STUDIES ON SUPERCONDUCTOR NANO …ethesis.nitrkl.ac.in/4474/1/Final_Thesis.pdf1.4 Short Overview on Superconductivity 1.4.1 Type1 & Type 2 Superconductors 4 1.4.2 Flux Quantization

i

STUDIES ON SUPERCONDUCTOR NANO

COMPOSITE OF Bi2Sr2CaCu2O8/BiFeO3

Thesis submitted to the

National Institute of Technology, Rourkela For award of the degree

of Master of Technology (Res)

By Sanghamitra Acharya

Under the guidance of

Dr. P. N. Vishwakarma DEPARTMENT OF PHYSICS

NATIONAL INSTITUTE OF TECHNOLOGY

ROURKELA, JULY 2012

ii

I Dedicate

MY HEARTFELT LOVE TO

His Devine Grace, The Saviour, Pujyapada

“SRIMAD BABA BALIA”

The Goal and Guide of my LIFE.

iii

CERTIFICATE

This is to certify that the thesis entitled “STUDIES ON

SUPERCONDUCTOR NANO COMPOSITE OF BSCCO/BiFeO3”,

submitted by Sanghamitra Acharya to National Institute of

Technology, Rourkela, is a record of bonafide research work

under my supervision and I consider it worthy of consideration

for the award of the degree of Master of Technology (Res) of

the Institute.

(Dr. P. N. Vishwakarma)

Supervisor

Date:

iv

DECLARATION

I certify that

a. The work contained in the thesis is original and has

been done by myself under the general supervision of my

supervisor.

b. The work has not been submitted to any other Institute

for any degree or diploma.

Signature of the Student

v

ACKNOWLEDGEMENT I would like to give my sincere gratitude to my advisor Dr. P. N. Vishwakarma

for his continuous support in my research work, motivated thought, patience

and immense knowledge.

I am extremely grateful to chairman and M.S.C members Prof D. K. Pradhan,

Prof S. Jena, Prof R. Majumder, Prof. S. C. Mishra for their insightful

comments and constructive suggestions to improve the quality of this research

work.

I would like to give special thanks to Jashashree & Achyuta for their continuous

encouragement and co-operation in my Cryo-Magnetism lab.

I am thankful to Senthil, Mousumi, Barun Kumar,Prakash Pallei, Rakesh,

Priyadarsini, Arnnapuna & Gitanjali for their valuable suggestion and

discussion at every step of my research work.

I would like to show my sincere thanks to Arpana Kujur for selfless help and

useful suggestions.

Thanks to all the faculty & staff of Department of Physics.

Thanks to Dept. of Ceramic & Metallurgy Engineering for allowing me for

XRD & SEM characterization.

At last but not least, I never forget to remember these members, my parents, in

laws, husband, Ashram Bhais, all my family members, well wishers, & loving

brother “BAPU” for their blessings, love, inspiration, encouragement, and

strong supports in every moments of my life.

Sanghamitra Acharya

vi

PREFACE

Classical theory of superconductivity predicts that, mixed state in homogeneous type-II

superconductors is associated with the flux-line lattice (FLL). Each FLL is associated with

quantized amount of flux due to presence of its normal core. But in case of an ideal

homogeneous defectless superconductor, FLL are not pinned. So critical current density Jc tends

to zero. Interaction of the FLL with crystal imperfections or pinning centers in type-II

superconductors are responsible for the existence of a critical current density Jc. The strength of

these pinning centers depend only on the nature, shape and physical properties of the defects.

These interactions arise from the difference in critical temperature (Tc), critical field (Jc) or

Ginsburg–Landau (G-L) parameter k. But pinning is very less in case of high Tc superconductors

(HTSC). So artificial pinning centers are needed to enhance the flux pinning in addition to

naturally occurred pinning centers. A direct correlation exist between defects with FLL (which

are moving) and simultaneously results in enhancing the Jc. As HTSC are granular in nature,

grain boundaries act as potential barrier which will block the flow of supercurrent in the adjacent

grains. Non superconducting grain boundary acts as a weak link between two superconducting

grains by forming a Josephson junction. So that cooper pair can easily tunnel through the

impurity.

This present work concerns with the knowledge of High Tc superconductor. Achieving

higher Tc and enhancing transport properties by increasing current carrying capacity (Jc)

becoming the major goal in the field of superconductivity. Discovery of oxide based

superconductors, around 1986 by Bednorz and Muller with higher Tc opened a new door for

scientists as well as researchers. Out of all oxide based HTSC the Bi based superconductor is

chosen as having comparatively higher Tc and more stable with oxygen concentration than

YBCO. Bi2Sr2Ca1Cu2O8 (BSCCO 2212) phase is chosen as it can be synthesized without lead

doping (non-eco friendly), which otherwise is essential for 2223 phase of BSCCO.

vii

Recent studies showed that magnetic nano-particles may act as efficient pinning centers

at much lower density. As BiFeO3 (BFO) being a multiferroic in nature have large magneto-

electric coupling takes place. Though ground state of BFO showing antiferromagnetic but its

nano form exhibits superparamagnetisim which is very suitable for pinning effect in

superconductor.

BSCCO 2212 phase is prepared by solid state reaction method from oxide and carbonate

precursor. Nano particles of BFO are synthesized via sol-gel auto combustion route from nitrate

precursors. After necessary heating cycle desired amount of BSCCO and BFO powders are taken

in various weight ratios for synthesis of composites to study the effect of nano particles in the

superconducting phase. Nano-composites of 1%, 2%, 3%, 4%, 5%, 10%, 15%, 20%, 25% and

30% BFO in BSCCO are prepared. Phase conformation is done by XRD which shows that BFO

peaks are clearly visible only in higher concentration without disturbing the parent one.

Microstructral analysis done through SEM clearly shows BFO nano particles homogeneously

distributed over the surface. Low temperature electrical transport property is observed by R-T

measurement & simultaneously critical current carrying capacity is measured at different

temperatures.

R-T measurement shows that, only parent BSCCO have sharp transition around 80K in

its derivative plot where all the grains become superconducting. Except BSCCO, all the

composites show double resistive transition in their respective derivative plot, higher one marks

the superconductivity in grains whereas the grain boundary still remains normal and the lower

one when the grain boundary also becomes superconducting.

Simultaneous increase in broadening also noticed in lower temperature region.

Throughout the composites two monotonically decreasing trend is observed. All the

superconducting parameters like Tc-on, Tc1, Tc2 & Tc0 all are decreasing upto 5% added BFO &

then suddenly increases in 10% BFO. For further higher and higher concentration the same

decreasing trend is maintained but with a faster rate.

viii

Exact nature of grain & grain boundary is studied by considering its pseudogap

temperature (T*) from its nonlinearity behavior. Conductivity study in normal region is studied

by Weak –link behavior as per this equation ρ(T) = ρ(0)+αT+bT1/2

.This shows that upto 10%

BFO added sample exhibit well fit to the -T plot in higher tempeature region. For 15% BFO,

the -T plot behavior is found to be better fitted with . For higher concentration of

BFO, -T plots are found to have good fit with . As β value nearly 0.22 signifies that

upto 30% still the composite is in the metallic side.

Excess conductivity or paraconductivtiy ( m n ) is studied by plotting a graph

between ln ε vs. ln Δσ which shows dimensional behavior. The parent BSCCO sample, λ3D is

found to be –0.47 and λ2D is –0.94, which fits well with the theoretical value. The cross over

temperature is found to be 85.12 K. For the BSCCO/BFO composites the respective values of

exponent determined from these plots are λ ~ – 0.5 and λ ~ –1.5. This higher value shows

percolative conduction in the inhomogeneous composites.

I-V measurement done at a particular temperature below its second transition to know its

maximum current carrying capacity i.e. Jc. Nonzero voltage drop in the I-V plot, gives Ic and by

dividing its cross-sectional area, Jc is calculated. As added BFO initially (upto 5%) goes to the

boundary, simultaneous decrease in Jc is noticed. Further added BFO goes to boundary as well

as inside the grain where added BFO act as a pinning center. So simultaneous increase in Jc is

noticed in higher concentration. Nature of the grain boundary is studied by Jc(T) = Jc (0) (1-

T/TC)n this equation. Though the higher concentration composites having n value nearly 2 gives

the impresion of superconductor-normal- superconductor (SNS) type, but under strong

proximity effect the junction becomes superconductor-insulator- superconductor (SIS).

ix

CONTENTS

Title page i

Dedication ii

Certificate iii

Declaration iv

Acknowledgement v

Preface vi

Contents ix

List of Figures xii

List of Tables xv

Symbols and Abbreviations xvi

CHAPTER 1

INTRODUCTION

1.1 Preamble 2

1.2 Zero Resistivity & Perfect Diamagnetism 2

1.3 High TC Superconductor 3

1.4 Short Overview on Superconductivity

1.4.1 Type1 & Type 2 Superconductors 4

1.4.2 Flux Quantization 5

1.4.3 Penetration Depth (λ) & Coherence Length (ξ) 6

1.5 Flux Creep, Flow and Pinning 6

1.6 Ginzburg-Landau Theory 9

1.7 Excess Conductivity or Para Conductivity 12

1.8 Josephson Junction

1.8.1 Weak Link Behaviour 14

x

1.9 Literature Survey 15

1.10 Role of Magnetism in Superconductor 18

1.11 Motivation 18

1.12 Crystal Structure of BSCCO 19

1.13 BFO Nano Particles 21

CHAPTER 2

EXPERIMENTAL TECHNIQUES & SAMPLE PREPARATION

2.1 Introduction 29

2.2 Various Types of Synthesis Technique for BSCCO

2.2.1 Solid State Reaction Route 30

2.2.1 (a) Reagents 30

2.2.1 (b) Weighing and mixing 30

2.2.1 (c) Calcination 31

2.3 Various Types of Synthesis Technique for BFO

2.3.1 Chelating Agent 33

2.3.2 Synthesis Process of BFO 33

2.4 Superconductor/Nano Composite of BSCCO/BiFeO3 34

2.5 Characterization Techniques

2.5.1 X-Ray Diffraction 36

2.5.2 Scanning Electron Microscope 37

2.5.3 R-T Measurement 38

xi

2.5.3 (1) Two Probe Method 40

2.5.3 (2) Four Probe Method 41

2.5.3 (a) Constant Current Source 42

2.5.3 (b) Digital Nanovoltmeter 42

2.5.3 (c) Data Acquisition 43

2.5.4 I- V Measurement

2.5.4 (a) Electric Field criteria 43

2.5.4 (b) Resistivity criteria 43

2.5.4 (c) Off-set Method 43

CHAPTER 3

PHASE CONFORMATION & MICRO STRUCTURAL ANALYSIS

3.1 XRD 45

3.2 SEM 48

CHAPTER 4

RESULTS & DISCUSSIONS

4.1 R-T Measurement 53

4.2 Excess conductivity or paraconductivtiy 79

4.3 Critical Current Density 89

CHAPTER 5

CONCLUSIONS 104

xii

List of Figures

Figure

No. Title

Page

No.

Fig. 1.1 Zero resistivity & Perfect diamagnetisms of Type I superconductor

3

Fig. 1.2 Meissner effect in Type I & Type II 5

Fig. 1.3 Quantized amount of flux associated in mixed region 6

Fig. 1.4 FLL Experiences Lorentz force 8

Fig. 1.5 Flux pinning in HTSC 9

Fig. 1.6 Variation of order parameter Ψ with T 10

Fig. 1.7 Variation of free energy F with order parameter Ψ at various temperatures

11

Fig. 1.8 Real Superconductor showing Excess conductivity region 13

Fig. 1.9 Grain boundary acting as Josephson weak link 14

Fig. 1.10 Crystal structure of BSCCO 20

Fig. 1.11 AFM Image showing BFO nano particles (Left) & M-H loop showing Superparamagnetisation(Right)

22

Fig. 2.1 Flow chart for BSCCO 2212 phase synthesis 30

Fig. 2.2 Flow chart for BFO synthesis 32

Fig. 2.3 Flow Chart For Superconductor/Nano Composite of BSCCO/BiFeO3

33

Fig. 2.4 X-Ray Diffraction Pattern 35

Fig. 2.5 Temperature dependence of resistivity plot for (a) metal, (b) semiconductor and (c) superconductor

37

Fig. 2.6 schematic diagram of two probe method 38

Fig. 2.7 Schematic diagram of four probe method 39

Fig. 2.8 Van-der Pauw measurement in different configurations 39

xiii

Fig. 2.9 Current – voltage curve and methods of determination of critical current density

42

Fig. 3.1 XRD pattern of composites of BSCCO, 1%, 2%, 3%, 4% and 5% added BFO composite

46

Fig. 3.2 XRD pattern of composites of 5%, 10%, 15%, 20%, 25% and 30% added BFO composite

47

Fig. 3.3

SEM images of BSCCO, 2%, 4%, 5%, 10%, 15%, 20% and 25% added BFO shows the variation of microstructure with wt. % of BFO

51

Fig. 4.1

R-T measurement of Pure BSCCO and (b - k) BFO composites (1%, 2%, 3%, 4%, 5%, 10%, 15%, 20%, 25% and 30% added BFO)

65

Fig. 4.2 Normalized r-T plot 68

Fig. 4.3 Fitted value of Power law of composite subtracted from parent as function of temperature and Pseudogap temperature T*

71

Fig. 4.4 R-T Plot of week localization behaviour 79

Fig. 4.5 ln (Δσ) vs. ln (ε) plot of BSCCO and its composites (1%, 2%, 3%, 4%, 5%, 10%, 15% and 20% added BFO)

86

Fig. 4.6 Current (I)-Voltage(V) curves of BSCCO, at 10 K, 20 K, 30 K, 40 K, 50 K, 60 K and 70 K

92

Fig. 4.7 Current (I)-Voltage(V) curves of 2% BFO, at 7 K, 10 K, 15 K, 20 K, 25 K and 30 K

93

Fig. 4.8 Current (I)-Voltage(V) curves of 3 % BFO, at 6 K, 10 K, 15 K, 20 K, 25 K, 30 K and 35 K

94

Fig. 4.9 Current (I)-Voltage(V) curves of 4% BFO, at 7 K, 10 K, 15 K, 20 K, 25 K, 30 K, 35 K and 40 K

95

Fig. 4.10 Current (I)-Voltage(V) curves of 5% BFO, at 5 K, 15 K, 20 K, 25 K, 30 K, 35 K and 40 K

96

xiv

Fig. 4.11 Current (I)-Voltage(V) curves of 10% BFO, at 6 K, 15 K, 20 K and 25 K

97

Fig. 4.12 Current (I)-Voltage(V) curves of 15% BFO, at 10 K, 15 K, 20 K and 25 K

98

Fig. 4.13 Variation of critical current density Jc(0) with BFO concentration 99

Fig. 4.14 Schematic grain/grain boundary arrangement for BSCCO/BFO composites

100

xv

List of Tables:

Sl. No. Contents Page No.

