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Available online at www.sciencedirect.com

Journal of the European Ceramic Society 31 (2011) 2005–2012

Enhanced piezoelectric properties of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 lead-freeceramics by optimizing calcination and sintering temperature

Pan Wang, Yongxiang Li ∗, Yiqing LuThe Key Lab of Inorganic Functional Materials and Device, Shanghai Institute of Ceramics, Chinese Academy of Sciences,

1295 Dingxi Road, Shanghai 200050, PR China

Received 8 January 2011; received in revised form 6 April 2011; accepted 17 April 2011Available online 14 May 2011

bstract

ead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 (BCTZ) piezoelectric ceramics were prepared by conventional oxide-mixed method at various calcinationnd sintering temperatures. Both calcination and sintering temperatures have a significant effect on the density and grain size, which are closelyelated with piezoelectric and other properties of ceramics. The calcination temperature has a great influence on the grain boundary, which also playsn important role in piezoelectric properties. With increased calcination and sintering temperature, the ferroelectric and piezoelectric properties

◦ ◦

ave enhanced significantly. The BCTZ ceramics calcined at 1300 C and sintered at 1540 C exhibit optimal electrical properties: d33 = 650 pC/N,31 = 74 pC/N, kp = 0.53, kt = 0.38, k31 = 0.309, sE

11 = 14.0 × 10−12 m2/N, εr = 4500, Pr = 11.69 �C/cm2, which is a promising lead-free piezoelectricandidate.

2011 Elsevier Ltd. All rights reserved.

eywords: A. Calcination; A. Sintering; B. Grain boundaries; C. Piezoelectric properties

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. Introduction

The Pb(Zr,Ti)O3 (PZT) based piezoelectric ceramics haveominated the commercial market of piezoelectric devices forore than half a century, owing to its superior piezoelec-

ric properties1. However, the environment pollution caused byighly toxic lead (Pb) has induced an urgent need in developingarious lead-free piezoelectric ceramics for the demands of var-ous applications.2–4 Therefore, much effort has been made tomprove the properties of conventional lead-free piezoelectriceramics, such as bismuth sodium titanate (BNT) and potas-ium sodium niobate (KNN) that could be comparable with theZT family.5–8 Nevertheless, their piezoelectric properties istill too low (d33 = 100–200 pC/N) compared with PZT. Throughhe combination of a morphotropic phase boundary (MPB) androcessing of highly textured polycrystals, the piezoelectric con-

2

tant d33 can be close to 400 pC/N, but it is still half of that ofigh-end PZT.

∗ Corresponding author. Tel.: +86 21 52411066; fax: +86 21 52413122.E-mail address: yxli@mail.sic.ac.cn (Y. Li).

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955-2219/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.jeurceramsoc.2011.04.023

The newly discovered lead-free BCTZ ceramics by Liu anden9 have attracted great attention10–15 due to the excellentiezoelectric properties (with d33 = 500–600 pC/N). The highest33 of these literatures is only 350 pC/N, which is far inferioro that of Liu’s report. The gap between Liu’s and the oth-rs’ is so big that doubt might arise. Although four elementselationship of composition–structure–properties–processing inaterials science has been the core of material for a long time,

nfortunately, most of the subsequent literatures seemed to payuch more attention to the compositions such as Ba/Ca ori/Zr ratio, and did not care so much about the processing. Theeramics with the same composition fabricated with differentrocesses, even if the same technique but different processingarameters, they can exhibit absolutely different microstructure,hen their properties may differ greatly. Before investigatinghe effect of compositions on the piezoelectric properties, its strongly advised the best process parameters of the con-entional ceramics should be chosen, because it makes a greatifference, which will be reported in this work. As for the con-entional solid-state reaction process, the most important would

e the calcination and sintering temperature.16–19 Therefore, its necessary to investigate the effect of calcination and sinteringemperature on the structure and properties of BCTZ ceramics.

2 an Ceramic Society 31 (2011) 2005–2012

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Fig. 1. X-ray diffraction patterns of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3: (a) powders,Tcal = 1000 ◦C; (b) powders, Tcal = 1100 ◦C; (c) powders, Tcal = 1200 ◦C; (d)p ◦ ◦ ◦ic

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006 P. Wang et al. / Journal of the Europe

In this paper, the dielectric and piezoelectric properties ofCTZ ceramics calcined and sintered at different temperaturesre studied. With the optimized processing, the highest d33n BCTZ system was obtained. In addition, electromechani-al coupling factor, mechanical quality factor and ferroelectricroperties of BCTZ ceramics are presented.

