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Journal of the European Ceramic Society 35 (2015) 125–130

Improving the functional properties of (K0.5Na0.5)NbO3 piezoceramics byacceptor doping

X. Vendrell a,∗, J.E. García b, X. Bril c, D.A. Ochoa b, L. Mestres a, G. Dezanneau c

a Departament de Química Inorgànica, Universitat de Barcelona, 08028 Barcelona, Spainb Department of Applied Physics, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain

c Laboratory of SPMS, Ecole Centrale Paris, Grande voie des vignes, 92295 Chatenay-Malabry Cedex, France

Received 25 April 2014; received in revised form 24 July 2014; accepted 25 August 2014Available online 7 September 2014

bstract

rO2 and TiO2 modified lead-free (K0.5Na0.5)NbO3 (KNN) piezoelectric ceramics are prepared by a conventional solid-state reaction. The effect ofcceptor doping on structural and functional properties is investigated. A decrease in the Curie temperature and an increase in the dielectric constantalues are observed when doping. More interestingly, an increase in the coercive field Ec and remanent polarization Pr is observed. The piezoelectricroperties are greatly increased when doping with small concentrations dopants. ZrO2 doped ceramic exhibits good piezoelectric properties withiezoelectric coefficient d33 = 134 pC/N and electromechanical coupling factor kp = 35%. It is verified that nonlinearity is significantly reduced.hus, the creation of complex defects capable of pinning the domain wall motion is enhanced with doping, probably due to the formation of

xygen vacancies. These results strongly suggest that compositional engineering using low concentrations of acceptor doping is a good means ofmproving the functional properties of KNN lead-free piezoceramic system.

2014 Elsevier Ltd. All rights reserved.

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eywords: Lead-free piezoceramics; Acceptor doping; Piezoelectric properties

. Introduction

The most widely used piezoelectric ceramics are Pb(Ti,Zr)O3PZT)-based materials, on account of their high piezoelectricesponse, large-scale production capability and the tailoringf their properties through composition. Due to the high tox-city of lead, a wide range of strict regulations concerningnvironmental preservation are increasingly being introducedorldwide. Many governments have therefore established leg-

slation regarding waste electric equipment (WEEE), restrictionsn hazardous substances (RoHS), and end-of-life vehicles (ELV)o introduce directives regarding environmental pollutants.1 Theearch for alternative lead-free piezoelectric materials is cur-

ently focused on modified bismuth titanates, alkaline niobatesKNN) and other systems in which a morphotropic phase bound-ry (MPB) occurs.2–4

∗ Corresponding author. Tel.: +34 934 021 270; fax: +34 934 907 725.E-mail address: xavier.vendrell@ub.edu (X. Vendrell).

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a)NbO3

Among the available lead-free piezoelectric materials undertudy, much attention has been paid over the last few yearso K0.5Na0.5NbO3 (KNN)-based ceramics as a result of thereakthrough made by Saito et al.,5 who obtained high d33∼400 pC/N) in the Li–Ta–Sb modified KNN. However, theajor drawback of KNN ceramics is the need for special

andling of the starting powders, sensitivity of properties to non-toichiometry, and especially a complex densification process.6

s regards PZT, sintering aids such as CuO, SnO2, ZnO ornO2 may improve sinterability and modify the dielectric and

iezoelectric behaviour of the KNN materials.7–11

Compositional modification by doping is a very activeesearch line for obtaining piezoceramics with enhancedroperties. The (K,Na)NbO3–LiTaO3–LiSbO3, particularlyhe composition (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3, isrobably the most workable lead-free piezoelectric systemnown to date. However, its properties are not suitable for all

nd use, e.g. for power devices where piezoceramics with lowosses and stable properties are required. In this perspective,ood results are expected by means of hardener substitutions,

1 pean Ceramic Society 35 (2015) 125–130

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uch as those that occur in other perovskites,12 although sometructural and electrical aspects remain controversial as regardshe role of dopants in the KNN system. It has recently beenhown that Cu-doped KNN-modified compounds may exhibitypical characteristics of hard behaviour.13–16 Hardener ionsn a perovskite compound (e.g. Cu2+ ions replace Nb5+ ionsn Cu-doped KNN) are most often acceptors, the introductionf which leads to the creation of oxygen vacancies, therebyorming the so-called complex defects.17 These defects oper-te as pinning centres by hampering the motion of the domainalls. This domain wall pinning effect is responsible for the

eduction in dielectric losses and stabilization of properties inerroelectrics.18 The goal of this study is thus to test the influ-nce of Zr4+ and Ti4+ acceptor-doping in (K0.5Na0.5)NbO3 onhe structural, dielectric, piezoelectric and nonlinear properties.he addition of ZrO2 and TiO2 is expected to improve functionalroperties of KNN ceramics for power application, i.e. reducehe losses and increase the properties stability.