Table 4.1 Variation in superconducting order parameters with BFO wt. %. 71

Table 4.2 Variation of a , n, 0

gb and T* (K). 74

Table 4.3 Fitted Parameters of Residual resistance ( 0), A and B of composite

up to 10% BFO. 75

Table 4.4 Various regions obtained from the ln (Δσ) and ln (ε) plots of BSCCO

and composites (1%, 2%, 3%, 4%, 5%, 10%, 15% and 20% added BFO). 90

Table 4.5 Variation of coherence length ( ) (Å) and Josephson coupling constant (EJ). 91

Table 4.6 Shows values of critical current density Jc (0) and ‘n’ in both the Regions. 102

xvi

List of Symbols and Abbreviations:

K Temperature in Kelvin scale

B External Magnetic Field

H Induced Magnetic Field

M Magnetization

χ Susceptibility

Φ Quantized Flux

h Planks Constant

λ Penetration Depth

ξ Coherence Length

Tc Transition Temperature

ns Superconducting Electron

vF Electron Velocity at Fermi surface

Δ Energy Gap

Ψ Super conducting electron Wave function

k =λ/ Ginzburg – Landau parameter

EJ Josephson coupling

T0 Cross over Temperature

C Degree Centigrade

F Atomic Form Factor

h, k, l Miller Indices

xvii

a,b,c Crystal Unit Cell Parameter

ρ Resistivity of Sample

R Resistance of Sample

Ω Ohm

σ Conductivity

Q Joule’s Heating

T* Pseudo Gap Temperature

HTSC High Tc Superconductors

LBCO LaxBa1-xCuO4

YBCO YBa2Cu3O7-x

BSCCO Bi2Sr2Can-1CunO8+x

TBCCO TimBa2Can-1CunO2n+m+2

HBCCO HgBa2Can-1CunO2n+2+x

FLL Flux-Line Lattice

G-L Ginsburg–Landau

L-D Lawrence-Doniach

JC Critical Current Density

FL Lorentz Force

β Phenomenological Coefficient

http://en.wikipedia.org/wiki/Mercury_(element)

http://en.wikipedia.org/wiki/Mercury_(element)

http://en.wikipedia.org/wiki/Calcium

http://en.wikipedia.org/wiki/Copper

http://en.wikipedia.org/wiki/Oxygen

Chapter 1 Introduction

~ 1 ~

CHAPTER 1


~ 2 ~

INRODUCTION

1.1 Preamble

The phenomena of superconductivity are low temperature business. The low temperature

works started with liquefaction of natural gases in the late 1800. Successful liquefaction of O2

was done by French scientist Caillettet in 1877. In 1911, superconductivity was discovered in

mercury at liquid Helium temperature by Dutch physicists H. K. Onnes [1]. He was awarded

with Nobel prize for his work. Since its discovery in 1911, even today, superconductivity is an

important area of research in solid state physics. The disappearance of resistivity in a material

is called superconductivity, and the temperature at which it occurs is called as critical

temperature (TC). The discovery of superconductivity opened a new area for research.

1.2 Zero Resistivity & Perfect Diamagnetism

Materials having zero resistance or infinite conductance at particular temperature (TC)

called Zero resistivity (Fig.1.1). Meissner and Ochsenfeld [2] in 1933 discovered that a

superconductor below its transition temperature expels all the magnetic fields below its critical

field i.e. perfect diamagnet (Fig.1.1). This phenomenon is called as Meissner effect which

distinguished a perfect conductor from a superconductor.

B = µ0 (H+M) (1.1)

i.e. 0 = µ0 (H+M) (or) H = – M (or) χ = –1

This observation was followed by other metals below a certain temperature. Magnetic

materials like Ni, Fe, etc. did not exhibit superconductivity. But with strong pressure Fe

exhibits superconductivity i.e. TC = 2 K. Soon after this, some alloys like Nb3Sn, Nb3Ge show

zero resistance with further higher temperature.


~ 3 ~

Fig. 1.1 Zero resistivity & Perfect diamagnetisms of Type I superconductor

1.3 High TC Superconductor

Achieving higher TC becomes the major goal in the field of superconductivity since its

discovery. Around 1980, exciting new superconductors came into picture with CuO2 plane

being origin of superconductivity. In the year 1986, J. G. Bednorz and K. A. Muller reported

superconductivity in LaxBa1-xCuO4 (LBCO) at nearly 30 K. Soon after this many other oxide

based superconductors were discovered and much importantly their TC and critical current

density (Jc) were much higher than alloys like NbTi and Nb3Sn. This field opened a new era in

field of High TC SuperConductor (HTSC) as they break the barrier of 30 K imposed by BCS

theory. LBCO (only insulator in HTSC) is the first oxide based HTSC material having TC equal

to 35 K. Further research on HTSC was carried on YBa2Cu3O7-x (90 K), Bi2Sr2Can-1CunO8+x

(110 K), TlmBa2Can-1CunO2n+m+2 (125 K) and HgBa2Can-1CunO2n+2+x (150 K). The values

provided in brackets are the respective transition temperatures.

Conventional Superconductor


~ 4 ~

FEW FETCHERS OF HTSC:

They have their structure derived from ideal perovskite structure.

Having layered crystal structure containing one or more CuO2 planes

Charge transformation can takes place in superconducting plane which is controlled by

the charge reservoir and insulating layer.

Small coherence lengths, large penetration depth, higher TC, Large energy gap are the

general fetchers of HTSC.

1.4 Short Overview on Superconductivity

1.4.1 Type I & Type II Superconductors:

On the basis of perfect diamagnetism superconductors are categorized into two groups

Type I and Type II. Type I superconductors having identical characteristic of zero resistivity

below critical temperature, zero internal magnetic field (Meissner effect) and critical magnetic

field Hc, above which superconductivity ceases. Here phase transition from superconductor to

normal is sharp. These superconductors are obeying the BCS theory of electron pairing

mechanism by lattice vibration. As the Hc and TC are small, their practical applications are

limited.

Besides normal and superconducting regions, Type II superconductors are having an

additional vortex state region. As the vortices of the superconducting current surround the

filaments or core of normal metal, complete exclusion of flux takes place up to Hc1 called lower

critical field. After that the complete flux penetration takes place up to higher field Hc2 and

superconductivity destroyed after Hc2 (Fig.1.2).

The variation of critical field with temperature can be represented by a parabolic law

Hc =H0 [1- (T/TC) 2] (1.2)

where HC –critical field strength at temperature T,

H0 - Maximum critical field strength occurring at 0 K


~ 5 ~

Fig. 1.2 Meissner effect in Type I & Type II

1.4.2 Flux Quantization

Beyond Hc1, the field penetrates as quantized flux lines or vortices. The basic unit of

flux vortex is one quanta of flux i.e. Φ = h / 2e. A flux vortex consists of a normal core of

radius ξ surrounded by a superconducting region, where a suppercurrent circulates around the

normal core to generate the single quantum of flux Φ (Fig.1.3). This superconducting region is

extended to distance λ i.e. penetration depth. Inside the vortex the order parameter is zero.


~ 6 ~

1.4.3 Penetration Depth (λ) & Coherence Length (ξ)

Magnetic field is exponentially screened from the interior of the superconductor with a

distance penetration depth (λ). The number density of superconducting electrons (ns), vary

continuously from zero at TC to n, the density of conduction electrons, at T<< TC. Thus the

parameter λ is a temperature dependant quantity described by

λ (T) = λ (0) [1- (T/TC) 4]

-1/2 (1.3)

The coherence length (ξ0) is the measure of distance between two superconducting electrons

and is given as

ξ0 = ћvF / 2Δ (1.4)

where, vF electron velocity at Fermi surface and 2Δ energy gap

1.5 Flux Creep, Flow and Pinning

Homogeneous Type-II superconductors are associated with the flux-line lattice (FLL)

[3]. Each FLL carry a quantized amount of flux (Φ) which is characterized by the presence of a

Fig. 1.3 Quantized amount of flux associated in mixed region

Φ=h / 2e


~ 7 ~

normal core. But in case of an ideal homogeneous defect less superconductor, the FLL are not

pinned and as a result the critical current density Jc tends to zero. The pinning or the interaction

of the FLL with various crystal imperfections, or pinning centers in Type-II superconductors

are responsible for the existence of a critical current density Jc, usually defined as the current

density at which an arbitrarily small voltage is observed. The strength of this interaction

depends only on the nature, shape and physical properties of the defects. These interactions

arise from the difference in critical temperature (TC), critical field (Jc) or Ginsburg–Landau (G-

L) parameter ĸ. In case of non-superconducting pinning centers this difference is large. But

pinning is very less in case of high TC superconductors (HTSC) [4, 5].

In presence of magnetic or electric field, magnetic lines of force experiences a Lorentz

force (FL) [6]

BJF CL

(1.5)

Force on a single vortex is J Cf (1.6)

For this force, flux lines tend to move transverse to current with a velocity V which

ultimately induce an electric field of magnitude

VBE

(1.7)

which is parallel to Jc (Fig.1.4). This acts like a resistive voltage and simultaneously power is

dissipated [7]. So decrease in critical current density takes place. To enhance critical current

density we require pining of vortices. Pinning can done through addition of impurities which

can block the vortices motion. Thus flux pinning is caused by forces between fluxiods and

inhomogeneties in the crystal known as pinning centers. Pinning of vortices can occur with

various types of microstructral inhomogeneties, such as dislocation, inter- and intra- grain

boundaries precipitation of secondary phases. So artificial pining centers are the more suitable

for superconductor technology [8-10].


~ 8 ~

Fig. 1.4 FLL Experiences Lorentz force

So pining is necessary to enhance Jc. In case of conventional superconductors like NbTi,

flux lines are pinned by dislocation where as in Nb3Sn flux lines pinned by grain boundary. But

in case of HTSC vortex pinning can takes place in the form of pancakes perpendicular to CuO2

layer (Fig.1.5).

Distance between double / triple layer of CuO2 superconducting planes is relatively

large which is responsible for anisotropy. As the decoupling between the layers is large in Bi &

Ti based superconductor, super-current are confined to the layers and the vortices are

segmented into vortex-pancakes with weak electromagnetic and Josephson coupling between

them [11].


~ 9 ~

Fig. 1.5 Flux pinning in HTSC

1.6 Ginzburg-Landau Theory

Researchers came forward with various theories to understand the mechanism behind

superconductivity. However none of them were suitable to explain complete zero resistivity

properly along with Meissner effect. The only successful theory put forwarded by BCS could

explain the mechanism of superconductivity satisfactorily and limits it to lower TC phase with

weak electron- phonon interaction.

Ginzburg and Landau (G-L) used a phenomenological approach to describe the normal

to superconductor transition on the basis of thermodynamically second order phase transition

(Fig.1.6). The GL theory introduced the order parameter Ψ for superconducting electrons (ns)

such that the local density of super electrons was given by ns= |Ψ|2. According to G-L theory,

variation of free energy (F) can be expanded as a power series of ψ,


~ 10 ~

i.e.

82

1

2

22

42 hAie

mFF n

(1.8)

where α = α0 (t-1), t=T/TC and β are phenomenological coefficients associated with variation of

free energy due to temperature and magnetic vector potential (Fig.1.7). When thermal

fluctuation rises from 0 →kβT, order parameter ψ decreases from ψ to ψ- δψ. Above TC, α > 0,

F = 0, when order parameter, Ψ = 0, for normal state. From G-L theory, magnetic penetration

depth (λ) over which magnetic field present and the superconducting coherence length (ξ) i.e.

maximum distance between electrons of cooper pairs is calculated as ξ2

(T) = ћ2

/ 4mα and λ2

(T) = mc2

/ 4πnse2. Both the parameters i.e. coherence length and penetration depth diverge in

the same way near T=TC. When T→TC the ratio gives finite parameter.

ĸ = λ/ ξ (1.9)

where ĸ is Ginzburg – Landau parameter and is the characteristic of a superconducting

material. The value of ĸ =1/√2 separates type I and type II superconductors.

Ψ

TC T

Fig. 1.6 Variation of order parameter Ψ with T


~ 11 ~

T > TC

F

T ≥ TC

T ≤ TC

T<TC

Ψ

Fig. 1.7 Variation of free energy F with order parameter Ψ at various temperatures


~ 12 ~

1.7 Excess Conductivity or Para Conductivity

The layered cuprates with complex crystal chemistry and inherent granularity leads to

strong structural disorder at the microscopic and mesoscopic level respectively. Mesoscopic

inhomogeneties like grain boundaries, cracks, voids, etc having much larger length scale than ξ

and being temperature independent and don’t influence the superconducting order parameter

region (SCOPF) [12]. These inhomogeneties strongly affect the zero resistance (TC) state of

mean field region. The microscopic inhomogeneties such as structural defects (twin boundary,

stacking fault), chemical imperfection (oxygen deficiency) occur inside the grain. The length

scale of these inhomogeneties is small w.r.t. mesoscopic, inhomogeneties but still larger than ξ

not affecting strongly SCOPF [13]. So in other term, impurities or the presence of secondary

phases expand the critical region in the composite and affect the Lawrence –Doniach cross over

temperature.

Shorter coherence length and higher value of anisotropy creates thermo dynamical

fluctuation in the bulk superconductors. Due to this superconducting cooper pairs are

annihilated and degraded much before the transition. As the temperature is further lowered, the

number of cooper pair density overcomes the electron density. Zero resistance (TC0) is achieved

at sufficiently low temperature where all the conduction fermions form bosonic condensate

cooper pair. This thermal fluctuation gives rise to anomalous increase in conductivity above the

TC i.e. called as excess conductivity or paraconductivtiy (Fig.1.8). This excess conductivity

region gives information regarding coherence length, dimensionality of conduction,

dimensionality cross-over (if present), etc [14 -15].


