Materials Science and Engineering B 132 (2006) 253–257

Microstructure and dielectric properties of PLZST ceramicsprepared by a modified coprecipitation

Lihong Xue a, Qiang Li a,∗, Yiling Zhang b, Xihe Zhen a, Rui Liu a, Lin Wang a

a Department of Chemistry, Tsinghua University, Beijing 100084, Chinab Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

Received 13 July 2005; received in revised form 13 January 2006; accepted 19 February 2006

Abstract

Lead lanthanum zirconium titanium stannate (PLZST) has been synthesized by a modified coprecipitation in a polyvinyl alcohol (PVA) aqueoussolution. X-ray diffraction (XRD) study shows single perovskite phase formation at 650 ◦C for 2 h. The average particle size of calcined powder wasabout 70 nm. The green pellets have been sintered at 1000 ◦C, 1050 ◦C, 1100 ◦C and 1150 ◦C for 2 h, respectively. Effect of sintering temperature onmicrostructure and dielectric properties has been investigated. Dense ceramics with average grain size about 2 �m were obtained at the relativelylow temperature of 1050 ◦C for 2 h and dielectric constant of 1633 at 1 kHz has been observed. Also, a comparison of particle size and sinteringp©

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rocess is made between the samples prepared by the modified coprecipitation and the conventional coprecipitation.2006 Elsevier B.V. All rights reserved.

eywords: PLZST; Chemical synthesis; Dielectric properties; Sintering

. Introduction

Lead lanthanum zirconium titanium stannate (PLZST)eramics have attracted much attention in recent years becausehey exhibit extremely high electric-field-induced longitudinal0.85%) and volume strains [1]. Such excellent performanceakes PLZST the electromechanical materials in a broad range

f advanced applications, such as actuation, energy conversion,nd charge storage [1–4]. For the purpose of improving theirroperties, these ceramics have been investigated by variousroups [5–10]. It is well known that the properties of ceramicsre strongly related to the purity, stoichiometry and grain size,hich are mainly controlled by preparation method. UsuallyLZST ceramics are prepared by the conventional solid-stateeaction. The inevitable inhomogeneity inhabits the composi-ional and microstructural homogeneity of sintered products.n order to overcome these drawbacks, wet chemical methodsave been developed for the synthesis of single-phase powderith controlled powder characteristics, including coprecipita-

ion [11–13], sol–gel [14], and other related methods [15,16].ompared with other wet chemical methods, coprecipitation

method favors for its simple processing. However, it is oftennot easy to synthesize monodispersed nanoparticles using thistechnique because of the coagulation of particles. In order toprevent powder agglomeration Lee et al. sprayed the precipitateinto liquid nitrogen and freeze-dried [11], which made the pro-cess complicated. To keep the process convenient, the addition ofwater-soluble polymers as a protective agent may be a promisingmethod to overcome this problem. Water-soluble polymers suchas polyvinyl alcohol (PVA), poly(N-vinylpyrrolidone) (PVP),and polyacrylic acid (PAA) used as a protective agent in the pre-cipitation of inorganic particles are effective for the synthesisof nanoparticles [17–19]. In this study, the synthesis of PLZSTnanoparticles by coprecipitation with PVA as a protective agentwas tried. The obtained powder and ceramic materials preparedby this modified method were studied. The results are comparedto that corresponding to a conventional coprecipitation method.

2. Experimetal

The composition of Pb0.97La0.02(Zr0.66Ti0.07Sn0.27)O3 waschosen for the powder preparation. The starting materials used

∗ Corresponding author. Tel.: +86 10 62781694; fax: +86 10 62771149.E-mail address: qiangli@mail.tsinghua.edu.cn (Q. Li).

were reagent grade lead nitrate Pb(NO3)2, zirconium nitrateZr(NO3)4·5H2O, lanthanum nitrate La(NO3)3·6H2O, stannicchloride SnCl4·5H2O, and titanium terachloride TiCl4. Indi-vidual aqueous solutions containing the required amounts of

921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2006.02.056

254 L. Xue et al. / Materials Science and Engineering B 132 (2006) 253–257

Fig. 1. XRD patterns of PLZST powders calcined at different temperatures: (a) precursor, (b) 500 ◦C, (c) 600 ◦C, (d) 650 ◦C and (e) 700 ◦C for 2 h.

metal ions were prepared by directly adding the reagents todistilled water and some amount of 30% aqueous H2O2 wasadded into the titanium source solution according to the pro-cedure described by Murata and Wakino [20]. Then they weremixed together to get the PLZST clear solution (the pH value wasadjusted to be less than one by concentrated HNO3 solution).A PVA aqueous solution (1 wt%) was prepared by dissolvingthe desired amount of PVA in distilled water and then addedinto the PLZST solution (the weight ratio of PVA to PLZST is3%). The process was carried out at 50 ◦C with vigorous stir-ring. The concentration of the solution of PLZST compositionwas 0.1 mol/dm3 and the pH value was controlled below one tokeep the solution transparent. When the transparent solution wasadded drop wise to a continuously stirring bath of NH4OH solu-tion, which was controlled at around pH 9, a white precipitatewas observed. During the coprecipitation, the pH of the bath wasmaintained at 9 by adding aqueous ammonia as necessary. Theprecipitate was then washed with distilled water until no reac-

tion for Cl ion with AgNO3 was observed and dried at 100 ◦C.During the drying, the release of O2 from H2O2 would result inloose product [20]. A conventional coprecipitation without PVAwas also performed with the same process.

