 1-3
tvhere p is the permeabiiihr of the material in the applied fieid, rvhich can bii rewritten to
refer to the contribution to the inductance from the material and the inductance h m the
field itself:
B = ,tr,,(H + L W ) = p , ,H + ,tr,,~bl
where BI is the magnetization of the material.
In this wav, the relative permeabiliv, p, can be ciefined as the ntio between I I and
p ~ . In other worris, a large C I , means a large reinforcement of the applied field inside the
material cvhereas for materials that oppose the applied field, 11, wiI1 be less than one.
1.1.1 Magnetic Materials
There are five categories oi materials in light of their behaviour in the presence of a
magnctic field: dimqnetic, paramagnetic, ferromagnetic, antifcrromagetic and
ferromagnetic materials.
Diamagnetic materials are materials which hme orbital magnetic moments tha t can
oppose an applied mdgnetic field, causing ilr to be less than one, and hence the
magnetization to be less thdn zero. For example, copper (Cu), silver (Ag) 'inci gohi (.AU) are
diamagnetic materials at room temperature. In CI paramagnetic matcrial, thcrr is CI net
magnetic moment from electron spin. Howcver, the individual atoms cio ncit intenct, anri
hence, reyuiring large magnetic fields to orient the dipoles. Furthermore, oncc the fielri is
removed, the rnagnetizing effect is lost. Paramagnetic materials genera1Iv have LLr behveen
1 and 1.01. Examples of panmagnetic materiaIs are aIuminum (Al) and titanium (Ti).
In ferromagnetic ma terials, the permanent magnetic Jipoles orient verv easih in the
presence of an appIied magnetic fieId due to mutua1 reinforcement. Large magnctizaticms
are tvpical of these rnaterîals, with p, as high as IOh. There are five ferromagnetic elements
in the periodic table, nickel (Ni), cobalt (Co), iron (Fe), ~ d o l i n i u m (Gd) m d dvsprosium
(Dy). The saturation magnekations for the technoIogicdIy important metals, nickel, cobatt
and iron are 0.6 Tesla (T), 1.6 Tesla and 3 2 TesIa, respectiveiy.
introduction
In Lintiferromagn~tic materials, the strength of the magnetic c l i ~ c i l c is quite higli.
However, in the presencc of an applied fi elci, adjacent dipo1t.s oricnt tlicrnsclve~ typosite to
cach othcr in ciirection, causing a zero net magnekation. Chrrmium (Cr), mangdncse
rixicie (MnO) m d nickel oxicie (NiO) art. examples of antiferrumdgnetic mdtcriLils.
Ferrimagnetic materi,ils art. similar to antiferromagnetic materials in thdt ncighbvuring
dipeiIcs ~ i r c aligner1 in oppositc directions whcn plCiccJ in magnetic ticl~ls. Howcver, thcre
arc more t h m one set of clipoles in the materials, and thev have difkrcnt strcrigtlis. As '1
result, h r c is a net magnet i~~~t ion although some cancellations uC mrignctic niliment clu
occur. Ceraniic matcrials dre tvpiallv ferromagnetic, where ciiffcrcnt ions have differt'iit
mdgnctic moments; dipoles uf ion X may line up tvith the appliecl fielci while tliosc cit ion B
iipposc it.
1.1.2 Characterization of Magnetic Materials
tn cliaracterizing LI magnetic material, a hvstercsis loop is uftcn obt~incci.
This is becausc d number of important properties of the matcnal c m bc rl~~t'rtdincd h m
such a lciup. T ~ v o of the must important properties to be considcrcci whcn cxamining CI sott
magnetic materid 'ire its saturation magnetization and its cvercivity (HL). The
saturation magnetization is defined as the maximum magnetic field inciucecl within LI
material in the presence oi an externally applied magnetic field. Saturation is reachcci
whcn a11 the magnetic moments in the material are aiigneti in cine dircctiun. i.e. paraIlcl tu
the applicd field. M e n the externa1 fieId is removeci, there wiII be some residuaI
magnetism ùi the material. The strength of the magnetic field that must be applied in the
oppusite direction in order to bring the residual magnetism in thc material to zero is thcn
Introduction
rictined as its ccwrcivitv (H,). Soli& exhibiting low values of c'ricriivity =Irp tcrmcci
"rnagneticalI\~ soft", while those rvith high çoercivib are gencrally knliwn '1s "magneticdtv
hdrd" niatcrL11s. Figure 1-1 shows a schernatic diagram of a hystertlsis Iuop.
Thermal demagneltration
Virgin magnetiralion f f c ,
A htpical hvstcresis loop showing internai domain alignmmts [Parker ( 19C)O)I.
- - - - ---=Y--]-] - H \ , Field dcrnagnelizalion
- 8,
Figure 1-2 shows the ctiffercncc in the hysteresis loops exhibitcd bv hard and '1 sott
mapet. The Iu»p for a tvpic-al hard rnagntit is much wider than for t h t ot a sott rn'~gnrt.
This is because of the Iow coercivitv generallv e,uhibited bv soft m,igntltic rnc~tt.riais in
cornparison to the high coerclvih. desired in a hard magnet. f i e satuntivn magnetkatiun,
Remagnetization samc direction
------A------+ {oooo] Rem~gnefization reverse direction
Introduction
hiç, o f a convcntional miqpctic material. whether hard cir s d t , cicpcnds m l v on t h t ~
c~impmitii"n ( ix . , the strcngth of the magnetic cIipoIes in h e matcrial).
Fiare 1-2
'%hcrndtic cliagnms of hvstwsis Ioops for a hard and soft rnagnct [Jlskcland (199b)j.
2.2 Motivation for the Present Work
Recent advances in the area of nanocryshiht! matenah have shrwn hcit grain s i x
reriuction in ferromagnetic materÏais mav rtisult in extremelv soft magnetic hliaviour. For
Introduction
cxampIc, Figure 2-3 shows coerciviw as a function of grain size tur several allws proclucd
hy crystallization 1) t c~mcirphous precursors.
Giain Size D
H~ 1 -
(A@)
0.1 -
0.0 1 -
0.001
Figure 1-3
, I ,
06 . , - , O
9: O‘, FeSi6.5
P d, 0
1 a',
.* I nono-
SONiFt d, O
'4. cryst. A d, \ T ,
perni- A ofr~~rphous 4 , -
A , olloy * ,
1 - - - -
. I i 1 I i i I
Ccwrcivitv '1s a functicin of gnin size for various soft magnetir ma tcrials;
(Iegend: A Fc-Nb-Si-6; Fe-Cu-Nb-Si-B; + Fe-Cu-V-Si-6; I Fe-Zr-B; V F e - G d r ;
l i +O Ni-Fe aiIoys; O Fe-Si) [Herzer (1990)l.
1 nm 'w' fmm
InitialIv, this cuNe shows inmeasing coercivitv with decrcasing gnin sizc (D).
tdowing a 1/D relationship tor p i n sizes between Imrn dncl about SOnm. This
relationship expIains the conventional approach to produce sott magnets with reiativctv
large grain size, i-e.. tu be in areas of Iow coercivitv [Herzer (1990)l. However, extremclv
soft materiais (magneticaily) are aiso observeci at grain sizes Iess than 51) nm. tn this rcgion,
introduction -
F- '
a Dn dependence on grain s i x was found which has been explainecl hv a ranclom
anisotropv mode1 for magnetkrn [Herzer (l990)]. in view of the trcmcn~lvus rcducticin in
coercivity at very Iow grain sizes, consicienble research efforts rverc initiateci in the late
L980s to the earlv 1990s to develop nanocrvstalline fcrromagnets kir soft magnctic
applications. This is becausc, combined cvith some of the other enl~ancccl properties
cibscrvcd on ncinucn~stallinc materials such as incrrased elcctrical rcsistivitv anri incrcasccl
meclianical hardness, CI nerv class of exceptional soft magnets could bc prciriucccl hv nùnrt-
prvcessing. Howtwr, the initial rcsciirch efforts using svnthcsis techniques othcr than
cnrstaIIizcitiun of ,ime~rphous precursors indicated that saturation rndgncti~~ition mcw Pt>
cornprumiscci when ferromagnetic materials are producerl with grain s i u in the nancinwtcr
nngc. Some «f the earlicr work inciicatcci CI consictcrablc dccreLisc in sdturcitiun
mLignctizati«n cvas cdxervecl when the cystal size cvas reduced to lcss than 1Wnm [Gong (f
d (LYYZ)], G,ingopdcfhvav . . tlt il/. (t992), Schdcfer cf d (1992), Cleitcr (198L))I for *is-
ci~ncicnscri nickel, cobalt dnct iron.
Figure 14 clcarly shows that the saturation magnctizatirm for nanii-pariiclvs iii
nickel produceel by inert g ~ s condensation decreased rapidlv to a value of about 3l)eniu/g
(113.3 T) as the crystal sizc approdches LOnm [Gong rY d ('1991)1. ln ccii-itrast, .-lus ct d.
Il9921 shotvcd that the scituration magnrtizdtion rcmains reldtivelv srinstant with grain sizc
when nanocryshliine nickel was produced in fu1Iv dense form by elt.ctrudepusitiun (results
of Aus ~'i II/. arc alsri included in Figurc 14). .A theoretical analvsis bv S~puna r ( Y d [19%I
has shown that the stnictunl ciisorder introduced in nickel bv grain boundaries sIiouIcI ncit
have a major effect on the saturation magnetization as Iong as the ma tericll is fullv riense,
thrts supporting fullv the resuits obtained bv Xus ~ f r d [1992]. In fact, it ~vds ccmcluctt'd thiit
the Iarge reduction in saturation magnetization reported for gas-condensed nanc~rvstiiLs
introduction P- 8
Synthesis m d SIicrostructur~I Chanttcriwtion of Electrodepositeci N.inocrystC~llineSolt JI~gnrb
was the rcsult of oxidation of the surfaces of the porcs betwwn the n,inii-siztld particlvs
mther than c-rystal sizc reduction [Xus rdd (LY92), &punar rVd (L99b)I.
10' 102 103 104
Grain Size (nm)
Fiwre 1-4
Saturation magnetization as a function ot p i n s i x
obtaincd bv Aus dd [19Y21 and Gong r p f d. [199LI.
The second drawback as far as saturation magnetization is crinc.erncd, evrn Lir
materials producd from amorphous precursors that exhibit vent Iciw ciierrrivity (set. Figurc
1-3), crimes from the dloy compositions themsclvcs. In ordcr to producc the amc~rphous
prticursor materials 6y npid solidification, ctinsi~ienble tontcntrations r)f a1loymg
elements such ùs copper (Cu), niobium (Nb), siLicon (Si), borun (8) and zirconium (Zr) wcrc
u s d to producc low-temperature eutectic compositions, a prerequisite for amurphous
Intro Juction
Syntlirsis and Xlicrostriicturid Char<icteriii*tiun of Electrodeposited Nanocrysbilinc Soit Xl.igncLç
structure formation [e.g. Yoshizawa (2988)l. However, these alloving elernents ,ire
~iiamagnetic cir panmagnetic and, thus, reduce the average magnetic moment, Le., thc
scituntion magnctization, of the alloy by ddution. For example, Figure 1-5 shows thc
saturation magnctization as function of coercivity for sevcral sot't magnets including sunw
of the nanocrystalline Fe-bascd alIoys çhown in Figure 1-5,
...- o. 1 I LO 100 LOlIll
Coercivity (H,), A/m
F i w e 1-5
%hematic diagnm çhowing the saturation magtwtization and cocrcivity combination c ~ t '
various soft magnetic materiah [adopted from Hdttendorf (l995)I.
This figure also shows the field of saturation miignetization anel cocrcivity fur " ieIecil
soft magnets" in terms of saturation magnetization and coercïvih, tvhich are ).et to be
devcloped. This icical soft magnet would contain the two elements iron anci ccibiilt, which,
in 'ln dl11w of composition b3Fe35Co, are knotvn to produce the highcst saturation
miignctization possible [Giles (1991)l. Furthemore, the ideal soft mcignetic materidl iv idd
have grain sizcs less than SOnm, i.e., to the Icft of the maximum dcpicted in Figure 1-3.
1.3 Obiective of the Present Work
Thc main objective of this work, whicli was initiateci in 1994, is tlic clevclupmcnt lit'
a pniducti~in rnethod fix svft m'~gnetic matcriiils that possess ptc~iti~ill!. IiigIi sC~turLitii)ii
rnagnctimtirm and smdll grain sizc, where ,i low aicrcivity vdue is rdcipritecf JS h ing J
constlqutlnce d the nanricrystallinc rnisrostructure. ln r)ther worcls, this thesis will f i ic~~s on
the cicvelopmcnt of nanlistructured soft magnets tvhich, bascd on thcir compositiun, WOL~IJ
move tlicir magnetization h m the current Iimit for amorphous i l h l t-basd alI«vs
tcwnrds the rcgion of ictcal soft magnets as inciicatod by the drrutv on Figure 1-5. T'irgtbt
materials inchde, in succession of higher saturation magnctizations, nickcl-iron dlIoys,
cobaIt and cobalt-iron allovs.
1.4 Nanocrys tailine Materiais
It \vas Cleiter [1981] who first introduced the concept of nanocnrstalline soli& to the
fietci of materÏaIs science approximately twentv - - vears ago. These nanocrvstalline materials
Introduction p- 11
Synthesis anri Microstructuni Charxtcrüdion of Elec.trodepositcd Nlinu'rystdline Sol'( hi~griets
are not unlike conventional - polvcrvstalline - solids, with the exception that their grain sizes
lie in the nanometer range. Nmocrvstaiiine materials are genenllv Jt.linc~l t d a y '1s
materials with a crystal size 1)i less than LOOnm. At these small grain sizcs, the volumt.
fraction of interfacial atoms in the material (grain boundaries and tripttl junctions), which
c m be ncglecterl in conventional polvcn~stalline - - rnaterials, is incrcascd ciciirniitic.allv.
Palurnbo LY tiL [lY90] prcscnted a geomebic mo~icl involving rcguhr shdpeci t'ourtcm-sicicd
cwstals thiit s l iowd the significancc of the interfaciai content of nlinocn~stallinc matcrids.
Employing this niode1 ,ind cissuming a nominal grain boundarv wicith of Lnm, the
interfaclal content of LI l ~ ~ m grain size material amounts to 0.3%, cvhcrcas tiiis vciIuc is
increased to «ver 30":~ for a material tvith a IOnm p i n size. At evcn smallcr gmin s i x
value of h m , the interfacial content in the matcrial inneases to over 50"L. daminciting thc
material microstructurc. It is for this reiison tha t materiaIs posscssing ultra-tinc. grain
structures beliavc, in man!. cases, differentlv than their convcntioncil cmrsc-graincd
counterparts. Examples of important properw changes in nanocxystailint. mcitcrials includc
increascd liar~lncss [Gleitcr (1989), Et-Sherik r 9 t rd (L992)], enhanced hvdrugcn si~lubilitv
and diffusivih [Gleitcr (1989), PaIumbo (1992)l and increased elcctricai rcsistivitv [Glcitcr
(2989), Aus L Y ~ (-1994a,b)].
As a result of these unexpected or Jifferent properties, nanocryçtallinc nic1tcrials
have beyorne the subject of a peat deal of interest, tndeed, in the two riecades sinw its
introduction, research in this area is stiII experiencing tremendous growtti.
Introduction
1.5 Synthesis of NanocrystaUine Materials
Nanocrvstalline materials are cssentidlv non-equilibrium structures. As ,i rcsult,
the production of these niaterials often involves apprciaches and techniques Ihat opera tc lar
h m cy uilibrium. 5inc.c the devclopment ci t the iirst svnthcçis ttxchniy uc 1lcvt.1c)pct.i h!.
Clciter, iithcr non-equilibrium techniques have bem utilized to prcirfuce mctals, alluvs,
ccraniics, semiconductors and corn positcç in nancicnrs tnllinc Eorms.
1.5.1 lnert Gas Condensation and 0 t h - Techniques
Thc firçt teclinique JcveIoped bv Gltliter t» pruducc nancicrystallinc matcrials was
the incrt gas condensation tcchniquc. Figure 1-6 shows a schcmatic ~ l i q p m irir ,in
appamtus uscd in tliis tccliniquc
The dpparatus consists of a vacuum chamber that contains the evaporation sciurc.c(s)
to bc u s d . Thc source is heatcd in an atmcisphcre of intirt gas (hclium (He) or x g m (Ar))
at CI partial pressure in the cirder ot 10-2 Torr and tiny crvstals are then torrneci in thc ~ C I S
phase, which arc collcçtd on a [iyuid nitrogtln (LX:) coolrd culd finger. Subscqucnk
ssra ping and consdidiiting is needed in order to obtain the nanocnrstallintl scilici. When
using hh'o or mure sources ilt the same time, nancic~stailine dliovs can hc prcirluced.
Thc inert gas cundensation technique marked an irnporL.int devt.Iuprncnt in the Ciel3
of nanocrystaiiine rnatcrialç. However, this tirçt technique has a numbcr r i t sliiirtcomings
and [imitations, the mus t important of which include:
(i) the technique is capital-extensive;
(ii) the comyacted solid retains porosih. (odv 75% to 9590 theoretical Liensitv);
(iii) t h e is Little control over the shape ot the Fina1 product.
Introduction
VACUUM PUYI'$- I l I
Figure 1-6
A schcmatic diagram showing an aFpirah1~ uscd in the inert gas crmrlcnsation tcchnicpt. to
produce nanoc~stallint. matcriais [SiegcI and Eastman (tYX9) 1.
Sincc thc introduction of the ïnert gas condensation technique, n imv otlicr n~cthriJs
were intruduccd t» ovcrcome some of the shortcomings inherent to this svnthcsis mcthu~l.
Somc of tliest. inclurfe sputtering [cg. Tsoukatos PI' ri/ . (1994)], ball-milling [cg. LI 1.1 IL!
(199311, rapid solidification [cg. Davies ~ V i i i ! (1Y93)I m d sol-gel prucessing [cg. ShuIl m c i
Bennett (1992)l. However, some of these aItemate techniques have Jisacivantagt's as wdl.
Sputtering and sol-gd techniques are rttlativelv inexpensive techniques, Eut thcv dre
Introduction
Synilirsis dnd Slicrostmitural Chdrxterizùtion of Eltitrodcposited Sùnoc~std i inc Suit bLtgnrts
difficuIt to control. In addition, sol-gel processcs are limited primarilv to the production of
ceramics, nthcr than rnetals and a1lws. Baii-milhg is also rather cconomica1; hritvever.
after prciccssing, the proctuct iç in pocvdcr hrm. As a rcsult, seconclan. con~olid~~tiiin
pruccsws arc still necessacy. tn addition, because of the nature of the technique,
contamination ot the product bv the niiiiing quipment is almost unavoidablc.
1.5.2 Electrodeposition
Anrithcr tccliniquc. lhdt c m dirni~~~itt! s ~ m e cif the drawb~cks ut tlic cifcircnwnticiiid
n,incicrystd synthcsis techniqucs is electrrid~psition. In compdrisun tci the various cithcr
procluction mcthods usccl to obtain ntinotrystaliinc mdterials, elcctrudcpcisitim is much
bettcr suitccl for large-scaIc industrial ,~ppIications, this bcing hascd upcm rts d ~ d n t ~ ~ g t ~ s tif
control. Clexibilitv, low cost and tugh rate oi production [Erb 1.f (lY93)l. Als~),
eIectrocteposition techniqucs c m be optirnized ter prvducing materials tvith different gnin
size anet shape.
The specific advmtages of electrorfeposition processes inclutle:
(i) the numbcr of potential materials (metais, ùtIoys and composites) thd t c m bc. nanii-
proccssed bv eltxtrocfeposition is v r - Iarge [Erb (1995)j;
(ii) continuous (strip-plating) a d batch (rack or barre1 piating) pr~icessing art. both
pssiblc;
(iii) CI 100% ~1ens.e end product can be obtdined tvithout secunclan prricessing stcps;
(iv) there arc fcwcr shapt. Iimitations;
Introduction
(v) al10y production is casier to control when compareri to somr of the cithcr
tcchniqucs;
(vi) the end product can be in f o m of a coating or bulk material;
(vii) ceramics can be codeposited in a nanocrystalline metal matnx cumpositc;
(viii) the sctup wst is low; existing plating lines can be morlified tri producc
nanocnrstalIine materials [Wood (1995)l;
(ix) transfcr ot the ctcveloped technologv
prortuction phase is relativelv easv [Erb (
from the resciircli stage tii inriustricil
L995)].
Becausc tif thcse cdvantagcs, in particular the fact that maicricils with tuil Jcnsitv
(i.e., negIigibIe porositv) can be produced, electrodcposition wùs sclcctcd '1s tiic
experimental tcclmique in the present work.
1.6 Theoretical Understanding of Electrocrystallization
Ln eLcctrodcpcisition prucesses, the theoretical understanlling rif crvstcilIitc tiirmatliin
is not cdsilv undcrstoud. This is because ot the intririsic complexih' uf sr~lutiun clicmistrv m
gcncral, and the numcruus operathg parameters wliose c k t s im thc ~icptisits x c niit
mutua& independent, whether eltxtrolvsis is camed out using direct current (D.C.) cir
pulsai cvrrent (P.C.) tcchniques. However, given the cidderi variables c~ssiiciatc~t tvitli P.C.
electrdeposition, tiie theory behind it becomes considenblv more intricatc.
During eIectrodcposition, the metaiiic sperries go through a rnuIti-stage process
before d Jeposit is eventually obtained. In p ~ c i p l e , the Jeposition process a n bc. broken
down into the following steps:
introduction
a driving force is introduced into thc sustcm (plating ceII), usuah in thc 6mn i i f ùn
impwci pcitentia1 Ieacling to a currcnt flcirv (D.C or P.C.);
m a s traiisfer -diffusion a€ metaIIic spcies h m buIk soIution to the vicinih' oi thc
ï a thocic (nega tivc tcrmina1) surface;
rerfuctiun of the metaIiic sp t~ i e s h m pcisitivt. to the cIcctrcinicallv neutriil statc;
dsorytion of the spccics unto the cathode;
,ictuiil ~tcpr)sition - cithcr nucleating ncrv grains or cimtributing hi t1w grciwth iii
cxis ting grains.
It is quitt. dt'ar that thc r1ectrciclcpcisition proccss is made up iii quite '1 numbtbr r i t
iridiviclud stages, anci wi thin t ' x h stage a rn ther complitx tcd pruccss tCAcs pIciiv.
Althuugh numcrrius pubIica tions art. availablt. to Jcscribe the prriccsscs thd t tnkc plxc. J t
the ca tkdc suricicc, rcpiirts pcrtaining to tlic thtiurctical ùspccts uf electrudcpiisitiun c i t
nancicrystals arc limiteri. In L980, IbI pubIisheJ a rather cmnprchcnsivc ùrticlc iin the
theoreticai dspt'cts ot' P.C. eIt'ctr~Jt'pcisiti~n [Ihl (148U)I. XIso, in ppers publislicd tiv
Pui~pc , qunlitiitivc exphnation of the phenurnena at the cathode surtùïc w.is givcn
[Puippc' ( 198ba)I and thc effccts of: various uperating prame tcrs on elcc trricrysta1lizatiur-i
urere ~lissusst.ii [Puippc (19Sbhjj. Foliorving thcse gentml clppr~dch~s, an attcmpt wt1l
made to dctclil thi! steps invoIvecl in eIectroJtpsition. Xreas of interest with respect tri thc
t'omatiun of nancicrystalline electrodeposits wiU then be pointd out.
For ùny rwction to occur, a driving force must be present. SimilarIy in
el~trodeposition, whtrre ehctrochemical reactions take place, a Jriving force is neelfecl. In
this casc, the Jriving force is applied in the form uf a potential ctifferen~e btitwccn thc
anode and the cathode-
Introduction
Since '111 metnls exhibit a potential (referenced to the standard hvdrogttn rcxtiun),
t» ensure elcctrricleposition, the appl id potenticil must be more negativc thm the potcntial
observecf if no extemal "hrce" rvas usecl- This is dcfincd as the overpotcntial. The highcr
thc overpotential, the liigher the iiriving force for the metallic specics in solution to bc
reducccf in an clcctrochcmical reaction.
The pottmtial ciifference behreen the anode and thc cdthod~ nc)t unlv sets ut1 the
appropriate ~iverput~mtid for elcctrodeposition tri occur, it ,ilso creatcs CI potcntidl gréclicnt
in the soiution, iausing the cations (piisitivc) to niuvc trwdrcls the icitl~ucl~ (ncg'itrvt*) ,incf
thc anions (negative) torv'irds the anode (positive).
Oncc the c-itions rcach the ~itthoelc wrtùce, thcv arc rcdclv to bc rcclucccl. Atter
rcceiving the nccded charge in the Row of eIcctruns from the cathode, it tvill becomc ,in
atom with no electronic charge. However, tlvcn at this stage, the ‘item is simply &~irh~cl
onto the cathocle surfcice, becoming xi adcitom. The addtom will travel '1 short clistmw
Liefore depositing onto the substratt! uf h e d h d d ~ elcposited laycrs. This distancc is
limitcct tiv the diffusion of the adsorbcd citom un khtt surface.
I t is cit this point O C thc dcpositicin procrss where either nucleatirm of CI nctu grain or
the growtli uf an existing grain occurs. During éIcctrodeposition, operating pdrarncters Ca l i
be acijusted t« optimize certain defusit qualitics of interest. In this study, the prima- icxxs
is grain size. A conceptualiy simpIe apprciùch tu producing srnaII-pincd clcposits r u s
cliscussed by Gabielson [19561. EssentiaIIv, it was reportcd thdt the grain size of ùn
electrorteposit depends on whether or not the svstem is activation-controllcd cir diifusion-
sontroiled.
In an activation-contro1h.i svstern, the c-ument density k Iow cotnpdrcd tu the
concentration of metailis species at the wtiiode surface. In other words, deposition is
dcpendmt on the "arrivai" of e l ~ t r o n s at thc cathodc. As a resuk, mtions wirl have ,i
rciatively long time tci miivc freelv about the crithde, "waiting" tri b r &pusittA. In
i~dJition, each depositd crystal is surruuncled by '1 large number of mctal1ic ions tu bc
Jepsitcd, hcnce producing a depiisit with to~irser grain silr.
In cuntrast, trir Jiffusirin-controiied svstems, the ntt&ttlrmining stcp in tlic
dcprisitiun process is the diffusion of cations to the cathode surface. This modc is ty piïaI '1 t
higli current densities. The ciepositiun ut the rnetallic ions is cuntrolIcd bv thc. rate tif
arrivai ul these species from the buik to the cathucle. In ddi t im. the dcptisitcci crystds do
not have an abunciance of ,idjasent metailic ions to "permit" thcir grotvth. As d rt'suit, ions
reaching the catliode surtace wiII get Jeposited tvithout much surfacc Jiffusicin as in the
activation-controlle~i casc. thcreby pro~iusing a finer-grained rieposit.
As sccn from thc qualitative liiscussirin, one factor that tvould prornlitc nucIc,itiun
and rctard grain grorvth is the surface ciiffusion experienicd bv an adatom hiitcm
Jcpositing ontri the cathodc. Thc use of saccharin, as sccn in the work cit ' El-Shcrik cm
nickel, inriicatcd this bath additive may furtition in just this way [El-Sherik (1993)I. In iact,
this phenomtlnrin was subsequentIv mocic1t.d bv Choo r f 111: [19951, validating this
hyothesis. i t shcdd I?e noteci h t the rcsuits prcsenteri bv Chcio tV d rvere the first to
svstcmatic-llv m d e I the dcpositiun l?chaviour in an attempt tu expiain the meci~anism iur
the electrodcposition of nanocrvstais. Hot\-evcr, the analvsis, thougli lugiml and quite
pwdictive trw the case of nickel eIectro&position, camot be easiiv transferrcd tu utlier
eiectropIating systerns.
From some rif the concepts prcsented in this section, it ts apparent that the ~Icsign ot
the piating bath as wetI as the openting conditions are both extremclv important in tiw
electrodeposition ot nanticwstaliine materiais. Care must bt! taken to ensure that the bah
Introduction p- 19
Syntlwsis .inci XlicrostructunI Chardcterr~idon rif Electrodeprisitcd N~nocrystrillinc Soft i h g n r h
bc Jesigned with ciiffusion-controlld deposition mechanism in rnind. hlorcwer. the
intelligent use of surface-active agents, such as saccharin, may prove beneficial.