~ 13 ~

Fig. 1.8 Real Superconductor showing Excess conductivity region

1.8 Josephson Junction

Around 1962, Josephson predicted that a zero voltage supercurrent Is = Ic sin ΔΨ should

flow between two superconductors separated by a thin insulating barrier. Here ΔΨ is the

difference in the phase of the GL wave function between two superconductors and Ic is the

maximum supercurrent that the junction can support is known as dc Josephson effect. And

further more if a voltage difference v is maintained across the junction, the phase difference ΔΨ

would evolve according to d (ΔΨ)/dt = 2ev / ћ. So an alternating current of amplitude Ic with

frequency ʋ = 2ev/ ћ. So that a quantum energy ћ ʋ equals to energy change of cooper pair

transferred across the junction. This is known as ac Josephson junction.


~ 14 ~

1.8.1 Weak Link Behaviour:

Conventional superconductors like Nb-Ti and Nb3Sn are not granular and having strong

pinning centres. Mainly granular superconductors suffered from weak link behaviour where

strong superconducting regions are separated by weakly coupled interface. So transmitting

super-current can flow over a small distance. As transport current mainly depend upon the

current between the grains (i.e. grains & grain boundaries).In high TC ceramics, the sintered

grain boundary behave as weak link i.e. Josephson junction shown in Fig. 1.9. The grain sizes

area large as compares to λ and ξ. So high TC superconductors are modelled by considering an

array of Josephson junction which links two superconducting grains.

Fig. 1.9 Grain boundary acting as Josephson weak link

A large number of studies have been done in the field of cuprate superconductors. Of all

RE-Ba-Ca-Cu-O cuprates, the most interesting system is Bi-based as it has some advantages

over other rare earth material i.e. no rare earth element is present leading to lower cost, higher

TC, lower reactivity to moisture, better mechanical properties for shaping and increasing critical

current values for practical application point of view [16].


~ 15 ~

1.9 Literature Survey

BSCCO compounds have three different superconducting phases, i.e. Ca free phase

having critical temperature (TC) value of 20 K (the 2201phase), 90 K (the 2212 phase) and 110

K (the 2223 phase). The 2201 and 2212 phases are comparatively easy to synthesize, as they

are thermodynamically stable over a wide range of temperature and Stoichiometric range

within the Bi2O3-SrO-CaO-CuO system. Bi –based superconductor is associated with liquid

phase by peritectic reaction among the elements with variation of temperature and

concentration. But on the other hand, the 2223 phase is stable only over a narrow temperature

range and in most cases Lead (Pb) doping is necessary. The 2212 superconductor have some

advantages over the 2223 phase. These include a considerably extended single-phase region for

BixSr3-yCayCu2O8+δ where x varies from 2-2.35 and y from 0.7-1 which have relatively rapid

preparation with higher degrees of phase purity and homogeneity with improved stability to

moisture [17-18].

Various experimental works suggested that Lead (Pb) doping stabilizes the phase

formation in Bi-based superconductor. But Lead (Pb) is harmful to human beings as well as

environment. So the main goal is to prepare an eco-friendly sample i.e. Bi -based 2212 phase

(comparatively stable) without Lead (Pb).

Like other high TC ceramic superconductors, Bi-based layered superconductors also

have randomly oriented grains with sharp grain boundaries connected by weak link or other

impurities secondary phases or defects. Such type of granular matrix leads lower value of flux

pining and low critical current density. Here critical current density experiences a Lorentz force

(FL) in the mixed state. This FL given by FL= (Jc x Φ) where Φ- quantized magnetic flux per

vortex. This force is opposed by pinning force E = - (B x V) where V is the velocity of vortices

in the transverse direction to both Jc and B. But unfortunately this pining force is very less in

HTSC [19].

Since the discovery of Bi-Based HTSC, vigorous works had been done to improve the

superconducting properties, particularly critical temperature and critical current density for

practical point of view. Insufficient flux pinning itself inside the crystal structure of BSCCO

(2D) limits the flow of critical current. Bi- based superconductors are highly anisotropic due to


~ 16 ~

alternate stacking of superconducting CuO2 layers and poorly conducting thick blocks, which

reduce the Josephson coupling between the superconducting CuO2 layers. Due to weak

coupling between CuO2 layers, 3D vortices changed to 2D pancake vortices which are easily

de-pinned at higher temperature and magnetic field and causes energy dissipation. Hence

numerous efforts have done to improve the Jc value by adding suitable impurities by doping,

irradiation and composite formation [20]. A. Crossley et.al. doped Pb on Bi-2212 phase and Pb

doping does not enhance the intra granular Jc in compared to undoped material due to miss

alignment of grains [21]. A. Ghattas et. al. successfully improves Jc by adding 2% Al nano

particles in Lead(Pb) based Bi-2223 phase [22]. M. Zouaoui added ZrO2 in Bi-Pb-2223 phase

and showed that 2% wt. successfully improves the Jc [23]. N. A. Hamid et. al. added TiO2 to

Bi-based 2212 phase decrease in critical current density was obtained [24]. W. Wei et.al.

successfully added MgO in 2212 phase which enhances irreversibly field [25]. Multilayer

structure of superconductor with other oxides, in particular, multilayers of high TC cuprates and

colossal-magnetoresistance manganites, has been paid special attention. Investigation of PLD

deposited YBa2Cu3O7/ La (Sr)MnO3 bilayers reveals the long scale proximity effect between

YBCO and LSMO layers[26]. Localized penetration of superconductivity into the

La0.7Ca0.3MnO3 up to distances much larger than that is possible for Cooper pairs in a singlet

spin state to exist is, observed in La0.7Ca0.3MnO3 films epitaxially grown on Pr(Ce)CuO4 [27].

Besides the above mentioned applications, one of the most promising and basic applications of

superconductors is in the electrical power transport. Hence there has been consistent effort in

the enhancement of current carrying capacity, which may be achieved by incorporating

extended defects acting as pinning centers. The effect of pinning centers is at its best when their

sizes are of the order of coherence length. It has been shown that introduction of high density

YBCO 211 nano particles (size 15nm) on the multilayers of ultrathin YBCO 211 and 123

increases the critical current (at 77 K) by a factor of two to three for high magnetic fields [28].

For the enhancement of current density, it is also important that the density of these pinning

centers should be as high as 1011

cm-2

. Many times in order to achieve this large density, large

number of defects are also created which degrades the superconducting properties. Recently it

is found that magnetic nano-particles may act as efficient pinning centers at much lower density

[19, 29 - 30]. Since then various magnetic nano-particles as pinning centers in the

superconductors, has been investigated [31- 34].


~ 17 ~

Another attractive feature is that the magnetic flux pinning and critical current density

of superconducting composites are enhanced by addition of lower concentration of

nonsuperconductors [35]. In the recent years, BiFeO3 (BFO) has emerged as potential candidate

for various applications due to its large magneto-electric coupling [36]. Ground state of BFO is

antiferromagnetic; however its nano form exhibits superparamagnetisim, which is very suitable

for pinning effect in superconductor [37]. So BFO may act as a good pinning centers in

Bi2Sr2CaCu2O8 (BSCCO-2212). Besides pinning effect, the conduction mechanism in these

materials is also investigated. So far very few works has been reported on BSCCO with

magnetic nanoparticles and moreover BSCCO is having elemental similarity with BFO. The

BSCCO-2212 phase is chosen because it can be synthesized without Lead (Pb) doping (non-

eco friendly), which otherwise is essential in BSCCO-2223 phase. As per H. Nadifi et al.,

successfully reported an active material that can exist either as a granular composite or as a

layered structure is BSCCO/BFO composite [38].

So interest is motivated towards the Bi- based supper conductor with BFO as pinning

center. Consequently it is of interest to study the superconducting behavior of these composites

with variation of BFO concentration. Due to the high anisotropy, shorter coherence length,

higher thermal energy, Bi- based superconductors shows broadening in resistive transition at

lower temperature region [39]. Thermodynamic fluctuation gives rise to anomalous increase in

superconducting properties far above the TC. This fluctuation induced conductivity (FIC) is

very important as it provides valuable information regarding conductivity, magnetization and

thermoelectricity [40]. Not only nano inclusion of magnetic impurities to superconducting

background affects the thermal fluctuation with interaction of vortices to superconducting

arrays but also provides the idea of Josephson like system which can be used as a tool to

modify the field dependent critical current of Josephson junctions. This allows the reduction of

noise in SQUID –based devices and in micro strip band pass filter and enhances critical current

and current carrying capacity of superconducting cable and wires [41].


~ 18 ~

1.10 Role of magnetism in superconductor

Magnetic impurities or ferromagnetic layer in a superconductor can act as a strong pair

breaker. Interplay between superconductivity and magnetism through proximity effect where

ordering in both the system is dramatically different from each other and generally they are

incompatible with each other [42]. The mutual interaction of two antagonistic phenomena like

magnetism and superconductivity in bulk materials, becoming the exciting topic in the field of

superconductivity [43]. Here the phonon-mediated attraction energy between electrons which

results the formation of Cooper pairs, is generally smaller than the exchange (Zeeman)

interaction between electrons which tend to align the electron spins. But, the total non-zero

momentum Cooper pairs can be accomplished even in the presence of an exchange field was

predicted first by Fulde and Ferrel [44] and independently by Larkin and Ovchinikov [45]

nearly 50 years ago. Coexistence between these contrasting phenomena has already been

observed experimentally in both bulk samples [46-47] as well as thin films [48-50]. After all in

ferromagnet superconductors where both superconducting and ferromagnetic order parameters

are present in a uniform hybrid material [51]. Recently neutron diffraction experiment shows

the suppression of the AFM ordering below TC suggesting the coexistence of these phenomena

[52]. Similarly ARPES [53], NMR [54] and TEM [55] analysis indicates the mesoscopic phase

separation between the superconductivity and the insulating antiferromagnetism state.

1.11 Motivation

HTSC superconductors are granular having sharp grain and grain boundary suffered

from weak link problem. And Bi-based systems are more disordered one among all HTSC. So

artificial pinning is very important to pin the vortex motion by addition of secondary phases.

Magnetic nano particles can pin more number of vortices with lesser concentration. As BFO

nano particles exhibiting superparamagnetisim behavior, they can act as a good pinning center

in BSCCO 2212 phase.

OBJECTIVES:

To synthesize BSCCO2212 phase and nano particles of BiFeO3 (BFO).

To prepare composites of different concentrations.


~ 19 ~

And study how nano particles play a major role and affecting the

superconducting nature of composites.

Study the pinning nature of BFO in the composites.

1.12 Crystal Structure of BSCCO

The general formula of this Bi based system is Bi2Sr2CanCun+1O6+2n where n is an

integer (Fig.1.10). In this system there are grouping of CuO2 layers, each separated by Ca atom

with no oxygen. The CuO2 layers are bound together by intervening layers of BiO and SrO.

Here the first member of this family 2201 compound with n = 0 has octahedrally coordinated

Cu and having TC nearly 20 K. The second member of this family with TC 90 K having general

formula 2212 with one layer of Ca. There are two (CuO2-) layers separated from each other by

the (--Ca) layer. The spacing between CuO to Ca is 1.66 A0.

Superlattice structure have been

reported along a and b, which mean that minor modification of unit cells repeated nearly every

5 lattice spacing. Third member of this family is 2223 having three CuO2 layers separated from

each other by Ca planes with TC having 110 K. This 2223 phase is very unstable as interlayer

coupling between BiO layers is very less. So Lead (Pb) is substituted for stability of phase.

Here BiO layer act as a charge reservoir layer that regulates the charge density in the

CuO2 layer. Holes are created by excess O-atoms in the BiO layers and by Sr deficiency.

Whereas charge conduction takes place through SrO layer. Here Ca atom acts as an insulating

layer which is sandwiched between CuO2 conduction layers.

To assess the effect of CuO2 planes on TC, it is necessary to consider the role of

interlayer coupling. TC is maximum for optimum number of CuO2 planes. Intracell and intercell

coupling can takes place in cuprates. But intercell coupling between two CuO2 plane is much

smaller because of spacers layer of Bi/ Sr/ Ca. Inter cell coupling is much larger as evidence by

smaller anisotropy of conductivity. Due to the broad range of composition and high defect

concentration, cuprates superconductors shows higher anisotropy, so inhomogeneity plays an

important role in Bi- based superconductors [11].


~ 20 ~

Fig. 1.10 Crystal structure of BSCCO

O

c

C

u

Cu

TC = 90K


~ 21 ~

1.13 BFO Nano Particles

BFO is a multiferroic (both ferroelectric & ferromagnetic) material where

magnetoelectric coupling takes place. Ferroelectrocity requires empty d orbital and

ferromagnetism requires partially filled d orbital. To achieve this contrasting behavior in a

single compound, elements form a distorted perovskite structure. Active lone pair is present in

Bi in 6s orbital which is the cause of ferroelectricity and B side cation Fe creates

ferromagnetism. Spiral spin structure where antiferromagnetic axis rotates through the crystal

with a period of ~ 62 nm. Spiral spin structure cancels out the macroscopic magnetization and

simultaneously inhibits linear magnetoelectric effect. But still a significant amount of

magnetization (~ 0.5 µB) is present in a unit cell and a strong magnetoelectric coupling takes

place [56 -57].

Here atomic force microscopy (AFM) image clearly shows nano particles of BFO

around 100 nm exist. Nano particles of BFO show superparamagnetisim behavior shown in

Fig.1.11. So that it can easily trap more number of magnetic lines of force which will act as a

good pinning center in BSCCO 2212 phase.

This thesis is organized as follows. Introduction provides the major role of impurities in

the vortex state and why magnetic nano particles of BFO are chosen as pinning centers in the

Bi-based cuprates. Chapter 2 describes the experimental techniques used for sample preparation

and their characterization. Chapter 3 provides XRD and SEM images of parent as well as

composites. In chapter 4 results obtained are discussed and analysised. Chapter 5 deals with

conclusion and important findings of present work.


~ 22 ~

0 2 4 6 8 10 12 14 16

0.0

0.5

1.0

1.5

2.0

2.5

M(e

mu

/gm

)

H(Tesla)

Fig.1.11 AFM Image showing BFO nano particles (Left) & M-H loop showing

Superparamagnetisation(Right)


~ 23 ~

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Chapter 2 Experimental Techniques & Sample Preparation

~26~

CHAPTER 2


~27~

Experimental Techniques & Sample Preparation

2.1 Introduction

The major challenges faced by researchers today are the synthesis of materials with

desired composition, structure, and properties for specific applications. There are developments

of new techniques to get high quality materials with affordable cost and minimum required

time. The synthesis of materials requires knowledge of crystal chemistry, thermodynamics,

phase equilibrium, and reaction kinetics [1].