. Experimental

The (K0.5Na0.5)(Nb1 − xMx)O3 − x compositions with x = 0.0nd 0.005 being M = Zr or Ti, hereafter abbreviated as KNN-, were synthesized by a conventional solid-state reaction.

he raw materials of analytical grade used in this study were2CO3 (99%), Na2CO3 (99.5%), Nb2O5 (99.9%), ZrO2 (99%)

nd TiO2 (99%). After separate milling, the powders wereeighed and mixed by ball milling using ZrO2 balls in abso-

ute ethanol medium for 3 h, then dried and calcined twice at00 ◦C for 2 h. The calcined powders were then milled againnd cold-isostatically pressed at 750 MPa into 7 mm diameterellets and sintered in air, without a binder, at 1125 ◦C for 2 h.are was taken to ensure that a high alkaline element pres-

ure is maintained during the process by surrounding the pelletsith powder of the same composition, and the pellets wereeposited on Pt foils to avoid reaction with alumina boats. All as-intered ceramics showed relative densities over 95% measuredy Archimedes’ method.

X-ray diffractograms were recorded at room temperatureRT) on a Bruker D2 PHASER equipped with a XFlash detec-or. A two-axis diffractometer in Bragg-Brentano geometry withu K�1,2 radiation was used for the in situ XRD characteriza-

ion. The control of temperature was provided by a furnace from0 ◦C to 500 ◦C. The cell parameters and their evolution withemperature were refined by a LeBail fitting procedure as imple-ented in the Fullprof suite.19 The Raman scattering spectraere obtained on a Labram (Horiba) spectrometer with a He–Ne

xcitation wavelength of 632.8 nm, coupled to a Linkam sam-le holder, with variable temperature from 30 ◦C to 500 ◦C. DSCeasurements were performed on a Seiko apparatus covering the

00–1000 K temperature range. Microstructure was evaluatedn polished and thermally etched samples (1000 ◦C for 5 min for

◦

ure KNN and 980 C for 5 min for doped KNN) using a Fieldmission Scanning Electroc Microscope, FE-SEM (JEOL JSM-001F). The micrographs were performed on the polished andhermally etched surfaces of the samples coated with graphite.

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ig. 1. XRD patterns (a) and Raman spectra (b) of the pure KNN and KNN-Meramics measured at room temperature.

he voltage conditions were set at 20 kV and the work distanceas established at 12 mm. In order to analyse the composition of

he material, an energy dispersive spectroscopy (EDS) analysisas performed using an OXFORD X-MaxN EDS detector pro-ided with INCA Energy software. A pure Co sample was useds a reference material. The average grain size was determinedrom the SEM images using an image processing and analysisoftware (ImageJ). The grain size was expressed as Feret’s diam-ter using more than 400 grains in each measurement. Electricalharacterization was carried out on ceramic discs with sputteredold–chromium electrodes on both parallel surfaces. The tem-erature dependence of the dielectric permittivity was measuredn unpolled samples using an impedance analyzer (HP 4192A)rom 30 ◦C to 600 ◦C. The measurements were done during theeating ramp at a rate of 2 ◦C/min. The permittivity dependenceith a sub-switching AC electric field was measured at 1 kHz

nd at RT, by means of a capacitance. The hysteresis cycles wereecorded using a modified Sawyer-Tower circuit, at 1 Hz and atT. The samples were polled in silicon oil at 80 ◦C for 30 minnder a 30 kV/cm DC electric field. Subsequently, the longitu-inal piezoelectric coefficient was measured using a piezo-d33eter (YE2730A, APC International) at RT. The piezoelectric

onstant d31 and the electromechanical coupling factor kp wereetermined at RT by the resonance/antiresonance method on theasis of the IEEE standards.