In summary, the understanding of electrocrystallization hùs receivcrl a great deùl of
attention ovcr rccent ycars, anci somc of thc funclamc.nta1 theories that erncrgerl swm to hc
sufficicnt for pmctical purposes. Howcvcr, when examining thc prucluction ut
nanocwstallinc mùterials using electrodeposition techniques, the theoretical bdsis for the
mechmisms involved rcmains empirical. Ln orrfer to turther our understanding in this
arca. a thorough electrwhemical studv ot the pl'iting solution involving mmy v,iridd~*s
and thcir dvnamics must be camecl out. However, this is bevond the scope of the prcscnt
research.
1.7 Research Strategy
This work is concemed with the devclopmcnt and producticin tif suft mùgnetic
materials that pusses nanocrystalline microstructure, From prcvious wiirk in thc.
~rociuction of nanocwstalline materials bt. electrocleposition processes, uperiting windows
t'or the svnthcsis of nanocrystallinc pure nickcl have ,ilrttacIv h c n cstablishcd [El-Slicrik
(1993)l. t-luwever, as a soft magnet, nickel is far from ideaI bccausc i t satumtcs
rnapcticallv at a venp low value of 0.6T. NcvcrtheIcss, the expericncc gùincd f r ~ m
previcius work involving nanoqstallint. nickel provideti an excellent basis for lurthcr
electrotieposition studieç. Chapter 2 wilI dcscribe the general expcrimcnta1 ipprï~ach
t.mpIovcci throughout the present work,
Introrluction
A Irigical Cirst step would be to increase the saturition magnetizùtirin of
nanocqstallinc nickel. One method is simplv to allov nickt.1 with dnothcr magnetic
element. PermdIoi@, 'it a nominal compusitiun of 80Ni-2UFe, is bcst knocvn for its hi$
penneabilitv as a soft magnct at '1 saturation magnetization 01 8.6 x 10jA/m (I-IT) [Bozurth
(2978)I. In view rit the large te~hnofogicaI importance of Pt.rmalIov@, an attcmpt cvill bc
mc& tu inciirporate iron mto nickel hv electrodepositicin. whilc prcscrving t h
naiicicrystallint. microstructure in the rieyusiteci ,illov. The gaa1 of this pirt ot the w~)rL. will
bc to produce nmocrystal1ir.c nickel-iron cillovs cvithin a compositird rmge cwcring t l i ,~t
of PermaIlovB (70wt.'':1 Fc), This task is the ~ U C U S of Chnpter 3.
The next step in achieving the icied sott rn'lgnct is tc~ mwe in thc clircction rit LI
highcr sc~turation magnetization. Cobalt h'is becn chosen as the logical subsequcnt stdgc ot
studv. This is becduse pure cobdlt saturates mL~gnctically at a highcr valuc (1.bT) wlicn
cornparcd witli bath pure nickel and nickel-iron al[oys. .b a tonsequcncc, tlic ncxt seetton
ot this work rvill bc cievotcd tci the Jcvcl»pment of an electrocfeposition proccss tci
svnthcsizc nc~nucrytallinc pure cobalt. This tlnJeavour comprises Chapter -4.
Thc tinaI stcp of this research strate^ is to then athv the ncincicn.stLiIlin~~ cribdt
with iron in ordcr to further inaease the saturation mdgnetization. Htlnctl, ChLIptcr 5 dcals
tvith Hie ~ievelopment of nanocrvstaiiine cobalt-iron cilloyç.
Aftcr the prcsentation of the individual svstems in seif-containeri cliLiptcrs, cLicli
with a littiraturc revicw, spec-ific experimenta1 Jetails as wdI as results and discussion,
genenl concIusions cvill be presented in Chapter 6.
tt shouIri be noted that this thesis deaIs mainiv cvih the Jevelopmcnt of suitalA~
techniques tu produce the magnetic materidis outlined above- The ïhdractcrization of
magnetic properties such as magnetization and coercivity were o u t d e the scopt. of this
Introduction p- 21
Syntliesis dnci .Iliçrostructuni Chançterization of EleçtrodepositeJ Ndnocystiiliinr Soit Xl.ignrts
research. However, after establishment of the prucessing windows for each group of
nanocrvstalline matcrials. samples in sulficient quanti? for magnetic chxactcrization in
another Ph.D. research program, cxrried out by MJ. ALE at Queen's Univcrsitv, wcre
produceci. in fact, the work of M-j. Aus m d the present work constituted two scparatc
research progranis conductd in panlie1 as part of a major reseàrch initiativc on tlic
clevel~~pnicnt of nmcistructured soit magncts. For tomp1t.teness in prcscntatiun, sumc c)t
the results cibtdincd by rL1.J. Xus wiIl be bRefIv discussd in Chapter 7 of this thesis.
Rcconimcnciiitions for future rescarch in the devclopment of ideùl suft nicigiietic
matcrials rvill 'ilsti be given in Chapter 7.
introduction
1.8 References
D.R. Xskcland, ;r/) . 5 7 ~ w i 2 * if,,/ fk@lr~r~qr o,f..L/r~f~~/ïidz, 3rd S.[. cclition, Chripmm
ancl Hall, New York ( 1996).
X1.j. Xus, B. S~punar , AM. El-Sherik, Lr. Erb, G. Palumbo and KT. Xust, "bIcignctic
Prr)ycrtics of Bulk Nanocrvstaiiinc Nickcï'. S7;rjv'u ib/L'f~///~/<pL/ rY . lhfr~/dl; l . \'d.21
( IqW), p.Ib39.
iL1.j. Xus, B. Szpunar, Li. Erb, X.M. El-Sherik, G. Palumbo and KT. =lust, "Electrical
Rcsistivitv of Bulk ,\i~ncicrvstallinc Nickd", ~ ~ ~ / ~ I I I I ~ ~ I # ; - I ~ ~ ~ / I ; ' I / P ~ I I . ~ I L : ~ ; Vri1.75 ( IYC&),
No.7, p. 1.
h1.J. Aus, B. Szpunar, U. Erb, G. Palumhi and K.T. Xust, "Electrical, Magnetic dnd
Mechanical Propertics u t Nanocn*staIline Nickel", .Lhft.lï~~h. R : + i ~ ~ r ~ f i jr)~7t'/l/
~ ' I / I I ~ I . W / I / I Prulri'~~fi/p, Vol. 3 18 ( 1994b), p 39.
RA[. Bozrirth, A . ~ ~ ~ ~ I I ~ ~ ! ~ ~ I I L * ~ I : ~ I I I , [EEE Press, N t w York (1978).
H.X. Davics, A. Manat', M. Ltionowicz. P.Z. Zhang, S.J. Dobson mcl R.=l. Bucklw,
"Nanocn~stallinc Structures ctn~I the Enhùniemcnt of Remmcncc ,in3 Encrgv
Produit in McIt Çpun Iron-Rarc Earth-Buron iVlovs for Permanent ;LLignt.Ls",
Km)>?ru~Yz/m/~ Mrfixmh; Vo1.2 ( 1993), p. 197-
AM. El-Shcrik, U, Erb, G- Palumbo and KIT. Aust, "Deviations Fnm Hall-Pctch
Bchaviour in As-Prepard Nanocm ta l l ic Nickel", Sirlifhr, Lkttd/r/<pif ( Y . L M t TIII~II,
VoI.27 (1992), p.1185.
A M El-Sherik, Ph.D. Thesis, "Svntheçiç, Structure and Propcrties d
Nanocrvstallinci Nickel", Department of hIaterids and iLIetal1urgic;il EngineerÎng,
Queen's Univcrsils, Kingston, Ontario, Canada (1993).
htro Juction
U. Erb. " EIcctrodeposited Nanucns tals: Svnthesis. Propertics and Indus trial
U. Erb, X.M. El-Sherik. G. Palumbo and K.T. Xust, "Synthesis, Structure and
C. G,ihiclson, "Production of Smooth, Fine-Gnineci Electro~lcposits", . ! L M
~ % I ~ I ~ ~ I I < ~ ~ Vo1.3 ( L%), No. 1 1, p . 3
S. GcingopacIhvav, . - G.C. Hadjipanavis, B. Dale, C.M. Sorcnscn and KJ. KIc~P~i-idc~,
" ht,ignctisrn of Ultrafine Particles", . L ~ ~ ~ s ' ~ ~ I / L . ~ ~ I I z ' ~ / I I I I I ~ L ' I ' I I I L ~ , Vol. 1 ( 19W), p.;.
Fkirscwcll, and H. Lilhott (ods.), Riso National Laborcitory, Dcnmark (108 1), p.15.
p.371.
W. Gong, H, Li, Z. Zhao and J. Shen, "L:ltrafine Particles of Fc, Cu a n ~ l Ni
Fcrromagnetic MetaIs", ~ T ~ I ~ I I I I / / ~ : - I ~ ~ I / ~ ; ~ / P / I ~ / ~ ~ ~ S , Vol.69 (199Z), No.& p.3 L 19.
H. Hattendorf, "Nanocrystaiiine Slagnets", intemal Report, Kmpr-VDhI GmbH
(1995).
G. Herzitr, "Grain Size Dependencc of Coercivihr and Permcabilih- in
introduction
N. Ibl, "%me Theoretical Aspects of Pulse EIectrolvsis", Si~rfri~z. % t h i / l q r r / , . . VoI. IO
(198U), p.8'1.
hl. Li, R. Birringcr, W.L. Johnson and R.S. ShuIl, "Nanoaystallinc Fe-Si Pliase bv
Mechanical A ttri t i m and its So tt Magne tic Properties", iLi1110~7ru~f~/nd . L4/kri1rI<,
v0r.n ( iwn), p.407.
G. Palumbo, S.J. Thorpe and KT. Aust, "On the Contribution of Triple Junctions tu
the Structure mcl Prcipertics of Ncinoi~stall ine Matcrialç", 5iïïjl/1 ; M Y t / / / f / ( \ i i , 1.t
;lfi/krt;/hi/, Vol24 (19YU), p.1347.
G. Palumbo, D.M. Do+, Ab[. El-Shcrik, U. Erb and KIT. h s t , "1ntercrystdIin~
HycIrcigcn Transport in X~ncicn~stdlitie Xickel", ky i~hi iLA,h///~~/,ym/ . L M ~ ~ ~ ~ ~ h ~ ~ ,
Vul.25 ( l Y Y Z ) , p.679.
R. J. Parker, .-Mvmw A 'r~mimwf ; C ~ ~ ~ ~ ~ ~ / ~ ~ ~ I L JO hn \ V i l q and Sons, Inc. ( LY 90).
JC1. Pu ipp , "Qualitative Xpproach tci Puhe Plating", m/ fmYri5' t~l'Prd+'
Phth(; J.CI. Puippe and F. Learnm (d~.), .-\merimn Electroplatm 'ind S u r t i u ~
Finishers Socie tv, Floriiia ( l'%a), p. 1.
J-C[, Puippe, "Idluence ot Puise PIating on CnstaLationW, ;r/rmrv md fn/~71;i' rV'
/'/~rI*i. Phth<\; J.CI. Puip pe an J F. Leaman (eiis.), American Electro p Ici tcrs mrf
Surfacc Finishers Socich., R o d a ('19Sbb). p.17-
H.E Schaefer, H. Kiker, H. kunmuller and R. h'ürschum, "Magnetic Propcrties of
Nanocrnstaiiint. NickcI", :Vi~~iuh'fri~~'f~md; th~tmizk Vol.1 (1992), p.533.
R.D* Shull and L.H. Bennett, "Nancxromposite hlagnetic ilIatt.rials", ; V / / / / i ~ > h / ~ f ~ / r i i t '
A.4~hni;k, Vo1.L (-1991), p.83.
Introduction
R.W. Siegel and J.A. Eastman, "Svnthesis, Characterization and Propcrtics of
Nanop hdsc Ccramics", Mlrr/;rk R w ~ r i f i 5;>cï2fy S,trpwli/ttrt Pro~rr.rr'j<p, Vd. 132
(LYS)), p.3.
B. Szpunar, U. Erb, G. Pahmbo, KT. Aust and L.J Lewis, "blagnetism in Cornplex
Atomic Stnicturcs: Grain Boun daries in Nickel", Phpi~.cr/ /l'miro 6, Vo1.53 (~19Yh),
?do.!), p.547.
A. Tsoukii tus, H. Wan anci G.C. Hacijipanavis, " Microstructurc d n J h1,ign~tii
H i fs tcrcsis 0 5 Co-Cu Films", ,L;IIcLIs~~I~L~~/I'L'~/:~/II~L*~I~~L~, VoI.4 (l994), p.49.
h1.X. CVhitc, Prrprthr. rblI.ht~~Ir/ll;, 0xI'i)r~t üniversihr Press, New York (LC)99).
D. Wcwcl, " Ndncicrvç talline EIcctrodepc)si ts", presen ted a t .Vu~o.+-trr/~;tr/n ?/ Chh T I I I ~
,trrl/Gt~ttli<~~.~ YÿJI Gorham / Lntcrtcch C~insuIting, Atlanta, Georgia (1995).
Y. Yoshizatva, S. Oguma and K. Yiirnauchi, "New Iron-Based SoCt Magnctic r \ I l l ) ~ ~
Composcd of C;ltntint. Grain Stmcturc" , /I)mrti/ of riky~hid Ph~/s.il;:s, V1iI.h-t ( 1988).
No. LU- 1 1, p.6UM.
Introduction
Chapter 2
Experimental
This chapter describes in generai terms the experimental approach used in this
research as weii as the techniques appüed for the synthesis and characterization of the
materiais produced in this study. System-specific experimental parameters (Le. bath
composition, pH, temperature, anode and cathode materiais) will be given in the chapters
d e a h g with the various materials svnthesized in this thesis.
2.1 Experimental Approach
Electrodeposition is the experirnental methodoIogy used throughout the present
work to produce various nanostructured materiais, in electrodeposition, an electrical
current is passed through an aqueous eiectrolyte that contains metaiiic ionic species. The
flow of electrons between the anode (negative terminal) and the cathode (positive terminai)
causes reduction reactions to occur at the cathode, thereby recovering the metal(s).
Depending on the various operating conditions, the resultant electrodeposit may have very
riifferent structurai characteristicç, çuch as composition, grain s ù e and shape,
Synt!!esis and .Clicrostructural Characterization of EIecirodeposited Nanocryshiiine Sort LIagnets
qstallographic textures, and hence, its properties might aIso varv sigruficandv [Dini
(l993)I.
Using electrodeposition methods, a number of experimental variables are available
for use. In the most general t e m , electrodeposition techniques cari be classified into two
categories, namely, direct current (D.C.) and pulsecl current (P.C.) electrodeposition.
RegardIess of whether D.C. or P.C. electrodeposition techniques are adopted, a number of
processing parameters may be used in order to optimize the operating window and
thereby faditate the control of the desired product properties. in this studv, the end result
of obtaining electrodeposits that possess a nanocrystalIine microstnicture is the primary
goal of varving the plating parameters, although attention is also given to certain other
microsûuctural characteristics as weil. in Sections 2.2 and 33, the general principles of
D.C. and P.C. electrodeposition wiii be described.
2.2 Direct Current (D.C.) Electrodeposition
In direct current eiectrodeposition, processing parameters avaitable for deposit
contro1 inchde the compositian, pH and temperature of the ektrolvte, the amount of
agitation O € the soIution as well as the current density to be used during electrodeposition.
Figure 2-1 shows the invariant nature of the current densitv used throughout a D.C.
experîment Since there is only one electrical processing parameters assoaated with D.C.
pIating (Le., current or current density), the success of any experimentai design that a b to
optimize the h a 1 deposit charader WU depend primarilv upon the proper control of the
elec trolvte composition.
Synthesiç and Microstructuraf Characteriution of Electrodeposited Nanocrysîailine Solt Magnets
Cwrent t Time
Schematic diagram showing the invariant nature of direct current density as a function of tirne.
- D.C. Powu Snpply
Schematic diagram showing a @ i d D.C. eiectrodeposition experiment setup.
Synthesis and Microsûuctural Characterization of Electrodeposited Nanoc~staiiine Soft Magnets
For the vanous systerns studied throughout this project, the D.C. electrodeposition
approach was utilized for the nickel-iron and cobait-iron binary systems. Figure 2-2 shows
a schematic diagram of a generai experimental setup using the D.C. electrodeposition
tedinique. The vesse1 containing the eIectroplating solution was either a 1 liter or 3 liter
beaker, which was situated atop a magnetic stirrer / hot plate. The magnetic stirrer kept
the solution agitated when desired, while the hot plate maintained the bath temperature at
a constant level at ail times.
in a D.C. setup, the current is supplied by a D.C. power supply, where the positive
and negative terminais are attached to the cathode and anode, respectivelv. The current
was monitored using a digitai mutlimeter (DMhll). The anode was usuallv made up of
electrolytic metal pieces, contained in an inert titanium (Ti) basket, whose purpose is to
replenish the bath with metal ions during reduction of the ionic species at the cathode. The
cathode was a square piece of titanium (2cni x 2cm unIess othenvise stated) embedded in
bakelite. Electrical contact was made by attaching a glass-covered steel rod through the
bakeiite, Figure 3-3 shows the schematic diagram of the cathode design [El-Sherik (2993)].
It is important to note that both the cathode and the anode are stored out of the
solution between plating euperiments. This is essentid because the cathode surface needs
to be cleaned and degreased each tirne before commencing an experiment. in addition,
since aii the sohtions used were aadic in nature, the anode basket was only submergeci in
the solution during eIectrodeposition, after which it was removed to prevent metai
dissoIution. This practice was to ensure that the composition of the platkg sohttion
remains under control, and more impor tan l no unintentionai changes occur.
Synthesis and Microstmctural Qiaracterization of EIectrodeposited Nanocrystalline S o f Magnets
Figure 2-3
X schematic diagram detaiüng the cathode design used in this work FI-Sherik (2993)].
2.3 Puised Current (P.C.) Electrodeposition
In pulsed m e n t dectrodeposition, the appiied current is no Ionger constant with
time, as is the case with direct current electrodeposition. Because of this ciifference, the
number of eZecû-icai p r o c e h g parameters avaiiable for pulsed current electrodeposition is
inmeased. in addition to *ose mentioned for D.C. eIectrodeposition, operator-controUed
Synthesis and S[icrostnictural Characteriration of Electrodeposited Nanocrystalline Soft Magnets
parameters now include the period of time when the current is applied (TCIN), the period of
time when the current is shut off (Tom), and the peak current density (IP,s).
Figure 2 4 is a schematic diagram showing some of the puise plating parameters
avaiiabfe for control. AIthough not shown in the figure, two other quantities relating to
P.C. elecbodeposition are also of some importance: the average current density (I,,,,) and
the d u l cycle (O). The dutv cycle is the percentage of the time when the current is on
during a m e n t cvcle, which is defined as:
Eq. 2-1
ïhe average current density insücates the "D.C." equîvalent of the peak cwrent
Jensity with the particular TON and TQFF, which may be defineri as follows:
= IPO& X O Eq. 2-2
Current t
Toff Ton Time
Fime 2-4
Schematic diagram showing the time-variant nature of a puIsed current experiment.
Synthesis and hticrostructuial Chamcterizatian of Electmdeposited Nanocrystaiiine Saft Magnets
Figure 2-5 is a schematic diagram showing a -pical experimental setup for the P.C.
electrodeposition process. Similar to the experifnental setup described for D.C.
electrodeposition, the beaker containing the ptating solution rests on a hot plate / magnetic
stirrer. The cathode and anode are treated in the same way, i.e. they are both removed
from the sohtion when experiments are completed. The primary riifference Les in the
power supply. in P.C. electrodeposition, the power supply is capable of producing
interrupted current in shapes shown in Figure 24. in the current study, a Puise Star puised
power supplv manufactureci by PWR, Wisconsin was used which deiivered a maximum
current of 30 amperes a t 20 votts.
anode 1
Magnetic StIncr (Hot Plate)
Ammctu P.C. Powu Supply
A schematic showing a typicai sehrp of a P.C. electrodeposition experirnent.
Experimen tal
Synthesis and Microstructural Characterimiion of Electrodeposited Nanocrystalline Soft Magnets
2.4 Anomalous Codeposition
in electrodeposition, the properties of the deposits may be a strong function of
certain operating variables. For example, without adequate buffer in the solution, the pH
ïncreases that occur near the cathode surface during electrodeposition as a result of
hydrogen evolution may result in the formation of hydroxide, the inclusion of which in the
deposit rnay adverse- affect its properties, in a single component plating system, the main
foms is often the quality of the deposit, as weli as the optimization of certain other
properties. However, in an alloy system, reaching the target composition range(s)
normally takes precedence over other plating consicierations.
in a typical alIoy-plating electroryte, more than one metal ionic species must be
deposited. It is reasonable to expect the composition of the aLIoy deposit to correspond to
the ratio of the metallic species in the solution. &O, it is Logical to assume that the more
noble of the components would be preferentiaiiy deposited over the other less-noble
speaes. However, experience proves that these assumptions are not necessary true for
most aiiov . svsterns * involving the üon group metais, e.g. nickel-üon and nickeltobalt
aiioys. This phenomenon is termed anomalous codeposition and was first f d y described
bv Brenner as the process by which the less noble of the metallic species is deposited
preferentiaiiy during d o y electrodeposition [Brenner (1963)l.
Of the systems to be studied in the present work, two of the three systems exhiiit
anomalous codeposition: the nickeI-iron and the cobalt-iron systems. Reports pertaining to
this phenomenon can be found in the Iiterature. in systems where the exchange current
densities of the components differ greaîiy, as in the zinc-nickel -stem, the kinetic activities
become the controiling factor in determinùig the reiative rate of deposition of the
Synthesis and ~IicrostructuraI Characterization of Etectrodepositeci NanocrystaIIine Soft Magnets
individuai metallic components [Brooks (1998)I. For thiç patticuiar example, the exchange
current density for zinc has been reported to be five orders of magnitude higher than that
of nickel [Mathias and Chapman (1987,1990)] and it is this characteristic that "destroysJ' the
themoilynamic nobiiitv of the nickel relative to zinc.
However, other explanations for the anomalous codeposition medianism have &O
gained widespread acceptance. Gangasingh and Tdbot, for example, studied this
phenomenon in the nickel-iron system [Gangasingh and Talbot (1991)]. It was suggested
tfiat a locai pH rise at the cathode surface due to hydrogen evolution causes the formation
of iron hydroxicie. This hydroxide formation and its adsorption on the cathode surface is
thought to suppress the reduction of nickel and promote the discharge of the iron (Fe)
ions. Essentiaiiy, thiç behaviour c m be Jescribeci as a cathodic çhift of the nickel
poIarization cunre [hcbicacos c t d (1989)j whose magnitude inmases with the amount of
agitation. hfodels for this mechanism have also been proposeci [cg. Hessami and Tobias
(1989)]. From this viewpoint, it is genera- accepted that the anomalous nature of
electrodeposition in the nickel-iron alfoy system is a mass-tram fer controiied process,
There are few publications in the fiterature ciirectiy ùivolving the studv of khis
phenomenon in the cobalt-iron system. dthough anomalous codeposition is &O observed
in the cobalt-iron system, it is not as not as pronounced as in the nickef-iron svstem. Once
again, a review of previous studies provides arguments for both of the aforementioned
explanations of anomaious codeposition. in a study conducted by Bertazzoii and PIetcher
in 1993, it was found that the composition of the ailoy deposits Jepends prirnariiy on the
ratio of the cobalt to üon ratio in the solution pertazzoli and Pletcher (1993)]. This
observation appears to confirm that the kinetic parameter of exdiange current densihr mav
play o d y a minor roIe in this aüoy system. However, it has ako been reported that this
Synthesis and hlicroshctural Charactecïzation of Electrodeposited NanocxystaILine Soft bfagnek
phenomenon ceases to exiçt when elctrodeposition occurs in electrolvtes with
temperatures above 80°C [Brenner (1963)]. in this instance, such an observation of a
temperature dependence of deposit composition may offer evidence for the kinetic
explanation of the anomalous codeposition mechanism [Roev and Gudin (1996)I.
Therefore, it mav be concIuJeJ that while both of the aforementioned theories have gained
widespread support, the exact mechanism by which anomalous codeposition occurs
remains a matter of ciebate.
2.5 Characterization of the Electroderrosits
AU electrodeposits produceci in this shidv were characterized in terms of surface
rnorphoIogy, grain size, microstructure, cr).stallographic texture, composition and
miaohardness. The surface morphoIogy of the eIectrodeposits was studied using a JEOL
JSM-840 scanning electron microscope (SEM). ha iys is of the deposit composition, in the
case of the ailoy systems, was carried out using an energy dispersive X-ray spectroscopy
(EDS) svstem attached to the aforementioned eIectron microscope. The EDS system was a
Tracor Norkhern mode1 TN-5500, equipped with a lithium-doped silican detector using a
12.5pm ttUck bervUium (Be) window. Using this equiprnent setup, the Jetection k t is
about 0.1% while elemental analvsis Jown to and inciuliing materiais with atomic number
(S) of 11 (sodium) was possible. Ail data presented in this study regardhg cornpositiona1
anaiysis using the SEM/EDS was an average of at least ten JiDS readings.
X-ray diffraction was canied out using a Rigaku D-Max lOClO ciiffractometer with a
8-28 geometry. The diffractorneter waç equipped wîth a copper tube and a graphite q s t a l
monodiromator in the diffracted beam generating diffraction patterns using Cu-Y,
Synthesis and Microstructural Characterïzation of Electrodeposited Nanocrystalline Soft Mapets
radiation with a wavelength of 1.54Lk (K, ,~wg) . CrystalIagraphic texture (preferred
orientation) was detennined using oniy the 8-28 geometrv and not by compIete pole figure
studies. This rationale was based on the observation previouslv made by Mchlahon and
Erb LI9891 that electrodeposited nanocrysta1s have fiber textures with certain planes
parder to the substrate materials, but they do not e-uhibit any preferential alignment about
the plane normal. As a resdt, the 9-20 diffraction geometrv was deemed mfficient to
de termine the stronges t Eiber texture component(s) .
Determination of the grain size of the etectrodeposits was camed out using X-ray
line broadening, utilizing the Scherrer formula in measuring the F d Width at Half
Maximum (FWHM) of a selected liiffraction peak (see Section 76). The grain size of some
of the eIectrocieposits was ais0 determined using a Philips CM-20 scanning transmission
eIectron microscope (STEM) operateci in the transmission mode. The acceleration voltage
used was 200kV. Brightfield (BQ and darkfidd (DF) micrographs as weU as eiectron
diffraction patterns were obtained for samples examined in the TEM Grain size
distributions were generated by measuring at least 150 grain diameters directIy on TEM
Jarkfield micrographs by the iinear intercept method, A grid is placed ont0 the negative
and the distance between the intercepts the grains make with the horizontal grid Iineç are
meamed as the grain size; the same is repeated with the vertical gri J lines.
Sampie preparation for transmission electron miaoscopy varied for the various
çvstems çtuciied- DetaiIs wiii be outlined in the respective sections dealing with these
sys tems.
Miaohardness meaçurements were done using a Leitz Vickers microhardness
indenter. The appiied IoaJ was varied from 50 to 200 gramç between the different senes of
eiectrodeposits, dependhg pr imdy on the thidcness of the eiectrodeposits. in aü cases
*
ExperimentaI p- 37
Synthesis and bIicrostructuraI Chamcterization of Elecfrodeposited NanocrystalIine Soft Magnets
the sample thidcness was at Ieast 20 times the depth of individual indentations. Exact
conditions for the measurements wiil be reported in the proper sections in later chapters.
Samples were epoxied ont0 a Bakelite mount More measurements were made. This
e m e d a proper support, thereby providing more accurate microhardness readings. It
shouid be pointed out that the reported microhardness value of each sample was obtained
from averaging at least 10 indentations.