A number of intensive research efforts have been directed at improving the properties of

bismuth-based superconductors soon after their discovery. It is evident that partial replacement

of bismuth by Lead (Pb) leads to enhance of superconducting properties such as Jc and Tc. To

augment the structural stability, to understand the nature of charge carriers, the effect of carrier

concentration on the superconducting properties of the system, the substitution of different

elements in the system and to study a number of related parameters are the subject of a number

of communications, thus, playing a vital role and arousing much interest in this field [2].

This chapter describes the various steps of the synthesis technique and different

methods of characterizations of the studied materials.

2.2 Various Types of Synthesis Technique For Preparation

Of BSCCO 2212 Phase

Synthesis of layered structure is the most important technology for practical point of

view. The basic idea is that synthesis technique should give uniformity in the microstructure of

a single phase ceramic for better properties. Numerous techniques are available in the literature

for the synthesis of oxide based superconductors. Selection of the synthesis route is crucial to

control the composition, structure, and morphology of a chosen material. It is observed that

there are several methods of preparation for BSCCO, i.e. solid state synthesis route,

melt process, pyrolysis and sol-gel synthesis route. In liquid phase technique the

constituents are mixed in the atomic level and then the lattice growth occurs. Though


~28~

liquid phase technique gives better homogeneity, yields less particle size and require

shorter heat treatment, Still the solid state reaction route method is well appreciated for a

large-scale production of layered structure. It requires low cost precursors which are readily

available and needs easier preparation technique and better homogeneity [3, 4].

2.2.1 Solid State Reaction Route

2.2.1 (a) Reagents

The solid state reaction route is the most convenient and widely used method for the

preparation of granular layered HTSC solids from a mixture of solid oxide and carbonates as

precursors. Heat is required for solid precursors for reaction. The samples of BSCCO 2212

phase were prepared from high purity chemicals of Bi2O3 (s-d fine, purity99%), SrCO3 (Finar

Reagent, purity98%), CaCO3 (s-d fine, purity98%) and CuO (Finar Reagent, purity 99.5%).

The nature of this raw material has a major effect on the properties of the final ceramic

material. The quality of raw materials depends upon the purity percentage and particle size. The

reagents are selected on the basis of reaction conditions in atmosphere.

2.2.1 (b) Weighing and mixing

The reactants are taken in a Stoichiometric ratio for a desired 2212 phase formation. Let

‘M’ be the molecular weight of the desired ceramic and ‘m’ be the amount of prepared

material. ‘Ma’ is the molecular weight of the oxide/carbonate used in the synthesis of BSCCO

and ‘z’ is the fraction of “a” metallic ion in the ceramic. Then required weight

Ma = (Ma z m)/M (2.1).

The Bi2Sr2Ca1Cu2O8+δ (BSCCO) samples are prepared from oxides and carbonates of the

respective metals, via solid state reaction method. The precursors used are Bi2O3 (s-d fine,

purity99%), SrCO3 (Finar Reagent, purity98%), CaCO3 (s-d fine, purity98%) and CuO (Finar

Reagent, purity99.5%). Stoichiometric amount of ingredients are weighted, mixed thoroughly

and ground in an agate mortar and pestle.


~29~

2.2.1 (c) Calcination

The heating of the mixture not only depends on the form of reactants but also its

reactivity. Calcination is used to achieve the desired crystal phase and particle size. For the

Heating of material high melting point container is used i.e. alumina crucibles (Al2O3). Then

the mixture is calcined in three stages at temperatures 700 0C for 12 hr, 770

0C for 12 hr and

820 0C for 20 hr in a silicon carbide programmable muffle furnace with a heating rate of 5

0

C/min. Intermediate grinding is done at the end of each heating cycle. After the third heat

treatment, the samples are allowed to cool under normal cooling rate and pelletized in a

cylindrical die of 10 mm diameter under 5 Kg/cm2 pressure. Then the pellets are subjected to

final heat treatment at 880 0C for 12 hr. Synthesis steps of BSCCO are described in fig.2.1.


~30~

Fig. 2.1 Flow chart for BSCCO 2212 phase synthesis

Heated at 770 0C for 12 hr.

Heated at 700 0C for 12 hr.

Bi2O3

SrCO3

CaCO3

CuO

Mixing & Grinding

Grinding

Grinding

Heated at 880 0C for 12 hr

Desired BSCCO

2212 Phase

Grinding

&Pelleting

Heated at 800 0C for 20 hr


~31~

2.3 Various Types of Synthesis Techniques For Preparation

of BiFeO3 Phase

As the chemical synthesis route is the best and simplest route which gives better mixing

and homogeneity with nano size particle, it is chosen for synthesis of BiFeO3 (BFO) for

composite formation.

2.3.1 Chelating Agent:

Chelating agent is a ligand which can chelate with metal atom. Urea, EDTA and

Glycine are few examples of chelating reagent. Here glycine is chosen as chelating reagent for

preparation of BFO.

2.3.2 Synthesis Process of BFO

Synthesis of BFO is done via sol-gel combustion route using Bi (NO3)3.5H2O (Merck,

purity 99%) and Fe (NO3)3.9H2O (Merck, purity 98%). Stoichiometric amount of metal nitrates

are weighted and dissolved in distilled water. As Bi (NO3)3.5H2O is insoluble in water,

appropriate amount of nitric acid is added to maintain pH = 1 of the solution. Glycine is found

to be the best chelating agent/fuel for the synthesis of BFO. The metal ions: glycine ratio is

maintained to be 1:1. Then the solution containing all the precursors is heated under constant

stirring with a magnetic stirrer. The solution slowly turned into brown colored gel and

ultimately burnt vigorously resulting in a brown color powder (xerogel powder). Then xerogel

BFO powder is calcined at 700 0C for 1 hr in a silicon carbide programmable muffle furnace

with a heating rate of 5 0C/min. Steps are shown in Fig. 2.2.


~32~

Fig. 2.2 Flow chart for BFO synthesis

2.4 Superconductor/Nano Composite of BSCCO/BiFeO3

Both BSCCO 2212 and BFO calcined powders are taken and manually grinded for 20-

30 min for homogeneous intermixing. Then the mixed samples were pelletized using a

cylindrical die of 10 mm diameter under 5 Kg/cm2 pressure. Then final heat treatment was done

at 800 0C only for 10 min. The composite of 1%, 2%, 3%, 4%, 5%, 10%, 15%, 20%, 25% and

30% BFO were prepared shown in Fig 2.3.

Xerogel Powder

Bi

(NO3)3.5H2O

12ml HNO3+38ml

H2O

Fe (NO3)3.9H2O

50ml H2O

Glycine

Heating & Stirring

700 0C 1 hr


~33~

Fig. 2.3 Flow Chart For Superconductor/Nano Composite of BSCCO/BiFeO3

2.5 Characterization Techniques

The above synthesized materials are characterized by various experimental techniques

at different measurement conditions to study its physical properties. The basic principle of X-

Ray diffraction (XRD) and Scanning electron microscope (SEM) is given below. Critical

temperature and critical current were measured using standard four probe method by Van-der

Paw method.

Heated at 8000C for 10 min

BSCCO 2212

Mixed & Grinded for

30 min

BFO

Pelletisation


~34~

2.5.1 X-Ray Diffraction

X-ray diffraction (XRD) characterization technique is used for structural study of the

material. Simultaneously it provides information about atomic arrangements of materials,

crystal structure, and identification of the chemical species present in materials which are the

origin of physical properties of materials. As the wavelength (λ) of X-ray is comparable to the

interplaner spacing (d) of crystals, diffraction occurs at particular angle (θ) (Fig.2.4). This

diffraction satisfies the Bragg equation,

2dsin θ = n λ (2.2)

Where n is the order of diffraction. The intensity of the diffracted beams depend on the

arrangement and atomic number.

As atoms are having radius which is of the order of X-ray wavelengths and the electrons

are distributed over the entire atomic volume. So a phase differences occurs between scattered

waves which decrease the intensity of the scattered wave with increase in scattering angle.

This decrease is described by the atomic form factor (F j) expressed as the Fourier sum

of f j all atoms j in unit cell, with the phase(δ) of the reflection.

F = ∑N

i=j fj exp [2πi (hXJ + kYj + lZj)] = F exp (iδ) (2.3)

The intensity of a diffraction peak is proportional to the squared value of the structure factor

I = Fhkl2

= ( ∑N

i=j fj exp [2πi ( hXj + kYj + lZ j)])2 (2.4)

The calculation of lattice constants from the line positions or d spacing can be found from a

general formula

1/d2hkl = (h

2 + k

2 + l

2) /a

2 (2.5)


~35~

Where h, k, l are the miller indices. The above formula is used to calculate lattice parameters

for all the compositions [8, 9].

Fig. 2.4 X-Ray Diffraction Pattern

Freshly prepared superconducting powder BSCCO as well as all composites are

characterized by powder x-ray diffraction at room temperature with Cu Kα radiation using

PHILIPS PW 1830 HT X-Ray Generator using Copper(Cu Kα) target with wave length 1.54

A0

and 2θ value ranging from 200 to 50

0 with scan rate 2

0 per min.

2.5.2 Scanning Electron Microscope

The scanning electron microscope (SEM) uses a focused beam of high energy electrons

which images the sample by scanning it with a beam of electron. The accelerated

electrons in the focused beam is dissipated by the electron-sample interactions when the

incident electrons are decelerated in the solid sample and produced a variety of signals like

secondary electrons which produce SEM images, backscattered electrons (BSE) and diffracted


~36~

backscattered electrons. Secondary electrons (gives morphology and topography) and

backscattered electrons (illustrate contrasts in composition in multiphase samples) are generally

used for imaging samples. Data are collected over a selected area of the surface of the sample,

and a 2-dimensional image is generated which displays spatial variations in these properties.

Scan areas ranging from approximately 1 cm to 5 microns where magnifications ranging from

20x to approximately 30,000x with spatial resolution of 50 to 100 nm [10-12].

All the samples BSCCO and BFO added composites are characterized by JSM 6480 LV

JEOL microscope. A very low magnification is taken to observe insulating BFO particles in the

conducting back ground of BSCCO matrix. And another high resolution magnification

(10,000X) is taken to study the behavior of grain and grain boundary.

2.5.3 R-T Measurement

The quantitative measure of a material’s opposition to the flow of current is called

resistivity. It depends only on the composition of the material and not on the shape and size.

Resistivity of various materials are plotted in fig.2.5.

Resistivity of metal can be formulated by

ρ = R x A/L (2.6)

Where ρ-Resistivity of metal

R- Resistance of metal

A-Cross sectional area

L- Length of the sample


~37~

Fig. 2.5 Temperature dependence of resistivity plot for (a) metal, (b) semiconductor and (c)

superconductor

Basically resistance R is determined by a voltage – current method. If a known amount

of current ( I ) is allow to pass through the sample and V is the potential difference, then the

resistance can be calculated according to Ohm’s law i.e.

R = V / I (2.7)

Temperature (K) Temperature (K)

Temperature (K)


~38~

But keeping I constant when the resistance of the sample is lowered then V is also lowered. So

it is very difficult to measure a very small potential difference. In case of metal, resistance is

very less. So simultaneously V is also very less. So in this case to get accurate value of V we

can’t increase current much more because of Joules heating i.e.

Q = I2RT (2.8)

Sample will heat. So in general there are two methods for calculation of resistance.

(1) Two probe method

(2) Four probe method

2.5.3 (1) Two Probe Method

In two probe method the voltmeter is directly connected to the wires through which

current is flowing (Fig.2.6). So there is a chance of short circuit.

V= (2r + R) I (2.9)

Where r - lead resistance.

This gives accurate result of the sample when resistance is high (106

Ω).

Fig. 2.6 schematic diagram of two probe

method.


~39~

2.5.3 (2) Four Probe Method

In four probe technique (2 probe for current and two probe for voltage), probes and

wires are not in direct contact with each other (Fig.2.7). If the resistance of the sample is very

less than current can easily pass through the sample.

Fig. 2.7 Schematic diagram of four probe method.

For the sample of irregular shape and size the above mentioned method may not work

and one to follow Vander Pauw method .This method consist of two measurement of resistance

RA and RB (Fig.2.8).

Fig. 2.8 Van-der Pauw measurement in different configurations


~40~

Where RA = V43/I12

And

RB = V23/I14 (2.10)

If RA and RB are similar then resistivity is

ρ= (πd /ln2) ((RA+RB)/2) (2.11)

Where d is thickness of the sample. The precession of such a Vander pauw measurements

depends on the flatness and parallelism of the surface of the sample and on the fact that the

contacts are point contacts. Again if RA and RB are not similar, then expression for resistivity

modifies to

ρ= (πd / ln2) ((RA+RB)/2) f (RA/RB) (2.12)

Where f (RA/RB) is the function of the ratio of RA/RB only.

R-T measurement is done via four probe method in a closed cycle refrigerator using

constant current source (Keithley model 6221), nano voltmeter (Keithley model 2182A) and a

temperature controller (Lakeshore 331). Critical temperature and critical current were measured

using standard four probe method with Helium compressor with a constant current source,

Nano voltmeter and a temperature controller. Here Helium compressor can capable of

compressing liquid He upto 5 K.

2.5.3 (a) Constant Current Source

Keithley constant current source (Keithley model 6221) is used to supply the DC

current to the sample. A voltage is developed across the two voltage leads due to the

application of constant current. Maximum 1mA current is passed through sintered circular

pallets.

2.5.3 (b) Digital Nanovoltmeter

The voltage developed in the sample by application of constant 1mA current

in the four probe setup, is measured by a digital nano voltmeter (Keithley Model 2182). This

is a highly sensitive nano voltmeter having low noise with a sensitivity of 10 nV with high

accuracy.


~41~

2.5.3 (c) Data Acquisition

The instrument such as the constant current source, the nano voltmeter and the

temperature controller are interfaced to a computer for automatic data acquisition as a

function of temperature. The resistivity of the sample is taken from room temperature (around

300 0C) to 6 K.