. Experimental procedure

.1. Sample preparation

The (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 (BCTZ) ceramics were pre-ared via conventional solid-state reaction process. The oxide orarbonate powders of BaCO3 (99.0%, d50 < 0.5 �m, Sinopharmhemical Reagent Co. China), CaCO3 (99.96%, d50 < 0.8 �m,uxi Kaimao Chemicals Co. China), ZrO2 (99.9%, d50 < 2 �m,

ishui Jixiang Co. China) and TiO2 (99.84%, d50 = 0.5–0.8 �m,iantao Zhongxing Electronic Material Co. China) were used

s raw materials. They were wet milled in a nylon jar with zir-onia balls for 5 h. Then the mixed powders were dried andalcined at 1000 ◦C, 1100 ◦C, 1200 ◦C and 1300 ◦C for 2 h inir, respectively. These calcined powders were ball milled againn anhydrous ethanol for 5 h, the dried powders were mixed withwt.% polyvinyl alcohol (PVA) and pressed into disks withdiameter of 15 mm by uniaxial pressing at 150 MPa. After

xcluding PVA binder at 800 ◦C for 1 h, the pressed pellets wereintered at 1500–1550 ◦C for 2 h in air.

.2. Characterization

For measuring electrical properties, silver paste was coated onoth polished surfaces of the sintered samples and fired at 700 ◦Cor 30 min to form Ag electrodes. Specimens for piezoelectriceasurements were polarized at room temperature in a silicone

il bath by applying a dc electric field of 3 kV/mm for 20 min.iezoelectric properties were measured after laying the polarizedpecimens for 24 h to release the remnant stress and charge.

The calcined powders and sintered samples were character-zed by XRD patterns using a Cu-K� radiation (λ = 1.54178 A).olume density (ρv) and relative density (ρr) of the sinteredpecimens were measured by Archimedes method using de-onized water as medium. The microstructure of the ceramicsas studied by electron probe microanalysis (EPMA, 8705QH2,himadzu, Tokyo, Japan) after polished and thermally etchedt 1400 ◦C for 10 min. Piezoelectric coefficient d33 valuesere measured by a quasi-static d33 meter (ZJ-3A, Institute ofcoustics, Chinese Academy of Sciences, Beijing, China). Elec-

romechanical coupling factor of thickness (kt), planar (kp) andransverse (k31), mechanical quality factor (Qm), piezoelectriconstant d31 and elastic compliance constants sE11 of the poledamples were measured by resonance–antiresonance methodith an impedance analyzer (Wayne Kerr 6500B, UK). Tem-erature dependence of dielectric constant (εr) and dielectric

3 6

oss (tan δ) of the poled samples at 10 –10 Hz was measuredith a multifrequency LCR meter (Hewlett Packard, HP4192A,SA). Ferroelectric hysteresis loops (P–E) were measured bymodified Sawyer-Tower circuit at 10 Hz at different temper-

cnρ

p

owders, Tcal = 1300 C; (e) ceramics, Tcal = 1000 C, Ts = 1520 C; (f) ceram-cs, Tcal = 1100 ◦C, Ts = 1520◦C; (g) ceramics, Tcal = 1200 ◦C, Ts = 1520 ◦C; (h)eramics, Tcal = 1300 ◦C, Ts = 1520 ◦C.

tures (TF Analyzer 2000, aixACCT Systems GmbH, Aachen,ermany).

. Results and discussion

.1. XRD characterization of calcined powders andintered ceramics

The XRD patterns of the powders and ceramics calcined andintered at different temperatures are illustrated in Fig. 1. Theaw material powders calcined at 1000 ◦C, 1100 ◦C, 1200 ◦Cnd 1300 ◦C, respectively, for 2 h did not form pure perovskitehase. It can be seen that the sorts and amount of impurityhases decrease with increased calcination temperature (Tcal),nd there is only a trace of impurity in the powders calcinedt 1300 ◦C. Nevertheless, all the ceramics sintered at 1520 ◦Cormed pure perovskite phase, implying that different Tcal doot lead to obvious change in the phase structure of the ceram-cs, i.e. the impurity phase will diffuse into BCTZ lattices toorm a solid solution finally. Accordingly, the ceramics sinteredt higher temperatures will also form pure perovskite phase.