2.6 Grain Size Measurement Using the Scherrer Formula
As the grain size of a material faiis below 1000A, the width of the peaks in an X-ray
diffraction pattern wilI increase [CuUity (1978)l. This phenomenon is known as peak
broadening or line broadening. Line broadening is commonIy used to measure the average
grain size of nanocrystaiiine materiais. Ushg the width of the diffraction peak of the
material of interest, the average grain size can be estimated using the Scherrer formuia:
d = 0.9A
B cos 0,
where d is the average grain diameter of the material (A);
h is the wavelength of the radiation used (A);
B is the tme broadening (radians);
OB is the Bragg angle (deg-rees).
2nd
Synthesis and Microstructural Characteriration of Eiectrodeposited Nanocrystalline Soft Magnets
where BM is the measured broadening (radians);
& is the standard (inûinsic) broadening due to equipment used.
It shouid be noted that peak broadening is measured at the Fuil Width at Half
Maximum (FWHM) in radians. Also, the true broadening (B) is dependent on the
equiprnent used. This is the reason it has to be corrected by measuring a standard peak
width (Bs). in practice, a polvcrvstaliine - - powder material with large grains (grain size >
O . 1 p ) is used for this purpose. Using a Rigaku D-Max 100 X-ray diffractometer, the value
of 3.8397~10-3 rad (0.22") adopted from the FWHM of the (111) peak of a nickel powder
diffraction pattern, at Bragg angle Be of 223" under Cu-K,, radiation (h = 1.54d), was used
as the standard, Bs, throughout the present work.
Figure - 6 shows the grain sue, as calculated using the S e r r e r formula, as a
function of the observed peak width (FWH&[) in the Rigaku diffractometer. XIthough the
rensitivity of the relationship is quite goad if the grain size is between IOOOA and 100A. the
sensitivitv is either too high or too low outside of this range. Since the reliability of the
relationship depends on the ability to measure the peak width correctiy, care shouid be
taken when extracting the peak width anci hence, interpreting the results of grain size
measurement using this method.
For comparison, the (111) peak of the nickel-iron d o y s were used to ascertain the
grain size whereas the (0002) peak refletions €rom pure cobalt were taken as the
cornparison peak. For the cobalt-uon system, the X-ray diffraction peak ciosest to the
diffraction angle 20 of 44.5" was used for this purpose-
Synthesiç and MicrostructuraI Characterization of Electrodeposited NanoqstalIine Soft Magnets
0.0 0.5 1 .O 1.5 2.0 2.5
Measured Peak Width (degrees)
Fime 2-6
Grain size as a h c t i o n of measured X-ray cliffiaction peak width (FWHhii)
as calculated using the Scherrer formula.
Synthesis and Microstnictud Characterïzation of Electrodeposited Nanocrystalline Soft blagnets
2.7 Ref erences
P.C. h~iricacos, C. h a , J. Tabib, J. Dukovic and L.T. Romankiw,
"Electrodeposition of Nickel-iron Movs. 1. fffect of Agitation", jormnf of the
E/ecfro~-/re~nIc~/ Su*, Vo1.136 (1989), No.5, p.1336.
R, Bertazzoli and D, Pletcher, "Studies of the hlechanism for the Electrodeposition
of Fe-Co Aliovs", Ei~~-fro~fiNrzicnAcfn, Vo1.38 (1993), No.5, p.671.
A. Brenner, E/r~~tru~Iupusffiot~ ufA//oys, Xcademic Press, New York (1963).
1, Brooks, "Synthesis and Characterization of Nanocrystailine SingIe y-Phase Zn-Ni
M o y Coatings", MSc. Thesis, Department of Materials and Metaliurgical
Engineering, Queen's University, Kingston, Ontario, Canada (1998).
B.D. CuUitv, E h ~ ~ r f OJ-X-Rq D@iitBon, 2nd edition, Addison Wesley Publishing
Company, Inc., Philippines (1978).
J.W. Dini, Efccfro~IrpositIo~z, Noves Publishing, New Jersev (1993).
A.M. El-Sherik, "Synthesis, Stntcture and Properties of Nanocrystailine Nickel" ,
Ph.D. Thesis, Queen's University, Kingston, Ontario, Canada (1993).
D. Gangasingh and J.B. Taibot, "homalous Electrodeposition of Nickel-Iron",
fo~~rnn~uffhe Ef~cfrn~-/rtrmi%~f Sont.&, Vo1.128 (1991), No.12, p.3605.
S. Hessarni and C.W. Tobias, "Mathematical Mode1 for homalous Codeposition of
Nickel-kon on a Rotaîing Disk EIectrode", jarmnI of th Ei'rcfroc/I~.~zicnf So&&,
VoI.136 (1989), No.12, p.3611.
ME. Mathias and T.W. Chapman, "A Zinc-Nickd Moy Electrodeposition Kinetics
Modd from niickness and Composition Meamements on the Rotating Disk
Electrode", fot~rnnf o f l e E f e d m d r m i c d S o ~ , Vo1.137 (1990) NOS, p.103
Synthesis and Microstructural Characterization of EIectrodeposited Nanoc~staliiie Soft Magnets
M.F. Mathias and T.W. Chapman, "The Composition of Electrodeposited Zinc-
Nickel AUov Coatings", / u m d of the Eiktrod~eniirnI Sane@, Vd.134 (1987) No.6,
p.1408.
G. McMahon and U. Erb, "Bulk Amorphous and Nanacrystalline Ni-P tUloys by
Elec trop la hg" , ~Mitmstr~~cti~rnI 5~7i.t-ILK Vol. 17 (1989), p.447.
V.G. Roev and N.V. Gudin, "New Aspects of Zinc-NickeI XUoy Codeposition",
Tri/~.m-ti~tis o f h his'iitid~ qfMetd fikzkhirzg, Vo1.74 (1996) No.5 p. 153.
Chapter 3
Nickel-Iron Binary System
3.1 Literature Review
Electrodeposition of the binary system of nickel-ùon has been studied
comprehensively, mainly because of its importance in the field of soft magnetism.
However, another reason for the interest in thk svstem is that the deposition of nickel-ùon
allovs demonstrates the phenomenon of anomalous CO-deposition (see Section 2.4).
There have been publications from as earIy as the early 2900s [e-g., Glasstone
(19W)I regarding studies conceming the electrodeposition of nicke1-iron alloys. However,
given the wide vanetv of pIating solutions and conditions employed throughout the earlier
literature, identification of the effects of speafic processing parameters on deposit structure
and properties is venr complicated. As a resdt, comparison between different studies is, in
general, rather difficult.
Since the thrust oE this part of the work focuses on the nickel-rich side of the nickel-
ùon bina^ system, in particular compositions near the 80Ni-20Fe benchmark, the Literature
review section will therefore concentrate mostiy on reports dealing with nickel-rich
deposits. That is, studies on the production of PermalIo@ type alloys will be reviewed.
Synthesis and Micros~ctural Characterization of EIectrodeposited NanocrystaIhe Soft Magnets
A survey of pre-1930 publications concerning nickel-rich Fe aiiovs shows that most
were dedicated to the prevention, rather than the promotion, of alioy CO-deposition as iron
impurities were considered defects in nickel deposits [Stdec (1969)l- However, there were
examptes that deserve atation. Glasstone and Svmes [1927,1928] produced, uçing a sulfate
bath, nickel-iron alioys with uon contents ranging from 20 to 57wt.C. Ah, in 1931, Burns
and Warner (19311 reported a concentration of 21wt.% iron in their electrodeposited binary
alIovs.
Throughout the 1940s to the 1960s, sulfate bath were most wiclely uçed (e.g.,
Aotani 119521, Wolf and McConneU [1956], Safianek [2957], EIsie d m ! [1966], Matuiis bd.
[1966], Giuiiani and Lazzari [1968], Khamaev and Krivtsov [19681), though mixeLi suifate-
chIoride ba th were emerging as the ba th of choice for some researchers (e-g., DuRose and
Pine [1944], Sysoeva [29591, Grilikhes and Sysoeva [1965], Koretzky [2966], Cisman rf rd
P967Ib
in these earlv works, some of the operating conditions varied a great Jeal. For
example, bath temperatures varied from room temperature to temperatures as high as 70°C
[Svsoeva (2959)] and 95°C [Koretzky (1966)j. Current demities rangeci from O.OIA/M-
[Cisman et d (1967)l to 40h/cW [Sysoeva (1959)] and aiioys with a wide range of iron
contents up to 94wrO6 [Safranek (1957)l were produced. Whiie nrch a generalized
compatison should not be taken out of context, it ilhstrates the point that in the eariy
investigations of this binarv system, few attempts to systematically deposit nickel-iron
d o v s within a spedic compositionai range were carried out
h the most generai terms, typicai nickel-iron plating soIutions Id Into either of the
foiiowing main varïeties: sulfate-based, chloride-bmed or mixed sulfatedoride-based and
damat+based b a h . Studies involving b a h containing a relativeIv Iarge number of
Nickel-Iron Binary Sys tem p. 4
Spthesis and ~Iicrostmctural Characterization of Etecbodeposited NanoctystaUine %ft Magnets
organic additives are also available. DjokiE and Maksimovif pubiished a topical review on
the eIectrodeposition of nickel-iron alloys in 1992 [Djokic and blaksirnoviE (1992)], and in
1994, hdricacos and Romankiw published a comprehensive paper induliing a literature
review of progress on the eIectrodeposition of nickel-iron aiiovs, specifically PermaUo@
type compositions, from the late 1970s to the early 1990s [hdricacos and Romankiw
(1994)l. The following is an expansion based on the literature reviews in the
aforementioned articles.
in 1978, Castellani rf (I/, produced PermalloyB type thin f h using a chIoride bath
[Castellani rt id (1978)I. Bielinski and Przyluski, in efforts to further the fundamental
understanding of the nickel-iron system, also successfuliy produced this binas. alloy
[Bielinski and Przyluski (1979a,b)l. The baths that were used, however, were formulated
riifferentlv than those utilized in most of the other studies; iron ammonium sulfate
(FeN-&(W~)~~nHzO) was used as a source of iron in the bath, in fact, this unusual iron salt
was associated onlv with the name PqIuski in the Iiterature review [B iehk i and
Przyluski (1979a,b), Przyluski and Madry (1981)]. Also reported in 1979 was Horkans's
success in producing the binary PermalioyB type nickel-iron aiIoy using a pure sulfate
bath [Horkans (1979)l. The reason for the citation here is that the bath consisted only of the
meta1 salts without any additives other than boric aad. Anderson and Grover, using a
mixed date-chIoride-based bath, produced nickei-iron alIoys with uniform nickei/iron
ratio that led to the issuance of a U.S. patent [Anderson and Grover (1981)].
Bv the late 1970s and earlv 1980s, suffisent studies had been performed to permit
researchers to address the more hdarnental aspects of the systerns and draw some
empùical condusions about the effect of various plating conditions and bath compositions
on the deposited alloy. One particular example is the presence of saccharin in the
Nickel-lron Binary System P. 4.5
Synthesis and blicrostructurat Characterization of Electrodeposited Nanocrystalhe Soft Magneis
formulation of these relativelv earlv plating baths, since most of the studies from this
period reported the use of saccharin as an additive to the plating solutions involved (e.g.
Bielinski and Przyluski (1979a,b)]. Beltowska-Lehman and Riesenkampf published two
closely related papers in the earlv 1980s detaiiing the kinetics of the electrodeposition of
PermalIo-y03 type aUoy Jeposits [Beltowska-Lehman and Riesenkampf (1980), Beltowçka-
Lehman and Riesenkampf (1981)], one of which specifically involved the effect of the
presence of saccharin in the plating solution [Beltowska-Lehrnan and Riesenkampf (1981)l.
At this tirne, a number of studies utilizing a more fundamental approach to the
study of these binary alloys produced using electrodeposition methodology began to
emerge. international Business Machines (IBM) cieveloped the first hard disk drive (HDD)
technoIogy in 1973 and it appeared comrnerciaily in IBM's first persona1 computers (PC) in
1983 in the form of the XT (eXTended) mode1 (although the very fùst P G that were
introduced in 1981 did not support HDDs). This advancement in magnetic storage
technology and the realization of the significance of Pennalloy43 in the magnetic recording
indusûy at approximately the same time probably led to the considerable increase in the
scientific interest in this allov - svstem. -
indeed, in the foiiowing years, publications involving the electrodeposition of those
aiioys close in composition to Permaliofl started to focus on their production for use as
magnetic recording heads. in particuiar, reports pertaining to pattern plating appeared in
the Iiterature (e-g. Wagner and Zilk [1982] and Duke rf nl. [1982]).
in addition to the increased number of fundamental studies on the nickel-iron
binary svstem, the use and studv of additives in the eIectrodeposition process received
more attention in the 1980s. In generai terms, up to this point, additives to nickel-iron
electrolytes were iimited to a smaii number of compounds, predomuiantiy saccharin and
Nickel-Iron Binary System p 46
Synthesis and Microstructural Characterization of Electrodeposited NanocqstaIline Soft Magnets
sodium l a w l sulfate (used as a wetting agent / surfactant). In the mid-1980s, researchers
started to employ various other chemicais in efforts to improve their eIectrodeposition
processes (i.e., bath iifetime and aiioy deposit quality). Lieder and associates published a
series of studies involving the use of potassium chloride (KCI) in their rnixed sulfate-
cidoride nickel-iron bath [Biaiiozor and Lieder (1983,1984), Lieder and Biailozor (19891.
Nakamura and Hayaski [19851 used sodium chloride (NaCl) as an additive in one of their
studies, whiie using citric acid (HOCOHK(OH)(COOH)CH~oOHo &O) in another çtudy
invoIving the nickel-iron sys tem [Nakamura and Hayashi (1985) 1. Srirnathi and Mayanna
[1985] used sodium sulfate (NazS04 in their study aiong with other Less conunon
compounds such as ascorbic acid (C6&06), pyrophosphate and ethyIene diamine. X year
later, in 1986, the same researchers studied the effects of d a m i c acid (HzN5û;H) and
su i€osa~c l ic acid ( H O C 6 H @ 3 0 H ) S 0 3 H ~ ~ ) on the eiectrodeposition of nickel-iron
alloys [Srimathi and Mayanna (1986)l. Grimmett rf n . studied vanous types of electrolytes
(mostiy sulfate and suifamate b a h ) and produced some excelient deposits, aIthough these
pubücations Jetaileci the synthesis of alloys on the ùon-rich side [Grimmett & r d
(1987,1988), Grimmett [1991]).
Towards the end of the 1980s, significant progress had been made in the
development of electropiating procedures to produce binary nickel-ùon aUoys as weli as in
the characterization of the d ~ y s . As a resuit, dong with continuing studies pertaining to
the plating process such as the effect of throwing power (e-g. Chomakova and Vitkova
[1986a,b,l989], White [1988]) and bath agitation during electrodeposition (e-g. hdricacos
et n/. [1988]), models desaibing va.rïous aspects of the alIoy deposition process aIso began
to appear; for instance, the codeposition process (e.g. Frederick and Landau [199û]) and
stress calcula tions (e.g. Young [1990]).
Synthesis and Microstnictural Characterizaiion of EIectrodeposited Nanocrystaihe Soft h1agnet.s
Since one of the main objectives for this work is the production of materials with
grain sizes in the nanometer range, a survey of the literature regarding this aspect needç to
be made. Throughout the hiçtory of the research conducted on thiç binary system, grain
size reduction has not been the primary focus. This is not to say that eIectrodeposited
nickel-iron with nanometer grain size has not been obtained; rather, the production of
nano-grained materials was mereiy coincidental as opposed to being the product of a
systematic study. Indeed, results presented in some of the literature avaiiable indicate that
the product was nanocxystaKne in nature. For example, Stefac produced a senes of iron-
nickel dey s with hardness over 700VHN [Stefac (l969)l. Ho wever, no microstructural
corroborations were given in support of this indication- Q u t e often investigations
reported on the production of "small-grained deposits", however, without givùig the
actual grain sizes [Djokic and Maksimovif (1992)l. Examples of other studies that do
report on grain sizes include the work of Grimmett [1991]. The researchers c1early had
obtained nanoqstalIine materials, however, Lhiç hding was not addressed directiy and
speficaiiy. As a result, one can Say that, at the outset of this research, no studv was
avaiiable that pertained specificaiiv to the production of nickel-iron aiioys by
eIectrodeposition techniques with the reduction in grain size as the main thrust of the
work.
The eiectroplating soIution, which was lound suitable for the svnthesis of
nanostructured nickel-iron ailoys, was a modified Watts nickel bath containing additions
Nickel-iron Binary System
Synthesis and Microstructura1 Characterizarion of Electrodeposited Nanocrystaliine Soit Magnets
of iron chlonde, sodium citrate and saccharin. in addition, the D.C. electroplating
technique was used throughout.
In terms of electrolyte development for the nickel-uon system, a Watts nickel bath
was used as a starting point [Brenner (1963)j. Nickel sulfate (NiÇOre6HzO) was the main
source of ionic nickel in the solution. Nickel chioride, while being a secondq source for
nickel cations, also served to aid in the ctissoIution of the anode [Brenner (1963)l. A source
of iron was supplied by the addition of some form of iron salt; in this case, iron chloride
(FeCl&HzO). The concentration of this salt was varied in order to produce deposits of
varving composition (ùon content). Banc a d (HsBG) was used for its buffering
properties as weii as to prolong the life of the plating solution [Sarojamma and Rama Char
(1972)l. Also, in a recuit publication [hdricacos and Romankiw (1994)], it was reported
that in electrodepositing PermaHoy@, deposits obtained from baths without the addition of
boric acid were gray and rough with higher coercivity than those from a boric acid-
containing solution, from which the elechodeposits were bright and reflective. In addition,
b r i c acid has been reported to increase the absolute iron deposition rate while inhibiting
the rate of nickel reduction [Yi L.f nl. (1995)], thereby having a nmilar function as a
complexing agent.
Containhg just the metafic salts and boric acid, this bath can be considered a
relative. simpIe formulation and some of the earlier attempts to produce aiioys from such
a bath, regardless of the resultant grain çize, have faiied. For example, solutions without
the addition of sodium atrate generdy produced sampIes that were of poor quality, Le.,
d d and brittie, and hence, unfit for further study. in addition, sustained deposition was
not possible from these baths. The details of the failed attempts wili not be given Iiere.
Nickel-iron Binary System
Synthesis and ~Iicrostmctural Characterïzation of Electrodeposited Nanocrystalline Soft Magnets
Sodium citrate has been reported to be a complexing agent for nidcel, iron, and in
fact, for diromium as well [e.g., [Domnikov (1964), Sarojamma and Rama Char (1970)l.
Plating soIutions developed to produce binary or temary d o y s of these elements often list
sodium citrate as a constituent. As a result, sodium citrate was incorporated into the bath
design.
The use of saccharin was adopted from previous work on the electrodeposition of
nanocrystaiiine pure nickel [El-Sherik (1993)], where it has been shown to be effective as a
stress reliever in addition to acting as a grain refinement agent. Since the electrolyte used
in the present studv and the target ailoy is nickel-rich, it was assumed that saccharin would
function similarly in this modified bath. Indeed, a recent study by Yin and Jan 119961
reported saccharin as a "surface-blocking" agent in the electrolyte, which confirmeri the
earlier finding that minimizing surface diffusion increases grain nucleation rather than
grain growth in an electrodeposition process [Choo rtnl., (1995)l.
it is important to note that d l nitrogen gas was purged through the soIution when
the bath was inactive or stored between plating experiments. This practice was found to be
crucial in proIonging the life of the solution,
in the 2-t- state without proper anti-oxidants, iron tends to transform into Fe)*,
foIiowed by the formation of iron oxide (ïe203) in the presence of ciissolved oxygen in
soIution. As a result, the life of Fez*-containing plaling solutions may be short-lived
fThompson (1996)l. in fact, some of the earlier experiments in this thesis performed
without the use of nitrogen were found to have a brownish precipitate forming in the
solution not Iong after moderate usage, whïch was believed to be iron oxide. Purging the
soIution with nitrogen reduced the iron oxidation quite considerably.
Synthesis and Microstructura1 Characterizaiion of EIectrodeposited Nanocrystalline Soft Lfagnets
Throughout the literature, typicai optimal pH values reported for nickel-iron
plating bath are in the range from about 2 to 5. The pH of the eIectroIyte for the present
study was kept around 4.0, which was f o n d to be optimal for the prescribed set of
operating conditions. The adjustment of pH was achieved by the use of suifuric acid. It
shouid be noted that, for the electrolyte type used here, an increase of temperature and pH
of the bath both would increase the nickel content of the deposit [Srirnathi and Mayanna
(i983)]-
Table 3-1
Composition of the electroplating solution developed for the nickel-iron bina- svstem. - - .
I Component Concentration I
Table 3-1 shows the composition of the solution used for this part of the study. In
summary, the developed electrolyte contains sources of the rnetallic species, buffer and
cornplexkg agents. In addition, saccharin, acting as a surface-biocking agent, promotes
grain nucleation during electrodeposition, m other words, increasing the likelihood of
nanocrystalline grain formation instead of growing existing grains. D.C. electrodeposition
was the technique used for thk part of the work, and in such a setup, only the current
density remains as an independent variable if the temperature is kept constant. A
Nickel-Eron Binary System p. 51
Synthesis and bIicrostructural Characterization of Eiectrodeposited NanocrystaIline Soft Magnets
relatively high current density of 200mA/cd was chosen. However, it was found that the
formulation of the bath chemisûy was suffiCient to handle such a high current density
without driving the system beyond the diffusion-controiied regime, leadhg to the
possibility of a " b u n e d deposit [Dini (1993)j.
The cathode was a 2crn x 2cm titanium square whiie the anode was electrolytic
nickel contained in a titanium basket (see Section 2.2). The anode to cathode surface area
ratio was about 10 to 1. ElectroIysis was carrieci out at approxirnately - 50°C, without any
bath agitation other than the intrinsic agitation resuiting from the hydrogen evolution on
the cathode. Electrodeposits of thicknesses around 300p were routinely obtained using a
D.C. current density of 200~/cm~. The e1ectrodeposiîs were subsequently stripped from
the titanium cathode for materials characterization. Thin foiis for transmission electron
microscopy examination were prepared using jet-polishing in a eiectroiyte comprising 75%
acetic acid, 15% methanol and 10% perdoric acid at a temperature of -10°C and a voltage
of 40V D.C..
3.3 Alloy Composition
Figure 3-1 shows the ùon content (wt-%) in the electrodeposits, determineci by EDS,
as a function of the üon d o r i d e concentration (g/L) in the solution. The samples obtaineci
with O g/L üon chloride correspond to deposits with 100% nickel. As expected, as the üon
chloride concentration in the soIution increases, the iron content in the aiioy also increases.
Nickel-bon Binary System
Synthesis and bIicrostructuraI Characteriration of Electrodeposited Nanocrystalline Soft blagnets
95% confidence intemal O -.
O 5 10 15 20 25 30 35 40 45
Iron Chloride Concentration (fi)
Fime - 3-1
Iran content (wt%) of the electrodeposited nidteI-iron alIoys as a function of uon chionde
salt concentration (g/L) in the solution; individual points represent the average of at least
10 EDS readings with the error bars representing one standard deviation.
It was observed that this increase in the ùon content in the deposit is linearly
related to the iron salt concentration in the solution, and this ünearity was preserved up to
an iron content of about 30 weight percent. Further increases in the iron chloride
concentration led to considerable deterioration in the quaiity of eiectrodeposits produced.
These eiectrodeposits were dull, generalIy stressed and premahuely peeled off of the
substrate surface pnor to compIetion of the pIating experiment. Compositional data for
these particdar deposits were not Înciuded in the resuits reported here. Furthermore, since
Nickel-iron Binary Systern p. 53
Synthesis and Microstnichiral Characterüation of Electrode~osited Nanocrystalline %ft Magnets
the primary goal of this part of the study was to obtain electrodeposits with a composition
dose to that of comrnerciaily available PermailoyB (80Ni-ZOFe), further optiinization of the
plating solution to accommodate higher iron contents was not continued.
35 0 Present Study - O Dennis and Such (1993)
NI / ~ e ~ + Ion Ratio
Fime - 3-2
Average iron content (wt.76) of the electrodeposited nickel-uon d o -
as a Function of the nickei to iron cation ratio in the solution.
Figure 3 - shows the iron content (wt-X) as a function of the Ni'+ to F$+ ion ratio in
the solution. At the highest N i Z + S + ratio of 50, the eIectrodeposits contain l e s than 5%
Nickel-bon Binary System
Synthesis and il.Iicrostmctural Characterization of Elechodeposited Nanoqstalline Saft Magnets
üon. With decreasing Ni>:F$+ ratio (moving left in the graph), the üon content in the
deposits inaeases rapidlv leading to an alloy of approximately Ni-28wt%Fe at the lowest
NikFe" ratio of 7.5.
The general trend obsemed in Figure 3-2 is consistent with that of previous studies,
as reported by Dennis and Such 119931 for example, whose results are ais0 reproduced in
Figure 3-2. It should be noted that, at Iow Ni2-:Fe=+ ratios, the iron content in the present
studv inaeases more rapidly than that reported bv Dennis and Such [1993]. This mav be
attributed to the differences in bath formulation and operating conditions, since their
pIating solution often contained a number of dilferent organic brighteners and stabilizers
[Dennis and Such (1993)] which would affect the bath chemiçhy. Conventional electrolvtes
used for binarv nickel-iron ailoys are generaiiy operated at higher temperatures than used
in this studv. Since increasing temperahtre causes a decrease in üon content in the
electrodeposited alloy [Romankiw and Thompson (1975)], the difference observed in the
data was more than reasonable.
TypicaI deposition rates of the alloys from the present soIution were in the order of
100 to 150 microns per hou. Mthough no actuai cdcuiations were carried out with
regards to current efficiencies, the pIating rate for this binary nickel-üon system is about 10
to 15 times higher than previoudy obtained for similar deposits produced from a
chromium-containing bath [Cheung efd (1994a,b)]. This is because of the intrinsidy Iow
current effiàency associated with chromium b a h . The electrodeposits (up to around
30wt.76 iron) showed no curling and buckhg after rernoval from the underlying Ti
Synthesis and Microstructural Characterization of Electrodeposited NanocrystaIline 5oR btagnets
substrate, which indicates that interna1 stresses were relativelv low. This is important
because stresses, in most cases, can cause adverse changes in the magnetic properties (in
particular, coerciviw) of electrodeposited magnetic fiims. h o , the minimal amount of
sbess observed is of large technologicai importance when producing relatively thick
sheets; for exampIe, transformer core sheets are typicaiiy 0.1 to 03mm thick.
Fime 3-3
A scanning electron miaograph of an electrodeposited nickel-iron d o y with a composition of Ni-20wt.XFe.
The surface morphoIogy of the eIectrodeposits was generaily very smooth, as
shown in Figure 3-3, which is a scanning electron miaograph of an electrodeposit with a
Nickei-Iion Binary System p. 56
Synthesis and Microstmctural Characterization of Electrodeposited Nanocrystalline Soft Magnets
composition of Ni-20wt.%Fe. The grain çize of the deposits, which cannot be resolved
using conventional scanning electron rnicroscopy even at very high magnifications, had to
be ascertained using X-ray diffraction iine-broadening and transmission electron
microscopy (see Çection 3.5).
3.5 Microstructures
The X-ray diffraction pattern (8 - 20 geomeby) of some of the electrodeposits are
shown in Figure 34. in addition, the diffraction pattern for a conventional polvcrystailine
nickel powder standard was obtained using the same equipment and is aIso included.
Figure 3-5 shows the JCPDS powder diffraction €de for nickel DCPDSPDF No.4-850
(l99O)I.
A comparison shows that the peak position and intensities of the diffraction peaks
of the powder standard sampIe agree verv weii with publiçhed values. in contrast to the
nickel powder standard, the diffraction patterns of the electrodeposited materials show
changes in peak widths and peak intensities, and slightiv in peak positions, which wiü be
adtessed in the foiiowing sections.
Firstiy, the X-ray diffraction peaks of the ailoy electrodeposits show considerabIe
broadening. For example, the F d i Width at Half Pvlaximum (FWHM) for the (111) peak of
the nickel powder standard is 0.22" using the Rigaku D-MAX 1000 diffractometer, On the
other hand, the pure nickel electrodeposit showed a (121) peak width of 0.474". With
increasing iron content in the deposits, the peak width increased to a value of O.ïi6" for the
aiioy containing 232wt.% iron. The grain sizes, ais0 induded in Figure 3 4 , were obtained
€rom these peak widths using the %errer foxmula (see Section -6).