2.5.4 I- V Measurement

From application point view, the most important parameter of any super conductor is its

ability to carry maximum electrical current i.e., critical current (Ic) or critical current density

(Jc) [13]. The critical current IC is defined as the transport current at which the flow voltage

distinctly appears. The critical current density is given by Ic divided by the cross sectional

area i.e. Jc=Ic / (b x t) Where (b x t) gives cross sectional area of the sample. As this

measurement is very sensitive to current and voltage, there is no clear point at which the

flow voltage will appear. Three different criteria are used for this measurement.

2.5.4 (a) Electric Field criteria

Here the critical current is measured where the electric field reaches a certain value. A

value of 100 μv m-1

or 10 μv m-1

is commonly used.

2.5.4 (b) Resistivity criteria

Here the critical current at which the resistivity of the superconducting sample achieves

a positive value. Generally a value of 10 -13

Ω or10 -14

Ω is used.

2.5.4 (c) Off-set Method

The current where tangential line form part of the current – voltage curve crosses zero

voltage [14 - 15].


~42~

Different methods such as DC magnetization method, Camp bell’s method and four

probe method are used for Jc measurement. But out of all measurements, DC four probe method

is the simplest and easiest one to measure Jc (Fig.2.9). As Jc inversely proportional to the

cross-sectional area of the sample, a very thin slice (as thin as possible) of rectangular bar

shaped piece is taken for this measurement. After ensuring the superconducting nature of

sample i.e., zero resistance, the critical current measurement is done in the superconducting

region at equal temperature intervals keeping temperature constant by temperature controller

(Lakeshore 331) and rising current.

Fig. 2.9 Current – voltage curve and methods of determination

of critical current density using respective criteria


~43~

REFERENCES

[1] K. C. Patil, Chemistry of nanocrystalline oxide materials - Combustion Synthesis,

Properties and Applications. World Scientific Publishing Co. Pte. Ltd. (2008).

[2] M. Arshad, A. H. Qureshi, K. Masudand, N. K. Qazi, J. of Thermal Analysis and

Calorimetry 89 (2007) 595.

[3] B. Heeb, S. Oesch, P. Bohac, L. J. Gauckler, J. Mater. Res. 7 (1992) 11.

[4] B. Lehndorff, D. Busch, R. Eujen, B. Fischer, H. Piel, IEEE Trans. on Appl. Supercond.

5 (1995) 2.

[5] Carvalo, P.B. Tavares, Mater. Lett. 62 (2008) 3984.

[6] A. Hardy, S. Gielis, H. V. den Rul, J. D. Haen, M. V. Bael, J. Mullens, Journal of

European Ceramic Society 29 (2009) 3007.

[7] D. Xue, J. Wei, Mater. Res. Bullet. 43 (2008) 3368.

[8] B. D. Cullity, S. R. Stock, Elements of X-Ray Diffraction. (3rd Edition) Prentice Hall,

(2001).

[9] C. Suryanarayana, M. G. Norton, X-Ray Diffraction A practical approach Plenum

Press (1998).

[10] D. Brandon, W. D. Kaplan, Microstructural characterization of materials. 2nd Edition,

John Wiley & Sons, (2008).

[11] J. Goldstein, Scanning electron microscopy and x-ray microanalysis. Kluwer Adacemic,

Plenum Publishers, (2003)

[12] P. J. Goodhew, J. Humphreys, R. Beanland, Electron microscopy and Analysis, (3rd

Edition) Taylor & Francis (2001).

[13] D. Dew-Hughes, Low Temp. Phys. 27 (2001) 713.

[14] T. Matsushita, Springer-Verlag Berlin Heidelberg (2007) 209.

[15] A. K. Saxena, High Temperature Superconductor, Springer-Verlag Berlin Heidelberg

(2010) 59.

Chapter 3 Phase Conformation & Micro Structural Analysis

~44~

CHAPTER 3


~45~

Phase conformation & Micro structural Analysis

3.1 XRD

Fig. 3.1 shows the XRD pattern of the pure BSCCO as well as compositesupto 5%

added BFO. All the XRD patterns are having well-defined sharp peaks.The indexedpeaks

corresponds to the2212 phase except a very few Cafree 2201 phase which aremarked with *

symbols. No other peak is detected upto 5% addition of BFO in the parent superconducting

powder, hence any othersecondary phases, due to BFO addition are ruled out.

With further addition of BFO, the peaks related to BFO are distinctly identified, marked

as ‘+’ shown in Fig.3.2. No apparent shift of BSCCO peaks is observed, which is a clear

indication that the BFO and BSCCO are in composite form, and BFO is not interfering with the

crystalline form of BSSCO.A very less no of Bi free superconducting phase Ca7Sr7Cu24O41is

also present in this XRD pattern marked as ♦symbol [1-2].


~46~

Fig.3.1 XRD pattern of composites of BSCCO, 1%, 2%, 3%, 4% and 5% BFO after

last stage of heating at 8000C for 10 min. Indexed peaks corresponds to 2212 phase

and of Ca free secondary phases are marked as *.

20 30 40 50

Inte

ns

ity

2

( 0

0 8

)

(1 1

3 )

(1 1

5 )

( 0

0 1

0 )

*

*

( 1

1 7

)

( 0

2 0

)

( 0

0 1

2 )

*

( 0

2 1

0 )

(1 1

13

)

*

( 0

2 0

)

( 0

0 1

7 )

2201

BSCCO

1% BFO

2% BFO

3% BFO

4% BFO

5% BFO


~47~

Fig. 3.2 XRD pattern of composites of 5%, 10%, 15%, 20%, 25% and 30% BFO after

last stage of heating at 8000C for 10 min. Presence of BFO peaks are marked as +.

Some extra peaks in composite samples marked (♦) is observed which resembles with

Bi free superconducting phase Ca7Sr7Cu24O41 (JCPDS: 48-1503 )

20 30 40 50

Inte

nsity

2

BFO

5% BFO

10% BFO

15% BFO

20% BFO

25% BFO

30% BFO


~48~

3.2 SEM

Since the sample under study is a composite, it is very essential to know the

microscopic details. In the Fig.3.3,the surface morphology, scanned via scanning electron

microscopy, of the BSCCO and its composite is shown.The surface of BSCCO shows the flaky

nature of its grains, owing to its layered structure. The clear and flaky grains of layered BSCCO

changeits grainboundary sharpness with slight addition of BFO.The poor-conducting BFO

(nearly1 μm) particles are homogeneously distributed in the conducting matrix of BSCCO

(shown in the lower magnification on the right).Large amount of pores is observed in 5% BFO

added sample. No clear image is observed in 10% added BFO. Again reconstruction of grain

and rounding up of grain boundary is observed in further higher concentration. In higher

concentration secondary phases form colonies which are isolated from the main

superconducting matrix.[3-6].

BSCCO


~49~

2% BFO

2% BFO

5% BFO

5% BFO

4% BFO

4% BFO


~50~

20% BFO

20% BFO

15% BFO

15% BFO

10% BFO

10% BFO


~51~

Fig. 3.3 SEM images of BSCCO, 2%, 4%, 5%, 10%, 15%, 20%and 25% added BFO shows the

variation of microstructure with wt. % of BFO. Lower magnification (shown on right side)

shows homogeneous distribution of BFO.

REFERENCES

[1] R. Shabna, P. M. Sarun,S. Vinu, U. Syamaprasad, J. Appl. Phys.105 (2009) 113925.

[2] V. P. S. Awana, J. K. Bains, G. S. Okram, A. Soni, P. K. Ahluwalia, H. Kishan, J.

Appl. Phys.106 (2009) 096102.

[3] R. P. Aloysius, P. Guruswamy, U. Syamaprasad, Supercond.Sci.Technol.18 (2005)

L23.

[4] W.Wei, J. Schwartz, K. C. Goretta, U. Balachandran, A. Bhargava, Physica C 298

(1998) 279.

[5] T. Haugan, P. N. Barnes, R. Wheeler, F. Meisenkothen, M. Sumption,Nature 430

(2004) 867.

[6] T. J. Haugan, P. N. Barnes, T. A. Campbell, J. M. Evans, J. W. Kell, L. B. Brunke, J. P.

Murphy, C. Varanasi, I. Maartense, W. Wong-Ng, L. P. Cook, IEEE Trans. on Appl.

Supercond, 15 (2005) 3770.

25% BFO

Chapter 4 Results & Discussions

~52~

CHAPTER 4


~53~

RESULTS & DISCUSSIONS

4.1 R-T Measurement

Temperature dependence of resistivity (R-T) below room temperature is one of the

experiment to know the superconducting behaviour of sample. Here, the R-T measurement is

done by Van-der Paw method (for circular pellets). For this measurement, constant current of 1

mA is passed through the sample through the two current leads and the corresponding potential

drop is measured by other two consecutive voltage leads. For this measurement pellets having

thickness 0.66 mm, 0.81 mm, 0.97 mm, 1 mm, 0.87 mm, 0.85 mm, 1.16 mm, 0.98 mm, 1.19

mm, 1.01 mm and 1.44 mm are used for BSCCO, 1%, 2%, 3%, 4%, 5%, 10%, 15%, 20%, 25%

and 30% BFO added samples.

Four electrical contacts over the sample are made with the help of conducting silver

epoxy. After putting the silver epoxy, the sample is kept under warm lamp light for one hour.

This ensures good adhesion of the contact to the sample. Then the sample is mounted over the

cold head of cryocooler with the help of a thin double sided tape. The cryocooler vacuum

shrouds are put and the sample area is evacuated with the help of a vacuum pump. After getting

the desired vacuum, the cryocooler is started and gradually the sample starts cooling. In the

meanwhile, the resistivity measurements process is started so that the resistivity data is

collected while cooling. The resistivity measurement is done with the help of constant current

source (Keithley 2161) and nano-voltmeter (Keithely 2182). All the instruments are interfaced

to a personal computer via IEEE interfacing card. A computer program is written using Lab

view software, to collect the data from the instruments. To obtain the true voltage from the

sample, the measurement is done for both positive & negative currents, which minimizes the

thermoelectric voltage.

The R-T behavior of BSCCO, as shown in Fig. 4.1(a), is almost linear as the sample is

cooled from room temperature. As the superconducting transition temperature TC is

approached, a slight non-linearity is seen in the data. Transition to superconducting state is

clear at the TC-ON and is complete (when resistance becomes completely zero) at TC0. The mean


~54~

field transition temperature, TC is obtained from the derivative of resistance vs. temperature i.e.

dR/dT, The addition of BFO in BSCCO induces non-linearity in the normal region of -T data

(Fig. 4.1(b-k)) and with increasing BFO content, the insulating nature increases, which is very

clear for higher concentration (after 10% BFO) samples.


~55~

0 50 100 150 200 250 300-0.5

0.0

0.5

1.0

1.5

2.0

Tc0=80.74K

Re

sis

tiv

ity

(m-o

hm

cm

)

T(K)

BSCCO

Tc=81.07K

Tc-on

=90.74K

-50 0 50 100 150 200 250 300

0.0000

0.0002

0.0004

0.0006d

R/d

T)

Tc = 81.07K

Fig. 4.1 (a)


~56~

0 50 100 150 200 250 300

0

1

2

3

4

5

6

7

Re

sis

tiv

ity

( m

-oh

m c

m)

T(K)

1% BFO

Tco=53.14K

Tc-on=83.47K

Tc2=57.72K

Tc1=70.63K

0 50 100 150 200 250

-0.0002

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

dR

/dT

(/K

)

Tc1

TC2

Fig. 4.1 (b)


~57~

0 50 100 150 200 250 300

0

2

4

6

8R

es

isti

vit

y(m

-oh

m c

m)

T(K)

2% BFO

Tonset=81.96K

Tc0=47.79K

Tc1=70.06K

Tc2=53.46K

0 50 100 150 200 250-0.0002

0.0000

0.0002

0.0004

0.0006

0.0008

dR

/dT

TC1T

C2

Fig. 4.1 (c)


~58~

0 50 100 150 200 250 300

0

1

2

3

4

5

6

7

8

9

Re

sis

tiv

ity(m

-oh

m c

m)

T(K)

3% BFO

Tc0=46.18K

Tc on=81.64K

Tc1=66.92K

Tc2=51.16K

0 100 200 300

0.0000

0.0003

0.0006

0.0009

dR

/dT

( / K

)

TC1T

C2

Fig. 4.1 (d)


~59~

0 50 100 150 200 250 300-2

0

2

4

6R

es

isti

vit

y(m

-oh

m c

m)

T(K)

4% BFO

Tc on=79.08K

Tc0=43.42K

Tc2=50.61K

Tc1=63.58K

-50 0 50 100 150 200 250-0.0002

0.0000

0.0002

0.0004

0.0006

0.0008

dR

/dT

(/K

)

TC1T

C2

Fig. 4.1 (e)


~60~

0 50 100 150 200 250 300-1

0

1

2

3

4

5

6

7

Res

isti

vit

y(m

-oh

m c

m)

T(K)

5% BFO

Tc0=37.77K

Tc2=36.87K

Tc-on=78.81K

Tc1=56.83K

-50 0 50 100 150 200 250-0.0002

0.0000

0.0002

0.0004

0.0006

dR

/dT

(/K

)

TC1T

C2

Fig 4.1 (f)


~61~

0 50 100 150 200 250 300

0

4

8

12

16

20

Re

sis

tan

ce

T(K)

10% BFO

Tc on=81.53K

Tco=34.16K

Tc1=67.6K

Tc2=44.34K

Resis

tivit

y(m

-oh

m c

m)

0 50 100 150 200 250 300-0.0002

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0.0014

dR

/dT

(/ K

)

TC1T

C2

Fig 4.1 (g)


~62~

0 50 100 150 200 250 300

0

5

10

15

Re

sis

tiv

ity

(m-o

hm

cm

)

T(K)

15% BFO

Tonset=77.43K

Tc0=20.24K

Tc1=63.73K

Tc2=36.55K

-50 0 50 100 150 200 250

-0.001

0.000

0.001

0.002

dR

/dT(

)

TC1

TC2

Fig. 4. 1 (h)


~63~

0 50 100 150 200 250 300-5

0

5

10

15

20

25R

esis

tivit

y(m

-oh

m c

m)

T(K)

20% BFOT

c1=60K

-50 0 50 100 150 200 250 300

0.000

0.001

0.002

dR

/dT(

/ K

)

TC1

Fig. 4. 1 (i)


~64~

0 50 100 150 200 250 300

30

40

50

60

70

80

Resis

tivit

y(m

-oh

m c

m)

T(K)

25% BFO

Tc1=55.83k

0 50 100 150 200 250

0.000

0.002

0.004

dR

/dT(

/ K

)

TC1

Fig. 4. 1 (j)


~65~

When BFO is added to BSCCO, the transition temperature splits in two, TC1 & TC2,

along with a broadening of the superconducting transition. In all BSCCO/BFO composites, the

superconducting transition occurs at two temperatures TC1 & TC2, revealed as double peak in

the dR/dT data. Similar behavior is also observed by Yi Bing Zhang et al, [1] and is accounted

to the weak-link nature of granular superconductors as the later is composed of

0 50 100 150 200 250 300

40

60

80

100

120

Resis

tivit

y(m

-oh

m c

m)

T(K)

30% BFO

Tc1

=56.22K

0 100 200 300

-0.002

0.000

0.002

0.004

dR

/dT(

/K)

Tc1

Fig. 4. 1 (k)

Fig. 4.1 (a) R-T measurement of Pure BSCCO and (b - k) BFO composites (1%, 2%, 3%, 4%,

5%, 10%, 15%, 20%, 25% and 30% added BFO) along with TC-ON, TC0, TC1 and TC2. Each

onset clearly showing TC1 and TC2 values in the plot.


