Materials Chemistry and Physics 75 (2002) 157–160

Ferroelectric lead scandium tantalate from mechanicalactivation of mixed oxides

Jonathan Lim, J.M. Xue, John Wang∗Department of Materials Science, Faculty of Science, National University of Singapore, Kent Ridge Road, Singapore 119260, Singapore

Abstract

Conventional mixed oxide preparation of lead scandium tantalate, PbSc0.5Ta0.5O3 (PST) powder, requires two calcinations steps, thefirst of which is to form a Wolframite precursor and the resulting PST phase is structurally long-range ordered. In this work, we report thesynthesis of perovskite PST via a novel one-step mechanical activation route, which leads to a long-range disordered structure as verifiedby X-ray diffraction. Sintered PSTs from the mechanically activated powders also retain the long-range disordered perovskite structureup to 1250◦C, while >95% theoretical density was achieved. They exhibit a spontaneous ferroelectric-relaxor transition at 15◦C with amaximum dielectric constant of∼13,850 and dielectric loss of<0.04. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: PST; Wolframite precursor; Mixed oxides; Mechanical aviation

1. Introduction

Lead scandium tantalate, PbSc0.5Ta0.5O3 (PST), is aPb-based electroceramic of perovskite structure with theformula AB′

1/2B1/2′′O3, where the ratio of B′ to B′′ cations

is 1:1. PST is known to be a normal ferroelectric whenchemically ordered. It is, however, a “relaxor” ferroelectricwhen disordered and exhibits a broad dielectric permittivitypeak against temperature [1,2]. The degree of B-site order-ing in PST can be controlled by thermal treatment and canbe used to modify its relaxor behavior [2,3]. In the past,perovskite PST derived from the conventional solid-statereaction among the three constituent oxides as well as fromWolframite precursor contains the undesirable pyrochlorephases [1–4]. Chemistry-based processing routes have beenattempted to synthesize sintered PST of high density atlower temperatures. These synthesis techniques, however,require several steps including refluxing, distillation, dry-ing and calcination to a temperature as high as 1400◦C, inorder to develop a single perovskite phase, when the highcost chemicals are employed as the starting materials [5–7].

Mechanical alloying was originally developed for synthe-sis of intermetallics, alloy compounds and nanocrystallinematerials [8,9]. Following this, mechanical activation wasemployed to alter and increase the reactivity of oxide con-stituents of electroceramics. For instance, BaTiO3 can beformed at lower calcination temperatures by mechanical

∗ Corresponding author. Tel.:+65-8742958; fax:+65-7763604.E-mail address: maswangj@nus.edu.sg (J. Wang).

activation [10]. More recently, novel mechanical activationtechniques have successfully been employed in the authors’laboratory to synthesize lead-based functional ceramics suchas PMN and PZN by skipping the formation of parasiticintermediate pyrochlore phases [11,12]. We report here thesuccessful synthesis and characterization of nanocrystallineperovskite PST with disordered structure and sintered PSTderived from mechanical activation. They were comparedwith the conventionally synthesized PST derived from aWolframite precursor via the Columbite route [13].

2. Experimental procedure

A mixed oxide composition equivalent to PbSc0.5Ta0.5O3was prepared by ball-milling appropriate amounts of com-mercially available lead (II) oxide (PbO, >99%, FlukaChemika), scandium oxide (Sc2O3, 99.9%, Aldrich) andtantalum oxide (Ta2O5, 99%, Aldrich) for 24 h in ethanol,followed by drying and passing through mesh-sieves. Fivegrams of the mixed oxide was weighed out and placed in asteel container (40 mm both in length and diameter) with a20 mm diameter steel ball weighing of 35.7 g. The mixedoxides was then mechanically activated at∼900 rpm in airfor 20 h in a high-energy shaker mill. A Wolframite precur-sor of PST was prepared by calcination of Sc2O3 and Ta2O5at 1350◦C for 4 h, followed by a second step of calcinationwith PbO at 1000◦C for 3 h. Post-calcination milling wasemployed to grind down the coarsened particles. PST pow-ders derived form mechanical activation and Wolframite