Nickel-Iron B h a r y System
Synthesis and MicrostmcturaI Characterization of EIectrodeposited Nanocrystalline b f t blagnets
1 Ni powder standard
OwW) Fe (20.8 nm) -
3.6wt'%1 Fe (18.9 nm)
14.1wt1!4) Fe (15.3 nm) A
22.2wt1!41 Fe (12.7 nm) - .-
40 50 60 70 BO 90 100
2 Theta (Degrees)
X-Ray diffraction patterns of a series of dectrodeposited nickel and nickel-iron
d o y s with compositions ranging up to 22wt.%Fe.
Nickel-iron Binary Sys tem
Synthesb and Microstnictural Characterization of Electrodeposited NanocrystaIline Soft Magnets
Nic k r l
Rad: CuKol Lambda: 1.5485 Filter: Ni b rp ; Cutaff: Ink Oiffractomwiwr Vicor: Rrf: Suonson. Totge. N d . 8ur. Stand. (US.), Circ. 539.113 (1953).
Sys: Cubic S.C.: Fm3mC225) a: 3.5238 b: C; A: Cc h 0: C; 2; 4 mp: Ref: Ibid.
01: 8.91 Dm: SS/FOfl: F8=87C011.8) €0: nue: *y: Sign: 2U:
Coloc: Uhitr. Pattrm ot 26 C. Somplr obtoinrd from Johnson. flanhry Company. CAS no.; 7448828. Sprdiagrophic onolyair show < @.@lx each of Mg. Si and Co. flerck Index. 8th Ed.. p 727. CU type. Gold gioup. gold rubgioup. PSC; cF4. nul: 58.70. Uolumr [CO]: 43.76.
- Int. - 108 42 21 2 8 7
4 14 1s
-
Figure 3-5
JCPDÇ powder diffraction fiIe for pure nickel (PDF No.4850) [JCPDS (1990)l;
20 d u e s üsted are for copper K.,, radiation.
It is important to point out that the CO-deposition of iron has a profound grain
refining effect as shown in Figure 3-6, which is a graphical representation of the grain sizes
of the nanocrystaIiine nickeI-iron electrodeposits as a hc t i on of the ùon content in the
doy . The grain size of the pure nickel deposit was found to be about 2 1 m As the iron
content increases, the grain size of the eIectrodeposits was found to decrease. The
obsemed decreasùig trend is neariy a h e a r inverse reIationship. Griuunett [19881 and
Cheung r f d [1994b] found simiiar hends, but appiïcabIe for other electroiytes.
Nickel-Iron Binary Sys tem
Synthesis and iv1icrostnicturaI Ci-taracterization of Electrodeposited NanocrystalIine Soft Magnets
8
O 5 10 15 20 25 30
Iron Content (wP/o)
Figure 34
Grain size of the nickel-uon alloys as a function of the iron content (wt%).
Figure 3-7 shows the brightfietd (BF) and darkfield (DF) transmission electron
micrographs, eIectron diffraction patterns and grain size distribution for the
electrodeposited nanocrystalline pure nickel; Figure 3-8 shows the same for the
nanocrystalline Ni-20wt.XFe dectrodeposib. The grain size distributions in Figures 3-7
and 3-8 are obtained from grain diameter measurements on the darkfieid miaographs on
at Ieast 150 grains.
Nickel-iron Binary System
Syntfiesis and Microstructural Characterization of Electrodeposited Nanoc~sta1Ime Soft Magnets
: (4 Average gnin size = 15.2 + 8.6nm
O 8 1 6 2 4 3 2 U J 4 8
Gain Size (nm)
Finue 3-7
(a) Brightfield transmission electron miaograph, (b) darkEield trammission eiectron miaograph, (c) electron diffraction pattern and (d) grain size distribution of the pure nanomstalline nickel electrodeposit (the uncertainty represents one standard deviation).
Nickel-Iron Binary System
Spthesis and Microsüuctural Characterization o f Electrodeposited Nanoqsfalline Çoft blagnets
(a) Brightfield transmission electron
micrograph, (c) electron diffraction
Average grain size = 9.7 : 33nm.
O 1 B l L I 6 2 0 L . l
Grain Size (nm)
Fimire - 3-8
micrograph, (b) darkfield transmission electron
pattern and (Li) grain size chtribution of the
n a n o q s talline eiectrodeposit with a composition of Ni-20wt% Fe (the uncertain? represents one standard deviation).
Synthesb and Microstructural Characterizatioion of EIectrodeposited Nanocrystdlluie Soft Magnets
The average grain sue as determined from grain size distributions based on TEM
observations are in good agreement with the values obtained frorn X-ray diEraction. The
second observation that c m be made in the X-ray chffraction pattern s h o w in Figure 3 4
is that the F.C.C. structure of nickel is preserved throughout the entire iron content range
in the electrodeposited alloys, up to approximatefy 30wt.Y~ iron. This indicates that the
elecfxodeposits are soiid soiutions of iron in nickel. This is &O confirmeci by the eIectron
di€fractiun patterns shown in Figures 3-7c and 3-8c which &O showed the characteristic
rings of an F.C.C. material,
Figure 3-9 is the equilibrium phase diagram of the b inw Ni-Fe systern [IMP~RIS
Hhw'booX- (1990b)l. At compositions dose to Ni-25wt.%Fe, an ordered NhFe phase c m be
seen on the phase diagram. However, th& phase is absent in the eIectrodep~sits, inciicating
that the designed experimentat conditions aiiowed the production of non-equiiibriurn
binarv allovs of nickel and iron. This observation is simiiar to that reporte4 by Ostrander
[19931 while studving the synthesis of nickel-phosphorus aüoys.
From the Wfraction pattern, it can ako be observed that the diffraction peaks shi€t
to siightiy srnailer 20 values as the iron content in the electrodeposits increases. For
example, the (111) riiffraction peak for the pure nanocrystalline nickel sample has a Bragg
angle of U.-Bo whiIe the corresponding peak is shifted to U . 1 8 O for the deposit conlaining
22wt.% iron. Athough unifonn stresses present in the electrodeposits may cause peak
sMtuig, the Iow Ievel of stress observed indicates that perhaps another mechanhm is more
likely to be responsible for this peak shift A shik in peaks to smaiIer 20 values wouid be
expected of the Ni Iattice expands upon alloying with Fe- Such a Iattice expansion with
inaeasing iron content has in fact been observed in numerous previous studies (e.g. see
Hansen (1958)l for Ni-Fe d o y s produced by conventional metaiiurgical means.
Nickel-iron Binary System P- 63
Synthesis and MicrosmcturaI Characterization of Electrodeposited NanocrystaIline Soft klagnets
Figure 3-9
Equilibnum phase diagram for the binary nickel-iron sys tem [rM~fnh Ha~dbouk (2990b)l.
Wdy, there are ais0 considerabIe changes observed in the peak intensities of the
electrodeposited materiais in cornparison with the nickel porvder standard. For al1
electrodeposits, the (220) peak intensitv is verv much reduced compared to a random
crystal distribution as per JCPDÇ reports. For the pure nickel electrodeposit, on the other
hand, the (122) and ( 2 0 ) peak intensities are enhanced over the random distribution,
indicating the presence of a strong (Ill), (200) doubIe fiber texture. As the iron content in
the electrodeposit inaeases, the (200) fiber texture cornponent decreases reIative ta the
(111) component, dong rvith decreasing grain size.
NidceI-Lon Bniary System
Spthesis and Microstructural Characterizaiion of Electrodeposited NanocrysbIIine Soft Magnes
ln addition to the overall texture changes observed in the X-ray diffraction patterns,
the electrodeposits also showed the presence of a mino-texture over short iiistances,
simiiar to what has been previously observed for other electrodeposited materiah [R-
Sherik ~f d (19921, Cheung d d (1994a,b)]. This is evident in the darkfield eIectron
micrographs shown in Figures 3-7b and 3-8b, where sorne of the larger bright areas consist
of clusters of grains in similar orientation probabiy separated by low angle boundaries as
previously observed for nanoc~stalline Ni deposik [Klement r.t t d , (1995)]. It should be
noted that thiç micro-texture cievelopment became more pronounced with increasing iron
content (ciecreasing grain size) in the deposits.
3.6 Microhardness
Microhardness measurements were carrieci out using a 200 gram Ioad on a Vickers
microhardness indenter. In Figure 3-10, the measured Vickers microhardness, based on 10
measurements for each sampie, of the eIectrodeposits is pIotted as a function of the iron
content in the d o v aIong with reported vaIues for conventiona1 polycxystalline materials
ob tained from Stizithefk r Z / I ~ t d Rt')m~tcr BooX- [l9831 and ~Mefah- H~mIbaok [IWOa 1.
The hardness of conventional polycrystalline nickel ( q s t a l size in the micrometer
range) is reported to be 85 W [511ifhdk (1983)l. As the iron content increases, the
hardness essentiaiiy remains constant up to an iron content of Slwt.% [ r M ~ f ( ~ / s k k ~ ~ ~ f i u ~ k
(1990a)I. On the other hand, aiI the eIectrodeposits have increased hardness (at Ieast a five-
foId increase) over the conventiona1 po@rystLilline materials For example, the hardneçs
of the e1ectrodeposited pure nickel sample is 4% W; and with increasing iron content,
the hardness increases to about 625 VHN at an iron content of about 16.5wt.%. At even
Nickel-Iron Binary Sys tem p- 65
Synthesis and Microsüuctural Characterization of EIectrodeposited Nanocrystalline Soft Magnets
higher iron contents, the hardness deaeases again siightiy to about 580 VHN for the
deposits containing 73 and 28wt.% uon. The observed variation in hardness in the
nanocrystaiiine materials is diffidt to explain in terms of conventional solution hardening
theory and mechanisms since a simiIar variation is not iound in the conventional
polycrystaiiine materials over the same composition range. Therefore, other factors must
be considered.
1 93% confidence interva1
nanocrystalline I polycrystalline
O 10 20 30 40 50
Iron Content (wt.%)
Fiwe 3-10
blicrohardness of the various eleckdeposited nickel-iron alIoys as a function of the iron content; error bars mdicate one standard deviation.
Synthesis and Microstructural Characterization of Electrodeposited Nanocwstalline Saft hIagnets
The hardness of nanocrystalline materiah has been the subject of numerous studies
with particular reference to the vaiidity of the Hall-Petch relationship for matends having
very s m d grain sizes+. This relationship, which was originaliy introduced for
conventional polycrvstalline materials [Hall (1951), Petch (1953)], States that the hardness
increases with ciecreasing grain size according to the foUowing equation:
H = Ho + kkL= Eq.3-1
where H is the hardness of the material with a grain sue ci, H, is the hardness of the
conventional coarse-grained material and k is a constant. The phvsical meaning of the
Hali-Petch relationship is simplv that grain boundaries act as barriers for dislocation
motion during plastic deformation of a metal or alloy. Decreasing the grain size in the
material results in an increased density of obstacles, Ieading to a higher required flow
stress for deformation, Le., a higher hardness of the material.
Figure 3-11 shows the microhardness data of the sarne electrodeposits, but now
plotted as a fünction of the inverse square root of the grain size (i.e. in the form of a Hall-
Petch plot). At a grain size of Xnm, the hardness of the eiectrodeposit is 490 VHN. The
hardness increases to a maximum of about 625 VHN at a grain size of l4nm. Foilowing the
maximum, the inuease in hardness deviates from regular Hall-Petch considerablv,
indicating a possibility of an inverse Hail-Petch behaviour. The transition from reguiar to
inverse Hall-Petch behaviour at very smaü grain sizes is presentiy an issue of great interest-
+ Et-Sherik et QL [1992], Hughes L.r aL [1986], Nieman t.t rl/. [1989], Jang t.f rd [2990], Ganapathi and Rigney [IWO], Cho and Koch [1991], Koch and Cho [1992], Nieman d RL [1992I, Kobelev r't A-! [19931, Recknagie d nL [1994], M&iahon and Erb [1989I, Chokshi rf d [1989], PaIurnbo rf id [1990], Lu t.r d [1990], Christman and Jain [1991], Chang rf aL [19911, Kim and Okazaki [1997], Liu &al: [1993], Scattergood and Koch [19921, Fougere b rtl: [1993], Lu and Sui [1993], Lian and Baudelet [19931, Lian ff QL [19931, Pande r t d [1993], Li &al: [1993], Nieh and Wadsworth [1991].
Nickel-Iron Binary System p. 67
Synthesis and MicrostmctunI Characterization of Electrodeposited NanocystalIine Salt Magnets
While some researchers have reported regular Haii-Petch behaviour down to the smalIest
grain sizes*, others observed a transition from reguIar to inverse Hall-Petch behaviour
after reaching a maximum hardness similar to the one fowd in the present study .
Grain Size (nm) 21 19 17 . . . - .- 15 13 11
/ .
450 - A 95% confidence intervai - - - * - - ,
F i w e 3-11
A Hali-Petch pIot for the eIectrodeposited nÏ&ei-iron aiioys; error bars represent one standard deviation.
+ Hughes c'ta(. [1986], Nieman rf al: [19891, Jang &al: [IWO], Ganapathi dd [IWO], Cho and Koch [ lWlk Koch and Cho [1992], Nieman r f al [1992], Kobelev rtnl: [19931, RecknagIe &al: [1994].
t El-Sherik rf nl: [1992], McMahon and Erb [19891, Chokshi bal: [1989], PaIumbo &al: [19901, Lu rf al: [IWO], Christman and Jain [1991], Changdal: [1991], Kim and Okazaki [19921, Liu rf al: [1W3].
Nickel-bon Binary Systern P- 68
Synthesis and blicrostnictural Characterïzation of Electrodeposited Nanoqstalline Salt Magnets
Athough this phenornenon is presentiv not fuiiy understood, a number of theories
considering different contributing factors to the softening observed at very smaU grain
sizes have been proposed. These factors include (gros) texture effects [McPvIahon and Erb
(1989)], diffusional creep [Chokshi ef nL (1989)], triple junction effects [Palumbo et nL
(1990)], &location nehvork formation [Scattergood and Koch (1992)l and pile-up [Pande rf
nl. (1993), Nieh and Wadsworth (1991)], annealing effects [Fougere rf d (1993)], decrease in
interfacial excess volume [Lu and Sui (1993)], bow-out of Frank-Read sources F a n and
Baudelet (1993), Lian ~f rd (1993)] and the grain boundary source mode1 [Li rfnL (1993)l.
MchIahon and Erb were the first to report the possible effect of texture on the
softening of nanocrystals at very smaii grain sizes [McMahon and Erb (1989)l. For
electrodeposited nanocrvstaiiine nickel-phosphorus aiioys that contained phosphorus in
soüd solution, thev observed that the deviation from regular Hd-Petch behaviour
coincided with the development of a strong (111) fiber texture component. This fiber
texture component increased over the entire grain size range for which considerable
softening was observed. McMahon and Erb argued that in a matenal containhg
nano-staIiine grains in sirnilar orientation, the observed softening could be indicative of
easy dislocation transfer from one grain to another.
A similar argument may be used for the softening observed in the present work.
Although the X-ray diffraction patterns show that the overd development of prderred
orientation in the nickel-ùon electrodeposits is not as strong as previously obsi-rved for
nickel-phosphorus [h/lcMahon and Erb (1989)], the ciarkfield electron dcrographs indeed
show considerable micro-texture deveiopment with deaeasing grain size (see Figure Mb).
in other words, for nanocrystalline Ni-Fe aiioys, the miaostnicture consists of regions of
Nickei-iron Binary System p. 69
Synthesis and blicrostructuml Characterization of Electrodeposited Nanonystalline Soft Magnets
grains in similar orientation embedded in a matrix of randomiv orientated nanocryçtais.
Therefore, the "effective" grain sue, as far as deformation by dislocation slip and slip
transfer is concemed, may be locally increased. As the density of regions exhibithg micro-
texture increases with decreasing grain sue, the volume fraction of materiai showing
reduced resistance to didocation slip increases and the overaii hardness of the material
decreases.
Clearlv, manv factors may contribute to the transition from regular to inverse Haii-
Petch behaviour, and more work is needed beiore this effect can be isolated and
understood. The presence or absence of macro-texture and/or micro-texture in materials
produced by different synthesis techniques may explain why softening at very srnaii grain
sizes are only observed in some but not ali stuhes. For exarnple, in materials produced
from other synthesis methocis such as cornpacted powder materials, this observation is less
likeiy to occur.
As shown in this chapter, the production of nanocrystaüine nickel-ùon d o y ,
espeàaiiv those aiiovs with compositions around the same as commerciaily available
PermailoyQ3, was very much feasible.
Using the eIectrochemica1 formuia and operating conditions ou thed in the chapter,
binary aiioys with iron contents up to about 30wt.% cari be readily produced in thicknesses
over 300pm. The rnorphology of these alloys as seen in the SEM was macroscopicalIy
smooth, and generally exhibited low internai stresses, X-ray diffraction revealed the
perseverance of the F.C.C q s t a i structure indicating a solid soIution over the entire range
Nickel-iron Binary Sys tem p. 70
Synthesis and Microsmichiral aiaracterization of Eiectrodeposited Nanoc~staliine Soft Magnets
of composition. Furthemore, fiber texture was present in the aüoy deposits with the fiber
component shifting from a (111)/(200) double fiber texture to an increase in the (111)
component as the iron content increases in the electrodeposited alioy.
The lowest grain size observed throughout this study was about llnm (Ni-
28wt.?/oFe) whiie the grain size for a Ni-20wt.XFe alloy deposit was about 12.5nm. It
should be noted that grain size measurement reveaied that ali the samples obtained
throughout this stuciy have grain çize Iess than 30nm. AIthough a deviation from the
traditional Hall-Petch behaviour was determined for the smallest grain sizes, the general
effect of grain size reduction in the produced materials caused a considerable uicrease in
their hardness as compareci to conventional polyqstalline materials with the same
composition.
NickeI-Iron Binary Sys tem
Synthesis and ~IicrostntctucaI Characterization of Electrodeposited Nanocrystalline Suft Magnetç
3.8 References
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Nickei-Iron Binary System
Synthesis and LIicrostmctural Characterization of Electrodeposited Nanocryshiiine Soft Xtagnetr
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NidceI-Lon Binary Sys lem
Synthesis and Minosüuctural Characterization of Electrodeposited Nanoqstaiiine Soft Pvlagnets
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Nickel-Iron Binary System p. 74
Synthesis and blicrostntctural Characterization of Electrodeposited Nanocrystalline Soft Magnets
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Synthesis and bIicrostructural Characterization of Electrodeposited NanocrystaIIine Soft Magnets
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Nickel-Iron Binary Systern
Synthesis and MicrostnicturaI Characterktion of Electrodeposited Nanocrystalline Soft btagnets
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Nanocrystaliine Materials", Scn,tn Metufhqkn rt MntenRhk, Vo1.28 (1993), No.12,
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J, Matulis et d, Karusr O r k rVL~ffer., Vol.10 (1966), No.4, p.75; as àted in Stefac (1969).
G. McMahon and U. Erb, "Buik Amorphous and Nanocrystaiiine Ni-P Moys by
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"Properties and Selection: Nonferrous iüiovs and Special-Purpose Materials", rM~t'n/s
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N. Nakamura, M. Urnetani and T. Havaski, "Eiectrodeposition of Iron-Rich Ni-Fe
Moys from Sulfate and Chlori J e B a h , St~@ce f i c ~ ~ n u f o ~ , Vo1.25 (1985a), p.111.
N. Nakamura and T. Hayaski, "Temperature and pH Effects on the homalous
Codeposition of Ni-Fe iUIovs", Ph&ignnd5f.fnceFinrShing, VoL72 (1985b), No.8, p.42,
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~ e ~ ~ f h i i ~ ~ ~ ~ f M u f Vo1.25 (1991), NoA. p-955.
G.W. Nieman, J.R. Weertman and R-W. Siegel, "hLiaohardness of Nanoqstalline
Palladium a d Copper Produced by inert-Gas Condensation", Sc-n>tn ~Mefnfh~rgca,
Vo1.23 (1989), p.2013.
Nickel-iron Binary Sys tem p. 79
Synthesis and Microskuctural (Jiaracterization of Electrodeposited Nanocrystaiiine Soft Magnets
G.W. Nieman, J.R. Weerûnan and R.W- Siegel, "Mechanical Behavior of
Nanoaystaiiine Metais", NnnoSfn~ch/redMnfenkfs, Vol.l(1992), No.2, p.185.
K. Ohashi, M. Ito and M. Watanabe, "Application of Bectroplahg to Thin Film Heads:
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L.T. Romankiw and D.A. H e m , Jr. (eds.), ï h e Electrochemicai scie-, New Jersey
(1988), V01.90-8, p.39.
G. Palumbo, U. Erb and K-T. Aust, "Triple Line Dischation Effects on the Mechanical
Behaviour of Materials", 5~n11.fetn//r~rgr'cnctMn~en~(in, Vo1.24 (1990), N0.12, p.2347.
CS. Pande, R.A. Masumura and R.W. Armstrong, ~Va~roStn~cfrm.. ~Mflerrid.., V01.2
(2993), p.323.
N.J. Petch, forurrdofihr fm miSfeef hrstihffe, Vo1.174 (1953), p .S.
J, Przyluski and K. Madry, "An investigation of the Deposition Kinetics of Ni-Fe M o y
Fi l i s " , 511r/nce fich~rohgy, VoI.13 (1981), p.177; as cited in hiincacos and Romankiw
[19941.
K. Recknagle, Q. Xia, J.N. Chung, C.T. Crowe, H. HamiIton and G.S. CoUins,
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Mnf~nk/s, VoI.4 (1994), p. 103.
L.T. Romankiw, "Use of =-Type Photoresist as insuiation in Multiturn Thin Film
Recording Hea ds", fBM TrchnIcnI Dr>c/osrue Brdi21ihI Vol23 (1980), p.2584; as àted in
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of Ek&odpposifs, Z7reir Mrnmrmmt nndSr@zr~cmrc~ The Electrochemicai Society, New
Jersey (2975), U.S.A..
Synthesis and I\iIicrostructural Characteriration of EIectrodeposited Nanocrystaiiine Soft Magnets
W.H. Safranek, "EIectrodeposition of Lon and iron Aiiovs", U.S. Patent No.2,809,156
(1957); as cited in Brenner [1963I.
M. Sarojamma and T.L. Rama Char, "Electrodeposition of S t d e s s Steel", t%rcfr~p/nhig
md~Mefd FInrsh~itg, Vo1.23 (1970), No.10, p.13.
M. Sarojamma and T.L. Rama Char, "EIectrodeposition OC Iron-Chromium-Nickel
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R.O. Scattergood and C.C. Koch, "A Modified Mode1 for Hall-Petch Behavior in
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Synthesis and bIicrostructura1 Characterization of Electrodeposited Nanocrystalline Soft Magnets
R.L. White, "Throwing Power Effects in Permdoy ElectropIating", Phhirg nndS&ce
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Chapter 4
Pure Cobalt System
Electrodeposition of cobalt has been reported as earIy as 1875 [Cobalt information
Center (1962)], though in 1974, klorral and Safranek stated that cobalt eIectrodeposition has
been known since arounci 1850 [Morral and SaEranek (1974)l. The earliest interest in cobalt
resided in the fact that cobalt and its aIioys demonstrate exceiient decorative properties and
corrosion resistance, in addition, the ease with which cobait can be electroformed [Morral
(1964)l aIso helped irtcrease Cocus on its production and applications. Moreover, cobait
stirndated much scientific interest because of its magnetic properties. in addition to the
relatively high saturation magnetization, one partidar aspect is the high Curie
(ferromagnetic to paramagnetic transition) temperature of cobalt at 1120°C. This
temperature is much higher than for nickel (358°C) or iron (770°C) DiIes (1991)].
in this section, the iiterature concerning various aspects of the development of pure
cobdt eIectrodeposition systerns wiII be reviewed. in general terms, most e1ectroIytes
involveci in the eIectrodeposition of cobait can be categorized hto three main groups:
sulfate-based and chloride-based baths, and bath comprised of mixed sulfate-chionde
sdts; sulfate bath being the most common. In 1964, MorraI included a fairly
Synthesis and Microstructural Characterization of Electrodeposited Nanocrystalline Çoft Magnets
comprehensive literature review of the electrodeposition of cobalt in an extensive
publication Florrai (1964)]. What foiiows is a summary of this literature review foiiowed
by an expansion to indude more recent studies.
Cobalt exists in two possible phases; the alpha (a) hexagonal closed-packed (H.C.P.)
phase and the beta (fi) face-centered cubic (F.C.C.) phase [Waiker and Cruise (1978)l.
Under equilibrium conditions, cobalt is H.C.P. below the transformation temperature of
417"C, and exhibits F.C.C. structure above this temperature. However, in electrodepositeci
form, both crystai s t r u c ~ e s can coeWst in cobalt. In 1971, Hull produceci cobalt
electrodeposits from a sulfate bath that contained both the H.C.P. and the F.C.C. phases
[Hull (1921)l. This abiiity to produce F.C.C. cobalt at room temperature, which is easier to
work (Le., roii, draw, etc.) than the H.C.P. counterpart is another reason for the interest in
electrodeposited cobalt [Walker and Cruise (1978)l. Xthough it was difficult to
quantitatively anaIyze the phases using X-ray diffraction [Kerstern (1932)], many
publications in this period did report the existence of both phases in the electrodeposits. in
fact, almost aU researchers had reported the coexistence of both cobalt q s t a l structures in
their electrodeposits. However, this observation could not be attributed to the tvpe of
electrolytes used; Finch et ni! [1947] studied the elecirodeposition of cobalt using an aii
sulfate, an a i i chloride and a Watts cobait bath operating at the same conditions for al1 baths
and observed mixed structures in their electrodeposits in all cases.
in 1935, Cochrane used a sulfate bath and produced cobalt electrodeposits with two
distinct structures [Codirane (1936)l- in addition to obtaining mixeci structures from the
bath, Cochrane was able to produce cobait with oniy an F.C.C. structure. Fisher and Sun
using a sulfamate bath 5 0 were able to eIectrodeposit cobalt Finch and Sun (1936)l.
Pure Cobait System
Synthesis and blicrostructural Characterization of Electrodeposited Nanonystaffuie Çoft Magnets
However, it was found that the cobalt dectrodeposits exhibited the same structure as the
various underlying platinum and goId substrates.
in 1956, Newman produced very thin (10 to 40nm thick) cobait deposits, also from a
sulfate solution [Newman (1956)]. These thin deposits also exhibited a mixed structure.
Reiner, using an electrolvte with a slightly different composition abo reported the same
[Reiner (1957)]. Fedotev rf [TL [1959] varied the cobalt sulfate concentration în an all-sdfate
bath and produced a series of cobalt deposits with varying deposit quality, aIthough no
qstailographic structure information was given. Similarly, Pangarov and Rashkov also
employed a sulfate solution in an attempt to produce pure cobalt dectrodeposits [Pangarov
and Rashkov (1960)l.
Llp to around the 1950s and the 1 9 6 0 ~ ~ most of the cobalt electrodeposition work
had focused on the cievelopment of sulfate-based b a h . However, eiectrolytes comprised
of cobalt chloride were not without consideration, in 1957, Reiner, using a very simple
chioride bath successfully obtained cobalt electrodeposits peiner (1957)]. In 1963, Fisher
produced electrodeposited cobalt from a number of chioride b a h with v-g cobait
chloride concentrations [Fisher (1962)l. It was found that for Iower concentrations of
chloride salt, mixed structures were observed in the deposits, whde for the baths having
high concentrations of cobalt chloride, deposits having the H.C.P. structure were obtained.
It was found that the addition of saccharin in the solution prornoted the formation of the
F.C.C. phase in the electrodeposits. A few years Iater, Abramova rf n/: &O empIoyed a
chioride bath to produce cobalt electrodeposits [Abramova rtnL (1969)l.