~66~

superconducting grains embedded in a nonsuperconducting host. Nonsuperconducting host may

be normal metal, an insulator, a ferromagnet, a semiconductor or a superconductor with lower

transition. Of the two superconducting transitions temperatures, the higher one stands for the

superconducting grains where the matrix still remains normal and the lower one when the grain

boundary also becomes superconducting. Such type of superconducting behavior is termed as

double resistive transition.

At TC-ON, the grains starts becoming superconducting, leaving the grain-boundary still

resistive. This results in sudden decrease in the resistance (due to zero resistance of grains), but

the value doesn’t reach zero (due to resistive grain boundary). The pair wave function is still

confined within the grain i.e., the coherence length is still smaller than the grain size. With

further decrease in temperature, the coherence length of the pair wave function grows. When

this length is large enough to penetrate the neighboring grains, the probability of tunneling of

cooper pairs through the grain boundary increases, provided the non-superconducting grain

boundary is sufficiently thin. This is called Josephson coupling. The Josephson coupling

between the grains is weak at temperature near TC-ON, so the temperature must be lowered to

TC0, where the Josephson coupling between grains overcomes the thermal energy, and the

grains are coupled. With increasing concentration of BFO, both the transition temperatures

shifts to lower values, but the shift in TC2 is found to be more rapid than TC1 (shown in Table

4.1) [2].

The appearance of additional transition temperature TC2 and its broadening clearly

reflects that the BFO added, goes to the grain boundary region, and becomes superconducting

due to proximity effect [3], at lower temperatures. With increase in BFO, the width of grain

boundary due to it increases, as seen in the broadening of TC2. Due to its broadening, the TC0

decreases very rapidly to the lower temperature. At 10% BFO, a sudden increase in TC, TC1 and

TC2 is seen. This may be due to, with further increase of BFO, the boundary being sufficiently

thick, the next batch of BFO particles mostly goes to the grains acting as flux pinning centers.

For higher concentration of BFO (more than 10%), the transition temperatures decreases but its

trend is different from the earlier decrease. This decrease is because of increasing non-

superconducting phase in the sample which ultimately leads to the decay of superconductor

ordering.


~67~

Here insulating BFO particles are embedded in metallic BSCCO grains. If the metallic

concentration is below the percolation threshold, an incomplete transition at TC2 is observed.

The metallic grains have a superconducting transition at TC1 and Josephson tunneling between

the grains forms a connected superconducting path across the entire sample at TC2. Transition at

TC1 includes significant amount of strongly coupled grains and the zero resistance state is

achieved when more and more junctions becomes superconducting as the temperature is

lowered.

From Fig.4.2 it is clearly seen that two different trends of monotonically decrease in

transition temperature for lower (up to 5% wt.) and higher (>5%) BFO concentration.


~68~

Intergranular broadening (ΔTC2) is observed due to addition of BFO where as intragranular

width (ΔTC1) remains almost nearly constant (shown in Table 4.1).

0 50 100 150 200 250

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

30%

25%

20%

15%

10%

5%

4%

3%

2%

1%

0%

/2

73

T(K)

Fig. 4.2 Normalized -T plot clearly showing two different trends of monotonically decrease

in transition temperatures(TC)


~69~

Table 4.1: Variation in superconducting order parameters with BFO wt. %

BFO conc. Tc-on (K) Tc1(K) Tc2(K) Tc0(K) ΔTC1 ΔTC2

0%(BSCCO) 90.74 81.07 81.07 80.24

1% 83.47 70.63 57.72 53.14 9.14 6.24

2% 81.96 70.06 53.46 47.79 9.35 7.35

3% 81.64 66.92 51.16 46.18 9.6 8.19

4% 79.08 63.58 50.61 43.42 9.54 7.66

5% 78.81 56.83 36.81 37.77 10.74 10.51

10% 81.53 67.6 44.34 34.16 9.81 47.43

15% 77.43 63.73 36.55 20.24 9.70 35.26

According to LD theory [4], the cooper pair formation is initiated much before TC-ON

[5]. However due to high temperature, the cooper pairs thus formed does not sustain for longer

period and dissociates into individual electrons. As the temperature is lowered, the cooper pair

formation rate as well as their lifetime increases. That’s why; the deviation from the usual

behavior in the temperature dependence of resistivity starts much before TC-ON. According to an

estimate, this deviation starts 2TC [6]. This temperature is called pseudo gap temperature (T*).

Below this temperature the resistivity deviates from its normal behavior and decreases slightly

faster. The temperature dependence of resistivity for parent BSCCO in the region 150-300K is

found to be well described by a linear behavior 0T T . Hence any deviation from

this behavior is marked as due to onset of T*. To obtain T*, the temperature dependent of

resistivity data is plotted as 0 /T T . At the onset of T*, 0 / 0T T ,

which otherwise is zero. For the composites, the exact nature of the grain boundaries

introduced by BFO is not known, hence a slightly different procedure is employed to obtain T*.


~70~

A best polynomial fit is done for parent BSCCO till TC-ON. Then this value is subtracted from

the resistivity data of all composites, to obtain the resistivity variation of BFO alone. In this

way the resistivity behavior of BFO grain boundaries in the sample are obtained. The

temperature dependence of these grain boundaries is found to obey a simple power law,

0( )gb n gbT aT shown in Fig.4.3 (a) whose respective parameters are listed in Table 4.2.

Hence for BSCCO/BFO composites, the T* is obtained by locating the temperature at which

0 / 0gbT T T . As a function of BFO composition, the T* thus obtained is

found to be decreasing to lower values (see Fig. 4.3 b).


~71~

0 50 100 150 200 250 300

0.0

0.2

0.4

0.6

0.8

1.0

gb = aT

n+ 0

gb

[/

273] s

am

ple

- [

/273] B

SC

CO

T (K)

Fig. 4.3 (a)

50 100 150 200 250 300

1.0

1.5

2.015%

10%

5%

4%

3%

2%

1%

0%[(T

) -

0 -

g

b(T

)]/

T

T (K)

Fig. 4.3 (b)

Fig. 4.3 (a) Fitted value of Power law0( )gb n gbT aT of composite subtracted from parent

as function of temperature and Fig. 4.3 (b) Pseudogap temperature T* from the dependence of

ρ(T) as function of temperature. The respective plots are off-shifted for better clarity. The

arrow marks the T*.


~72~

Table 4. 2: Variation of a , n, 0

gb and T* (K)

BFO conc. a n 0

gb T* (K)

0%(BSCCO) -- -- -- 178

1% -0.00007(0.1) 1.538(2) 0.3704(3) 124

2% -0.00016(1) 1.397(1) 0.4103(2) 115

3% -0.00022(1) 1.354(4) 0.4416(8) 131

4% -0.00018(1) 1.387(1) 0.4291(3) 117

5% -0.00032(1) 1.310(3) 0.5059(8) 104

10% -0.00119(1) 1.140(1) 0.7120(4) 124

15% -0.00376(3) 0.987(1) 0.9575(8) 100

20% -0.0166(1) 0.783(1) 1.350(1)

To know more about the composite system, the temperature dependence of resistivity is

correlated with the various existing models. From the R-T plot, it is clear that increasing BFO

concentration in BSCCO increases the resistivity and brings insulating nature. Since the grain

boundaries have disordered nature until a critical concentration of BFO is reached, it is much

plausible to consider the weak localization theory for disordered media as the conduction

mechanism [7]. According to the weak localization theory, the temperature dependence of

resistivity follows behaviour at high temperature (T > 100 K) [8]. Hence -T plot is fitted

with above TC-ON. The first term is the residual resistivity, the

second term is linear metallic behavior as seen in parent BSCCO and last term ( is due to

weak localization. It can be seen in the plot that the fitting is reasonably good, which supports

our weak localization conduction. However the fit is found to be good only till 10% BFO

(Fig.4.4 (a-g)) shown in table 4.3. For 15% BFO, the resistivity behavior is found to be better

fitted with (Fig.4.4 (h)). Such behavior has been observed in granular metals and

is ascribed to the tunneling of carriers through the grain boundaries. Considering the granular

nature of our samples, we too ascribe this behavior to the tunneling of carriers through the grain

boundary. Further higher concentration of BFO (20% , 25% and 30%) (Fig.4.4(i-k)), -T plots

are found to have good fit with proposed by Macmillan [9] and Larkin and

Khmel’nitskii [10] for the samples on the boundary of metal-insulator transition or critical

region. The exponent takes the value 0.33 < β < 1, with β ~ 0.33 on the metallic side and β ~ 1


~73~

on the insulating side. However, experimentally the values of β beyond the above range has

also been reported and has been ascribed towards the metallic side (β < 0.33) [11] and the

insulating side of the boundary (β > 1) [12]. In our samples the respective values of exponents

are 0.12, 0.15 and 0.22 for (20% BFO, 25% BFO and 30% BFO) respectively. These values of

exponent indicates that the samples are towards metallic side of the M-I boundary and with

increasing BFO concentration, the behavior is becoming more insulating. Hence in conclusion,

we can say that the parent BSCCO is metallic in nature and with increasing BFO content, the

sample drift towards insulating side via critical regime. However the insulating regime is not

reached even for 30% BFO added sample.

Table 4.3: Fitted Parameters of Residual resistance ( 0), A and B of composite up to

10% BFO

Conc. of BFO 0 A B

0%(BSCCO) -0.00005(2) 0.00002 0.0062(2)

1% 0.01386(2) 0.00002 -0.0338(2)

2% 0.01510(1) 0.00002 -0.0270(1)

3% 0.01619(5) 0.00002 -0.0235(4)

4% 0.01219(1) 0.00002 -0.02103(6)

5% 0.01250(4) 0.00001 -0.0176(3)

10% 0.03005(1) 0.00001 -0.00791(1)


~74~

100 150 200 250 300

0.0006

0.0012

0.0018

Resis

tivit

y (

-m)

T (K)

BSCCO

Fig. 4.4 (a)

100 150 200 250 3000.004

0.005

0.006

Re

sis

tiv

ity

(-m

)

T (K)

1% BFO

Fig. 4.4 (b)


~75~

100 150 200 250

0.006

0.007

0.008

0.009

Re

sis

itv

ity

(-m

)

T (K)

2% BFO

Fig. 4.4 (c)

100 150 200 250

0.006

0.007

0.008

0.009

Resis

itvit

y (

-m)

T (K)

3% BFO

Fig. 4.4 (d)


~76~

100 150 200 250 300

0.004

0.005

0.006

4% BFOR

es

isit

vit

y (

-m)

T (K)

Fig. 4.4 (e)

100 150 200 250

0.0045

0.0050

0.0055

5% BFO

Re

sis

itv

ity

(-m

)

T (K) Fig. 4.4 (f)


~77~

100 150 200 250

0.016

0.017

10% BFOR

es

isit

vit

y (

-m)

T (K)

Fig. 4.4 (g)

0.06 0.07 0.08 0.09 0.10 0.11

-1.915

-1.910

-1.905

-1.900

-1.895

-1.890

log

(

-m)

(1/T)0.5

15 % BFO

Fig. 4.4 (h)


~78~

0.51 0.54 0.57 0.60

0.020

0.022

0.024

(-m

)

(1/T)0.12

20 % BFO

Fig. 4.4 (i)

0.425 0.450 0.475 0.500 0.525

0.04

0.05

0.06

0.07

(-m

)

(1/T)0.15

25 % BFO

Fig. 4.4 (j)


~79~

0.30 0.33 0.36 0.39

0.06

0.08

0.10

0.12

(-m

)

(1/T)0.22

30 % BFO

Fig. 4.4 (k)

Fig. 4.4 (a-g) -T Plot is fitted with up to 10% BFO, Fig. 4. 4

(h) 15% BFO plot is fitted with and Fig. 4.4 (i-k) up to 30% BFO is fitted with

.

4.2 Excess conductivity or paraconductivtiy

In case of conventional superconductor, while cooling, zero resistance appears suddenly

at the superconducting transition temperature, TC. Here free energies of both superconducting

and normal state of the material are equal. But in case of HTSC, initiation of thermodynamic

fluctuations much before the transition temperature gives rise to an anomalous increase in

superconducting properties even at temperature above TC. These fluctuations induced

conductivity is less than the normal state conductivity by a factor 2

1

(0)F

kwhich comes out to

be 10–5

(for conventional Type I) superconductors [13]. However for Type II superconductors,

the coherence length (0) is small and hence the excess conductivity is no longer small. Study

of this fluctuation induced conductivity provides a great deal of information such as coherence

length, dimensionality of conduction, dimensionality cross-over (if present), etc. [14].


~80~

Ginzburg-Landau theory explains the superconductivity by introducing a complex order

parameter ψ= ψ0 exp (iφ) such that |ψ|2 represents the local density of superconducting

electrons. As per Ginzburg-Landau theory, free energy has minimum at ψ0 for calculation of all

the physical parameters. But thermodynamic fluctuation allows the system to have another

order parameter ψ(r) which considerably increase the free energy by ~ kBT. Due to the

fluctuation, superconducting droplets are formed and annihilated near to the mean field

transition temperature and produce excess conductivity [15]. At the onset of superconductivity,

the order parameter ψ attains a finite value, which otherwise is zero. If the temperature is

lowered further, ψ increases and attains a maximum value at T = 0. In the case of weakly

coupled granular HTSC, at superconductive transition (TC), all the individual grains become

superconducting and ψ = ψ0. In this situation ψ = ψ0 everywhere inside the grains but its phase

φ varies from grain to grain. The phase coherence (same φ in all grains) is achieved when

temperature is lowered to TC0 where the electrical resistivity of the material becomes zero.