. Results and discussion

Fig. 1(a) shows the XRD patterns of the pure KNN andNN-M ceramics measured at RT. For all ceramics, a pure per-vskite phase indexed in the Amm2 orthorhombic space group isbserved. In contrast to the results obtained by Ramajo et al.,20

o parasitic phases are observed at low concentration of dopant.ur results suggest that taking special care of the reagents andf the synthesis method plays an important role to avoid theormation of parasitic phases. The cell parameters vary by lesshan 0.5%, indicating that the structure does not change sig-ificantly upon doping with ZrO2 or TiO2. Since the Ti4+ and

r4+ ionic radii, 0.61 A and 0.72 A, respectively, in a six-foldnvironment,21 are close to that of Nb5+ ion (0.64 A), dopanttoms should essentially locate on the B-site, which by charge

pean Ceramic Society 35 (2015) 125–130 127

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X. Vendrell et al. / Journal of the Euro

ompensation gives rise to the creation of oxygen vacancies.he Raman spectra of the sintered samples (Fig. 1(b)) present

he typical vibrations corresponding to a perovskite phase, asso-iated with the BO6 octahedra.22 No extra signals are detected,hus, no parasitic phases are observed, as confirmed by XRD.he vibrations of the BO6, A1g (�1) and F2g (�5) are rela-

ively strong scatterings in systems similar to KNN, becausef a near-perfect equilateral octahedral symmetry. Furthermore,he stretching modes (�1, �5) shift to lower frequencies. Thisffect may be attributed to a weakening of the bond strength,robably caused by the oxygen vacancies.

The transition temperatures from orthorhombic to tetragonalhase and from tetragonal to cubic phase are observed at around00 and 420 ◦C, respectively, in pure KNN ceramics.23 Theseransitions are confirmed here by different techniques. The anal-sis of temperature-dependent Raman spectra, XRD patternsnd DSC data allows the two transitions for KNN and KNN-Meramics to be identified without ambiguity, as shown in Fig. 2.ll techniques show practically the same transitions tempera-

ures, although the DSC data show lower transition temperaturesue to the dynamic acquisition process. When adding 0.5% of

i4+ and Zr4+-ions on KNN B-site, we observe a decrease of

he cubic-to-tetragonal transition temperature of 45 and 25 K,espectively, which is similar to what was previously found

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Fig. 3. Microstructure and grain size distributions of polished and thermally

ig. 2. Temperature-composition evolution of the pure KNN and KNN-Meramics, based on DSC, Raman spectroscopy and XRD measured from 30 ◦Co 500 ◦C.

n acceptor-doped KNN.24–26 The tetragonal to orthorhombicemperature transitions decrease only slightly, this effect beingigher when doping with TiO2.

The variation of the microstructures and the grain size dis-ributions of the KNN and KNN-M ceramics is shown inig. 3(a)–(f). All the samples exhibit the typical morphol-gy of the alkaline niobate ceramics. Although the pure KNN

eramic micrograph is over-etched, the surface reveals a bimodalicrostructure with cube-shape grains ranging from about

00 nm to a few micrometers, with an average grain size of

etched surfaces of pure KNN (a,d), KNN–Zr (b,e) and KNN–Ti (c,f).

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3.9 ± 2.3 �m (Fig. 3(a) and (d)). The micrograph and the grainize distributions of the pure KNN also show the presence ofome large abnormal grains, when doping with Zr4+ or Ti4+

eramics show a decrease of the grain size and an increase of therain size distribution uniformity, Fig. 3(b), (c), (e) and (f). Theverage grain size of the KNN–Zr ceramics is ∼2.1 ± 1.2 �m,nd for the KNN–Ti ceramics is slightly lower ∼1.7 ± 1.1 �m.s can be observed in Fig. 3(d)–(f) the grain size distributions

hift toward smaller grain sizes when doping and causes the grainize distribution to become much narrower. Therefore, the addi-ion of acceptor dopants inhibits both the grain growth and theormation of abnormally grown grains, as already observed.20,27

he limited grain growth observed when doping could be relatedith a non-uniform distribution of the Zr4+ or Ti4+ dopantsetween the grain and the grain boundary. Higher concentra-ion of the dopant in the grain boundary region could hinder therain growth, as observed for doped BaTiO3 ceramics.28,29 TheEM studies presented by Malic et al. confirmed the presencef ZrO2 inclusions boundary region30 that could avoid the grainrowth. In our case, there is also a decrease in the grain growthhen doping, although the presence of ZrO2 inclusions couldot be observed in our case. Probably, it can be attributed tohe different synthesis method used by Malic et al. where theopants are added in the powder mixture after the solid stateynthesis, thus, in our case the dopants are incorporated in theattice instead of forming inclusions in the boundary region.