Chiginally, plating electrolytes for pure cobaIt were quite simple and the eartier
studies showed that cobalt couid be eleckodeposited with rdative ease and without drastic
modifications to the plathg solutions. However, as the demand for better properties of the
Pure Cobait System p- 85
Synthesis and rClicmstructural Characterization of Electrodeposited Nanocrystalline Soft Magnets
metal deposits increased, investigators began to tailor their electrodeposits with additives
to the electroplating bath (e.g., Lindsey and Read (1970)l. With respect to bath additions,
publications conceming cobalt pIating ekctrolytes containhg additives started to appear in
the 1940's. The foilowing is a bnef survey of this stage of development.
Duval and Liger [19431 added potassium chloride (Ka) to their aii-sulfate soIution
in their attempt to electrodeposit cobalt. Hothershaii ako iisted KCI as an additive to a
sulfate cobalt bath [Hothershaii (2947)l. in addition to KCI, Kochergin added sodium
sulfate (NazSOa) to a sulfate bath to produce cobalt eIechodeposits [Kochergin (19JZ)l.
Sodium sulfate has alço b e n reporteci as an additive in conjunction with magnesium
sulfate (MgSO4) used by Kaznachei rt d [1957]. Cobalt fluobonte and naphth01
(GIZIOC~H~OOH) were used in the investigation by Fegrado and Balachandra [1934]. A
few years later, naphthol was used, dong with Co(SO&Hz)z as additives by Barrett [19601.
CoSOKo(NE2i)2(SO& was among the Iess common additives to a cobalt pIating
electrolyte [Okund (1956)l. Bursuc c . t d aIso added (NtL)2ç0J to a relativeIy simple sulfate
bath as an additive [Bursuc r f d (2964)j.
in the 1970s, the "cobalt crisis" caused a considerable increase in the pnce of cobalt.
As a result, considerations for the use of cobalt in many applications diminished around
this time. Fortunately, certain applications in the aerospace and cornputer industries
required cobalt for very specific properties (e-g., mechanical and magnetic properties),
which took priority over econornic constraints [Safranek (1986)l. indeed, by the late 20th
century, there was a renewed interest in cobalt because of its magnetic properties.
At around the same tirne, investigators were engaging in studies involving various
aspects related to the structure of cobalt eIectrodeposits. Matulis rf d, using two different
ba th (a reiatively simple sutlate bath and a atrate compIex bath), studied the effect of
Pure Cobalt System p. 86
Synthesis and blinostructuml Characterization of Electrodeposited NanocrystalIine Soft Magnetç
current density on the structure of cobalt eIectrodeposits platulis dd (1972)l. It was noted
that in sulfate b a h , the H.C.P. phase was prevalent, while the F.C.C. phase was
predominant in the complexed citrate bath. They &O concluded that both the Iowering of
the operating temperature and the pH of the eIectrolyte suppressed the formation of the
hexagonal phase. Sadance, using a simdar complex sulfate bath, studied the effect of citrate
additives and operating conditions on the structure of the cobalt deposits and reported a
similar trend [Sadance (1977)j. in addition, it was found that increasing the current densi?
resulted in a decrease in the H.C.P. phase, whiie favouring the F.C.C. structure formation.
Chen and Sautter, in an investigation on the effect of pulse current on the mechanical
properties [Chen and Sautter (1976)], found that increasing the pulsed current density
improved the strength and hardness of the cobalt electrodeposits. in 1983, Riveiro
produced verv thin electrodeposited cobalt samples and studied their properties in
cornparison with those of bulk materials [Riveiro (198311. It was found that the first
deposited layers were different in properties from the buik material. The study was carried
out on both cobalt and nickel, and the same observation held tnie for the both metals.
in 1984, Cavallotti rt d studied the growth of electrodeposited cobalt and
concluded tha t a t low to moderate m e n t densities columnar growth predominated, whiie
at high current densities the growth "perfection" is lost, resulting in a "dendritic" growth
pattern [Cavailotti et nl. (1984)], Berezina ef a(. [1986] studied the effect of cobalt complex
formation in a chioride bath on the deposition of the metal. It was reported that the
formation of the cobalt comptex promotes ddocalization of surface electrons in the final
phase of metal reduction, In the earIy 1990s, Cui et n(. pubiished a series of studies
regarchg the deposition mechankm in various cobaIt dectrolytes; from deposition from a
simple cobalt chioride aqueous solution [Cui ef a/. (1990)], to deposition £rom a cobalt
Pure Cobalt System p. 87
Synthesis and Microstructural Characterization of Electrodeposited Nanocrystalline Soft Magnets
thiocvanate (Co(NCS)?) / N,N-di-methvl-fornamide @MF) solution with the addition of
chioride ions [Cui e t d (1991a)11 to deposition from a cobalt chloride / DMF electrolyte [Cui
r t d (1991b)j. It was found that the complexing of metal speaes pIays an important role in
the eiectrodeposition process, and the addition of anions (chioride in thiç case) &O affects
the degree to which the metal complexes. This effect is greater in non-aqueous sohtions
than in aqueous ones. Abyaneh and Pour [1994] published a paper reporthg the very
initial stages of electrodeposition of cobalt. It was found that after a monolayer was formeci
on the cathode, subsequent deposited layers were produced by way of nucleation and 3-
dimensionai growth. Under the range of operating conditions used, Xbyaneh and Pour
postulated the formation of both cobalt phases in the very initial stages of
electrocrystallization. This hypothesis was supported by the occurrence of two anostic
dissolution peaks when an anodic potential sweep was appüed to the electrode.
Quite recentiy, Abi dm! studied the eIectrodeposition of both nickel and cobalt in
low gravity [Abi (1996)I. No structural ciifferences were observed for cobalt when
compared to conventionally deposited cobalt.
A review of pertinent literature has shown that, in terms of engineering the
minoscaIe of the structure of electrodeposited cobdt, no systematic study has previously
been carried out. in particdar, no focus on the reduction of grain size has been noted, i-e.,
no generai trends in the grain size of eIectrodeposited cobdt were observed in previous
studies [Walker and Cruise (1978)J. The pin size depended very much on the operating
conditions and the type of electrolyte empIoyed for a particular investigation and many
investigators did not specificaiiy mention the grain size in their work. nius, the present
work is the first attempt to concentrate specifically on a drastic grain çize reduction in
Pure Cobalt System
Synthesis and Microstructural Characterization of Electrodeposited Nanocrystalline Soft Magnets
electrodeposited cobalt as a means to potentialk r n o q critical magnetic and mechanical
properties.
4.2 Experimentai Details
In this chapter, the formulation of an electrodeposition process and an optirnized
operating wùidow for pure nanocrystalline cobalt wiiI be discussed. The eIectropIating
solution selected for this system was a sulfate based bath containing cobalt sulfate, sodium
chloride and saccharin. Table 4-1 shows the composition of the solution used throughout
this part of the study.
Table 4-1
Composition of eIectropiating solution used in the pure cobalt system.
Component 1 Concentration 1
Cobalt sulfate (CoçO.p6H20) was the main source of cobalt available for reduction
in the solution. Borïc acid (&Ba) was used again for its bufferîng properties as weii as to
proIong the üfe of the pIating soIution [Sarojamrna and Rama Char (1972)l. Sodium
diloride (NaCI) was used to maintain the overaii quaiity of the eIectcodeposits. It should
Pure Cobalt Systern
Synthesis and Microstnictural Characterization of Elecîrodeposited Nanocrysîaiiine Soft Magnets
be mentioned that during the developmental stages in this part of the study, solutions
without the addition of sodium chloride yielded on$ brittle simples with extremely high
stresses and of very poor physicai quality. For some deposits, saccharin (GECCONHÇO-)
was added as a stress reiiever and a grain refinement agent [n-Sherik (1993)l.
The cathode materid was titanium (2cm x 2cm) while the anode was electrolytically
r;iissolvable cobalt (see Section 2.2). Electrolvsis was carried out at room temperature (=
33°C) for an average of 60 to 90 minutes per experiment. The solution was agitated with a
magnetic stir bar at approximately 200 R.P.M. during electrodeposition. !3ectrodeposits of
thicknesses around 100 to 150 microns were routinely obtained. M e r eIectrodeposition,
the samples were mechanically s trip ped from the t i tanium cathode for charac terization.
Pulseci current was used throughout this part of the study because pulsed
deposition was found to be very effective in reducing the grain size of electrodeposited
nickel [EI-Sherik (1993)l. However, when using pulse electrodeposition, the numkr of
control variables becomes rather large. in addition to bath composition, pH and temperature,
the peak current density (IP~J~) as weii as the pulse on (Tm) and puise off (Tm) t h e s can be
varied independentiy. For such complex -stems, the use of bctonai design to establish an
operating window for the svnthesis of nanqs ta iüne structures has proved successlul in
previous studies pertaining to the production of nanocrystaiiïne cobait-tungsten alloys
[Osrnola (1993)l. h approach similar to the one desaibed by OsmoIa bd. has been adopted
in the present work.
A 2 factond design was utilized to study the effects of TON, TOFF and IP& on the
miaostnicture and grain size of deposits produced fiom the saccharin-fiee and saccharin-
containing bath. The Iow (-) and high (+) d u e s for TON, TOFF and IR:& given in Table 4-2
were selected based on preliminary eIectropIating experiments using dired current (DC) and
Pure Cobdt Sys tem p. 90
Synthesis and blicrostmctural Characterization of Elechodeposited NanocrystaUine Çaft Magnets
previous experience with pulse-plated pure nickel pl-Sherik (1993)J and Co-W aiioys [OsmoIa
(lg!B)].
Table 4-2
Low (-) and high (+) values for puise on time (TON), pulse off tirne (Torr), peak current density (TF&) and the saccharin concentration in the plating solution.
Table 4-3
Parameter
TON (mec)
Ton (mec)
TON, Ton and I P ~ settings for electrodeposition experirnents 1 to 8 (saccharin-free bath) and 1' to 8' (saccharincontaining bath).
L m (-1 2
10
Pure Cobait Systern
(+) 8
40
Synthesis and blicrostructura1 Characterhtion of Elecîrodeposited Nanoctystaltine Soft blagnets
The particular settùigs of the 16 experimental runs were broken down into two groups
of 8. Experiments 1 to 8 were performed in the saccharin-free bath and experirnents 1' to 8'
were carried out in the saccharin-containing bath. This configuration of qeriments
essentially divided the experiment design into two series of 2 factorial design rum. The
conditions of the remaining three parameters and th& rotation sequence are listed in Table 4-
3. Figure &l is a graphicaI representation of the experimental parameters used for either of
the two 2 factorial design.
Graphical representation of the three variabies in the F factorial design experîments.
Foiiowing eIectroIvsis, the deposits were mechanîcaUy stripped from the titaniurn
substrate for structural anaiysis. Surface morphotogies were examined using sçannulg
electron microscopy whiIe grain size measurements were carrîed out using X-ray h e
broadening and transmission eIectron microscopy. Hardness measurements wing a
Vickers microhardness indenter were performed with a 200-gram Ioad, and the average of
ten measurements was taken as the f i d hardness value for the specixnen. 3mm riiscs for
Pure Cobalt Systern
Synthesis and Microstructural Characterization of Elechodeposited Nanoc~stalline Soft Magnets
transmission electron miaoscopy were mechanidv punched foiiowed bv ion milling
using a Gatan Duomiil with the folIowing parameters: 4 volts; two guns at 0.5d/gun; gun
angle - 12'; sample rotates at iiquid nitrogen (LN4 temperature.
A n additional series of experiments was carried out in the presence of an extemallv
applied magnetic field. Thiç series of experiments was perfanned in order to study the
effect, if any, of the magnetic field on the structure and qstallographic texture of the
electrodeposits. Plating parameters in the factorial design that produced a nanocrystalline
microstructure were used throughout tfüs part of the study. The extemal magnetic fieici
was applied using a Varian Associates mode1 V4055 eIectromagnet The nomina1 magnetic
field present in the gap was measured to be 3500 Gauss.
4.3 Results and Discussion
The fuiI analvsis of the factorid design includes details of morphology,
crvstallographic texture, microstructure and grain size as a function of the pulse plating
parameters. in the foUowing sections, the resuits are divided into two main parts, nameiy
those for the saccharin-free bath and the saccharin-containing bath, respectivelv.
4.3.1 Saccharin-Free Bath
SampIes obtained from the saccharin-free bath had very different morphoIogies and
textures depending on the plating conditions. Figure 4-2 shows the three different
morphologies observed in this series of experiments- Weil, in an extensive study of the
structure of electrodeposits, devdoped a comprehensive ciassification of the morphoIogies
Pure Cobalt Systern p. 93
Synthesis and Microstmctural Characterization or Electrodeposited Nanocrystalline Solt Magnets
obtained for various electrodeposited metais [Weil (1931)j. Since th% is one of the mosk
complete and in-depth studies of its kind, an attempt was made to catalogue the
morphologies observed in the present study as Fer Weil's classification (see Appenciix A for
a bnef summarv and description of Weii's dassification scheme) in addition to the more
intuitively ob~lous descriptions, e.g. needles, pyramïds, and so on.
AU the cobalt electrodeposits possessed one or a combination of the foilowing three
main morphology mes : type II-A (neede structures), type 1-B (uniform grain structures)
and a mixed type 1-A/[-B (Iarge areas consistuig of smalier grains) surface structures.
Figures 4-2 contains scanning electron micrographs of the three morphoIogy tvpes
observed in the cobalt electrodeposits. The deposit shown in Figure 4-21 iç from Run #2
(produced at Tc)N = Smsec, TOFF = ZOmsec and [ ~ rak = 0.x/crn2); the deposit s h o w in Figure
4-2b is from Run #3 (produced at Tm = 2msec, TOIT = 40msec and Ircak = O.N/m=);
whereas for Figure &2c, the sampIe is from R u n #4, produced at TOX = 8rnsec, Ton =
4Omsec and I P , , ~ = O.U/crn?, which shows the mi-ed type morphology.
ïhe grain size of the type LA structures (pyrarnids) was found to be around Spm
while the tvpe II-A structures (needes) have grain sizes around 2pm (measured across the
short axis of the needle features). Furthemore, the grain dusters measured around 5pm to
8pm in diameter. It shouId be noted that the mixed morphology deposits generdy consist
of relatively large areas (in the order of 5pm) with inter-dispersed smaiier features less than
one micron in size, as shown in Figure 4-2c.
AU electrodeposits obtained hom the saccharin-free bath were duiI in appearance /
lustre, macroscopicaiiy smooth, except for occasionaI hydrogen pits, and were quite brittie-
Using scanning eIectron microscopy, ali of the dectrodeposits obtained in this series of
experïments were found to be conventionally potycrystalline Le., their grain sizes were in
Pure Cobalt System p. 94
Synthesis and Microstructural Characterization of Electrodeposited Nanoqstalline Soft hrlagnets
the micrometer range. A s u m m q of the results is presented in Table 4-4, shotving the
predominant crystallographic texture component dong with the morphology type from
which "grain size" was measured.
F i m e 4-2
Çcanning dectron miaographs showing exampks of different morphologies observed on
electrodeposited pure cobait produced from the saccharin-hee bath; (a) TON = 8msec, TOR =
IOmsec and beak = 0.2A/cm2, (b) TON = timsec, Tom = ZOmsec and Ipe& = O.M/cd, and (c)
TON = amsec, Ton = 40msec and Ipr& =O0.2A/cmt.
It should be mentioned that the grain size was measured on scanning dectron
miaographs (at Ieast I O readings for individual crystaIs) and it is essentiai& a function of
Pure Cobalt System p. 95
Synthesis and Microstructuml Characterization of flectrodeposited Nanoc~stalline Soft Magnets
the respective morphology. Table 4-4 lists the dominant texture components, as
cietermined by X-ray ciiffraction.
Table 44
Response variables for experiments 1 to 8 as a fünction of the
TON, TOFF and I P ~ A setthgs conducted in the saccharin-free bath.
Run # 1 TON
l - 2
Tom 1 Ihak
- I -
4
5
Figure 4-3 contains graphical representations of the response variables as a Çunction
of the pulse plating conditions. Observing the r e d t s from the saccharin-free bath, it can be
concluded that variations in the pulse plating parameters, namely, TON, TOFF and I P ~ ~ have
a profound effect on the response variables: qstalIographic texture, deposit growth
morphology and grain size. It should be noted, however, that aü the electrodeposits
obtained from this particuiar series of experiments have grain sizes in the conventional
p o l y a y talline range.
Morpholog Texhve 1 1 GrainSize
(1 1 A (N) / 1 - (P) N - 2pm; P - 3 pm
+
8 i + + i + i
Pure Cobalt Sys tem
+
LA (P) M + 1-B (c) / P PT 5 . 3 . ~ ~ I
3
-
i
N = Needles; P = Pyramids; C = Grain Clusters.
- + 1 *
- l - +
(i0i0) 1 U-A (N) I N - 1.5 pm
-
+ 1 (11%)
1 1 6 1 -+ / - / + I ( 1 1 2 0 ) ! 1-A (P) 1 P - 4.5pm
-
1-A (P) I
I
(1110) 1 LA (F)
1 l P - 4.5pm
P - 5 p m
(1 150) 1-A (P) / 1-A + 1-B (C) P = 6 pm; C = 8.5 pm
Synthesis and ~licrostmctural Characterization ~FEiectrodeposited Nanoc~staiiine Soft Magnets
- Ton
Fimire 4-3
GraphicaI representation of the various response variables for the saccharin-free bath as
f ict ions of the pulse plating conditions; (a) texture response, (b) morphology response
and (c) grain size response.
Figure II shows two diff'raction patterns of cobait samples obtained from the
saccharin-free bath with the accompanying scanning electron rniaograph endosed as weil.
The type LA surface (pymmids) structure exhibits a strong (11%) texture cornponent
(Figure M a ) whde the type II-A (needles) morphology shown in Figure 42b is associated
with the texture component as obsenred From the X-ray diffraction pattern in Figure a b ,
As a result, an empiricai correlation can be found between the morphoIogy and the
crystailographic texture in the poly-stalIùle cobait eiecfrocieposits obtained h m the
saccharin-Free bath. This observation can be important when tailoring cobait
Pure Cobalt Sys lem p. 97
Synthesis and bIicrostnictural Characterïzation o f Electrodeposited Nanocrystalline Soft Magnets
electrodeposits for specific magnetic properties (e.g. coeravity) that are dependent on the
crystallographic texture.
3 JL! 30 10 50 60 70 80 90 LOO 110 120 130 1. iO 150
Fiwe 4-4
The two typical X-ray diffraction pattems obtained fiom cobaIt eiectrodeposits produced from the saccharin-free bath and its associakd surface structure; (a) pyramid type (II-A) has
a (150) prefened texture while @) needle surface structure (14) has a (10i0) texture.
Pure CobaIt System
Synthesis and MicmstructuraI Characterization of Elecîrodeposited NanocrystaIIine Çoft bfagnets
4.3.1.1 Microhardness
Vickers microhardness was carried out on the electrodeposited cobalt using a 200-
gram load. Table 4-5 shows the resuIt of these measurements. The hardness values range
from about 240 to just over 300. These values are consistent with hardness values
previously reported for conventional polycrystalline cobalt [SnzifIIe//s (1983)I.
Table 4 5
Results of microhardness measurements for cobalt samples obtained [rom experiments 1 to
8 (the saccharin-fiee bath); uncertain. represent one standard deviation.
Run # 1 Viclrers MiCi.ohudness [
1 Average: 1 276 VHN 1
4.3.2 Saccharin-Containing Bath
AU the samples obtained fiom the saccharin-containing bath - the second series of
23 factorial designs - were macroscopidy smooth and semi-bright. Using scanning
eiectron microscopy, al1 the eiectrodeposits obtained from this series of experiments
exhibited the same, rather featurdess morphology, as shown m Figure 45. The surface of
Pure Cobalt System
Synth&s and Microstructural Characterization of Electrndepasited NanocrystaUine Soft Magnets
the electrodeposits revealed features of around 300nm to 5OOnm in diameter. Howtiver,
individual grains could not be criticaiiy resolved.
Fiwe 4-5
Scanning eIectron micrograph of a tvpical cobalt eIectrodepos
obtained from the saccharin-containing bath. iit
A sumrnary of the results obtained from the saccharin-containing bath iç presented
in Table 46 in a simiIar fashion as m Section 4.3.1, and the results are &O reproduced in a
graphical form in Figure 4-6.
Figure 4-7 shows the difhcfion patterns obtained from a cobalt powder standard as
weiI as from rui electrodeposited cobalt produced under the folIowing conditions: TON =
Pure Cobalt System p. 200
Spthesis and rliiaostnictural Characterizaiion of Electrodeposited NanocrystaIIine M t Mapets
h e c , TOFF = ZOrnsec and i p , ~ = O.W/&. The cobalt powder standard contains
reflections from both the F.C.C. and the H.C.P. structures possible for cobalt, as welI as
some minor peaks hom cobait oxide.
Table 44
Response variables for experiments 1' to 8' as a Function of the TON,
Ton and i ~ , , k settings conducted in the saccharin-containing bath.
Comparing the locations of the peaks observed in the nanocrystailine cobalt
diffraction pattern, the main observation is that the (111) and (311) F.C.C. peaks are not
f o n d in the nanoqstaiiine pattern. Therefore, it may be concluded that the
nanocrystaliine sampIe is predominantiy comprïsed of the H-CP. Iattïce structure. In other
words, although cobaIt can exist in F.C.C+, H.C.P. or mixed structure components, the
existence of peak triplets ~LrongLy indicates a strong H-CJ?. component. Figures 4-8 anci 4-9
show the JCPDS powder diffraction Mes for cobdt in both structures IJCPDSPDF N o 5
727, N0.15-8061.
R u n l 1
2
6 ' + I + 1 (0002) 1-B/ smooth ' 8 3 m 1
Pure Cobalt System
O
- +
- I + 1 f 1 (0002) 7 1-B / smooth
(0002) 1 1-B / smooth 1 15nm 3 ~ - ~ +
B r + i + I + 1 (0002)
- 11.3nm
Tom 1 k 1 Texture
-
1-0 / smooth 1 llnrn
Morpholog 1-B / smooth - 1 -
q c I + 5 1 - 1 -
Grain Size llnm (0002)
- 1 (0002) I-B / smooth 125~11
+ 1 (Oûû2) / 1-8 / smooth \ IOnm 1
(0002) 1 1-B / smooth 10.5nm - -
Spthesis and Micrastnictural Characterization of Electrodeposited Nanocrystalline Solt Magnets
smooth
smoo smooth
- Ton
Graphical represenhtion of the various response variables for the saccharin-containing bath as functions of the pulse plathg conditions;
(a) texture response, @) morphology response and (c) grain size response.
Pure CobaIt System
Sythesk and b[iaostnictural Characterhtion of EIectrodeposited Nanocrystalline Soft Magnets
3 - F.C.C. ZJ - H.C.P. 3 - Co oxicie
powder standard
30 40 50 60 70 80 90 100 110
2 Theta (Degrees)
X-ray diffraction patterns fiom a typicai electrodeposited n a n o j s talhe cobait
as weil as a cobalt powder standard.
Pure CobaIt Sys tem
Synthesis and rCliaostnrctura1 Characterization of Electrodeposited Nanoaystaiiine Soft Magnets
From the analyses of the X-ray diffraction patterns, aii the cobalt eiectrodeposits
from the saccharin-containhg bath exhibited a (0002) crystallographic texture. This
observation can be explained by the fact that saccharin, a weii-known grain-refining agent,
acts as a surface diffusion bamier retarding lateral adatom migration on the cathode
surface. As a result, the presence of saccharin dong the cathode surface promotes the
formation of close-packed planes parallel to the surface of the substrate.
5-m
Co
Cobalt
Rad; FeKa Lambda: 1.9373 fiiter: d-apz O.S. -114.6 Cutoff: Int: Uisual VIcor: Rd: Hofer. Peebles. J. fim. Chem. Sac, 69 847 0
Sys: Hexagonal S.G.; P6Wrnmc (194) O: 2585 b : e: 4.868 A: G 1.62m fi: 8, CL t 2 mp: Rd: Ibid.
On: 8.871 Om: çS/FOHs fl0mBC130.18) €ax nu& *Y' Sign: ni: Rif:
US no.: 7448-484. Uiriures noimolly coeaist and puis cubic specimena con be retained at roorn tirnpemture. This transition ia mortinritic uith flr-338 CTioiono and TaLich. Tronc HUE m. 72û (1948). Filter ii U n 0 2 Rifennce tipenr: a=êSh37. c 4 . W . Cobalt has a face- caninrd eubic (Cu type) structure ( h a Tom) aboue458 C and a cp. hexagonal structure Colpha form) stable at room temperoture. Mg type. PSG h P 2 UutS8.93. Volume [CD]: 22.86.
h k l 1 0 9 8 0 2 1 0 1 1 8 2 1 1 8
1 0 3 2 8 0 1 1 2 201 0 8 4
JCPDÇ powder diffraction fiIe for H.CP. cobalt (PDF No. 5-727) [JCPDS (1990)I; 20 values iisted are for copper K, radiation.
Pure Cobah System
Synthesiç and Microstnictural Characterization of Electrodeposited Nanocrystalline Soft bfagnets
Cobalt
Rad: CoKo Lambda: 1.78841 Filtor, h h p : Cutoff: h t : Oiffioctometir VIcor: Roi: Noil. Bur. Stand. (US.) flonogr. 25.25 10 [1965)
Sya: Cubic S.G.: P63/mmc (1W)
a: 3 . M b: C; A: C: 1.6288 A: 8: c, 2: 4 mp: R i 6 ibid.
Or: 0.789 Om : SWFOU: R=224CBW.5) €O: nu& *y: Sigm 2U R.f:
Colot: Dok gray Pattein at 25 C. CAS no.: 7440484. Sample uor ptoparod at NBS by heoting cobolt oxalate in H 2 for 10 minutor ot 300 C. Spoaiorcopic onalyrir: 0.1 to 1.0% each o f Ni and âb; and 0.01 to 0.B rach o f M and Fe. florch Index. 8- Cd.. p. 272 Cu type. Tungrten u r d as intotnol rtondoid. PSC: cF4. flutE8.93. Volume [Ca: 44.54.
JCPDS powder diffraction file for F.C.C. cobalt (PDF No. 15-806) [JCPDS (1990)j;
20 values Listed are for copper K, radiation.
As shown in Table 4-6, the grain size of these deposits varied from about 8 to 15nm
as per X-ray line broadening measurements. For cornparison, Figure 4-10 shows the
brightfield, darkfieId transmission electron micrograph as well as the eIectron ciiffiaction
pattern of a nanoaystalline cobalt electrodeposit produced from the saccharin-containùlg
bath with the foffowing conditions: TON = h e c , Torr = lOmsec and r p , ~ = 00.2A/& The
average grain size as determined from the darklïeld trammission electron rnicrograph is
about 1Onm (based on a grain count of 200), which is consistent with the vdue obtained by
X-ray h e broadening (llnm) for this partidar deposit-
Pure Cobalt Systern
Çynthesis and ;LIicrostructural Characterization of Electrodeposited NanocrystaIIine Soft Magnets
Average Gnin Size 1 = 9.8 r 3.6nml
O 5 1 0 1 2 0 5
Grain Size (nm)
(a) Brightfield, (b) darkfield transmission cIectron micrograph and (c) eIectron diffraction
pattern (order of rings in increasing radik (LO~O), (OZ), ( loi) , (loi?), (1 1%). (LoÙ) )
and (d) grain size distribution @ased on 200 grallis) of a nanoqstaiiine cobalt
electrodeposit with an average grain size of 10 nm; the error represent one standard deviation.
Pure Cobalt Sys tem
Synthesis and Micmstruchimi Characterization of Electrodeposited NanocrystaIIine Soft Magnets
Vickers microhardness measurements were also perfomed on the electrodeposited
cobalt €rom the saccharin-containing bath using a 200-gram load. Table 4-7 shows the
results of these measurements.
Table 4-7 shows that the hardness vaiues ranged from about 375 to 64OVHN, which
is considerably higher than the range observed for the polycrystalline electrodeposits
produced from the saccharin-free bath. The high harriness values reflect the smaii crvstal
size obtained in these deposits.