From TC to TC0 the magnitude of order parameter ψ0 assumes an identical nonzero value in the

grain, but the phase φ of the order parameter fluctuates from grain to grain. The fluctuation in

the phase of the order parameter gives rise to excess conductivity or paraconductivtiy (∆σ) in

the above temperature range and is given by

m n (4.1)

Where σm is the measured conductivity and σn is the extrapolated conductivity drawn above the

2TC. Actually the resistivity is measured first and it is converted to conductivity via σ= 1/ρ

[16].

Aslamazov and Larkin provide the following expressions for excess conductivity in the

Paraconductivtiy region i.e. [13]

(4 )/2A A d

(4.2)

Where ε is the reduced temperature (T–TC)/TC (4.3)

Here TC is mean field critical temperature (obtained from the derivative of R-T data) and is

dimensionality parameter. More elaborately [17],


~81~

For 3D case, 2

0.5

3

1 e

32 (0)d

(4.4)

For 2D case, 2

1

2

1 e

16d

d

(4.5)

Where (0) is the coherence length at T = 0, in the stacking direction of multilayer structure

and d is the superconducting layers periodicity length.

In the layered superconductor, the superconducting properties are highly anisotropic.

Mobility in the CuO2 conduction plane depends on the charge reservoir layers MBa2O4-δ

(M=Y, Bi, Hg, Ti, etc.). The charge reservoir layers are known to supply the carriers to the

conducting plane where the superconductivity occurs. In order to reach the CuO2 plane, the

charge carriers have to overcome each time the thickness of charge reservoir layer barrier

which is mostly resistive. When the superconductivity starts at TC-ON, the cooper pairs are

confined only to CuO2 plane and hence the superconductivity is 2D in nature [18]. The charge

carrier from the reservoir crosses the layer barrier only, when the temperature is further

lowered. At this point the superconductivity is spread in the entire crystal; hence its nature is

3D. A careful observation of excess conductivity as a function of temperature often reflects the

transition from 2D to 3D, as one approaches TC from high temperature [14]. Usually 2D to 3D

cross over temperature (T0) is found above the critical temperature TC. This analysis of excess

conductivity above the mean critical transition temperature is performed as per Lawrence-

Doniach (L-D) model [4], for layered superconductor.

Once knowing the cross over temperature (T0), the other parameters such as coherence

length ( ) and Josephson coupling constant (EJ) may be calculated as

T0 = TC [1 + (2ξ /d) 2]

(4.6)

And EJ = (2ξ /d) 2

(4.7)


~82~

In order to study the excess conductivity in our samples, the data is plotted as ln (Δσ)-ln

(ε). Two distinct changes in slope for each plot have been observed with gradual increase in

temperature. The slope value gives the value of λ; hence in order to know it, the two regions are

linearly fitted. The exponent value λ ~ –0.5 indicate 3D fit and λ ~ –1 corresponds to 2D fit.

Cross over temperature (T0) is calculated by finding the common solution for the two linear fit

equations.

In the as prepared BSCCO sample, λ3D is found to be –0.47 in the region –5.52 < ln(ε) <

–1.40 which fits well with the theoretical value. And λ2D is –0.94 in the region -3.97 < lnε < –

0.55. The cross over temperature is found to be 85.12 K. Similarly for all other composites the

values of exponent determined from these plots along with 3D fitted region and 2D fitted

region which is tabulated in the Table 4.4.

-7 -6 -5 -4 -3 -2 -1 0 1-8

-6

-4

-2

0

2

4

6

8 T0=85.12K

ln(

)

ln( )

ln( ) 2D FIT

3D FIT

BSCCO

Fig. 4. 5 (a)


~83~

-8 -6 -4 -2 0-6

-4

-2

0

2

4

6

1% BFO

T0=73.66K

ln(

)

ln( )

ln( ) 2D FIT

3D FIT

Fig. 4. 5 (b)

-8 -6 -4 -2 0 2

-6

-4

-2

0

2

4

6

2% BFO

T0=72.86K

ln(

)

ln( )

ln( ) 2D FIT

3D FIT

Fig. 4. 5 (c)


~84~

-7 -6 -5 -4 -3 -2 -1 0

-4

-2

0

2

4

6

3% BFO

T0=70.53K

ln(

)

ln

ln( ) 2DFIT

3D FIT

Fig. 4. 5 (d)

-7 -6 -5 -4 -3 -2 -1 0 1

-2

0

2

4

6

4% BFO

T0

=66.63K

ln(

)

ln

ln( )

2D FIT

3D FIT

Fig. 4.5 (e)


~85~

-7 -6 -5 -4 -3 -2 -1 0 1

-3

-2

-1

0

1

2

3

4

5

6

T0

=61.03K

ln(

)

ln( )

ln( ) 3D FIT

2D FIT

5% BFO

Fig. 4.5 (f)

-10 -8 -6 -4 -2 0-8

-6

-4

-2

0

2

4

T0

=70.57K

ln(

)

ln

ln( ) 2D FIT

3D FIT

10% BFO

Fig. 4.5 (g)


~86~

Fig. 4.5 (h)

-7 -6 -5 -4 -3 -2 -1 0

-1

0

1

2

3

4

5

6

7T0=63.54K

ln

ln(

ln( )

3D FIT

2D FIT

20% BFO

Fig. 4.5 (i)

Fig. 4.5 (a-i) ln (Δσ) vs. ln (ε) plot of BSCCO and its composites (1%, 2%, 3%, 4%, 5%, 10%,

15% and 20% added BFO). 2D and 3D Fitted regions are shown along with T0

-8 -7 -6 -5 -4 -3 -2 -1 0-8

-6

-4

-2

0

2

4

T0

=66.66K

ln(

)

ln

ln( )

2D FIT

3D FIT

15% BFO


~87~

Similarly for the composite, the exponents are found to be –0.41, –0.41, –0.5, –0.41, –

0.42, –0.38, –0.42 and –0.48 in the respective reduced temperature range of –6.13 < lnε < –

1.44, –6.45 <lnε < –2.08, –6.68 < lnε < –2.14, –6.22 < lnε < –0.88, –6.34 < lnε < –1.53, –6.17 <

lnε < –1.59 and –5.24 < lnε < –1.84. Simultaneous decrease in 3D region is observed except for

5% BFO. This decrease in 3D fitted layer is associated with the weaker inter layer coupling due

to increased impurity scattering at the intergranular region. Above the cross over temperature

2D exponent is observed whose values are almost unity. The 2D exponent for parent

superconducting system is found to be –0.94 best fitted with the region –3.97 < lnε < –0.55.

Similarly for the other systems it is found to be –1.61, –1.31, –1.24, –1.37, –1.75, –1.53, –1.34

and –1.48 with lnε values varies from –3.94 < lnε < –0.43 , –4.32 < lnε < –0.25, –4.14 < lnε < –

0.22, –3.68 < lnε < –0.20, –2.70 < lnε < –0.48, –3.80 < lnε < –0.22, -3.17<lnε <-0.77 and –3.31

< lnε < –1.25. Simultaneous variation of temperature in 3D and 2D region is listed in the Table

4.4.

In the as prepared BSCCO sample, λ3D is found to be –0.47 and λ2D is –0.94, which fits

well with the theoretical value. The cross over temperature is found to be 85.12 K. For the

BSCCO/BFO composites the respective values of exponent determined from these plots are λ ~

– 0.5 and λ ~ –1.5. Value of λ ~ –1.5, gives an impression of 1D nature of conduction.

However similar values have also been found in films having percolative conduction [19-20].

Using numerical simulations technique, Seto et.al [21], has shown that for inhomogeneous

superconductors, the exponent takes the value λ = 1.33. Being composite nature of our samples,

the λ ~ –1.5 is attributed to the percolative conduction due to inhomogeneity in composite

samples.

Coherence length ξ is calculated using T0 = TC [1 + (2ξ /d) 2] Where T0-Cross over

temperature and d–thickness of superconducting layer (15A0) is taken for calculation. And here

for TC, TC2 which is taken for calculation as at TC2 grains as well as grain boundaries are

superconducting.


~88~

Table 4.4: Various regions obtained from the ln (Δσ) and ln (ε) plots of BSCCO and

composites (1%, 2%, 3%, 4%, 5%, 10%, 15% and 20% added BFO)

Conc of BFO 3D REGION

[in Kelvin]

ΔT3D(K) 2D REGION

[in Kelvin]

ΔT2D

(K)

T0 (K) λ3D λ2D

0%(BSCCO) -5.52<lnε<-1.40

[81.4-100.9K]

19.5 -3.97<lnε<-0.55

[82.6-127.8K]

45.2 85.12 -0.47 -0.94

1% -5.64<lnε<-1.40

[70.9-87.9K]

17 -3.94<lnε<-0.43

[71.9-116.5K

44.6 73.66 -0.41 -1.61

2% -6.13<lnε<-1.44

[70.2-86.6K]

16.4 -4.32<lnε<-0.25

[70.9-124.6K]

53.7 72.86 -0.41 -1.31

3% -6.45<lnε<-2.08

[67.0-75.7K]

8.7 -4.14<lnε<-0.22

[67.9-120.3K]

52.4 70.53 -0.5 -1.24

4% -6.68<lnε<-2.14

[63.6-71K]

7.4 -3.68<lnε<-0.20

[65.2-115.6K]

50.4

66.63 -0.41 -1.37

5% -6.22<lnε<-0.88

[56.9-80.4K]

23.5 -2.70<lnε<-0.48

[60.5-93.1K]

32.6

61.03 -0.42 -1.44

10% -6.34<lnε<-1.53

[67.7-82.1K]

14.4 -3.80<lnε<-0.22

[69.1-121.8K]

52.7 70.57 -0.38 -1.53

15% -6.17<lnε<-1.59

[63.9K-76.7K]

12.8 -3.17<lnε<-0.77

[66.4K-93K]

26.6K

66.66 -0.42 -1.34

20% -5.24<lnε<-1.84

[60.3-69.5K]

9.2 -3.31<lnε<-1.25

62.18K-77.18K

15

63.54 -0.48 -1.48


~89~

Table 4.5: Variation of coherence length (ξ) (A0) and Josephson coupling constant (EJ)

Conc. of BFO Coherence length, ξ (A0) Josephson coupling constant (EJ)

0%(BSCCO) 1.67 0.19

1% 3.94 1.10

2 % 4.51 1.45

3 % 4.61 1.51

4% 4.21 1.26

5% 6.07 2.62

10% 5.76 2.36

15 % 6.80 3.29

1.

4.3 Critical Current Density

The most important parameter of any super conductor from application point view is its

ability to carry maximum electrical current i.e. critical current (Ic) or critical current density (Jc)

[22]. The critical current is generally measured from the Current (I) –Voltage (V)

characteristics using four probe method. For this measurement the sample is cut into a bar

shape keeping the cross section area (A) as small as possible (because Jc = Ic / A). In our

samples, the thickness (t) are 0.66 mm, 0.7 mm, 0.9 mm, 0.73 mm, 0.77 mm, 0.81 mm and 0.88

mm and breath (b) are 1.5 mm, 1.58 mm, 1.88 mm, 2.25 mm, 1.63 mm, 1.79 mm and 1.72 mm)

for 0%, 2%, 3%, 4%, 5%, 10% & 15% BFO respectively. Hence the critical current density

(Jc) is obtained from the critical current as

Jc = Ic / (b x t), (4.8)

Where (b x t) gives cross sectional area of the sample.

Four linear electrical contacts over the sample are made with the help of conducting

silver epoxy. After ensuring the superconducting nature of sample i.e., zero resistance, the


~90~

critical current measurement is done in the superconducting region at equal temperature

intervals.

In the following figures (4.6 – 4.11), the V-I characteristics of all the samples are

shown. At a fixed temperature, the voltage across the sample is measured for the regular

increase in current. In all the plots, no voltage is observed, until a critical value of current Ic is

reached, at which a non-zero voltage starts appearing. Beyond this value, the voltage increases

non-linearly initially and then becomes linear. This measurement is repeated at temperatures

intervals of 5 K below the critical temperature, till the lowest temperature 6 K. In this way the

critical current Ic for various temperatures is obtained. These entire sets of experiments are

done for all the samples including parent BSCCO. It must be mentioned here that the Ic value

measured for 2% BFO to 15% BFO at 6 K is beyond the limit of our current source, which is

105 mA. Hence Ic at 6 K for these two samples is considered as 110 mA. This assumption may

result in slight error while the fitting of equation mentioned below. But in any case the value of

parameter ‘n’ won’t be less than the observed one. The critical current Ic thus obtained is

converted to critical current density Jc by using the relation Jc = Ic /A. This temperature

dependence of critical current density is fitted to

( ) (0) 1 /

n

c c cJ T J T T . (4.9)

The exponent ‘n’ takes the values depending on the type of Josephson junction. The value of n

= 2 for superconductor–normal metal–superconductor (SNS) junction [23], and n = 1 for

superconductor–insulator–superconductor (SIS) junction [24]. The value 1 < n < 2 indicates the

superconductor–insulator–normal metal-superconductor (SINS) junction. Sometimes n < 1

which indicates, the SIS junction but the junction is thin enough so that the quasiparticles may

tunnel through it [25].

While fitting the above equation to the Jc-T plot, it is found that for the data for parent

BSCCO fits very well in the entire temperature range, whereas for BSCCO/BFO composites,

the equation fits in two different regions (Region 1 & Region 2) with different values of ‘n’ in

the respective region. The region near the TC (Region 1) still holds n 0.5, where as the

temperature approaches T = 0 (hence called as Region 2), the value of ‘n’ diverges from 0.05 to

2 for 2% BFO to 15% BFO. Hence from these results, it is concluded that the grain boundary of


~91~

the composites are of two kinds. The first kind of grain boundary is the intrinsic kind having

the parent BSCCO composition but in disordered form. This grain boundary is very thin and is

present in all samples including parent BSCCO. The second kind of grain boundary is formed

when BFO is introduced in the sample. This grain boundary is having composition of BFO and

is very thin for lesser BFO content (the value of n = 0.05, indicating tunneling). As the BFO

content is increased, the thickness of this grain boundary increases and the junction becomes

SNS type (n 2). But being insulating nature of BFO, SNS junction seems confusing. It has

been shown by Golubov et .al [26], that n = 2 for SIS junction is possible under strong

proximity effect. In the temperature dependence of resistivity, we have already seen that BFO

contribution to resistivity is not insulating but is of critical behavior due to proximity effect.