No evidence of secondary phases was observed by BSE orDS analysis, as confirmed by XRD and Raman spectroscopy.he EDS analysis of the matrix reveals that the atomic percent-ges of elements do not differ significantly from the nominalomposition, as reported in Table 1. It should be noted that Zrnd Ti elements could not be detected due to the low content ofopants. Finally, the matrix grains present a Na/K ratio around, which is close to the nominal composition, confirming thathe low synthesis temperature and the special care taken whenintering avoid the evaporation of Na or K elements.

Fig. 4 shows the temperature dependence of the real andmaginary parts of the relative permittivity of pure KNN andNN-M ceramics, measured at 10 kHz from RT to 450 ◦C.ermittivity versus temperature shows two anomalies, one atround 200 ◦C associated with the orthorhombic to tetrago-al phase transition, and the other at higher temperatures,t which a clear maximum of the permittivity is shown forll ceramics and is associated with the tetragonal ferroelec-ric to cubic paraelectric phase transition. In doped samples,he orthorhombic-to-tetragonal (O–T) phase transition shiftslightly toward lower temperatures, while the decrease in tem-erature of the tetragonal-to-cubic (T–C) phase transition isuch more significant. The temperature change of O–T and T–C

hase transitions for TiO2 or ZrO2 doped samples may also bettributed to the acceptor doping effect.24,30

As already shown in Fig. 4, for pure KNN a typical nor-al ferroelectric to paraelectric phase transitions is observed

howing a narrow phase transition. Meanwhile, the ferro-lectric to paraelectric phase transitions broadens on doping,uggesting the appearance of diffuse phase transition. Thisehaviour may be induced in many ways, such as by microscopic

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ig. 5. P–E hysteresis loops of the pure KNN and KNN-M ceramics measuredt room temperature and at 1 Hz.

ompositional fluctuation, by the merging of micro-domains intoacro-domains, or via a coupling of the order parameter and

ocal disorder mode through local strain.31 The diffuse phaseransition is observed for both TiO2 and ZrO2 doped KNNeramics. Thus, one probable cause of this behaviour is localuctuations induced by the incorporation of Ti4+ or Zr4+ ions,hose valence is different from the Nb5+ one, into the crystalline

attice of the perovskite.Polarization versus electric field, (P–E) hysteresis loops for

ure and doped KNN ceramics measured at 1 Hz and at RT arehown in Fig. 5. Only KNN ceramic possesses well-saturated–E loop under an electric field of 30 kV/cm. A higher elec-

ric field must therefore be applied to doped KNN ceramics tobtain a well-saturated hysteresis loop. The coercive field Ec

nd the remanent polarization Pr of pure KNN are 8.5 kV/cmnd 11.4 �C/cm2, respectively. These two values increase with

oping. The increase in Ec means that the material becomesarder, which is consistent with the assumption that oxygenacancies are formed, and the so-called complex defects are

X. Vendrell et al. / Journal of the European Ceramic Society 35 (2015) 125–130 129

Table 1Elemental composition of KNN and KNN-M ceramics determined by EDS analysis. This table represents the atomic percentages of elements.

Na K Nb M Na/K

KNN 10.18 ± 0.32 10.24 ± 0.41 20.44 ± 0.27 – 0.99KNN–Ti 10.04 ± 0.18 9.98 ± 0.34 20.12 ± 0.16 – 1.01KNN–Zr 10.09 ± 0.34 10.18 ± 0.27 20.34 ± 0.21 – 0.99KNNNominal composition 10 10 20 – 1

Table 2Properties of pure KNN and KNN-M ceramics at room temperature.

