Hydrothermal Synthesis and Spark Plasma Sintering of (K, Na)NbO3

Lead-Free Piezoceramics

Nan Liu,z,y Ke Wang,z Jing-Feng Li,*,w,z and Zonghuai Liuy

zKey Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education; School of Chemistry and MaterialsScience, Shaanxi Normal University, Xi’an 710062, China

yState Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering,Tsinghua University, Beijing 100084, China

A facile hydrothermal route was adopted for synthesis of lead-free piezoceramics (K, Na)NbO3 powders. The influences oftemperature and KOH/NaOH concentration on the resultantpowders were investigated. Although two similar perovskitephases appeared when K/Na ratio tended toward 1:1, the two-phase coexistence tendency was weakened by increasing hydro-thermal reaction temperature, and consequently only one phasecould be obtained after spark plasma sintering. Reasonablygood ferroelectric and piezoelectric properties were obtainedfor the samples after postannealing, whose piezoelectric con-stant (d33) reached 135 pC/N. The optimal remnant polarization(Pr) and mechanical quality factor (Qm) were 26.2 lC/cm2

and164, respectively, which were both twice as much as those of thesamples using powders prepared from solid-state reaction.

I. Introduction

BECAUSE of environmental protection concern, lead-free pi-ezoelectric ceramics are calling for substitution of conven-

tional lead-based counterparts, which are represented by thePb(Zr,Ti)O3 (PZT) family. (K, Na)NbO3 (KNN)-based piezo-ceramics have become one of the most promising candidatesbecause Saito et al.1 made a breakthrough in the textured KNNceramics with codopants of Li, Ta, and Sb, which shows a d33constant as high as 416 pC/N. To date, reported synthesis pro-cesses are generally conventional solid-state reaction routes,which usually operate at elevated temperatures in the range of7501–9501C.2,3 The high-temperature heating inevitably resultedin the volatilization of alkaline species, leading to a stoichiomet-ric deviation in composition. Besides, impurities and composi-tion inhomogeneity are easily introduced during time-consumingball milling process.4 In order to avoid the drawbacks of solid-state reaction, chemical solution synthesis routes are oftenapplied, such as sol–gel and precipitation methods. Besides,hydrothermal synthesis has been proved as a facile and effec-tive route to fabricate ceramic powders. It is reported that thed33 of BaTiO3 ceramics could reach as high as 350 pC/N usinghydrothermal powders and microwave sintering, compared withmerely 190 pC/N obtained in samples by solid-state reaction andconventional sintering process.5 Although the piezoelectric andferro-

electric properties are influenced by many factors, such as grainsize, domain size and so on, it is reasonable that at least thesynthesized powders based on chemical solution processing havebetter compositional homogeneity and less impurities, whichshould benefit the performances of resultant ceramics.

Hydrothermal synthesis is favored for compounds with un-complicated constitutions, such as KNbO3 and NaNbO3,

6 dueto the easy control of stoichiometric ratio between correspond-ing elements. However, for hydrothermal synthesis of solid so-lution (K, Na)NbO3, it is not easy to determine the KOH/NaOH ratio for desirable products, such as (K0.5Na0.5)NbO3,because both KOH and NaOH act as mineralizers and rawmaterials, respectively.7,8 As a result, The possible approach isto investigate the relationship between resultant products andstarting KOH/NaOH ratio in a wide range. However, it seemsthat it was feasible to obtain single perovskite phase of compo-sitions near either end members in KNbO3–NaNbO3 systemby hydrothermal synthesis at temperatures of 2201–2301C, whilesingle-phase solid solution near the composition (K0.5Na0.5)NbO3 was not easy to acquire.7,8

Nevertheless, the present study shows that single-phase KNNceramics near the composition (K0.5Na0.5)NbO3 could be ob-tained directly by spark plasma sintering, using two-phasehydrothermal precursors. After postannealing in air, the resul-tant (K0.5Na0.5)NbO3 ceramic samples showed good piezoelec-tric constant of d335 135 pC/N, with twofold higher remnantpolarization Pr and mechanical quality factorQm than that fromsolid-state reaction synthesis.