0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0254-0584(02)00046-9

158 J. Lim et al. / Materials Chemistry and Physics 75 (2002) 157–160

precursor, respectively, were then characterized for phasedevelopment using XRD (Philips X’Pert, PW3040, Cu K�).Particle morphology was investigated using both a scanningelectron microscope (Philips, XL30-FEG) and a transmis-sion electron microscope (JEOL, 100CX). The two pow-ders, derived from mechanical activation for 20 h and fromWolframite precursor, were compacted in a 10 mm diameterhardened steel die at a uniaxial pressure of 50 MPa, followedby isostatic pressing at 350 MPa. Sintering of these PST pel-lets was conducted in covered alumina crucibles containingbedding powder (PbZrO3) at various temperatures for 2 hin air and were furnace-cooled to room temperature. Thephases developed in sintered PST were analyzed using XRDand sintered densities were measured using the Archimedesmethod and by weight-dimension measurements. The di-electric behavior of sintered PST was measured using aprecision inductance–capacitance–resistance (LCR) meter(HP, 4284A) from−50 to 100◦C at 100 Hz to 10 kHz.

3. Results and discussion

Fig. 1 shows the XRD traces of PST powders derivedfrom 20 h of mechanical activation (MA) and from Wol-framite precursor, respectively. It can be seen that the com-position derived from mechanical activation has the sameperovskite structure as that synthesized conventionally fromthe Wolframite precursor. The observed peak broadening inthe XRD trace of PST derived from 20 h of mechanical acti-vation is due to the nanocrystalline nature of the PST phase.A slow XRD scan between the 2θ angles of 15 and 25◦ of

Fig. 1. XRD traces of PST phases derived from 20 h of mechanicalactivation (MA) and from Wolframite precursor, respectively.

Fig. 2. Low-angle XRD traces of PST phases derived from 20 h ofmechanical activation (MA) and from Wolframite precursor, respectively.

both samples as shown in Fig. 2, however, suggests that the(1 1 1) superlattice reflection does not occur in the PST de-rived from 20 h of mechanical activation, implying that theactivation-derived perovskite phase possessed a long-rangestructural disorder. Using the XRD technique used by Setterand Cross [3], it was determined that the order parameterS of the PST phase derived from Wolframite precursor is0.729, in contrast to an unmeasurable order parameter forthe PST derived from 20 h of mechanical activation.

It was observed using both SEM and TEM that PSTsynthesized from 20 h of mechanical activation comprisedparticles of rounded morphology and∼10 nm in size butoccurring as∼100 nm particle agglomerates due to the highspecific surface area possessed by the nano-particles. Onthe other hand, the PST derived from Wolframite precursorexhibited highly faceted particles with a wide size distribu-tion ranging from∼80 to 500 nm despite a post-calcinationmilling for 100 h.

XRD phase analysis conducted on the sintered PSTderived from 20 h of mechanical activation as well as fromWolframite precursor revealed that the sintered perovskitephase was stable up to 1250◦C for 2 h. It was furtherobserved that the PST synthesized from 20 h of mechan-ical activation retained its long-range structural disorderafter sintering as confirmed from the weak intensity ofsuperlattice XRD (1 1 1) reflection. This is unusual as thelong-range structural disorder in sintered PST normallyrequires sintering at 1560◦C, followed by rapid quenching[3]. The order parameters had however increased to 0.23and 0.764, respectively, for the PST derived from mechan-ical activation and that from Wolframite precursor after

J. Lim et al. / Materials Chemistry and Physics 75 (2002) 157–160 159

Fig. 3. Dielectric spectrum as a function of temperature for the PST derived from Wolframite precursor and sintered at 1200◦C for 2 h.

sintering at 1250◦C for 2 h. In addition, the highest sintereddensity measured at the sintering temperature of 1250◦Cwas 97.1% theoretical density (TD) for the mechanicalactivation derived PST, in contrast to 92.6%TD for thatderived from Wolframite precursor. It is thus apparent thatmechanical activation produced highly a sintering-active,nanosized PST, which possesses high sinterability.