.2. Density and microstructure

Fig. 2 shows the effect of sintering temperature (Ts) on theolume densities (ρv) and relative densities (ρr) of BCTZ ceram-cs calcined at different temperatures. As the ceramics sinteredt different temperatures all formed MPB between tetragonalnd rhombohedral phases at room temperature,9 it is not easy toetermine the accurate value of the theoretical density (ρt), ando relative or theoretic density is reported for MPB composition

eramics. However, since the distortion between the tetrago-al and rhombohedral phase is tiny, herein, we substitute thet (5.687 g/cm3 with a = 4.0149 A, c = 4.0394 A) of tetragonalhase calculated from XRD results for that of BCTZ ceramics,

P. Wang et al. / Journal of the European Ce

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BIoAsgtebsacaWbaawdt

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csTmbp“gciaTagamfiH

ptscmsga

impihfjKcut1Htv=

cifimTs = 1540 C. The maximum kp and kt were 0.53 (Tcal = 1300 C,

ig. 2. Effect of sintering temperature on the volume density of BCTZ ceramicsalcined at different temperatures.

hich provides a very close value for comparison study. Withhis approximation, we can see that ρr of all the ceramics exceeds5%. The density increases significantly as the Tcal increases,hile the density first decreases as Ts increases to 1540 ◦C, and

hen increases slightly as Ts further increases. We can see thatcal has a greater influence on the density of ceramics than Ts.o the increasing calcination temperature is beneficial to obtainense BCTZ ceramics.

Fig. 3 shows the EPMA micrographs of polished surfaces ofCTZ ceramics calcined and sintered at various temperatures.

t can be seen from Fig. 3(a)–(d) that Tcal has varying influencesn the pore size and distribution, grain size, and grain boundary.s the Tcal increases, the pore size tends to increase, but there

eems to be less pores with higher Tcal. The influence of Tcal onrain size is shown in Fig. 4(a). The grain size increases a lit-le with Tcal. The influence of Tcal on grain boundary should bemphasized here. As Tcal increases, the grain boundaries becomelurred. All the polished samples were thermally etched at theame condition (1400 ◦C for 10 min), however, the grain bound-ry is more difficult to be etched for higher temperature calcinederamics. For the sample with Tcal = 1000 ◦C, the grain bound-ries are etched into grooves and they can be easily recognized.

hile as the Tcal increases, the grain boundary could hardlye etched at 1400 ◦C for 10 min. Then, the grain boundariesppeared blurred and narrowed as Tcal increases. Because thetoms at grain boundary can diffuse into the adjacent grainshen sintered at high temperature, and with higher Tcal thisiffusion becomes much easier due to the shorter diffusion dis-ances between grains.

From Fig. 3(c and e–h) it can be seen that the Ts has annfluence on the grain size and pore size, but almost no influencen the grain boundary. The average grain size as a function ofs is shown in Fig. 4(b). As the Ts increases, the grain size

ncreases obviously. It can be concluded that the grain size isainly controlled by Ts, while the grain boundary is dominated

y Tcal. These two parameters of solid-state reaction process

ave an important influence on the microstructure of the BCTZeramics, and hence influence the electrical properties of theeramics.

Tto

ramic Society 31 (2011) 2005–2012 2007

.3. Piezoelectric properties, electromechanical couplingactor and mechanical quality factor

Fig. 5 shows the effect of Ts on the d33 of BCTZ ceramics cal-ined at different temperatures. On the one hand, for the ceramicsintered at the same temperature, d33 increases steadily as thecal increases. Furthermore, the effect of Ts on d33 becomesore obvious as Tcal increases. This result indicates the grain

oundary basically controlled by Tcal could greatly affect theiezoelectric properties. Because the atomic arrangement at theblurred” grain boundaries is much more similar to that in therains, resulting a decrease in internal stress, which will induceontinuity of strain in the grain boundaries and result in continu-ty of domains across the grain boundaries.20 While the atomicrrangement at the grain boundaries of the ceramics with lowcal is relatively irregular and domains across the grain bound-ries are not continuous. The continuity of domains across therain boundaries is beneficial to the piezoelectric properties,nd then the d33 of the ceramics with “blurred boundaries” isuch higher. However, further work to clarify the domain con-guration and microstructure of blurred grain boundaries usingRTEM is needed.On the other hand, for the ceramics calcined at the same tem-

eratures, the d33 increases to the maximum value as Ts increaseso 1540 ◦C and then decreases as Ts further increases. Both den-ity and grain size have an important effect on the piezoelectriconstant.21 The ceramics sintered at 1540 ◦C do not have theaximum density but have the largest d33, that is because grain

ize affects d33 more markedly than density.22 In order to getood piezoelectric properties, Ts must be optimized to obtain anppropriate grain size.