Table 47
Results of microI-tardness €or cobalt sampies from experiments 1' to 8' (the saccharin-
Pure Cobalt System
containing bath); uncertain. indicates one standard deviation.
Rrm # 2'
Vi&m Micnihardness 0 602 I 21
Synthesis and h,licrostnichirai CharacterÏzation of Etec*odeposited NanocrystalIine Soft Magnets
Although the grain sue range for this series of electrodeposits is rather smd,
covering from about 8 to 15nm ody, a Haü-Petch plot simiiar to the one shown in Figure 3-
11 for nickel-iron was made for the nanocrystaiiine cobalt electrodeposits obtained for this
study, as shown in Figure 4-11.
Grain Size (nm) 15 13 11 10 9 8
95% confidence interval
Fime 4-11
A Haü-Petch plot for the electrodeposited nanocrystaiiine cobait obtained from the
saccharïn-containing bath; enor bars Ïndicate one standard deviation.
At 15nm, the hardness of the electrodeposited nanocrystaiiïne cobait is around
575VHN. As the grain sue decreases, the miuohardness inaeased slightiy, to a maximum
Pure Cobalt System
Synthesis and Microstructurai Characterization O€ Uectrodeposited Xanocqstalline Soft hlagnets
vaiue of about 64OVHN at a grain size of l0.5nm. Further deaease in the grain size is not,
however, accompanied bv the same rate of increase in the microhardness. In fact, it can be
argued that there is a deaease in the hardness of the material. Although this observation is
indicative of a simiIar transition from regular to inverse Hall-Petch behaviour often
observeci for nanocrystaüine materials [e.g. El-Sherik d al: (1992)], the iimited grain size
range rendered a conclusive observation rather difficult,
4.3.3 Synergistic Effects of Pulse Plating and Saccharin Additive
The conditions at which experiments 1 to 8 were carried out were exactly the same
as For the corresponding experlments '1' to 8'. However, the Iatter series of experiments
were carrieci out in a pIating solution containhg the addition of saccharin, unlike the
saccharin-free bath used for the first series. Although puise eIectrolysis has had some
success in reducing grain size during eiectrodeposition, the use of pulse current
eiectrodeposition alone was not sufficient to produce nanostmctured cobalt. ïhe use of
saccharin as a surface-active species was apparently needed to cause an additional effect
during electrocrvstallization, thereby producing nanocrystalline eIectrodeposits.
To prove the hvpothesis, four more experiments were devised in addition to the
ones in the 23 factoriai design, These experimentai conditions were designed to separate
and/or identily the individuai effects of pulsing m e n t and saccharin use. AU four
expwhents were carried out using direct m e n t (D.C.) pfating instead of pdsed m e n t
(P.C.) plating. The independent experimental variables were saccharin concentration (O or
35g/L) and current density.
Pure Cobalt Sys tem
Synthesis and klicrostmctural Characterization of Electrodeposited Nanocrystalline Soft Magnets
The "low" current density value was obtained From the average current density
(Section 2.3, Eq. 2.2) using the low values of TOM, Ton and I ~ r a k in the P.C. experiments (see
TabIe 4-2 and Table 4-3). The "high" current density value was obtained in a similar
fashion using the high values of TON, TOR and [ P ~ I ~ in Tables 4-2 and 4-3. in other words,
the low curent densiiy for this part of the study is the average m e n t density calculated
from the TON# TOR and bt.& combination prescribed for Run #l and #1'. Similarly, the high
current density is extracted from the plating condition iisted for Run #8 and W. Table 4 4
shows the cietails of these additional experiments.
Table 4-8
Parameters for experiments to separate elfects of pulsed m e n t and saccharin.
Run # A B
Of these four additional experiments, only three yieIded satisfactory samples for
subsequent characterization, Run #D repeatediy resuited in samples which were of very
poor physical attributes; Le., cracked, curIed and peeling off the cathode surface.
Apparently, such a high current ciensity in the presence of saccharin produces
unsatisfadory deposits. As a resuit, oniy three conditions (A, B and C) yielded sampIes for
W e r anaIysis.
C D
Pure Cobait System
Saccharin (#LI O O 35
Cunent Density (A/cm*) Iow high low
0.03 0.10 0.03
35 I high 0.10
Synthesis and Microstmctural Charactecïzation of Electrodeposited Nanocrystalline Soft Magnets
Figure 4 2 2 shows the three samples avaiiabIe after the additionai experimentation.
Ali thee sarnples exhibit similar morphologies, as was the case for experimental series 1 to
8. The size of the surface features observed was slightly mialler; both the type 1-A
(pyramids) and the type II-A (needles) surface shuctures are of a finer scale.
Fime 4-12
Scanning electron micrographs of sarnpIeç obtained
under conditions A, B and C in TabIe 48.
Figure 4-13 shows the three X-ray diffraction patterns obtained for the sarnples
produced in this section of the study. A iew observations cari be made regarding these
cobait deposits.
Pure Cobait System
Synthesis and Microstructural Characterization of Electrodeposited Nanocrystaiiine Soft Magnets
X-ray diffraction patterns of cobait ciepositç produced for the study of the combined effect
of s a c c h a ~ and pulse pIating; (a) low current without sacdiarin, (b) high current without
saccharin and (c) Iow current with sacdiarin addition.
Firstiy, ai i of them have an H.C-P. crystai structure evident from a cornparison
between the peak positions in the diffraction patterns and those listed for H.C.P. cobait in
JCPDS powder iiiffraction Me 5-727 (see Figare C9). Çecondiy, for the two sampIes
produced without the use of saccharin in the solution, the diffractions patterns are very
rimilar (Figures 413a and C13b, respectively) and exhibit a very strong (1 1%) prefmed
crystallogaphic texture- in tab. other Uw the (11%) peak refiection, the intensity of al l
Pure Cobdt System p. 112
Synthesis and Microstnictural Characterization of EIectrodeposited Nanoaystalline Soft Magnets
the other peaks is ahos t zero. Unfortunately, o n . one out of the two conditions produced
samples from the experiments carried out with saccharin in the solution, Nevertheless, the
diffraction pattern shown in Figure M 3 c suggests a preferred qstallographic texture.
Comparing the theoreticai intensity ratios iisted for H.C.P. cobalt with this sample, it can be
conduded that there exhts a (10i0) / (0002) ; (1 150) triple texture.
Combining the results from the factorial design for puise plating (Sections 4.3.1 and
4.3.2) and the additionaI direct current plating experiments presented in this section, the
foliowïng conclusions can be drawn. From experiments 1 to 8 (pulse current without
saccharin), only conventionai polyqstaihe electrodeposits were obtained; in the direct
current plating experiments with and without saccharin, electrodeposits exhibithg oniy
conventional polycrystalline microstructures were produced. It was oniy from experiments
1' to 8' (puise current and saccharin) that nanoqstalline cobaIt could be synthesized. in
other words, for pure cobalt, neither the individuai use of puise current during plating nor
the addition of saccharin alone suffices to produce nanocrystaiiine cobalt samples. The two
operaîing parameters interact synergistidiy in faciiitating the production of
nanocrvstalline cobalt, at least for the piating bath used throughout this part of the study.
4.4 Plating in the Magnetic Field
in conventionai processing routines for soft magnetic materiaIs, extemal magnetic
fieids during various stages of production are sometimes applied to manufacture products
with more desirable structures and properties [hdncacos and Romankiw (1994)l.
Examples uiciude annealing and deposition (evaporation as weiI as electrodeposition) of
Pure CobaIt System p. 113
Synthesis and Microstructural Characterization of Electrodeposited Nanocrystalline Saft Magnets
thin fifrns in a magnetic field. Even in the early literature, publications cieahg with the
presence of an extemal magnetic fieid during electrodeposition can be fourid (e.g. Rentsch
[1%21).
The chief purpose of the applied extemal field during the Jeposition of soit
magnetic materiais is to affect the qstallographic texture of the deposits in order to Iower
the overall coercivity of the material. in other words, since cobalt is an anisotropic material,
sûucturally and magnetically, its magnetic properties depend highiy on the
a-ystaiiographic textures that devdop during its production and electrodeposition within
an extemal magnetic field is one way to affect the preferred texture exhibited by these
materiais during the production stage.
The purpose of the ~urrent set of experiments was to stuciy if an extemal magnetic
fieId also has an effect on the structure (in partidar, crystailopphic texture) of
nanocrvstalline cobalt.
Figure 4-14 shows the experimental setup for this part of the work; Figure &24a
shows the setup for the mapetic field parallel to tbe deposit surface whereas Figure C l l b
shows the same for the field perpendicular to the deposit surface- The speciaiiy designed
plexiglas vesse1 containhg the platine solution, anode and cathode was pIaced in the
middle of the field gap. The operating conditions shown in Table 4-9 ( h m the saccharin-
containing bath) were adopted from the puIsed m e n t experiments (Section 4.3). These
were operating conditions for which nanocrystalline cobalt sampies were produced. A 2crn
x 4 m Ti cathode was used throughout this series of experïmentation instead of the 2cm x
2cm Ti square previous ernpIoyed.
Pure Cobalt System
Synthesis and Microstmctuml Characterization of Electrodeposited NanocwstilIine Soft blagneb
s applied f i l d
power b- h u m t e r
Schematic diagram (top view) showing setup of the experiments camed out in the presence of an extemaiiy appiied magnetic fieid (3500G);
(a) field paraHe1 to the deposit surface and (b) field perpendidar to the deposit surface.
Table 4-9
Plaihg conditions used for the study of eIectrodeposition of cobalt in a magne tic field.
Pure Cobalt System
- -
Parameter TON TOR I~eiik
Saccharin Temperature
Vaiue 2mec IO mec
200~1A/ cmz
25 g/L 23 2 2OC
Synthesis and i\iIicrostructural Characterization of EIectrodeposited Nanocrystaiiine Çoft Magnets
Before experiments were conducted in the two field configurations outlined, a
number of electrodeposition runs for the production of nanocrystalline pure cobalt were
repeated in the plating vessel, without the application of magnetic field. The sampIes
obtained fiom the repeat experiments were anaiyzed in t e m of structure and hardness
and it was confirmed that the developed electrodeposition process were reliable under
these speaal experimental conditions and that the results were reproducible. Table CIO
shows the various microhardness measurement results of these pure cobait deposits
obtained from the special vessel without the application of any magnetic field. The
microhardness results were in the same range as the values obtained in Çection 4.3.2.
Table 4-10
Microhardness of cobalt deposits in the absence of applied magnetic field;
error represents one standard deviation.
[ Microhardness (VHN) 1 1 no field 1
Sample #1 SamaIe #7
The X-ray diffraction patterns of these electrodeposits exhibit the same
crystaiiographic textures as previousiy observed, Le., they have the (0002) basai plane
parailef to the surface of the deposits (see Figure 4-15c). The simiIaritv indicates that the
changes in the experimental setup did not significantiy change the structure of the deposits
597 2 19 623 2 42
Sarnple #4
Pure Cobalt System p. 116
390 ~ 2 . 5 Average: 610 VHN
Synthesis and Microstnictural Chamcterization of Electradepasited Nanoqstalline Soft bfagnets
and that the results are easiiy reproduable even though the geometry of the plating ceU is
changed.
The in-field sarnples were also anaiyzed in t e m of crystallographic texture as weli
as hardness. Figure 4-15 shows the diffraction patterns of the in-field plated cobait deposits
produced from the two magnetic field orientations.
'100 - 0 400 - G 700
2 PCO - C- : iua
= LI10 - 100
Typical X-ray diffraction patterns of in-fieid plated cobdt samples with the applied field
(a) parailel to the deposit surface, and (b) perpendicdar to the deposit surface; (c) Pattern
for a sampIe plated without a field using the same plating c d .
Synthesis and blicrostnictural Characterization of EIectdeposited Nanoccystalline Soft Magnets
After examining the X-rav diffraction patterns shown in Figure 4-15, one can
conclude that the H.CS. q s t a l structure observed for cobalt deposited without the
magnetic field is preserved for in-field eIecûodeposition. in addition, both of the in-field
samples are of nanocrystalhe microstructure and exhibited a (0002) prefened
crystallographic texture. However, for the sampie plated with the magnetic field
perpendidar to the cathode, an additional weak (1 1%) texture component appeared.
Results of the hardness measurements carried out on the study of rnagnetic field on the
electrodeposition of nanoqstaliine pure cobdt; enor represents one standard deviation.
no field Average Vickers hardness 1 Averaee: 610 VHN 1
1 ~ a r d e l field 1 Sample #1 616 = 35 Sample #2 Sample #3
612 2 32 594 24
Sample #4 625 2 20- Average: 612 VHN
perpendicular field Sample #l Sample #2 Sample #3
The Vickers microhardness measurements on the in-fieid pIated cobaIt
electrodeposits were carried out as w d , Table 441 shows the r e d t s of these
measurements. The r e d t s for the no-fieId samples have been induded for comparison.
586 r 14 624 t 15 615 2 39
Sampie #4
Pure Cobalt System p. 118
602 = 28 Averaee: 607 VHN
Synthesis and klicrosûuctura1 Characterization of Electrodeposited Nanocrystalline Soft Magnets
Mthough the microhardness of the cobaIt deposits pIated pardel to the magnetic field was
slightiy higher than those plated with the cathode surfice perpendicular to the field, both
values are essentidy the same as the reference cobaIt deposits obtained without an
application of magnetic field. From the resuit of thk analysis, it was concluded that at the
Ievel of the applied magnetic field used for this study, the microhardness was not
significantiy affected by the presence of an extemal magnetic field.
4.5 summary
From the results presented in this chapter, it can be concluded that the production
of pure nanoqstalline cobalt by electrodeposition methods is feasible. [t was found that
neither pulse plating nor the addition of saccharin alone was sufficient to facilitate an
operating window for the production of nanocrystaiiine cobalt. However, the synergistic
effect of the two factors provided the necessary approach for the synthesis of
nanocrystalline cobalt electrodeposits. These were produced from the saccharin-containhg
eIectrolvte. The smaliest grain sue of the electrodeposits obtained was f o n d to be about
lOnm as per transmission electron microscopy, and the maximum hardness measured was
about 64OVHN. Since the experîmentation was essentidy divided into two separate
sections involving different plating b a h , s p d c condusions are given in the sections
below.
Pure Cobalt System
Synthesis and Microstructural Characterization of Electrodeposited Nanocqstdlline Saft Magne ts
4.5.1 Surnmary for the Saccharin-Free Bath
From the analysis of the various response variables hom the saccharin-free bath, a
number of conclusions c m be reached. Firstiy, ai i of the cobait electrodeposits were
conventional polycrystaiiine in nahue regardless of the plating conditions employed, Le.,
their gain size was in the micrometer range. Secondly, this series of cobalt electrodeposits
exhibited a variety of surface structures, mainly the [-A (pyrarnid) and II-A types (needes)
of morphology, with some regions in some deposits showing a type 1-B (clusters) surface
structure. One significant finding was that the crvstalIographic texture of these deposits
could be correlated to the morphology Qe; type CA structure is related to a (1 1%) texture
while the type II-A structure is associated with the (10i0) texture.
The microharciness of these deposits fali into the same range as reporteci for
conventional polyqstaliine cobalt.
4.5.2 Surnmary for the Saccharin-Containing Bath
iU1 cobalt e1ectrodeposit.s obtained from the saccharin-containing bath had a
nanocrystaiiine microstructure, with a 1-5 type surface structure. Even under high
magrufication scanning electron miaoscopv, individuai grains codd not be discerneci.
Transmission electron miaoscopy c o n f i e c i the nanocrystalhe structure of aii the cobalt
samptes in this senes, and a smaiIest grain size of about 1Onm was found for these samples.
One interesting point to note is that, not on* did the electrodeposits exhibit a
nanocrystalline miaostructure, ai i the cobait deposits presented a (0002) basai plane
crystaliographic texture upon anaiyçis by X-ray cliffraction. The Vickers microhardness
values for these samples were in the range of 575 - 64OVHN. The miaohardness values
Pure Cobalt Systern p. 120
Synthesis and b!icrostructural Characteriration of Electrodeposited Nanocrystailine Saft Magnets
indicate a possibility of a transition from regular to inverse Hall-Petch relationship,
dthough hrther investigation is needed in order that a more definitive and conclusive
statement can be justifiably made.
4.5.3 Summary for the In-Field Platina Experiments
From the analysis of the results, the application of a magnetic field during
eIectrodeposition did not have any noticeable effects on the deposit crystai structure and
hardness. However, it was observed that plating with a magnetic field perpendicular to the
cathode prornotes the formation of a weak (1 120) texture component.
Pure CobaIt System
Synthesis and Microstructural Characterïzation of Electrodeposited Nanocrystaltine Soft Magnets
4.6 Ref erences - -
AH. Abi, C. Rilev and G. klaybee, "Electrodeposition of Ni and Co in Low Gravity",
/ozmznluf iMntrnhfs SnCirm, Vo1.31 (1996), No.7 p. 1767.
N.I. Abramova, N.N. Balashova and E.I. Meshcherskaya, "Porosify, Brighhiess and
Hardness of Cobalt-Nickel-Phorphorus Electrolytic M o y Prepared in an üitrasonic
Field", E/rctrochr~~&ty, Vo1.5 (1969), No.5, p.528.
M.Y. Abyaneh and T. Pour, "Initial Nucleation and Growth of Electrodeposits of
Cobalt", forrnrr/of L ~ P hrstitzdr ufMefi/fin~>hlirg, Vo1.72 (1994), Pt.1, p.19.
P.C. hrincacos and L.T. Romankiw, "h/Iagneticaily Çoft Materials in Data Storage:
Their Properties and Eiectrochemistry", Ahnncrs in E / r . r r ~ ~ / r ~ ~ t ~ i i ~ / S ~ f r m ~ iznd
Engtir~enk~ K. Cerischer and C.W. Tobias (eds.), Vo1.3, VCH Publishing inc., New York
(1994). p.27 .
R.C. Barrett, "Plating of Nickel, Cobalt, iron and Cadmium from Suifamate %lutions",
P r u c r e d ~ ~ s of the A~llrrri-n E/tctrop/ntm'Soc12& Vo1.47 (1960), p.170.
S.1, Berezina, R.M. Sageeva, L.G. Sharapova and V.G. Shtyrün, "Electrodeposition of
CobaIt from Glvcina te Electrolv tes", Prof~ctiutr gf-Mfff& Vo1.2(1986), No.?, p.228-
1. Bursuc, A. Ojog and V. Tutovan, "Contributions to the Study of Magnetism of
Electrodeposited Cobalt", Physki Stnft~s Soi& Vo1.4 (1964), p.161.
P.L. Cavallotti, D. CoIombo, E. Gaibiati and R, MartineUa, "Ceiiuiar Growth of
Transition Me taIs by Electrodeposition", Pmtecdnys of Lhr I l f i Worfd C~ngess orr rLkfd
fiizrLsInnirg (XVZERFN'ZSN '9'9, Jerusalem, IsraeI, Metal Finishing Congress (1984), p.235.
ES. Chen and F.K. Sautter, "The Effeds of Puise Current Plating on the Mechanicd
Properties of Cobdt and Cobait-M24", P/RhhgndS~r~cefikrSh~hrii.g, VoL63 (1976), p.38.
Pure Cobait System p.122
Synthesis and Micmstructural Characterization of Electrodeposited Nanoqstalline Çoft Magnets
Cobalt information Center, "Bibliographv on Eiectroplating of Cobalt and Cobalt
Ailoys", Cobn/t /nfinr.nfbn Ce& BibIrOgrqh~ No.lOr, Cobalt information Center,
Columbus (1961); as cited in M o r d [1964].
W. Cochrane, Praceedhgs ofthe Phpfs 50ai.t~ V01.48 (1936), p.723; as cited in Morral
[1964].
C.Q. Cui, S.P. Jiang, A.C.C. Tseung, "Electrodeposition of Cobalt from Aqueous
Chloride Solutions", /or~mn/#f the ~~ectrod~e~t~ic~f SonP&, Vo1.137 (1990), No.11, p.3418.
C.Q. Cui, S.P. Jiang, A.C.C. Tseung, "Mechanism of the Electrodeposition of Cobalt(1I)
Chloride in NIN-Dimethy iformamide (D MF) %lu tion", /orm/ of 1% E/~cfroc/rrr~zica/
Çocietv, Vo1.138 (1991a), No.1, p.94.
C.Q. Cui, S.P. Jiang, A.C.C. Tseung, "Mechanimi of the Electrodeposition of Cobalt(I1)
Thiocyanate in N, N-Dimethylfomamide (DMF) Solution and Effect of Chioride Ions",
/ornznf of fhlr E/ect~~dz~tr~icn/Society~ VoL138 (1991 b), No.4, p.lOO1.
C Duval and C. Liger, Car@ Rmd, Vol.216 (1943)) p.249; as cited in M o d [1964].
A M . El-Sherik, U. Erb, G. PaImbo and KT. Aust, "Deviation from Haii-Petch
Behavious in As-Prepared Nanocrystalline Nickel", S c t r j ~ Metrnl~rgici rf rtlncnir/.h,
Vo1.27 (1992), p.1185.
A M . El-çherik, "Synthesis, Stnrcture and Properties of EIectrodeposited
Nanocrvstalline Pure NickeI", Ph.D. Thesis, Queen's UniversiSf, Kingston, Ontario,
Canada (1993).
N.P. Fedotev, 2 z r PnXhd Mrrk, VoL32 (1959), No.7, p.1542; as àted in Morral[1944].
D.M. Fegrado and J. Balachandra, "Measuement of internat Stress of Cobalt Deposited
EIectroIyticaiiy hom Fluoborate Baths", /or~md~~Snkn/r~c~nd~nd(~stnhIRese~r~/I fhd,ir)r
Vol.13B (1954), p.753.
krre Cobalt System
Synthesis and LIicrostructural Characterization of Electrodeposited Nanocrystaiiine Soft Magnets
G.I. Finch and CH. Sun, T~RIISRCLIOIIS qfihe FFnray Sune& Vo1.32 (1936), p.832; as ated
in Monal [1964],
G.I. Finch, H. Wilman and L. Yang, ihRdRy Sonkfy D15~11ssions; Vo1.l (1947, p.14; as
Qted in Mond [1964].
R.D. Fisher, "The Muence of ResiduaI Stress on the Magnetic Characteristics of
EIectrodeposited Cobalt and Nickel", fo~m~nioffh~ E[Pctroc/rrt~~ic~i So&&, Vo1.109 (1962),
p.479.
T.C. Franklin and SA. Matthew, "Effect of %me Additives on Volumes of Activation in
the Electrodeposition of Cobalt, Nickel, and Silver", ju~~mni~ffhr E/rctro~-/rrr~~kniSonB~
Vo1.136 (1989), No.12, p. 3627.
A.W. Hothershail, "Stress in Electrodeposited Metais", h~frrnai S . s s r s hrir Mrfnis nnd
.-l/ioys S ~ y l r o s i ~ t ~ P ~ C P P I ~ I ~ S , institute of Metals monograph and Report Çeries No.5
(1947, p.107.
A.W. Huii, Physrcn/Rmiii~, Vo1.17 (1921), P.57l; as cited kt Morral [1964].
D. JiIes, /rrtroCrrrrbbn tu rkhgnrticfn nmi'~Mngn& ~MnCmkk Cha p man and Haii (l99'1).
B.Y. Kaznachei and V.M. Zhogina, ~Mefn~/mtdrnie i Obmbofkn TL?P~- MF. i Spfr~nr,
Sbornik Statei (2957) , p.77; as cited in MorraI[1964].
H. Kersten, Plysii, VoD (1932), p.274; as cited in M o r d [19û4].
S.M. Kochergin, at~r, fi2 ah., Vo1.26 (1952), p.1610; as cited in Morral[1964].
J.H. Lindsey and HJ* Read, "Some Properties of Electrodeposite J Cobalt", Phtzhg,
Voi.57 (1970), p.497-
J, Matulis, A. Bondevas and K. Gaglas, E/ecfrodtposr?hn nrrd Srf@ce %m?nt'nf, VOIS
(1972), p.123; as ated in Waker and Guise [19781.
Pure CobaIt System
Sy nthesis and Microstructural Characterization of Electrodeposited Nanoc~stiiiine Soft Magnek
F.R. Morral, "Electrodeposition of Cobalt", Metn/Rrzikhin& Vo1.63 (1964), No.6, p.82.
F.R. Morrai and W.H. Safranek, ~Mo~r'rnr Ekfrophhkg 3rd edition, F.A, Lowenheirn
(ed.), John Wiley and Sons, New York (1974).
R.C. Newman, "The Lattice Spachgs of Thin Electrodeposits of Cobalt and Nickel on a
Copper Single Crystal", Pm'~en'r'trgsofthe Phy~r'csSu~7e~v) Vo1.69B (1956), pA32.
G. Okund, "Crys ta1 S tmcture of Electrodeposited Cobalt", Bc~iietriz u f th~ Lhizwszii offhr
Os& Pr1./i,~-ficre, Series A, Vo1.4 (19561, p.89.
D. Osmola, "Sdynthesis of Nanostructureci Co-W Moys", MSc. Thesis, Queen's
Universihr, Kingston, Ontario, Canada (19%).
MA. Pangarov and St. Rashkov, "Electrolytic Deposition of Alpha and Beta
Modifications of Cobalt", Cu~npfrs RPIII~~IS L& ~Aciderr~iBu/Bnre rirs S ~ ~ ~ L ' C ' S , Vo1.13 (1960),
p.439.
L. Reiner, "Magnetic investigations of Electrolyticdy Deposited ïhin Cobalt Layers",
Zm'rsc/frr~/7~r ~Vnft~@nchiiq, V01.12a (1957, p.1014.
M. Rentsch, P!ysici SInfi~s SuhIr; 1, K105-6 (1961); as Qted in Morral(1964).
J.M. Riveiro, "Optic and Magnetic Properties of Very Thin Electrolytic Co and Ni
F W , E/rcfroc/rNrzri-rrilcfrr, VoI.28 (3983), No.6, p.813-
Y.N. Sadance, 5 4 % ~ Trc/rno(Dyy, VoI.5 (1977), No.2, p.97; as ated in Waker and Cruise
[1978].
W.H. Safranek, l71e Pntprtks of Elcfrndios~Ped MrtnIs nnu'A//ays - A Xrrndbuuk, 2nd
edition, American Qectroplaters andsurface Finishers Society, Orlando (1986).
M. Sarojamrna and T.L. Rama Char, "Electrodeposition of Lron-Chromium-Nickel
Aiioys", Mefui Enkhing Voi.70 (1972), No.% p.36.
Pure Cobalt Systern
Synthesis and LIicrostructuraI Characterization of EIectrodeposited NanocrystaIIine Soft Magnets
SzniII,eI/s Meta1.c Re#r~ncr Book, 6th edition, E.A. Brandes (ed.), Butterworth & Co. Ltd.,
London (1983).
R. W a e r and B. Cruise, "The Use and Production of Electrodeposited Cobalt", ~Metd
Fr'nis/tri-r$ Vo1.76 (1978), No.6, p.45,
R. Weil, "The Structure of Electrodeposited Metals", Ph.D. Thesis, The Pennsyhania
State CoUege, Pennsylvania (2951).
Pure CobaIt System
Chapter 5
Cobalt-Iron Binarv Svstem
5.1 Literature Review
Cobalt aiIoys have been electrodeposited for about 100 years [Morral and Safranek
(1974)], and the earliest references of the binary cobalt-iron system were cited by Brenner
[1963]; the most important ones inciude the papers by Glasstone and Speakman
[1932,1933]. Other earlier works that deserve reference indude and Aotani [1950] and
Rotinvan rt nl, [19591.
in the most general terms, d a t e bath were most commonly used for the
electrodeposition in the bùiary cobaIt-iron .stem, although others such as chioride and
Watts type solutions were sometirnes utiiked.
in 1974, Sadak and Sautter produced cobalt-iron alloys with iron content up to
about 12wt.X from a Watts type bath in an effort to study the mechanicai properties of
these alloys [Sadak and Sautter (1974)], A few years later in 1978, Sadak &al, studied the
production of cobalt-üon doys from the same bath with the use of insoluble anodes
[Sadak rt d (1978)l. hound the same time, Armyanov t.t n/, produced binary cobalt-iron
d o y s from a sulfate bath with compositions up to about 50 wt% iron [Armyanov rt d
(19ml-
Synthesis and Microstructural Characterization of Electrodeposited Nanocrystalline Soft Magnets
in a sulfate bath, binarv aiiovç with iron contents as high as 80 at.% were obtained
by Srimathi and Mayanna [Srimathi and Mayanna (1982)J. From another sulfate bath,
Admon and Itay [1984] produced cobalt-iron alloys with compositions up to about 20 wt.%
bon.