II. Experimental Procedure

Reagent grade KOH, NaOH, and Nb2O5 were used as raw ma-terials. For the preparation of powders, Nb2O5 (0.125 mol) wereadded in a mixed solution of KOH and NaOH (30 mL), withdifferent KOH/NaOH ratios but a total alkalinity of 10 mol/L.After magnetically stirring for half an hour, the resultant sus-pension was transferred into a p-polyphenylene lined stainlesssteel autoclave (50 mL) and heated in an oven at 1801–2201C for24 h without stirring. After cooling down to room temperature,the precipitates were washed several times with deionized waterand ethanol, and then dried at 801C. The synthesized powderswere charged into an SPS graphite die 15 mm in diameter.After the SPS chamber was evacuated (B10�2 Pa), the tem-

perature was raised to the predetermined peak temperature at arate of 1001C/min and kept for 1 min, under a constant pressureof 50 MPa along the Z-axis. The as-sintered SPS samples wereannealed in air at 9001C for 4 h.9

The bulk densities of the SPS samples were measured by theArchimedes method. The crystal structure was determined byX-ray diffraction (XRD) using Ni-filtered CuKa radiation(Rigaku, D/Max 2500, Tokyo, Japan). The constitution ofhydrothermal powders were measured by the X-ray fluorescence

K.-i. Kakimoto—contributing editor

This work was supported by National Nature Science Foundation of China (GrantsNos. 50772050, 50672040, and 50621201) and the Ministry of Science and Technology ofChina under the grant no. 2009CB623304 as well as by Specialized Research Fund for theDoctoral Program of Higher Education (grant no. 20060003023).

*Member, The American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: jingfeng@mail.tsinghua.

edu.cn

Manuscript No. 25651. Received December 16, 2008; approved March 24, 2009.

Journal

J. Am. Ceram. Soc., 92 [8] 1884–1887 (2009)

DOI: 10.1111/j.1551-2916.2009.03126.x

r 2009 The American Ceramic Society

1884

spectrometer (Shimadzu, XRF-1800, Sequential, Kyoto, Japan),using solid-state reaction powder (K0.5Na0.5)NbO3 (7501C 4 h)as the standard. The microstructure was observed using fieldemission scanning electron microscopy (FE-SEM S-4800, Hita-chi, Tokyo, Japan).

For measurement of dielectric and piezoelectric properties,silver paste electrodes were formed at the two surfaces of disc-shaped specimens (1 mm in thickness and 15 mm in diameter),which were fired at 5501C for 30 min. The dielectric propertieswere measured in the temperature range from room temperatureto 6001C, using an Agilent 4294A precision impedance analyzer(Agilent, Palo Alto, CA). The ferroelectric hysteresis loops weremeasured at room temperature using a ferroelectric tester(RT6000HVA, Radiant Technologies Inc., Albuquerque, NM)at 1 Hz. The piezoelectric properties were measured using aquasi-static piezoelectric constant testing meter (ZJ-3A, Instituteof Acoustics, Chinese Academy of Sciences, Beijing, China) forthe samples that were poled under 30 kV/cm bias at 1301C for10 min in a silicone oil bath.