Large grain size is another characteristic of the BCTZ ceram-cs sintered at such a high temperature.13,15 The grain size is

uch larger than 10 �m, while the grain size of other lead-freeiezoelectric ceramic systems such as BNT and KNN systemss only 3 �m.23,24 That is because the sintering temperature isigh enough for the grain growth of the BCTZ systems, whileor BNT or KNN system, the sintering temperature is usuallyust adequate for densification due to the volatilization of Bi2O3,

2O and Na2O at higher temperature. Generally speaking, thealcined powders with the same composition of ceramics aresed as supporting powders for ceramic pellets when being sin-ered. These powders begin to melt when the temperature rises to450 ◦C, limiting Ts not high enough for grain growth.10,12,13,15

ere, ZrO2 powders were used to support the pellets andhe Ts can be elevated to as high as 1550 ◦C. A maximumalue of d33 = 650 pC/N was obtained at Tcal = 1300 ◦C and Ts1540 ◦C.Fig. 6 shows the effect of Ts on kp, kt and Qm of BCTZ

eramics calcined at different temperatures. Both kp and ktncrease significantly as Tcal increases. The value of kp increasesrstly and then decreases with increasing Ts, while kt increasesonotonously as Ts increases. The maximum kp was obtained at

◦ ◦

s = 1540 ◦C) and 0.44 (Tcal = 1300 ◦C, Ts = 1550 ◦C), respec-ively. However, the variation of Qm with Tcal and Ts waspposite to those of d33, kp and kt. Qm decreases as Tcal and

2008 P. Wang et al. / Journal of the European Ceramic Society 31 (2011) 2005–2012

Fig. 3. EPMA micrographs of polished surfaces for BCTZ ceramics calcined and sintered at various temperatures: (a) Tcal = 1000 ◦C, Ts = 1540 ◦C; (b) Tcal = 1100 ◦C,Ts = 1540 ◦C; (c) Tcal = 1200 ◦C, Ts = 1540 ◦C; (d) Tcal = 1300 ◦C, Ts = 1540 ◦C; (e) Tcal = 1200 ◦C, Ts = 1500 ◦C; (f) Tcal = 1200 ◦C, Ts = 1520 ◦C; (g) Tcal = 1200 ◦C,Ts = 1530 ◦C; (h) Tcal = 1200 ◦C, Ts = 1550 ◦C.

P. Wang et al. / Journal of the European Ceramic Society 31 (2011) 2005–2012 2009

Fig. 4. Average grain size of BCTZ ceramics (a) sintered at 1540 ◦C and calcinedat different temperatures and (b) calcined at 1200 ◦C and sintered at differenttemperatures.

Fig. 5. Effect of sintering temperature on the d33 of BCTZ ceramics calcined atdifferent temperatures.

Fc

Tg

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ig. 6. Effect of sintering temperature on the (a) kp, (b) kt and (c) Qm of BCTZeramics calcined at different temperatures.

s increase, it tends to have a smaller value when d33, kp and ktet a larger value. This result is similar to Ref. 25.

The effect of Ts on d31, sE11 and k31 of BCTZ ceramics cal-ined at 1300 ◦C is shown in Fig. 7. It can be seen that as Tsncreases, d31, sE11 and k31 all increase to the maximum val-

es at Ts = 1540 ◦C, and the maximum values are 74 pC/N,4.0 × 10−12 m2/N and 0.309, respectively. While d31, sE11 and31 of the ceramics calcined at 1000 ◦C and sintered at 1520 ◦C

2010 P. Wang et al. / Journal of the European Ceramic Society 31 (2011) 2005–2012

Fc

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Fig. 10(a) shows the ferroelectric properties of BCTZ ceram-ics sintered at 1540 ◦C with different calcination temperatures,which is measured at 25 ◦C. The ferroelectric hysteresis loops

ig. 7. Effect of sintering temperature on d31, sE11 and k31 of BCTZ ceramicsalcined at 1300 ◦C.

re 24 pC/N, 11.97 × 10−12 m2/N and 0.140, respectively, whichs much lower than those calcined at 1300 ◦C and sintered at540 ◦C.