In 1987, Liao, in an effort to produce 9Ko-lOFe, a high moment, zero
magnetostrictive dey, electrodeposited cobalt-iron alioys with iron content up to about 10
at.%. However, no bath composition was given p a o (1987)1. Chang et d [1990] used a
chloride-based electroIyte to produce soft cobalt-iron alloys as possible replacement
coatings for Permaiiov@ deposits. In the course of the study on interna1 stresses in binary
aiioys of uon-group metals, Çotirova-Chakarova and h y a n o v produced cobalt-üon
aiioys (iron content from 32% - 75%) from a hybrid suIfate bath containing iron ammonium
sulfate (Fe(NH&(ÇOLL)r) [Çotirova-Chakarova and h y a n o v (Z990)I.
El-Halh and Fawzy [1993] produced dovs with ùon contents up to about 80 w t.%
from a series of sulfate b a h . Shinoura and Kamijima aIso produced cobait-iron alloy films
in 1993 [Shinoura and Kamijirna (1993) J. The main focus of this study was on the effect of
some Iess common additives (e.g. Zprop yn-1-01 (Pm) and na phthdene-l,3,6-Wonic
hisodiumsait ( N E ) ) on the magnetic properties of the ailoy tïims. In that same vear,
Partain d 17[. reported on a study involving electrodeposited binary and ternary d o y s of
transition met& partain et RL (1993)]+ From a sulfate sdution, cobalt-uon ailoys were
obtained and the degree of anornatous codeposition of iron-cobalt was found to be the
lowest In a similar investigation, krtazzoli and Pletcher studied the effect of various
pIating and operating conditions on the composition and morphobgy of iron-cobait doys
[Bertazzoli and Pletcher (1993). in a study to evduate the performance of cobalt-ùon for
S-mthesis and klicrostnictural Characterization of EIectrodeposited Nanocrystaiiine Soft Magnets
the oxvgen evolution reaction (OER), aiioys with iron contents between 28wt.% and
43wt.% were produced [Brossard and Lessard (1993)l.
In 1994, kcos d aL produced, dong with cobait-nickel ailop, cobalt-iron aiioys
from a sulfate soIution [Arcos rt d (1994)l. The focus of the work was on the
electrochemistry, e.g., effect of rotational speed (R.P.M.) of a rotating electrode on current
waveforrn and efficiencies, etc.. No structural data was given for the cobalt-ïron alloys in
this report.
Kakum rf d producecl cobait-uon allovs and studied their structure and
morphology in 1997 [Kakuno ff nl: (1997)1, and found that the structure of the a b y
changed from hexagonal at low üon contents to a body-centered cubic (B.C.C.) structure at
high iron contents.
As for the nickel-iron system and the cobalt etectrodeposits, previous efforts in the
electrodeposition of cobalt-iron d o y s ciid not speciEicaUy address the grain size of the fina1
alloys produced.
5.2 Experimental Details
in t e m of the development of the pIating bath for nanostructured cobalt-iron
aiioys, the saccharin-containing electroplating sohtion used throughout the work on
nanocrystaiiine cobalt (nanoshcture-producing plating bath) was taken as the starting
point for the cobalt-iron system. iron d a t e (E:eSO4 was incrementally added to the
solution to effect various aiioy compositions (percent iron). Table 5-1 shows the bath
formulation used for this part of the research. D-C- plating technique was used throughout
this part of the work.
Cobalt-lron Binary System
Synthesis and MicrostructuraI Chamcterïzation of Electrodeposited Nanocrystaliiie Soft Magneis
Table 5-1
It should be noted that when the solution was not in use, nitrogen gas was purged
continuously through the solution in order to avoid oxidation of Fet- ion in the presence of
oxvgen residual in the solution. As was the case for the nickel-uon plating baths, the Me of
the cobalt baths could not be sustained for extended periods of time without nitrogen
purging. Otherwise, the degradation of the plating solution was visiblv observed, as
brown precipitates form in the solution, which was believed to be iron oxide (see Section
Composition of the eIectropIating solution used in the cobalt-iron system.
km? titanium square
Component CoÇO~*6H~O
&Bo3
NaCi
C&&CONHS02
FeS014Hz0
was employed as the cathode while electrolyticaiiy
de cobalt squares encased in titanium mesh were used as the anode. The range of
iron sait concentration was between Og/L to 5Og/L (in 5-gram increments).
Eiectrodeposition was carried out using D.C. plating at two current densities: 501nA/cmt
and 100mA/&, respectiveiv. The temperature of the eiectroiyte was maintained at 40 t
2°C while the pH was monitored and adjusted (using sulfunc aad) to about 4. EiectroIysis
was carried out for approximately 45 minutes and the deposits were mechanicaiiy stnpped
from the substrate for stnicturai analysis.
Concentration 350 g/L
30 g/ L
25 g/L
75 g/ L 0-50 g/L
Cobalt-iron Binary System
Synthesis and Microstructural Charactecization of Electrodeposited Nanoc~sîalline Soft Magnets
It should be noted that deposits obtained with üon sulfate concentrations greater
than 50g/L were of poor quality, and unsuitable for further analysis; results pertaining to
these deposits are not included in this report. This observed deterioration of Jeposit
quality is beiieved to be due to the nature of the plating solution itself, which is essentiah
a cobalt-based formulation.
Scanning electron microscopy was wed to study the surface morphologies while
grain size measurements were camed out using X-ray line broadening and transmission
eIectron microscopy. Hardness measurements using a Vickers rnicroharciness indenter
were performed with a 50- to 100-gram load, and the average of at least ten measurements
was taken as the final hardness vaIue for the deposit. 3mm discs for transmission electron
microscopy were mechanicaiiy punched foiiowed by ion milling using a Gatan Duomiii
with the foiiowing parameters: 4 volts; two guns at 05mA/gun; gun angle = 12"; sample
rotates at iiquid nitrogen (LN?) temperature.
5.3 Resdts and Discussion
5.3.1 Alloy Composition
Figure 5-la shows the iron content of the binary alIoys as a function of the uon
sulfate concentration in the solution for the experiments carried out using a m e n t density
of 50mA/cm2- As the concentration of üon sulfate was inaeased, the iron content of the
aiiov increased, as expected. For Iow concentrations of iron sulfate, the initial trend is
hem. However, the inaease in iron content starts to deviate from the linear rdationship
at higher concentrations of iron saIt in the solution. A simiiar overaII trend can be seen
CobaIt-Lon Binary System
Spthesis and bficroshctirrai Characterization of Electrodeposited Nanocrystalline Soft Magnets
with the electrodeposits obtained using a m e n t densitv of 100 rnA/cmz, as shown in
Figure 5-Tb.
Edo4 Concentration (fi} Fe504 Concentration (gL)
O IO 20 30 40 50 60
FeSO* Concentration (@)
Fime S-1
Iron content of the electrodeposited cobdt-iron d o y s as a function of the iron d a t e saIt
concenûation in the solution: (a) ment density i = 5 0 d / a n ' . @) i =100mA/cm~ and (c) a composite plot for both current densities sitown in (a) and @); error bars represent one standard deviation.
Cobait-lron Bïnary Systern
Synthesis and Microstructural Characterization of Electrodeposited NanocrystaIIine Soft Magnets
Figure 5-le, a composite of Figure 5-la and lb, induding both sets of data points
shows that, given the same set of plating parameters and bath formulation, current densihf
had very Little effect on the composition of the aiioy. k c o s e t d [1994] observed a similas
current density independence with pulse plated cobait-iron alloys. This observation may
prove important when considering plating rate in a commercia1 operation as the plating
rate at 100mA/cxr$ is expected to be twice that for the rate at 5OmA/an2, assuming similar
m e n t efçiciencies.
5.3.2 X-Ray Diffraction Analysis
Throughout the series of aiioy deposition experiments, the crystdographic
structure changed considerablv. These transitions wiii be described in the foiiowulg in
t e m of the ùon content in the aiioy deposits.
Fus*, it shouid be pointed out thiat samples obtained from the two series of
experiments have verv similar X-ray diffraction patterns. For example, Figure 5-2 shows a
cornparison for X-rav diffraction patterns of samples obtained with the two different
cunent densities while keeping the other operating conditions constant Figure 5-?a is the
pattern hom a deposit produced at s'OmA/m2, while Figure 3-2b is from a deposit
produced at 100mA/d . The composition of the two samples was 11.2wt% iron for the
alloy deposit produced at 50mA/an2 (Figure 5-2a) and 10.5wt.% iron for the speàmen
deposited at ZOOrnA/cm~.
The two diffi-action patterns essentidy show the same peak positions and similar
relative intensities. This is not surprisùig, given the results presented in the previous
section, which showed that current density had no apparent effect on the composition of
Cobalt-Iron Binaxy System p. 133
Synthesis and Microstructural Characterïzation of Electrodeposited Nanocrysttlline %Et Clapets
the ailoy deposits within the range of operating conditions used. In the folIowing section,
the details of the structural changes observed in the d o y deposits with inneaçing iron
content will be described.
X-ray diffraction patterns of two cobait-üon d o y eiectrocieposits hom a bath containing
20g/L of iron sait obtained using current densities of (a) 50mA/& and (b) l O O m A / d .
.- 'S;
I s e C - QO
m .
Cobait-iron Binary System
i
O
2 theta (ciegree)
Synthesis and ~Iicrostructural Characterization of Electrodeposited NanoaystaIline Soft Magneb
Figure 5-3 shows the equiiibrium phase diagram for the iron-cobalt svstern. At
room temperature, it c m be seen that cobalt-uon allovs c m exist in three distinct phases:
narnely, the e (H.C.P.), y (F.C.C.) and a (B.C.C.) phases, respectively. Table 5-2 hts the
phase fields at room temperature as a hc t i on of irm content in cobalt-iron ailoys under
equilibrium conditions.
Equilibrium phase diagram of the iron-cobalt sytem [Bozorth (1978)l.
Table 5-2
Equilibrium crystal structures of cobaIt-iron ailoys.
Cobalt-Iron Binary System
F.C.C. 5-21 Y 11 - 24 , Yfa F.CC + B.C.C.
Synthesis and blicrashuchiraI Characterization of EIectrodeposited Nanocrystalline Soft hlagnets
Figure 5-4 shows the X-ray diffraction patterns for the complete series of cobalt-üon
alloys that were obtained using a m e n t density of lOûtnA/cm=. It is immediately
apparent that the diffraction patterns experience considerable changes as the iron content
inaeases in the d o y deposit. Table 5-3 shows the -theta values for H.C.P. and F.C.C.
cobalt exkacted from the JCPDS powder diffraction fiies No5727 (H.C.P.) and No.15-806
(F.C.C.), respectiveIy, while Table 5 4 shows the same for B.C.C. Von given by the JCPDÇ
powder diffraction fiie No.6-676 [JCPDS (1990)l. By comparing the various sets of
diffraction peaks and positions with these reference values, it can be seen that the
electrodeposited cobalt-uon aiioys produced in this study generaily foiiowed the phase
transitions detailed in the equilibrium phase ciiagram shown in Figure 5-3.
Figure S4a is, in fact, an X-ray difhction pattern for a pure cobalt deposit. By
comparing the positions of the peaks in the diffraction pattern with the JCPDS powder
diffraction file (see Figure 4 4 , it is apparent that the pure cobalt deposit exhibited the
H.C-P. crystal structure. This was anticipated, based on the results presented in aiapter 4.
For the Co-3.4wt.YhFe d o y deposit (Figure Mb), this structure is still dominant, though a
smaii F.C.C. component has begun to appear.
As the iron content increases to 5.83wt.%, this alloy is at a composition
corresponding to the H.C.P./ F.C.C. two-phase region indicated by the equilibrium phase
diagram in Figure 5-3 (at 3 to 5wt-%Fe). indeed, the diffraction pattern shows F.C.C.
refiections dong with the H.C.P. peaks. The resdts of this anaiysis confirm that the
structure change in the cobdt-iron ailoy systern occurs even when the ailoy is produced
using a non-equilrbrium technique-
Cobait-iron Binary System
Synthesiç and Microstmctural Characterization of Electrodeposited NanocrystalIine Soft Magnets
!
O g/L FeSO4 1 O wt% Fe I
Ail peaks are 1 H.C.P. reflections 1
, 8
JO 4 YI W 70 M W 1W t10
2 theta (degrees) B rg 5a :a :J ro HI :a0 v u
2 theta (degrees)
- - --
15 g/L F-4 7.15 wt.% Fe Al1 peaks are
F.C.C. reflections
30 4) 9 ai n m ni lm 110
2 theta (degrees)
Al1 peaks are F.C.C. reflections
I
20 a 9 eo n ia ;a :Cu 't3
2 theta (degrees)
7 theta (degrees)
25 g/L F e 5 0 4 1766 wt-X Fe AU peaks are
1 I F.C.C. refiections
2 theta (degrees)
Fiaure 5 4
X-ray diffraction patterns for cobait-üon electrodeposits produced at lOûmA/cmt; (a) Og/L FeÇO4 (pure cobalt), (b) 5g/L F m 4 (Co-3.4Fe), (c) IOg/L Fe334 (Co-5.83Fe), (d) ls'g/L FeSOa (Co-7-&Fe), (e) 20g/L Fe504 (Co4O.MFe) and (F) 25g/L FeSO4 (Co-1266Fe).
Synthesis and Microstructural Characterization of Electrodeposited Nanocvstaiiine Soft Magnets
i (h) 1
h
1 e- 3.1 - .
a 4 ca a m *i G Q I W T I O YI UJ 50 t4 7 w w 100 110
3 theta (degrees) 7 theta (degrees)
(4 , 40 g/ L FeSOa (il 17.4 wt.% Fe
I 1 = residual F.C.C. A
bmponent -a L. 'd V .
2 . .- % S . 2 1 .
C -
J) 40 90 BO n, m n i i a , 1 r o 30 40 9 BO a l 9 0 1 O l 1 1 0
2 theta (degrees) 2 theta (degrees)
(k) 50 g/L FeKh 71.08 w t % Fe A peaks are
-z r, B.C.C. reflections Ci Y Y
J) 41 9) m n m 90 la0 Il0
2 theta (degrees)
Figure 5-4 (continued)
X-ray diffraction pattern for cobalt-iron dectrodeposits produced at 100mA/cm~;
(g) 30g/L Fe% (Co-14.5Fe), (h) 35g/L Fe% (Co-KaFe), (i) 40g/L FeçO4 (Co-l7.4Fe),
(j) Gg/L Fe504 (Co-19.69Fe) and (k) SOg/L F m 4 (Co-21.08Fe).
Synthesis and Microstmctural Characterization of Electrodeposited Nanocrystalline Soft Magnets
Table 5-3
X-ray diffraction peak locations for H.C.P. and F.C.C. cobait using copper Y, radiation.
[ H.C,P. Cobalt (JCPDSPDF NOS-727)
h k i l 2-theta (degrees)
F.C.C. Cobalt (JCPDSPDF No.15-806)
Cobalt-iron B k a r y System
h k l 111
Table 5-4
X-ray diffraction peak locations for B.C.C. iron under copper K., radiation.
B.C.C. Iron (JBDSPDF No.6-767)
2-theh (degrees) 44.2
hkI 2-theb (degrees)
I 110 1 44.7
Synthesis and kIicrostmctura1 Characterization of EIeckodeposited Nanocrystalline M t hlagnets
At 7.45wt-% uon, the reflections observed in the diffraction pattern for this d o y are
entirely from the F.C.C. phase, as shown in Figure 5-3d. As the iron content continued to
increase in the binary d o v , the structure continued to exhibit the F.C.C. stnicture, as
shown in Figure 5-3e. Upon examination of the peak positions in the X-ray diffraction
patterns, it is interesting to note that even the alioy composed of 1266wt.% uon produced
diffraction peaks that are verv close in position to those of F.C.C. pure cobalt (Figure 5 4 .
This observation is indicative of the higruficant changes to the lattice parameter of cobalt
with the addition of iron as an alloving element, at Ieast up to about 13wt.%
in the nickel-iron system, the ailoying of iron in nickel caused the lattice parameter
of nickel (F.C.C.) to change. in cobalt-iron, an attempt was made to resoIve the validity of
Vegard's law in this svstem. Vegard's Iaw predicts a shift in the peak locations in the X-ray
diffraction patterns when a solid solution of two elements is involved [CuUity (1978)l.
Eçsentidy, the shift is caused by the change in the lattice parameter of the material in the
form of a solid solution. With 13wt.% iron in the alloy, there is only 0.377% decrease in the
lattice parameter of F.CC. cobalt (a = 3.mA). In other words, even if Vegard's law appiies
in the electrodeposited cobalt-iron allov . svstem, . the change may be too smaU to cause a
s i w c a n t change that is observable in the X-rav diffraction pattern.
Figure 5-3g, 33h and 5-3i show the X-ray diffraction patterns for three aiioys with
compositions between 14.5wt.% and 17.4wt.% iron. halysis of the diffraction peaks led to
the conclusion that there are, once gain, two distinct phases present in the materiai. As the
iron content incIeased further, the peaks for the F.C.C. phase began to deaease, giving way
to a new set of peaks observed in the &fraction pattern. This new set of diffraction
maxima was indexed and f o n d to have originated fiom a B.C.C. crys td structure- Figure
Cobait-iron Binary System p. 140
Synthesis and Microstructural Characterization of Electrodeposited Nanocrystalline SoCt Magnets
5-3j and 5-3k are diffraction patterns for the electrodeposited cobalt-iron alloys with the
composition 19.7wt.% and 21.lwt.% iron. These deposits, with the highest iron content in
this senes of experirnents, cIearly exhibited an entire B.C.C. crystal structure that was
consistent with the reference data for B.C.C. iron listed in Table 5-4.
An analvsis was carried out regarchg the phase fields observed on the
electrodeposited cobalt-ùon alloys. Fram the equilibrium phase diagram, the H.C.P. (E)
phase is found between O to 3wt.X uon. This was consistent with the eiechodeposited
aiioys. From about 3 to 5wt.%, a H.C.P./F.C.C. rnixed-structure (E + y) phase field was
inclicated by the phase diagram. For elech.odeposited alIoys, this phase field is slightly
extended to an iron content close to 7wt.s.
For the F.C.C. phase field shoun in the phase diagram, the composition range
extended from 5wt.% to llwt.%. For the electrodeposited cobalt-iron alloys produced in
this study, this phase field exists between approximately 7wt.X and 13wt.?6. in other
words, this F.C.C. region is slightly shifted to higher uon contents. The biggest difference
was observed for the region of the mixed F.C.C./B.C.C. (y + a) structured cobalt-iron
eIectrodeposits, which is greatly reduced in size, giving way to a much Iarger region where
the B.C.C. (a) structure was dominant.
Figure 3-3 shows the phase fkId regions for equilibrium cobalt-iron ailoys in
comparison with resuIts obtained from this study. It should be noted that the
compositions quoted in Figure 5-5 (wt.% iron) are average values obtained by EDS. The
observed shifting of the phase fields is not uncornmon in eiechodeposited alloy systems.
For exampIe, the same phase field shifting phenornenon has been observed for the zinc-
nickel system @aU (1983)l.
Cobalt-iron Binary Sys tem
Synthesis and Microstructural Characterization of EIectrodeposited Nanoqstalline Soft Magnets
Eauilibrium Phase FieIds
HCP FCC fl HCP- + + FCC + 4 BCC -
FCC BCC
L -
+--- nano - - - * Co-Fe
a
B.C.C.
E
H C P
Fime 5-5
Cornparison between the equilibrium phase fields for cobalt-iron ailoys hom the cobait- iron phase diagram and resdts obtained in thk study, showing changes in the width and
region of occurrence for these phases.
E
H.C.P
in çummarv, the eiectrodeposited cobait-iron exhibited the three distinct phases
normally observed for p p m e t d u r g i c d y produced alIoys. However, these phase regions
were shifted with respect to each other.
Y
F.C.C.
E+Y + G u $ - -
Non-Equdibrium Phase FieIds m
wt.%Fe
€ + y
H.C.P. +
F.C.C.
Cobalt-Iron B k q System
Y +a
F.C.C. + B.C.C.
Y
F.C.C.
Y +a
F.CC -t B.CC
a
B.C.C.
Synthesis and hIicrostruchiraI Characterization of Elecîrodeposited Nanocrystalhe Çoft Magnetr
5.3.3 Microstructure
The morphology of these alloys was observed to Vary as a Function of iron content.
The types of morphology exhibited by the binary d o y electrodeposits are simiiar to those
obsemed for pure cobalt electrodeposits. Generally speaking, there are three major types
of morphologv observed in this bina- system, regardless of the current density used
Figure 56
High magnification scanning dectron mïaograph showing an example of the nodd
structures for sorne of the b i n q cobalt-iron electrodeposits for whïch grain size couid
be resolved in the SEM. in this example, a deposit with 10.3wt-% iron ïs shown.
Cobalt-iron Binary Sys tem
Çynthesis and BIicrasbuctuml Chamcterization of Electrodeposited Nanoc~staIline Soft Magnets
Tables 5-5 and 5-6 summarize the resdts on the morphology and grain size of the
d o y deposits produced for the current densities of 50mA/Lm2 and 100 mAlcm2,
respectively. The ciassification scherne is aciopted from Weil [1951] (see Appendix B).
Grain size measuements were carried out ushg scanning eIec!xon microscopy whenever
possible. However, for several deposits, individual grains could not be criticaiiy resdved
in the scanning eleckron microscope, As an example, Figure 5-6 shows a scanning elecîron
micrograph of such a cobalt-iron aiioy with an iron content of 10.5wt.X produced at a
current densi- of 100~/crn~. For aiioy deposits that feu into this category, an atternpt
was made to ascertain grain sue values using X-ray he-broaderting câlculations.
However, the results were unsatisfactory. This issue will be fuily addressed Iater in this
section.
Table 5 6
Morphoiogy and grain size (as per SEM) as a hc t ion of iron sulfate in the solution (i = 50 mA/cmz).
20 20.6 1-0 (C) unresolved 1
FeSOI(@) O
5
10
15
N = Needes; P = Pymnïd shapes; C = Clusters.
Cobait-Iron Binary System
Morphology
E-A (N)
II-A (N) II-A (N) + EB (C)
II-A IN) + 1-B (C)
wt.% Fe
O
3.5
63
7.5
30
35
40
45 50
Gnin Size
N = 2-20pm
N = 1-1Opm
N = 1-4pm; C = unresolved
N = 1-5p; C = unresolved
15.2
163
1-B (C) + 1-A (P)
1-A (P)
unresolved
P = 1 - 2 ~
16.5 LA (P) P = 1-2pm
285
19.6
1-A (J? P = 1-2pm
LA (P) P = 1-2pm
Synthesis and PrIicrostructuraI Characterization of Electmdeposited Nanocrysblline Soft Bfagnek
Table 5-6
hilorphorogy and grain size (as per SEM) as a h c t i o n of iron suifate in the solution (i = 100 mA/cm').
i = 1 w r m 2
FeSO, (fi) O
L
5
10
15
20
25
Figure 5-7 shows the various rnorphoIogies exhibited by the electrodeposited
cobalt-iron aiioys. At low ùon contents, the type II-A surface structure (neede-Iike
features) was obsewed. These are verv similar to hose exhibited by the pure cobalt
electrodepositç, which is to be expected. This morphology extends into the bina^ cobalt-
iron d o y s up to about 3wt.% üon.
As the iron content increases hiher, the morphoIogy of the deposits changes €rom
the neede structure to a grain duster / noddar topography (type 1-8). These noduiar
featwes were typicaiiy around 1 micron in size or smailer, and they were observed at iron
contents between 5wtwt% to approximateiy 1Swtio. Above 15wt-ls uon, the nuface
morphology of the aiioy eIectrodeposits changed to a pyramidal-shaped morphoIogy (type
1 1 1
I
Cobalt-lron B i System p. 145
wt.% Fe O
3.4
5.8
7.5
10.4
137
50 I 21.1 1-A (P) I P = -3pm
Morphology II-A (N)
(N) A (hl) + 1 - (C)
1 ( ) + 1 - (
El3 (C)
1-B (cl pP
30
35
40
45
I N = Needles; P = Pvrarnid shapes; C = Quçters.
Grain Size
N = -10pm
N = 2-15pm
N = 14pm; C = unresolved
N = 2-5pm; C = unresolved
unresolved
unresolved
21.5
16.4
1 7.4
'19.7
1-B (C) + LA (P) 1-A (P)
LA (P) 1-A (P)
unresolved
P = 1-2pm
P = 1-2pm
P = 2-3pm
Synthesis and iClicrostnrctural Characterization of Electrodeposited Nanocrystalline Soft Magneis
LA). These pyrarnidal-shaped grains are a few micrometers in size, and are analogous to
those observed in pure cobalt electrodeposits, showing an apex on top with facets and/or
edges on the sides. It should be noted that the pyrarnids observed at high iron contents
(see Figure 5-6c) are simiiar, but not the same, to the pyramids observed in the pure cobalt
electrodeposits (see Figure 4-21 and U a ) .
Fime 5-7
Representative scanning electron miaographs showing the various morphoIogies obser
on the binary cobaIt-iron eiectrodeposits: (a) low iron contents - type II-A (needles),
intermediate iron contents - type I-B (dusters) and (c) Eügh iron contents - type
(pyramids) -
ved
(b) LA
Spthesis and Microstructuml Characterization of Electrodeposited Nanocrystalline Soft blagnets
Samples prepared for transmission electron microscopy were taken horn those
deposits whose morphology inciicated possible small grain sizes, Le., the nodular features.
Figure 5-8 shows the brightfield (BF), darkfield (DF) transmission electron micrographs as
weii as the electrcn diffraction pattern of one such deposit. This particular electrodeposit
contains approximately 14.5wt.76 iron, and was produced using a current densitv of
lOOrnA/ cm=.
I . : 1 :il
G ra in 5 ize Inm 1
Fime 5-8
(a) BrightfieId trammission electron micrograph, (b) darkfieId transmission ekctron
micrograph, (c) dectron difhction pattern and (d) grain size distribution (based on >
200 grains) of a binary cobalt-iron deposit having noduIar morphotogy (Co-14SFe);
uncertainty represents one standard deviation.
CobaIt-iron Binary System
Synthesis and Micmstnictural Characterization of Eleclrodeposited Nanocrystalline Soft Magnets
From the darkfield electron micrograph (Figure 5-ûb), the average grain size of this
deposit was measured to be about 4 7 m by counting more than 300 grains. However, by
examining Figure 5-8d, it is clear that the grain size distribution is quite different than for
nanocrystaiiïne nickel-iron and pure cobalt, There are large grains (>lOOnm) embedded in
a ma& of much smaiier grains (about 20nm). Some of these large grains can ais0 be seen
in Figure 5-8b.
Nthough the average grain size of this deposit was in the nanocrystalline regime,
its grain sue &tribution was such that grain size measurements using üne-broadening
calculations were not reiiable. As a result, grain sizes Cor these deposits obtained from X-
ray line-broadening were not included in TabIes 5-2 and 2-3.
Based on the results obtained for the cobalt-iron system, a correlation can be
reached between the morphology and the crystal structure exhibited by these aiioys found
by the X-rav diffraction analysis outiined in Section 5.3.2. Table 5-7 includes the observed
morphology and the crystaliographic texture exhibited by the cobalt-iron ailoys.
Firstiy, the pure cobalt deposit and bina. aiioys at low iron contents (up to about
5wt,%), the H.C.P. crvstal structure is observed in the X-ray diffraction, and it is
responsible for the II-A type (needles) morphobgy obsewed. Secondly, as the iron content
increases, the pure F.C.C. phase becomes dominant, and it is associated with the t p e 1-B
(grain dusters) morphology exhibited by these d o y deposits. The same is true for the
pure B.C.C. d o y deposits, which is coupled with the development of the pyramid type (1-
A) morphoIogy observed in the electrodeposib.