III. Results and Discussion

The starting KOH/NaOH ratio has a profound influence on thecompositions of hydrothermal products (K1�xNax)NbO3. It wasfound that single-phase perovskite with compositions near eitherend members in KNbO3–NaNbO3 was easy to be synthesized,while two phases were obtained with composition close to(K0.5Na0.5)NbO3. As shown in Figs. 1(a) and (b), single-phase(K0.83Na0.17)NbO3 and (K0.08Na0.92)NbO3, were directly syn-thesized at 1801C, using the starting KOH/NaOH ratios of 8M/2M and 7M/3M, respectively. However, two similar perovskitephases, with a total K/Na ratio of 36/64, were obtained whenstarting KOH/NaOH ratio was 7.6M/2.4M, as shown inFig. 1(c). Increasing reaction temperature effectively promptedthe reaction toward one single-phase; however, secondary phasetraces were still observed when the reaction temperature wasraised to 2201C, as indicated in Fig. 1(e). Because of the limi-tation of our autoclaves, the reaction temperature could not befurther increased. Besides, compositional shift took place whenelevating reaction temperatures. Hydrothermal precursors withcompositions close to (K0.5Na0.5)NbO3 were successfully syn-thesized at 2201C, as listed in Table I.

Single perovskite phase was obtained after the hydrothermalprecursors were sintered by SPS, as shown in Fig. 1(f). It is as-sumed that during high temperature sintering process, single-phase solid solution with minimum Gibbs free energy could beformed with the aid of solid-state mass transfer. Figure 2 showsthe shrinkage curves of hydrothermally synthesized powders(2201C, 24 h) during SPS process, with comparison with pow-

ders prepared by conventional solid-state reaction. For thesethree species of hydrothermal powders, shrinkage occurred inthe temperature range of 8501–9301C, which is more than 1001Chigher than that of conventional powders by SPS.2 Although thepowders synthesized by the two different methods had compa-rable grain sizes around 1 mm, the hydrothermal powderssuffered severe glomerations, as seen from Fig. 3, which is re-sponsible for the increase of sintering temperatures. The glome-ration morphology is probably caused by the large amount ofKOH/NaOH additives, because both NaOH and KOH act aseffective mineralizers. SEM micrographs of the SPSed KNN ce-ramics are also shown in Fig. 3. The fractured surfaces show thatthe samples were very dense and almost no visible pores could befound. The relative densities measured by Archimedes methodswere all around 99%. Because the fracture was in a transgran-ulation mode, the exact grain size could not be confirmed.

Table I lists comparison of various properties of KNN ce-ramics fabricated by SPS with different powders prepared byhydrothermal synthesis and solid-state reaction, respectively.The piezoelectric constant d33 of the ceramics synthesized byhydrothermal methods reached 135 pC/N, comparable with 148pC/N of the ceramics prepared by solid-state reaction. Thedifference could be caused by two major reasons. The first oneis compositional inequality, because (K0.5Na0.5)NbO3 locatesaround a morphotropic phase boundary (MPB) which is verysensitive to piezoelectric properties. The second one is the grainsize. Although the grain size of KNN ceramics by hydrothermalprecursors could not be deduced from the SEM micrographs, itcould almost be confirmed that the grain size must exceed 1 mm,which was the level of hydrothermal powders. However, ac-cording to our previous reports, the KNN ceramics prepared by

Fig. 1. X-ray diffraction patterns of (a–e) hydrothermal powderswith different KOH/NaOH ratios and reaction temperatures, and(f) as-SPSed ceramic sample with powders (e).

Table I. Various Properties of SPS-Sintered (K, Na)NbO3

Ceramics Using Powders by Hydrothermal Synthesis and SolidState Reaction

KOH/NaOH ratio 7.8/2.2 7.6/2.4 7.4/2.6 KNNw

K/Na (powders) 58/42 57/43 53/47 50/50r (g/cm3) 4.54 4.52 4.53 4.47d33 (pC/N) 135 133 134 148kP (%) 44.9 45.1 45.6 38.9e 473 445 436 606tan d 0.03 0.02 0.03 0.04Pr (mC/cm

2) 23.5 23.6 26.2 13.6EC (kV/cm) 12.3 13.6 14.9 10.3TC (1C) 451 407 420 395Qm 149 151 164 78

wKNN, (K0.5Na0.5)NbO3, powders by solid state reaction, see Li et al. 2

Fig. 2. Shrinkage curves of (K, Na)NbO3 ceramics during SPS process,using powders by hydrothermal synthesis (colored solid lines) and solid-state reaction (black dash line), respectively.