The dried precursor was then crushed and calcined at varioustemperatures. The crystalline phase of the powder was identifiedby X-ray diffraction (XRD, D8 ADVANCE) analysis using CuK� radiation. The particle size of the powder was measured byscanning electron microscopy (SEM, JSM-6301F). After calc-ing the powder was pressed at 100 MPa in a stainless steel moldto form green bodies with a diameter of 10 mm. Then the greenbodies were sintered at different temperature in sealed aluminacrucibles. To limit the loss of PbO, a lead rich atmosphere wasprovided by PbZrO3 + 8 mol% ZrO2 powders. The density ofthe samples sintered at various temperatures was determined bythe Archimedes method in water. The microstructure and grainsize of the sintered body were analyzed by using SEM (JSM-6460LV). Dielectric properties of the samples were measured

cipitat

Fig. 2. SEM images of the powders prepared by (a) modified copre ion and (b) conventional coprecipitation calcined at 650 ◦C for 2 h.

L. Xue et al. / Materials Science and Engineering B 132 (2006) 253–257 255

with an LCR meter (HP4194A) controlled by a computer. Thefrequencies used were 1 kHz and the temperature range were−50–200 ◦C.

3. Results and discussion

Fig. 1 shows XRD pattern of the precursor and powders cal-cined at different temperatures for 2 h prepared by two methods.For the modified coprecipitation method (Fig. 1(a)), the precur-sors are amorphous, and the PLZST perovskite phase is initiatedat 500 ◦C. The heat treatment of the precursors at 650 ◦C for 2 hresults in the formation of single-phase PLZST. The studies alsoreflect the growth of crystallinity in the powders with the increas-ing heat-treatment temperatures. Similar result was observed forthe conventional copreicpitation (Fig. 1(b)).

The calcined powders at 650 ◦C for 2 h were characterizedby SEM to investigate their morphology (Fig. 2). The modifiedcoprecipitation method yielded very fine powder with a medianparticle size of around 70 nm. Precipitation of inorganic parti-cles in a cross-linking polymer matrix or network of gel oftenprevents coagulation of particles, giving rise to monodisperseparticles and uniform size distribution. The powder obtainedby the conventional coprecipitation contained a large portion ofagglomerates with particle size of 0.2 �m.

Fig. 3. Density of the PLZST ceramics as a function of sintering temperatures.

Compacted samples of the calcined powder prepared by themodified coprecipitation were axial pressed at 200 MPa and sin-tered at different temperature for 2 h. The density of PLZSTceramics as a function of sintering temperatures is plotted inFig. 3. As it can be seen, the density reaches a maximum valueat 1050 ◦C and decreases a little after further sintering at higher

Fig. 4. SEM images of the PLZST ceramics sintered at various temperatu

res for 2 h. (a) 1000 ◦C, (b) 1050 ◦C, (c) 1100 ◦C and (d) 1150 ◦C.

256 L. Xue et al. / Materials Science and Engineering B 132 (2006) 253–257

Fig. 5. SEM images of the PLZST ceramics prepared from (a) modified coprecipitation and (b) conventional coprecipitation, respectively and sintered at 1050 ◦Cfor 2 h.

temperature. The decrease in density of PLZST ceramics sin-tered at high temperature may be due to the grains growth andloss of PbO.

Fig. 4 is SEM of fracture surface of PLZST ceramics sinteredat various temperatures. It indicates that the sintering temper-ature has influence on the morphology and microstructure ofsintered PLZST ceramics. Sample sintered at lower tempera-ture (1000 ◦C) contained smaller grains but also a considerableamount of pores in their structure, which explains the relativelylow density measured for this sample. The grain size increasedwith an increase in sintering temperature. Dense pellets with-out pores were obtained at 1050 ◦C and uniform grain-sizegrowth was observed. With an increase in grain size, the frac-ture characteristics of the samples change from intergranular totransgranular fracture.

Fig. 5 compares the microstructure of ceramic samples madefrom powders obtained by using the two studied processingroutes and sintered at temperature of 1050 ◦C for 2 h. For thesame sintering conditions, PLZST ceramics prepared by themodified method have noticeably higher sintered density and

Fs

much more homogenous grain-size distribution than those pro-duced from powder prepared by the conventional method, wherea considerable amount of pores and some abnormally largegrains are observed. In a normal growth process, the grainsgrow along the grain boundaries through a grain boundary dif-fusion process. The driving force causing the growth is interfaceenergy (negative capillary pressure) between the grain bound-aries. The interface energy is dependent on the radius of thegrain and increases with a decrease in radius, meaning that thedriving force for grain growth is bigger for smaller particles. Itis obvious that the smaller particle size is beneficial to sinter-ing and results in higher density after the sintering process iscompleted.

A typical variation of dielectric constant with temperature forPLZST sintered at various temperatures is displayed in Fig. 6;they were measured at 1 kHz. In all the cases a broad peak,characteristic of relaxor is observed. From Fig. 6, it is clearthat the variation in dielectric constant is in agreement with thevariation in density. The highest dielectric constant (εmax) 1633was observed for PLZST ceramic sintered at 1050 ◦C.

4. Conclusions

PLZST with perovskite structure has been successfully syn-thesized by a modified coprecipitation in the presence of PVA.S ◦pticcc

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ig. 6. The temperature dependence of dielectric constant of PLZST samplesintered at various temperatures.

ingle-phase synthesis can be performed at 650 C for 2 h. Theowder shows nanometric-scale size (∼70 nm) and uniform par-icle distribution. The powder has better sinterability character-stics, and allows us to obtain dense, homogeneous grain-sizederamic bodies at temperature of 1050 ◦C lower than that of theonventional coprecipitation powders. The maximum dielectriconstant was found to be more than 1600.

cknowledgement

The authors thank the National Natural Science Foundationf China, NNSFC 50272030, for the financial support of thisork.

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