Hence n = 2 behavior due to proximity effect is quite reasonable.

A plot of Jc obtained as the intercept of the above equation fitting in the low temperature

range (Region 2) as a function of BFO content in the sample is shown in Fig 4.6 to 4.13. From

the plot it is evident that the Jc first decreases on BFO incorporation in the sample, attains a

minimum value at 5% BFO and increases thereafter. This is because, for higher values of BFO,

the BFO goes not only in the grain boundary region, but also enter inside the grain [27].

The BFO nano particles inside the grain act as pinning centers and hence an increase in

Jc is observed.


~92~

0 20 40 60 80 100 120

0.0

0.5

1.0

1.5

2.0

2.5

3.0BSCCO

Vo

ltag

e(m

V)

Current(mA)

10K

20K

30K

40K

50K

60K

70K

Fig. 4.6 (a)

0 10 20 30 40 50 60 70 80

0

10

20

30

40

50

60

70

80

n = 0.42

Jc

(0)

x 1

03

Am

-2

T (K)

BSCCO

Fig. 4.6 (b)

Fig. 4.6 (a) Current (I)-Voltage(V) curves of BSCCO, measured in a pressurized liquid Helium

cryo cooler, at various temperature of 10 K, 20 K, 30 K, 40 K, 50 K, 60 K and 70 K. Fig. 4. 6

(b) shown below is the variation of critical current density (Jc) plotted against temp (T) which

is fitted with the equation ( ) (0) 1 /n

c c cJ T J T T and values obtained are Jc =

75.81x103(Am

-2) and n = 0.42.


~93~

0 20 40 60 80 100 120

0

5

10

15

20

25

30 2% BFO

Vo

ltag

e(m

V)

Current(mA)

7K

10K

15K

20K

25K

30K

Fig. 4.7 (a)

5 10 15 20 25 30 35

0

10

20

30

40

50

60

n = 0.08

Region 2

n = 0.2

Region 1

Jc

(0)

x 1

03

Am

-2

T (K)

2% BFO

Fig. 4.7 (b)

Fig. 4.7 (a) Current (I)-Voltage(V) curves of 2% BFO, measured in a pressurized liquid

Helium cryocooler, at various temperature of 7 K, 10 K, 15 K, 20 K, 25 K and 30 K. Fig. 4.7

(b) shown below is the variation of critical current density (Jc)-plotted against temp (T) which

is fitted with the equation ( ) (0) 1 /n

c c cJ T J T T and values obtained are Jc = 50.73x103Am

-

2 and n = 0.16 in Region 1 and Jc = 57.09x10

3Am

-2 n = 0.05 in Region 2.


~94~

0 20 40 60 80 100 120

0

5

10

15

20

25

3% BFO

Vo

lta

ge

(mV

)

Current(mA)

6K

10K

15K

20K

25K

30K

35K

Fig. 4. 8 (a)

0 10 20 30 40 50

0

10

20

30

40

50

n = 0.04

Region 2

n = 0.39

Region 1

Jc

(0)

x 1

03

Am

-2

T (K)

3% BFO

Fig. 4. 8 (b)

Fig. 4.8 (a) Current (I)-Voltage(V) curves of 3 % BFO, measured in a pressurized liquid

Helium cryo cooler, at various temperature of 6 K, 10 K, 15 K, 20 K, 25 K, 30 K and 35 K. Fig.

4.8 (b) shown below is the variation of critical current density (Jc)-plotted against temp (T)

which is fitted with the equation ( ) (0) 1 /n


41.43x103Am

-2 n = 0.39 in Region 1 and Jc = 50.97x10

3Am

-2n = 0.04 in Region 2.


~95~

0 20 40 60 80 100 120

0

5

10

15

20

25

30 4% BFO

Vo

ltag

e(m

V)

Current(mA)

7K

10K

15K

20K

25K

30K

35K

40K

Fig 4.9 (a)

5 10 15 20 25 30 35 40 45

0

10

20

30

40

50

n = 0.41

Jc

(0)

x 1

03

Am

-2

T (K)

4% BFO

Fig 4.9 (b)


Helium cryo cooler, at various temperature of 7 K, 10 K, 15 K, 20 K, 25 K, 30 K, 35 K and 40

K. Fig. 4. 9 (b) shown below is the variation of critical current density (Jc) plotted against

temp (T) which is fitted with the equation ( ) (0) 1 /n

c c cJ T J T T and values obtained are Jc

= 44.90x103(Am

-2) and n = 0.02


~96~

0 20 40 60 80 100 120

0

5

10

15

20

25

30

35

5% BFO

Vo

lta

ge

(mV

)

Current(mA) X 10-3

6K

15K

20K

25K

30K

35K

40K

Fig 4.10 (a)

0 5 10 15 20 25 30 35 40 45

0

10

20

30

40

n = 0.54

Region 2

n = 0.49

Region 1

Jc

(0)

x 1

03

Am

-2

T (K)

5% BFO

Fig 4.10 (b)


Helium cryo cooler, at various temperature of 6 K, 15 K, 20 K, 25 K,30 K, 35 K and 40 K. Fig

4.10 (b) shown below is the variation of critical current density (Jc) plotted against temp (T)

which is fitted with the equation ( ) (0) 1 /n


30.24x103Am

-2 , n = 0.49 in Region 1 and Jc = 43.77x10

3Am

-2 , n = 0.54 in Region 2.


~97~

0 20 40 60 80 100 120

0

10

20

30

40

50

60

Vo

lta

ge

(mV

)

Current(mA)

6K

15K

20K

25K

10% BFO

Fig 4.11 (a)

5 10 15 20 25 30-10

0

10

20

30

40

50

60

70

80

n = 0.55

Region 1

n = 2.44

Region 2

Jc

(0)

x 1

03

Am

-2

T (K)

10% BFO

Fig 4.11 (b)


Helium cryo cooler, at various temperature of 6 K, 15 K, 20 K and 25 K. Fig. 4.11(b) shown

below is the variation of critical current density (Jc) plotted against temp (T) which is fitted

with the equation ( ) (0) 1 /n

c c cJ T J T T and values obtained are Jc = 141.95x103(Am

-2) and

n = 2.44 in Region 2 and Jc = 20.27x103Am

-2 and n = 0.55 in Region 1.


~98~

0 20 40 60 80 100 120

0

5

10

15

20

25

30

Vo

lta

ge

(mV

)

Current(mA)

10K

15K

20K

25K

15% BFO

Fig 4.12 (a)

8 10 12 14 16 18 20 22 24 26 28-10

0

10

20

30

40

50

60

70

80

n = 2.01

Region 2

n = 0.36

Region 1

Jc(0

) x

103 A

m-2

T (K)

15% BFO

Fig 4. 12 (b)


Helium cryo cooler, at various temperature of 10 K, 15 K, 20 K and 25 K. Fig. 4. 12 (b) shown

below is the variation of critical current density (Jc) plotted against temp (T) which is fitted

with the equation ( ) (0) 1 /n

c c cJ T J T T and values obtained are Jc = 25.44x103Am

-2, n =

0.36 in Region 1 and Jc = 297.99x103Am

-2, n = 2.01 in Region 2.


~99~

This result is supported by B. Zeimetz et al., in HTSC the superconducting properties

are considerably weakened at grain boundaries as compared to intra grain values [28].

-2 0 2 4 6 8 10 12 14 160

50

100

150

200

250

300J

c(0

)(*1

03)(

Am

-2)

Conc(wt%)

Jc(0)

Fig. 4.13

Fig. 4.13 Variation of critical current density Jc(0) with BFO concentration


~100~

Table 4.6: Shows values of critical current density Jc (0) and ‘n’ in both the Regions.

Conc. of BFO Region 1 Region 2

Jc (0) in Am-2

n TC Jc (0) in Am-2

n TC

0% (BSCCO) -- 75.81 x 103

0.42 78.002

2% 55.62 x 103

0.2

32.02 58.71 x 103

0.08

15.001

3% 41.43 x 103

0.39 45.003 50.97 x 103

0.04

20.004

4% 44.9 x 103

0.41

44.001

5% 30.24 x 103

0.49

43.02 43.77 x 103

0.54

32.25

10% 20.27 x 103

0.55 30.06

141.95 x 103

2.44 26.20

15% 25.44 x 103

0.36 27.00 297.99 x 103 2.01 23

Fig. 4.14 Schematic grain/grain boundary arrangement for BSCCO/BFO composites. The inner

most circles are the superconducting grains and second one with light shade are the intrinsic

grain boundaries. The additional grain boundary due to BFO (shown with dark shade circle) is

present in second fig. The dark spots in the second fig, stands for the BFO nano particles inside

the grains acting as pinning centers.


~101~

REFERENCES

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Yao, C. Wang, J. Appl. Lett. 98 (2011) 222504.

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[9] W. L. Macmillan, Phys. Rev. B 24 (1981) 2739.

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5578.

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[13] Michael Tinkham, Introduction to Superconductivity, 2nd Edition, Dover publication.

[14] N. A. Khan, N. Hassan, S. Nawazj, B. Shabbir, S. Khan, A. A. Rizvi, J. Appl. Phys. 107

(2010) 083910.

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Yoon, K. Y. Choi, Y. H. Jo, J. Appl. Phys. 108 (2010) 063916.

[16] G. Balestrino, M. Marinelli, E. Milani, Phys. Rev. B 46 (1992) 14919.

[17] R. S. Thompson, Phys. Rev. B 1 (1970) 327.

[18] W. Lang, G. Heine, Phys. Rev. B 51 (1995) 9180.

[19] Q. Y. Ying, H. S. Kwok, Phys. Rev. 8 42 (1990) 2242.

[20] X-F Chen, M. J. Marone, G. X. Tessema, M. J. Skove, M. V. Nevitt, Phys. Rev. B 48

(1993) 1254.


~102~

[21] R. Seto, R. Botet, H. Kuratsuji, Phys. Rev. B 73 (2006) 012508.

[22] D. Dew-Hughes, Low Temp. Phys. 27 (2001) 713.

[23] P. G. Degennes, Rev. Mod. Phys. 36 (1964) 225.

[24] P. G. De Gennes, Phys. Lett. 5 (1963) 22.

[25] V. Ambegaokar, A. Baratoff, Phys. Rev. Lett. 10 (1963) 486.

[26] A. A. Golubov, E. P. Houwman, J. G. Gijsbertsen, V. M. Krasnov, J. Flokstra, H.

Rogalla M. Yu. Kupriyanov, Phys. Rev. B. 51 (1995)1073.

[27] H. Hilgenkamp, Rev. Mod. Phys. 74 (2002) 485.

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in superconductivity, Cambridge university, Pembroke Street, Cambridge, CB 2 3QZ,

U.K

Chapter 5 Conclusion

~103~

CHAPTER 5


~104~

CONCLUSIONS

An environmental friendly lead free Bi based superconducting oxide i.e. BSCCO 2212

phase is prepared. Ceramic oxide BFO is chosen as the composite material as it is Bi

based. Synthesis of BFO is done by chemical synthesis route (sol gel synthesis).

Different compositions of BSCCO/BFO composite are prepared by adding different

wt. % of BFO to BSCCCO which is finally sintered at 800 0C only for 10 min.

XRD confirms the presence of BFO phase in the higher concentration (10% - 30%).

SEM image of BSCCO shows plate like grains owing to its layered structure. Grain

boundary sharpness changes when BFO is added to the superconducting oxide. Lower

magnification confirms that white dots of BFO particles are homogeneously distributed

on the conducting surface of BSCCO.

R-T measurement of parent BSCCO 2212 phase having Tc at nearly 80 K in dR/dT

plot. In all other composites superconducting transition occur at two temperatures Tc1

and Tc2. Complete zero resistivity is not achieved after 15% added BFO.

Except BSCCO, all the composites have double peak (Tc1 & Tc2). Tc1 corresponds to

superconductivity inside the grain & Tc2 is due to where boundary becomes

superconducting. Broadening in the intergranular region confirms that added BFO

directly goes to the boundary upto 5%. Further added BFO goes to boundary as well as

inside the grain.

Conduction mechanism studied in normal region confirms that added BFO brings

insulating nature through a critical region. Upto 30% added BFO still the composite is

in the metallic side as having β value nearly 0.22.

Excess conductivity or Para conductivity is studied near Tc. Cross over temperature

(T0) is calculated by LD theory where superconducting cooper pair formation takes

place in 3D plane. As parent compound BSCCO 2212 phase having λ values 0.47 &

0.97 which are nearer to theoretical value. But higher value in the composites confirms

the percolative conduction due to inhomogeneity.


~105~

I-V measurement done at different temperatures after second critical temperature is

achieved to know its maximum current carrying capacity of composites. Initially

degradation of Jc is noticed at lower concentration of BFO content. Nearly three times

Jc is enhanced by adding 15% BFO content where BFO particles go into the grain act

as a pinning center.

By studying the nature of grain boundary we emphasizes on two kinds of grain

boundaries are present in sample under consideration. First one is intrinsic in nature

which is present parent as well as composites in a disordered form. And the second

kind is due to when BFO is added.

Initially added BFO goes to boundary forming a thin insulating layer through which

quasiparticles are tunneling. Further added BFO going to its boundary as well as inside

the grain. Under strong proximity effect SNS type grain boundary becomes SIS type.

FURTHER STUDIES CAN DONE

Low temperature XRD can be done to study its structure at low temperature.

Better micro structural analysis can done by AFM, TEM.

AC Susceptibility measurement can be done with and without magnetic field to

know its superconducting behavior.

Pinning nature and grain allignment can be studied.

A comparative study can be done by Fe doping in Cu side.

For further higher concentration of BFO proximity effect and percolation

behavior can be studied.

This study can further extended to the 2223 phase.

Documents

STUDIES ON SUPERCONDUCTOR NANO …ethesis.nitrkl.ac.in/4474/1/Final_Thesis.pdf1.4 Short Overview on Superconductivity 1.4.1 Type1 & Type 2 Superconductors 4 1.4.2 Flux Quantization