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solid-state reaction and SPS had a grain size in the range of 200–500 nm.2 Such ultrafine grains are considered to be responsiblefor the enhancement of its piezoelectric property, as pointed outby Shrout and Zhang,10 similar to the case found in fine-grainedBaTiO3.

5

It is interesting that both the remnant polarization (Pr) andmechanical quality factor (Qm) values are almost doubled usinghydrothermal powders. Figure 4 compares the P–E hysteresisloops of the KNN ceramics prepared by hydrothermal synthesisand solid-state reaction. The Pr was increased from 13.6 mC/cm2

Fig. 3. Scanning electron microscopic micrographs of powders and fracture surface of SPSed (K, Na)NbO3 samples after being annealed at 9001C for4 h. For hydrothermal powders, starting KOH/NaOH ratios are (a) and (e) 7.8/2.2, (b) and (f) 7.6/2.4, (c) and (g) 7.4/2.6, respectively; (d) and (h) arefor powders by solid-state reaction.

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of KNN ceramics by solid-state reaction to around 25 mC/cm2

of those by hydrothermal synthesis, which is a result of en-hanced grain size. It should be noticed that the P–E hysteresiscurve for KOH/NaOH ratio of 7.4/2.6 shows asymmetric char-acteristic, which is possibly related to internal field, due to re-maining oxygen vacancies formed during SPS process and leftafter postannealing in air.9 And the Qm values also reached ashigh as 164 for KNN ceramics synthesized from hydrothermalprecursors, compared with 78 for KNN made by solid-state re-action. BecauseQm is closely related to mechanical losses duringworking cycles of piezoelectric devices, the increase of Qm hasalways been an insistent demand of industrial applications. Theincrease of Qm may be attributed to better compositional ho-mogeneity and less impurities using hydrothermal powders. Thephase angle y between the resonance and antiresonance fre-quencies were 79.2–82.61, and the DC resistivity were 462–760MO �m, which are within the same range reported for KNN-

based ceramics prepared by normal sintering in air.11 Figure 5shows the temperature dependence of relative dielectric con-stants for the present KNN ceramics. The two dielectricanomalies correspond to the phase transitions of orthorhom-bic—tetragonal (at TO�T) and tetragonal-cubic (at TC), respec-tively. At the Curie temperature, TC, the phase transition fromferroelectric to paraelectric occurred. This result indicates thatthese KNN ceramics synthesized by hydrothermal reactions be-haved the same with KNN ceramics prepared from conven-tional solid-state reactions.

IV. Conclusion

Single-phase lead-free KNN ceramics with compositions around(K0.5Na0.5)NbO3 have been prepared using hydrothermal pre-cursors and SPS, although the hydrothermal powders consist oftwo similar perovskite phases. After postannealing treatment inair, the samples showed typical ferroelectric and piezoelectriccharacteristics. Good piezoelectric constant of 135 pC/N wasobtained, which is reasonably high for undoped KNN ceramics.Large remnant polarization Pr of approximately 25 mC/cm2 andmechanical quality factor Qm above 150 were about twice asmuch as those of SPSed (K0.5Na0.5)NbO3 ceramics from solid-state reaction, which was a result of homogenous composition.The present study proved that hydrothermal synthesis and SPSare effective for the fabrication of KNN-based lead-free piezo-ceramics.

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Fig. 4. Polarization hysteresis curves of SPSed (K, Na)NbO3 samplesafter annealing at 9001C for 4 h, using powders by hydrothermalsynthesis (colored solid lines) and solid-state reaction (black dash line),respectively.

Fig. 5. Temperature dependence of dielectric constant for the SPSedand annealed (K, Na)NbO3 ceramics using hydrothermal powders at2201C for 24 h.

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