ρrelative (%) ε′ (10 kHz) tan δ (10 kHz) (−)d31 (pC/N) kp (%) d33 (pC/N) α (10−3 mV−1)

KNN 95.1 ± 0.4 338 0.061 19 28 96 0.452KNN–Ti 97.4 ± 0.2 510 0.032 35 33 124 0.385K 37 35 134 0.323

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roduced by these acceptors additives. However, Pr shows anncrease when doping with ZrO2 up to 19.8 �C/cm2 and anncrease to 12.4 �C/cm2 when adding TiO2. This fact can bexplained taking into account that the density of the materials enhanced by doping, as is reported in Table 2. The piezo-lectric properties are also improved, as shown in Table 2. The33 value for the undoped KNN is 96 pC/N, which is close tohat reported by Guo et al.32 The d33 and kp are significantlynhanced when doping with TiO2 and ZrO2, reaching higheralues of the two coefficients reported by other authors whenoping with a higher concentration of acceptor dopants.33 KNN-

ceramics also show an improvement in dielectric properties atT. Results point out that low amounts of acceptor dopants pro-ote the densification; thus, enhanced remanent polarization,

iezoelectric coefficient and dielectric constant are obtained inNN-M ceramics. The lower values of relative permittivity and

he higher dielectric losses observed for the pure KNN ceramicould be related with its lower density, but a slight deficiency oflkaline metals could not be discarded since the functional prop-rties of the ceramics strongly depends on the stoichiometry.34

lthough in our case, as observed by EDS analysis, the ratioa/K is close to 1. Moreover, as observed in BaTiO3-based

eramics,35 the relative permittivity increases when the grainize decreases, reaching a maximum of permittivity at ∼1 �m.herefore, in our case, the increase of the relative permittivityan be attributed not only to the slight increase of the relativeensity, but also to the decrease of the grain size when doping.

The dielectric and piezoelectric responses of piezoelectricaterials are nonlinear under a high electric field. This nonlinear

ehaviour is satisfactorily described by the Rayleigh model onZT36,37 and KNN-based38–40 ceramics. The nonlinear dielec-

ric and piezoelectric behaviour is principally caused by therreversible movement of the ferroelectric domain walls. Theayleigh relation ε′(E0) = ε′(0) + αE0 is typically used in order

o analyse the nonlinear dielectric response, E0 being the ampli-ude of the electric field, α the Rayleigh coefficient, ε′ (0) theelative permittivity at very low E0, and ε′(E0) the measured

elative permittivity as a function of the electric field amplitude0. The slope α of the �ε′ (�ε′ = ε′(E0) − ε′(0)) versus E0 plot

s used to evaluate the nonlinearity numerically; therefore, the

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ig. 6. Dielectric constant as a function of the electric field amplitude of theure KNN and KNN-M ceramics, measured at 1 kHz and at room temperature.

igher the Rayleigh coefficient the higher the instability of theielectric response. Fig. 6 shows ε′ versus E0 plot for pure KNNnd KNN-M ceramics. A linear relation is verified for all sam-les, as predicted by the Rayleigh model. Rayleigh coefficientsre calculated and reported in Table 2. As one may observe,NN-M ceramics show better stability than pure KNN. Conse-uently, acceptor dopants may lead to the formation of oxygenacancies, and thereby facilitate the creation of the complexefects that may act by hampering the domain wall motion.

. Conclusions

Oxygen vacancies are induced by introducing Ti4+ or Zr4+

ons into the B-site of the KNN perovskite, which is preparedy conventional solid-state reaction. The addition of TiO2 orrO2 shifts the phase transitions toward lower temperatures,s confirmed by different techniques, this effect being higherhen doping with TiO2. The addition of low amounts of accep-

or dopants promotes the densification, improving functionalroperties of KNN ceramics. The piezoelectric properties of theeramics are greatly enhanced when doping with small concen-rations of TiO2 or ZrO2. The coercive field increases and the

ielectric losses decrease when KNN is doped, which is consis-ent with the hypothesis that oxygen vacancies are formed bycceptor doping. Acceptor-doped KNN piezoceramics exhibitn increase in the stability of the properties (nonlinear behaviour

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eduction) probably as a consequence of the domain wall pin-ing effect. Therefore, doping with low amounts of acceptoropants should be taken into consideration in order to improvehe functional properties of KNN-based piezoceramics, i.e. anncrease of the dielectric constant, the piezoelectric coefficients,nd the coupling factor as well as a decrease of the losses andhe instability of the properties.

cknowledgements

The authors would like to thank the Spanish GovernmentAT2010-21088-C03 Project, the Catalan Government PIGC

roject 2009-SGR-0674 and the French Agence Nationale de laecherche, Surffer project, for their financial support..

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