CobaIt-hon Knary System
Synthesis and Microstructural Characterization of Electrodeposited NanocrystaIIine Saft Magnets
Table 5-7
Correlation between surface morphology and structure of electrodeposited cobalt-iron aiioys produced at 100 mA/cn$.
i = 1oOmrycm2
In summarv, for the compositional ranged produced, the grain sizes of the cobalt-
iron deposits are large (i.e. conventionai polyc~vstalline) at low and high concentrations of
iron. However, for the intermediate range (about 7 to lSwt.X), the grain size of the al1oy
deposits was observed to be in the upper range in the nanocrystahe regime.
45
50
Vickers miaohardness measurements were carried out on the two series of
electrodeposits. Figures 5-8a and 5-8b show the miaohardness of the electrodeposits as a
function of the iron contents for the two m e n t densities respectively. Both plots are
presented again in Figure 5-8~. From Figure H c , it is dear that the two curves coincide,
Gystal Structure H.C.P. H.C.P.
H.C,P. + F.C.C.
H.C.P. + F.C.C.
F.C.C.
F.C.C.
F.C.C. + B.C.C.
F.C.C + B.C.C.
F.C.C. + B.C.C.
FeSOa ($) O
5
10
15
20
25
30
35
40
CobaIt-1ron Binary System p. 149
N = Needles; P = Pyramid shapes; C = Clusters.
wtwtOh Fe O
3.4
5.8
7.5
10.4
12.7
14.5
16.4
17.4
B . C C
B.C.C.
19.7
21.2
Morphology II-A (N)
II-A (N)
II-A(N) + 1-B (C)
II-A (N) + 1-B (C)
1-B (C)
1-B (C) 1-B (C) + 1-A (P)
1-A (P)
, 1-A (P) 1-A (P)
1-A (P)
Synthesis and bIicrostructura1 Characterization of Electrodepasited Nanocrystalline Soft bfagnets
suggesting that within the vaIues of current densities chosen for this study, there is no
effect on the hardness of the d o y deposits. This is as expected, however, since the current
density was previously shown to have no signhcant effect on the aiioy composition, the
morphology or the q s t a l structure of the electrodeposited cobalt-iron ailoys.
As shown in Figure 5-8, the pure cobalt electrodeposits possess microhardness
values of just above 210VHN. This value is in accordance with published vaIues for
conventionai polycrys taiiine cobalt [Srr~i~hrLs (1983) 1,
initiaiiy, as the iron content increases, the hardness of the cieposits aiso increases.
This could be the result of at least three hardening mechanisms: (i) solution hardening, (ii)
phase distribution hardening and (üi) grain size hardening. Unfortunately, there are no
published values on the hardness of even conventional polycrvstaliine cobalt-iron aiIovs in
this compositiona1 range to decide which of these is the dominant hardening mechanism.
Perhaps the overaii shape of the curve could be indicative that grain size hardening is the
con t rohg factor for these alloys. As can be seen in Figure 5-9, the hardness of the
deposits increases with increasing iron content reaching a maximum of about 380VHN at
around 10 to llwt.% iron. This maximum coinudes with the center of the F.C.C. phase
field for which deposits with very srnail grain size were observed. For higher uon
concentrations, the a phase with Iarge grain size became the dominant structure.
Consequentiy, the hardness decreases again. It is important to note that this "softening" at
high iron content should not be assoaated with the inversed Hail-Petch behaviour
observed for other nanocrystaliine materiah.
Cobalt-Iron Bmary System
Microhardness (VHN) ...
. \ .-.
Microhardness (WIN)
Synthesis and blicrostructural Characterizaiion of Eleckodeposited Nanocystalline Soft Magnets
From the results presented on the production of nanocrystalline cobalt-iron alloys,
it can be seen that the morphologies of these alloys are generally very similar to those
observed for the pure cobalt system. This is probably because the d o y s are primarily
cobalt-based.
&O, the cobalt-iron alloys go through several phase transitions as indicated by the
equilibrium phase diagram. However, the compositions at which the transitions occurred
have shifted, in particular as far as the cornpositional range of the u + y phase field is
concemd. A correlation was found between the phases present and the morphology
observed, denoting a close relationship between the deposit surface structure and its
intemal crystal structure.
Frorn the grain size analysis, it was found that oniy a portion of the allov deposits
possessed a nanocrystahe microstructure. SpecificalIy, nanocrystaliine cobalt-ùon alloys
were only found in the midde of the composition range (Le., 3 to 13wt.76) covered in this
study. Grain size measurement based on grain count indicated a non-uniform distribution
of cmstal size where few relative1 large grains (grain size > 1OOnm) are embedded in a
mabW of the nanocrystalline grains of about 20nm in diameter.
Cobalt-iron Binary Systern
Synthesis and iClicrosûuctura1 Characterization of Electrodepasited NanocryshlIine Soft Magnets
5.5 References
K. Aotani, "Studies on Electrodeposited Ailoys (Second Report). Structure and
Electrode Potentials of Electrodeposited Iron-Nickel, Nickel-Cobalt and Cobalt-Iron
Alloys", ~ ~ ~ r n ~ f o f t h e j q n n fnsfi'tra'e of Mefnfs (Sendnrl, Vo1.814 (1950), No.5, p.53; as
cited in Brenner [19631.
C.E. Arcos, M. Schwartz and K. Nobe, "Pulse Electrodeposition of Cobalt-Rich
Co Ni and Co Fe Movs", Procerd~rgs ofthe ~E/frochenricnf SonCrfy on Efecfrodrn/~icnffy
Dpposited i'Zlljl Fhhnsf/, Vol. 94-31, The Electrochernical Society (1994), p.193.
U. Admon and T. Itav, "The Microstructure of Thin Co-Fe Films", Pro~'~enihgs offh~
11//1 kt/urhi Corrgiess mr rMernl Fhfrr5hi~rg (fNTERFBVlSN Y4), Jenisalem, Israel, Metal
Fuiiçhing Congress (1984), p.168.
S.A. h y a n o v , S.D. Vitkova, ZZ.V. Semenova, Polukarov and M. Yu, "Structure
and Physicomechanical Properties of Electrodeposited Iron-Cobalt Aiioys",
Trn/~(incfio~c ufEleckfroM~limyn, VoI,13 (1977, No.3, p.418; ci ted in Safranek [19861.
R. Bertazzoli and D. Pletcher, "Studies of the Mechanism for the Eiectrodeposition
of Fe-Co Movs", E/ectrochrinicaArfn, Vo1.38 (1993), NO,.^, p.6/1.
A. Brenner, Efectrodrposihun ofAfhys, Academic Press, New York (1963).
L. Brossard and M. Lessard, "Preparation of Co-Fe Eiectrodeposits and Their
Performance in Relation to Oxvgen Evolution in 40wt.76 KOH at 7O0C", fntenrnfionnf
/ozmnf uJHhgen Enerm, VoI.18 (1993), N0.10, p.807.
J.W. Chang, P.S. hdricacos, B. Petek and L.T. Romankiw, "Electrodeposition of
Soft CoFe AIloys", Proceenings of th Ekdmdrtrnicd Sonefyr Vo1.90 (1990), No&
p.361.
Cobalt-iron Binary System
Synthesis and Microstructural Charactecization of Electrodeposited Nanocrystalline Soft Magneis
AM, Abd El-Hahn and M.H, Fawzy, "Electroplating of Co-Fe Alloys from
Aqueous Suifa te Ba th<, lrnrzsn.bns o . j e hsIrIrhte of Mefnf fi'r~~Shihg, Vo1.71 (1993),
Pt.4 (Nov.), p.125,
S. Glasstone and J-C. Speakman, "The Electrodeposition of Iron-Cobalt AIIoys. I",
Trnt~sncfz-ons of the Fnrnd+ySoc12& Vo1.28 (1932), p.733; as ated in Brenner [1963].
S. Glasstone and J.C. Speakman, "The Electrodeposition of iron-Cobalt Uoys. II",
Trnnsncfz-uns @the Ffrndiy SonéS, VoI.29 (1933), p.426; as cited in Brenner [1963].
D.E. Hall, "Electrodeposition of Zn-Ni U o y Coatings - A Review", PfntNg nmt
S I~&P firr~s/rir,il Vo1.70 (1983), N0.11, p.59
E.M. Kakuno, D.H. Mosca, 1. Mazzaro, N. Mattoso, W.H. Schreiner, M.A.B. Gomes
and MO. Cantao, "Stmcture, Composition and Morphology of Electrodeposited
Co,Fei., M o y s", /uwr~d qf'rhc EZ~'ctrodrewricn/ Srne&, Vol.144 (1997), No.9, p.3322.
S.H. Liao, " High Moment CoFe Thin Films by Electrodeposition", /EEE Trnr~snd~bns
on MngnehIs, VoLMAG-23 (1987), N0.3, p.2981~
F.P. Mord, "Electrodeposition of Cobaitl', ~MrrnlFriz1k4N'g, VoL64 (1964), No.7, p.82.
P. Partain, MB. Balmas, M. Schwartz and K. Nobe, "Electrodeposition of Iron,
Nickel and Cobalt Ailoys", Pnrcrdings ofthe SOf/r AESFAnnmf Technitif Conférence
h e r i c a n EIectropIaters &Surface Finishers Soaety, inc., Fiorida (1993), p.707.
A.L. Rotinyan and E.N. Molotkova, "Cathode Polarkation in the Deposition of
Iron-Cobalt Moys and Causes of Depolarization and Superpolarization", a~~~
Pn;(-nd mh., VoL37 (1959), p.302; as ated in Brenner (1963).
J.C. Sadak and FX, Sautter, "Composition, Mechanical Properties and Structure of
Electrodeposited Co-Fe iUIoysf', /ormd of Ynnmn So2m.e 8 Tecrnofoafy, Vol.11
(1974), N0.4, p.m.
CobaIt-Iron Binary System p. 154
Synthesis and b~icrostructural Characterïzation of Electrodeposited Nanoqstalline Soft Magnets
J-C. Sadak, E.S. Chen, F.K. !%utter and S.G. Lakshminaravann, "Use of insoluble
Anodes in Co-Fe EIectrodeposition from a Sulfate Electrolyte", Phfzi/g md St~rfice
finzshr'ng, Vo1.63 (19781, N0.4, p.34.
W.H. Safranek, 771r Propcfz2s ufE/~ctrodqosif~d~MrfnIs nmf A//oys - A An~z~fbuok 2nd
edition, American Electroplaters and Surface Fuiiçhers Society, Orlando (1986).
O. Shinoura and A. Kamikirna, "Magnetic Properties of Electrodeposited CoFe
Films", /mntnf of fhr 511fncr fi~z13hziig Sonëtyof,fdnpn~t, Vol.44 (1993), No.12, p.1114.
Slzr'ihr//s hfrfn/s R@renCce Book, 6th edition, E.A. Brandes (ed.), Butterworth & Co.
Ltd., London (1983).
G.S. Sotirova-Chakarova and S.A. Amyanov, "The interna1 Stress in NickeI, NiFe,
CoFe, and CoNi Layers Measured by the Bent Strip Method, /oz~mn/ of t h
E/~C~~OL./~PIII~~-I~/SDCI~&, Vo1.137 (1990), ;"\Io.ll, p3351.
S.N. Srimathi and S.M. Mayama, "Eiectrodeposition of Iron-Cobalt Alioys:
Mluence of Alternaîing Current", /a~~nzn/ qf- the Elt '~~tru~fic~~zkd So~~2tv (hzdirl,
Vo1.31-3 (1982), p.59.
R. Weil, "Structures of Electrodeposited Evletals", Ph.D. Thesis, Rte Pennsylvania
State Coliege, Pemylvania (1951).
Cobdt-Lon Binary Systern
Chapter 6
Conclusions
6.1 Generai Conclusions
From the results presented in the previous chapters, it c m be concluded that the
production of nanocrystalline ferromagnets bv eIectxodeposition for possible application as
soft magne& is a viable proposition. in partidar, thoughout the course of this work,
nanocrysta1he nickel-iron, pure cobaIt and cobalt-iron alioys have been synthesized.
Operating windows for the synthesis of each of the mentioned systems have been
identified and correIatiuï-s between miaostnicture, morphology and q s t a l structure were
presented. Furthemore, correlations behveen microstructure and microhardness were
given.
6.2 Nickel-Iron Binary System
Nanoayslalline nickel-iron alloys were obtained from a modified Watts type
eiectroIyte using direct current eiectrodepositiom The composition of the d o v deposits
ranged from pure nickel to an aüoy with an ùon content of just under 30wt-%. The range
Synthesis and CvIicrostnictural Characterization of Electrodeposited NanocrystaIline Soft Magnets
of compositions available includes that of PermaiioyB (nominal composition 80Ni-20Fe),
which is an important soft magnet because of its high magnetic permeability.
Ni of the obtained d o y s have nanocrystalline microstructure. The grain size was
found to decrease as a function of the iron content in the d o y deposits to a minimum of
about llnm (as per X-ray diffraction) for an aiioy with iron content of 28wt.% iron. From
transmission electron microscopy carrieci out on the Ni-22wt-%Fe alloy, the average grain
size of a nanocrvstalline PermalloyO-type binary alloy produced was measured to be
1om
The maximum hardness observed for this aiioy system was about 6OOVHN with a
softening behaviour noted for the smallest grain sizes, Ieading to an inverse HaU-Petch
relationship.
6.3 Pure Cobalt System
Cobait was produced using pulse current eIectrodeposition from two sulfate-based
electrolytes, a saccharin-free bath and a saccharin-containing sotution. From the saccharin-
free bath, ail cobait samples deposited were of conventional polycrystaIiine structure. in
contrast, deposited cobalt obtained from the saccharin-containing bath showed the
nanocrystalline microstructure.
An exh-a set of experiments was devised to ascertain the necessity of either the
presence of saccharin or the use of pulse eiectrolysis for the production of nanocrvstalline
cobalt. From this analysis, it m a s concluded that, either one of the two conditions aione
was not suffisent to faalitate the production of nanoaystalline cobait in fact, the two
Condusions
Synthesis and Microstructural Characterizalion of EIectrodeposited Nanocrystalline Soft Magnets
conditions act synergisticaiiy to open up an operating window for the synthesis of
nanocrystalline pure cobalt.
The conventionai polycwstalline cobalt deposits obtained from the saccharin-free
bath exhibited a varietv of morphologies and crystallographic textures and the correlation
between them was presented. On the other hand, the nanocrystalline samples obtained
from the saccharin-containing bath al1 exhibited a similar rather featureless morphology in
the scanning electron microscope where the individuai grains codd not be critically
resolved. Ali the n a n o q s t a h e cobalt sampies have the same H.C.P. basal plane
preferred c~staiiographic orientation and the smdest grain size produced was about
1Onrn. The application of an extemal magnetic field up to 3500 Gauss was shown to have
some minor effect on the crystailographic texture of n a n o q s t a h e Jeposits.
6.4 Cobalt-Iron Binarv Svstem
Using a sulfate-based solution, cobalt-iron alioys with iron content up to around
22wt.% iron were obtained with different crystal structures, Jepending on the iron content.
From transmission electron microscopv carried out on the deposit with the smaiiest grain
size, an average grain size of 47nm was found, aithough with a non-uniform grain size
distribution. Not al1 of the cobalt-uon d o y deposits were of nanoaystalline nature.
Indeed, throughout the series of samples obtained (O to 22wt.% uon), nanocrystahe
cobalt-iron aiioys were found to be in the middie of the compositionai range (5 to 15wt.%
iron). Outside of this range, the cobalt-ùon aiioys euhibited conventionai polycrystalline
rrüaostcucture and presented surface morphologies similar to the pure polycrystaiLine
cobalt samples.
Condusions
Synthesis and Microstructural Characterization of Electrodeposited Nanoc~staI1in.e Suft Magnets
6.5 Outlook for Electrodeposited Nanocrystaliine Soft Magnets
In the previous work on the magnetic properties of nanocrystalline nickel produced
by electrodeposition, it was shown that grain size did not greatiy affect the saturation
magnetization [Aus rf R/. (1992)], which was in contrast to reçults reported eariier for
materiais produced by other techniques (see Çection 1.1). Combined with some of the
other beneficiai properties such as increased sirength, hardness and increased etectricai
resistivitv, nanocrvstalline ferromagnetic structures could therefore lead to acivanced soft
magnetic materials (referred to as ideal soft magnets in Section 1.1). The basic objective of
the research effort at Queen's University was to see if eIectrodeposition teduiology is a
suitable approach in the development of such materials
Whiie this thesis has cleariy demonstrateci that nanostructured cobalt, nickel-iron
and cobalt-iron alloys with various compositions can be svnthesized using
electrodeposition, the research by MJ. Aus, whidi was camed out in parailel, was
concerned with the assessment of the magnetic properties of materiais produced bv the
synthesis route developed in the m e n t work. The details of the work by M.J. Aus are
summarized in his Ph-D. thesis [Aus (1999)I. For the purpose of assessing the out[ook for
eIectrodeposited soft magnets, some of the critical results are considered here.
Shown in Table 6-1 are saturation magnetization and coercivity data for several of
the materiaIs produced in this study. &O shown are saturation magnekations for some
of their conventional polycrystaiIïne counterparts found in the fiterature. As far as the
saturation magnetization is concerned, hvo important results can be seen. First, the
saturation magnetization values measured on nanocrystalline materi& are very dose to
pubiished d u e s for conventional polycrystals. This confimis eariier experimentai
Condusions p. 159
Synthesis and bIicrostructuraI Characterization of EIedrodeposited Nanocrystaliiie Soft Magnefs
tïndings [Aus rf ~ t ! (1992)] and a theoretical analysis [Szpunar CL ut! (1998)] that saturation
magnetization is not strongly affected by grain size reduction. Second, because nano-
processing has Little effect of saturation magnetization, the saturation magnetization can be
increased by going from pure nickel to ninickel-uon alIoys, then to pure cobalt and M y
cobalt-iron aiioys to approach the composition of the ideal soft magnetic materials just as
outlined in Section 1.1. In this regard, the combined efforts of this work and the work by
M.J. Aus have acivanced the field considerabty by approaching the structure and magnetic
saturation requireci for ideal soft magnets.
Table 6-1
EvIagnetic properties measured for the various nanoqstalline soft magnetic materiab
produced in this work by bu. Aus [Aus (2999)I dong with published Evk values
for their conventiond poIyqstalline counterparts.
Soft Magnet 1 PublishedMr 1 Measured Ms 3
Table 6-1 also shoes that the coeravity of the materials produced in this work are
still too high, at least by one order of magnitude, before they ~m be considered for
industrial applications. However, it shoutd be pointed out that coeravity is known to be
Coercivity (a) 3
Ni-Z.?Fe
Ni-27.9Fe
Co
Co-15Fe
Co-17.5Fe
Conclusions p. 160
1 jiles [199l]; Bozorth [1978]; 3 Aus [19991.
LIT' -
1.7T 1
LOT
LIT
035 x 103 A/m
0.26 x 103 A/ m
1.95T 1
LOT 1
1 -7T 1 0.48 x 103 A/ m
1 .87T
1.90T
7.61 x 103 A/m
3.65 x 203 A/m
Synthesis and iCIicrrostnictumlCharacterization of Electrodeposited Nanocrystalline %ft hilagnets
stronglv dependent on microstructural defects such as grain boundaries, dislocations and
impurities Diles (1991)]. More specificdy, coercivity, which is a measure of the interaction
of moving domain waiis with the materials defect structure, c m be adjusted in a
ferromagnetic material by controlling factors such as crystallographic texture, interna1
stress, solute redistribution by anneahg, etc.. The goai of this research was to develop
synthesis methoris to produce nanoqstaiiine ferromagnets with specific grain sizeç,
which was largely achieved. However, few attempts were made to control the
microstructural features that strongly influence coerciviS. Follow-up research is requiseci
to address the issue of coercivity before the field of the ideal soft magnet shown in Figure
1-5 c m be approached both in terms of saturation magnetization and coerciviv.
6.6 Contributions to the Field
CompIetion of the present work has brought about several contributions to the
field. This work is the first in-depth study deaiing with the synthesis of nanocrvstalline
soft magnetic materials bv the eIectrodeposition processes. Opetating windows were
found for the production of these materiais, which can be addeci to the growing Iist of
nanocrystalline material for future applications. Two such potential applications are the
next generation of power transformer cores and candidates for advanced
microelectromechanical (hiLEbIS) components.
The technotogy developed in this work formed part of a successfd application for a
U.S. patent. A Queen's University spin-off Company, NanoMetals Corporation, was
formed in 1995 to comrnercialize the nanomateriais technologies based on lhis anci one
previous patent Ontario Hydro TechnoIogies, and more recently lntegran Technologies
Conclusions p. 161
Synthesis and bIicrostructural Characterization of Electrodeposited Nanocrystaliiie Soft Magnetç
Inc., applied the basic operating windows for nanocrystailine Ni-Fe, Co and Co-Fe alloys
estabiished in this work in a scaled-up continuous plating technology to produce
nanoqstalline materials in large quantities.
This thesis also conûibuted to four 4th year undergraduate theses and two Ph.D.
theses at Queen's University. For the latter, materials were provided, using the teduiology
deveioped in this thesis, for extensive structure-property measurements of nanocrystaiiine
materiais including magnetic properties and thermal / thermodynamic properties.
Condusions
Synthesis and MicrosûuctumI Characterization of Electrodeposited Nanocryshlline Çoft Magneb
6.7 References
M.J. Aus, B. Szpunar, AM. El-Sherik, U. Erb, G. Palumbo and K.T. Aust,
"Magnetic Properties of Bulk Nanoc~ystaüine Nickel", Sr-npfn Mefdhir-i-n d
~Mdenk/rk, Vo1.27 (1992), p.1639.
MJ, Aus, "Magnetic Properties of Nanocrystaiiine Transition Metais", PkD.
Thesis, Queen's University, Kingston, Ontario, Canada (1999).
R.M. Bozorth, Frrrot1rng/t~tz3m, iEEE Press, New York (1978).
D. Jiles, /nfroC/rin'iott fo rMngtrrf&.cn n d Mngtfi ii-Lhf~rthh, îhapman and Haii
(1991).
B. Szpunar, hl- AUS, C. Cheung, U. Erb, G. Palumbo and J.A. Szpunar, "Magnetism
in Nanos truc tured Ni-P and Co-W Nioy s", /omtnI of ~Lhgn~fts~~t md rMnptcf~i-
rMnfen;7/s, VoL187 (1998), No3, p.325.
Conclusions
Chapter 7
Recommendations for Future Work
7.1 General Recommendations
Mthough the prixnary objective of the present work, the deveiopment of
electrodeposition processes to produce nanocrystalline soft magnetic materials, has been
accomplished, more research is needed in order to further the understanding of these new
magnetic materials tha t possess this unconventional microstructure.
As pointed out in Chapter 6.5, a major concern with these materia15 is stiii their
relatively high coercivity, Future work should addtess the issues of magnetic domain
structure in these materiais and how dornain wall motion can be facilitated by controhg
qstaIiographic texture, grain size distribution and internal stress. Very littIe is known to-
date on the domain structure of n a n o q s t a h e materials, not only for materials produced
by eIectrodeposition, but also for materials produced by other methods.
in the experiments mvolving eIectropIating of cobaIt m an extemal magnetic field,
some changes in aystdographic texture were observed- in-field plating experiments
shouid be m e r devdoped for the purpose of contcolling texture to a larger extend than
those achieved in this research. in addition, annealing m an e x t e d magnetic field, a
pracüce commonly used m the manufacture of soft magnetic recordhg heads, couid reduce
Synthesis, Structure and Properties of EIectrodeposited Nanonystaiiine Soft Magnets
the coeravitv, However, such experiments must be carried out at temperatures below the
omet of grain growth to preserve the nanostnicture in the materials. h attempt shodd be
made to study the domain structure of these materiais using, for exampte, Lorentz electron
rnicroscopy. To-date, no such measurements have been done on electrodeposited
nanoqstais and, consequentiv, no information on domain size, shape and domain wall
thickness is currently available.
Specific recomrnendations for the svstem studied in this research incIude the
Eollowing.
Modification of the bath chemistry in the nickel-iron system is recommended to
accommodate other alloys having higher iron contents that are of scientific and
engineering interests. For example, i n v d (Fe-Ni36), &O known as invar36, is a low
expansion d o y that lend itseif very weii in applications where dimensional stabiiity is a
very important property, such as in McroelectromechanicaI systems (MJ3E).
In the cobalt system, a more detailed stuciy needs to be deviseci in order that
materials with larger variations in grain size could be produceri. This wil1 aid in the
understanding the behaviour of the plathg bath as weU as providing a better spectntm of
samptes for subsequent acadernic study.
For the cobalt-iron system, hrther experimentation is needed to produce cieposits
not oniy with smailer grain size, but also with more uniform grain size distribution.
Similar to the nickel-iron system, the bath chemisûy of the cobait-iron bath aiso needs to be
modified in order that other ailoys with higher bon contents may be attained. In this
partidar system, such an aüoy is at the composition of c ~ b a l t ~ w t . l o Fe, where the
saturation magnetization of the materiai is above each of the individual elements,
Recommendations for Fuhne Work
Synthesis, Structure and Properties of Electrodeposited Nanouystalline Soft Magnets
exhibithg a synergistic effect. Production of such an aiioy in nanocrystalline fom would
constitute the ultimate goal towards the ideal soft rnagnet outlined in Chapter 1.
Recornmendauons for Future Work
Chapter 8
Appendices
8.1 Appendix A Weil's Classification of the Structuie of Electrodeposited Metals
in 1951, Roif Weil studied the structures of electrodeposited metals for his Ph.D.
thesis [Weil (1951)l. The met& shidied inciuded nickel, copper, zinc, cadmium and cobdt.
Mi electrodeposits were produced from commerciaily avaiiable electrolytes under various
operaling conditions. AU observations of the as-pIated surface morphologies were made
using the repiica technique and a transmission eIectron microscope. Among other results,
one of the major achievements that arose €rom this work was a comprehensive
categorization of the as-plated surface structure of these metais, which can be genedized
into a ~Iassification scheme. This is one of the most elaborate reports of its kind.
in Weil's dassification system, a i i the observed as-pIated (surface) structures can be
divided into two dasses. a a s s 1 indudes structures made up of crystals that have no
prefened growth direction in the pIane of the deposit surface, while structures consisting
of crystals having growth direction preferences comprise aass II. in each dass of
structure, subdivisions can be made according to grain size and type. Moreover, a
subdivision of partide size is aIso included in the designation of the structure dassification.
Synthesis and MicrostructuraI Characterization of EIectrodeposited Nanocrystalline Soft Magnets
The foilowing table is a summary of the classification developed bv Weil.
Table 8-1
Sumrnary of the classification scheme for as-pIated surface shctures of electrodeposited metais developed by WeiI.
Qass 1 - no directiod p w t h preference in the pIane of the deposit. 1 large, rather smooth area with regions of fine grains' in between;
can be rtivided into two sizes, i.e. LAI and LM.
[-Al - mainly cornposed of fine grains;
1 LA2 - mainly plane areas of large grains.
1-B
1-C
Qass II - presence of directional growth preference in the plane of the deposit,
d o r m f i e grain structures;
can be divided into three sizes, i.e. 1-83, i-34 and 1-B5.
large grains with lieii.de platelet structures;
no grain size subdivisions. 1 -
II-A
1-D
a a d a r stnictures; either has platelets perpendicular to the
preferred direction, or shows no plateIets at aii;
c m be divided into two sizes, i.e., &A6 and II47
structures with small, uniform, Jorne-shaped grains;
1 no grain size subdivisions.
"ine grains in the context of Weil's work, would be refened to conventional polycrystalline grain size in the present work, Le., miaometer-sized grains.
II-B platelets are parallei to the preferred direction; share size subdivision with [I-A, Le., II-86 and iLB7
Spthesis and biicrostnictural Characterization of Elecîrodeposited Nanocrystaiiine Soft Magnets
8.2 Ref erences
R. WeiI, "The Structure of Electrodeposited Metah", Ph.D. thesis, The Pennsylvania
State Universily, Pennçyivania, U.S.A- (1951).
Appendix A