.4. Dielectric properties

Fig. 8 illustrates the effect of sintering temperature on (a)ielectric constant (εr) and (b) dielectric loss (tan δ) of BCTZeramics calcined at different temperatures. We can see that withncreasing Tcal, εr increases obviously. While as Ts increases, εrrst increases to its maximum value at Ts = 1540 ◦C and thenecreases. At Tcal = 1300 ◦C and Ts = 1540 ◦C, εr = 4500. Thishenomenon is in accordance with piezoelectric constant resultsn Fig. 5. While tan δ increases as Ts and Tcal increase. Thencrease in tan δ might be ascribed to the presence of oxygenacancy generated at higher temperature.

Fig. 9 exhibits the temperature dependence of dielectric con-tant and dielectric loss of BCTZ ceramics calcined at 1300 ◦Cnd sintered at 1540 ◦C at different frequency. As it can be seen,wo peaks are observed on the dielectric constant versus temper-ture curves (εr − T) in the measured temperature range between5 ◦C and 200 ◦C. The first peak corresponds to polymorphichase transitions from rhombohedral phase to tetragonal phaseTr−t) at 32◦C. The second peak corresponds to transitions frometragonal phase to cubic phase (Tc) at 85 ◦C, which is lower thanef. 9, but much larger than Ref. 13. There are also two peaks on

he dielectric loss versus temperature curves (tan δ − T), whichs similar to the curves of εr − T. However, the temperatures ofhe peaks of tan δ − T is lower than that of εr − T.

FB

ig. 8. Effect of sintering temperature on the (a) dielectric constant, (b) dielectricoss of BCTZ ceramics calcined at different temperatures.

.5. Ferroelectric properties

ig. 9. Temperature dependence of dielectric constant and dielectric loss ofCTZ ceramics calcined at 1300 ◦C and sintered at 1540 ◦C.

P. Wang et al. / Journal of the European Ce

Fig. 10. (a) Effect of calcination temperature on the P–E hysteresis loops ofBCTZ ceramics sintered at 1540 ◦C and measured at 25 ◦C; (b) temperatureds

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ependence of P–E hysteresis loops of BCTZ ceramics calcined at 1300 ◦C andintered at 1540 ◦C.

ith a good square shape for all the ceramics are clearlybserved in Fig. 10(a). Remnant polarization Pr increasesrom 7.18 to 11.69 �C/cm2 as the Tcal increases from 1000 ◦Co 1300 ◦C, which is much higher than previously reportedBa1−xCax)(Ti0.9Zr0.1)O3 ceramics.13 As the Tcal increases, theoercive field Ec first decreases to 182 V/mm at Tcal = 1200 ◦Cnd then increases slightly to 190 V/mm, which is a little higherhan Ref. 9. Fig. 10(b) illustrates the P–E loops of ceramicsTcal = 1300 ◦C, Ts = 1540 ◦C) measured at different tempera-ures. Pr decreases from 11.69 �C/cm2 to 6.55 �C/cm2 as the

easured temperature increases from 25 ◦C to 70 ◦C.

. Conclusions

The electrical properties of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3ead-free piezoceramics prepared at different calcination tem-eratures from 1000 ◦C to 1300 ◦C, and at different sinteringemperatures from 1520 ◦C to 1550 ◦C were investigated. The

-ray diffraction analysis showed that Tcal has a significant

nfluence on the phase structure of the calcined powders,ut there is almost no influence on the phase structure of

ramic Society 31 (2011) 2005–2012 2011

nal sintered ceramics. Tcal has a significant influence onensity and grain boundary, while Ts has an obvious influ-nce on grain size. With optimized Tcal and Ts, we canbtain desired microstructure, which is beneficial to piezoelec-ric properties of the ceramics. The BCTZ ceramics calcinedt 1300 ◦C and sintered at 1540 ◦C exhibit excellent prop-rties of d33 = 650 pC/N, d31 = 74 pC/N, kp = 0.53, kt = 0.38,

31 = 0.309, sE11 = 14.0 × 10−12m2/N, εr = 4500, tan δ = 0.009,c = 85 ◦C, Pr = 11.69 �C/cm2 and Ec = 190 V/mm. The presenttudy demonstrates that the calcination and sintering tempera-ures play a very important role in the microstructure, dielectricnd piezoelectric properties of BCTZ ceramics, which is impor-ant to guide the design of BCTZ-based lead-free ceramics withnhanced piezoelectric properties.

cknowledgements

This work was supported by the Ministry of Sciences andechnology of China through 973-Project (2009CB613305),he Major Program of the National Natural Science Foundationf China (50932007), and The Science & Technology Commis-ion of Shanghai Municipality (10XD1404700).

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