The sintering temperature at which the highest dielectricconstant was measured for sintered PST derived from 20 hof mechanical activation was 1200◦C. This temperaturedid not coincide to that where the highest sintered densitywas obtained (1250◦C). This is a result of the occurrenceof a small amount of pyrochlore phase present in the PST

Fig. 4. Dielectric behavior as function of temperature for the PST derived from 20 h of mechanical activation and sintered at 1200◦C for 2 h.

sintered at 1250◦C. Pyrochlore phases have been shownto be responsible for the reduction in dielectric constant ofsimilar perovskite compounds used in multilayer ceramiccapacitors [14]. Figs. 3 and 4 show the dielectric constantand dielectric loss as a function of temperature for the PSTderived from Wolframite precursor and from mechanicalactivation, both of which were sintered at 1200◦C for 2 h.The dielectric behavior of sintered PST derived from Wol-framite precursor in Fig. 3 resembles that of conventionalfirst-order ferroelectrics that have dielectric constants risingto a maximum (εmax) at the characteristic Curie tempera-ture (TC) and then falling down to a lower value as a resultof phase conversion to a paraelectric state. The dielectric

160 J. Lim et al. / Materials Chemistry and Physics 75 (2002) 157–160

spectrum for such ferroelectrics is rather symmetrical aboutthe Curie temperature. Its maximum dielectric constant(εmax) is 4535 at 100 Hz and dielectric loss (tan�) is 0.112at the Curie temperature (TC) of 23◦C. As a result of itsorder parameter S of 0.75 and thus not possessing the fulllong-range structural order, a slight dielectric relaxation wasexpected.

Fig. 4, which shows the dielectric behavior of sinteredPST derived from 20 h of mechanical activation, howeverexhibits an asymmetric dielectric spectrum as a functionof temperature. Increasing the temperature from its ferro-electric state results in a rather steep rise in the dielectricconstant, which is followed by a slight relaxation below theeventual dielectric maximumεmax. A maximum dielectricconstant of 13,847 was measured at 100 Hz with a corre-sponding dielectric loss of 0.032. Beyond the temperatureof 15◦C at which the maximum dielectric constant occurs(denoted asTmax), the dielectric constant starts to fallgradually with further increase in temperature. Mechani-cal activation has therefore been successful in synthesis offerroelectric PST powders and sintered PST, whereby theformation of parasitic transitional pyrochlore phases wereskipped. Sintered PST derived from 20 h of mechanicalactivation also demonstrates an improvement in dielec-tric properties over that synthesized from the conventionalWolframite precursor.

Dielectric relaxation occurring beyondTmax is a commoncharacteristic of the relaxor ferroelectrics and is observed inmany mixed B-site, Pb-based ferroelectrics (e.g. lead mag-nesium niobate, Pb(Mg1/3Nb2/3)O3) [15]. However, unlikewhat is observed in Fig. 4, the dielectric relaxation in con-ventional relaxor ferroelectrics extends to temperatures waybelow Tmax, such that the dielectric spectrum is somewhatsymmetric about theTmax. The asymmetric dielectric be-havior in Fig. 4 is neither a first-order nor relaxor transi-tion and was described to occur in PST by Chu et al. [2]as exhibiting a spontaneous ferroelectric-relaxor transition.The possible factors contributing to this unique phenomenonwere further described by Chu et al., who suggested that themajor contributing factor was the presence of a sufficientlong-range structural disorder. It is therefore demonstratedthat from the XRD and dielectric studies performed in thiswork, that the PST derived from mechanical activation does

indeed possess a long-range structural disorder. In fact, it isfurther considered that the PST derived from 20 h of me-chanical activation and sintered for up to 6 h at 1200◦C hadretained a sufficient long-range structural disorder exhibit-ing the spontaneous ferroelectric-relaxor behavior. Furtherstudies are however required to elucidate the mechanismsbehind the observation.

4. Conclusions

Ferroelectric PST derived from 20 h of mechanical ac-tivation of mixed oxides possesses a long-range structuraldisorder. Sintering at 1250◦C retained the long-range disor-dered perovskite structure and gave rise to a sintered densityof 97%. They exhibited a maximum dielectric constant of>13,000 and a dielectric loss of 0.032 at the sintering tem-perature of 1200◦C. The dielectric spectra of PST derivedfrom 20 h of mechanical activation as a function of test tem-perature demonstrated a spontaneous ferroelectric-relaxortransition at temperatures in the vicinity of the dielectricmaximum.

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