NANOCRYSTALLINE FERROELECTRIC PHASES FROMMECHANICAL ACTIVATION OF OXIDE COMPOSITIONS

Jonathan Lim, Xue Junmin and John WangDepartment of Materials Science, Faculty of Science, National University of Singapore, Singapore

119260

(Received August 21, 2000)(Accepted December 5, 2000)

Abstract—Successful synthesis of nanocrystalline perovskite lead scandium tantalate,PbSc0.5Ta0.5O3, was achieved via a novel one-step mechanical activation route. The as-formed PSTexhibits a disordered structure and nanocrystalline domains of;8nm. Sintered ceramics made fromthese powders retain long-range disordered perovskite structure up to 1200°C and a dielectric constantof ;23,000 is obtained. Sintering above this temperature results in the formation of undesirablepyrochlore phases that degrades dielectric properties.© 2001 Acta Materialia Inc. Published byElsevier Science Ltd. All rights reserved.

Keywords:Powder processing; Transmission electron microscopy (TEM); Functional ceramics;Dielectric permittivity; Order-disorder phenomena

Introduction

Lead scandium tantalate, PbSc0.5Ta0.5O3, hereafter-named PST, is a member of the Pb-based electro-ceramics with perovskite structure corresponding to the formula AB9xB01-xO3. PST is known to be anormal ferroelectric when chemically ordered. It is however a “relaxor” ferroelectric when disorderedand exhibits a broad dielectric permittivity peak against temperature (1,2). The degree of B-site orderingin PST can be controlled using thermal treatment and can be used to modify its relaxor behavior (2,3).In the past, perovskite PST formed from the conventional solid–state reaction among the threeconstituent oxides as well as using wolframite precursor contains the undesirable pyrochlore phases(1-4). Chemistry-based processing routes have been attempted to synthesize PST at lowered temper-ature so as to produce a sintered ceramic of higher density. These techniques however require severalsteps including refluxing, distillation, drying and calcination to temperature as high as 1400°C in orderto develop a single perovskite phase (5,6,7).

Mechanical alloying was originally developed for synthesis of intermetallics, alloy compounds andnanocrystalline materials (8,9). Mechanical activation was devised to improve the reactivity of oxideconstituents such that electroceramics, e.g. BaTiO3 are formed at a lowered calcination temperature(10). More recently, novel mechanical activation techniques have successfully been employed in theauthors’ laboratory to synthesize lead-based functional ceramics, such as PMN and PZN, skipping thepyrochlore phases (11,12). We report in this work the successful synthesis and characterization ofnanocrystalline perovskite PST with disordered structure and sintered ceramics derived from mechan-ical activation.

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Experimental Procedure

The starting materials used in this work are commercially available lead (II) oxide (PbO,$99%, FlukaChemika), scandium oxide (Sc2O3, 99.9%, Aldrich) and tantalum oxide (Ta2O5, 99%, Aldrich).Mixed-oxide powder of PbSc0.5Ta0.5O3 was prepared by ball-milling desirable amounts of the threeoxides for 24 hours in ethanol and subsequently dried and sieved.

A batch of approximately 5grams of mixed-oxide was weighed out and placed in a steel containerof 40mm both in length and diameter with a 20mm diameter steel ball weighing of 35.7 grams. Thecontainer was mounted in a high-energy shaker mill and mechanically activated at;900rpm in air for20 hours. The mixed oxide and mechanically activated powders were then characterized for phasedevelopment using XRD (Philips X’Pert, PW3040, CuKa). In addition, their particle sizes weredetermined from the specific surface areas obtained using BET specific surface area analysis (Quan-tachrome, Nova-2000). Particle morphology was investigated using SEM (Philips, XL30-FEG). Atransmission electron microscope (JEOL, 100CX) as well as a high-resolution transmission electronmicroscope (Philips, CM300FEG) was employed to study the particle and structure characteristics ofthe activated powder. Several pellets weighing approximately 0.4grams each of powder mechanicallyactivated for 20 hours, were compacted using hardened steel die of 10mm in diameter under a uniaxialpressure of 50MPa and then isostatically pressed at 350MPa. Sintering of PST was then conducted incovered alumina crucibles containing PbZrO3 at temperatures ranging from 1000°C to 1400°C in airand furnace-cooled to room temperature. The phases in PST pellets were analyzed using XRD while thesintered density was measured using the Archimedes Method or calculations based on mass-dimensionmeasurement. The dielectric behavior of the sintered PST was measured using a precision inductance-capacitance-resistance (LCR) meter (HP, 4284A) from250°C to 100°C at 100Hz to 10kHz.

Results and Discussion

As seen in Fig. 1, the sharp peaks in XRD trace show PbO, Sc2O3 and Ta2O5 in the starting oxidemixture, indicating that no reaction occurred among the constituent oxides during conventional

Figure 1. XRD traces of mixed oxides of PbO, Sc2O3 and Ta2O5, before and after 20 hours of mechanical activation. (Π- PbO,f - Sc2O3, < - Ta2O5, F - PST)

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ball-milling. However, upon 20 hours of mechanical activation, the formation of nanocrystalline PSTwas triggered and became the only XRD-detectable phase. The apparent broadening in XRD trace wasdue to the nanocrystalline nature of the PST derived from mechanical activation. A long-rangedisordered structure was detected in the as-formed PST powder using XRD study technique employedby Setteret al. (3). As observed from SEM and TEM studies and gas adsorption analysis (BET), thePST powder derived from mechanical activation consists of nanosized particles (;20nm) of roundedmorphology with a degree of agglomeration. Fig. 2 is a high-resolution transmission electron (HRTEM)micrograph of PST formed at 20 hours of mechanical activation. The material consists of nanocrys-talline domains of disordered structure in the size range of;8nm, in an amorphous matrix. This isconsistent with what has been observed from other Pb-based electroceramic powders previouslysynthesized in the author’s laboratory (12).

Phase development in sintered PST derived from mechanical activation was studied using XRD andit was observed that perovskite PST phase was retained upon sintering at temperatures up to 1200°C.Sintered density reached.95% theoretical density (relative density). Further XRD studies showed thatsintered PST was of long-range disorder, which was previously developed. Development of this phaseusually requires sintering above 1550°C followed by rapid quenching. The dielectric constant for PSTsintered at 1200°C is shown in Fig. 3 with peak dielectric constant of;23,000 at 100Hz. Whensintering time was further extended, the sintered density and dielectric properties may be enhanced.However, as mentioned by Chuet al. (2), the dielectric properties of PST relaxors were not alwaysconsistent. A high sintering temperature of 1300°C and above resulted in the formation of undesirablepyrochlore phases, accompanied with a drop in the relative sintered density and dielectric constant. Thedecrease in sintered density was attributed to loss of lead at high temperatures and change in thechemical stoichiometry. Degradation of the dielectric properties is due to the formation of pyrochlorephases.

Figure 2. High-resolution transmission electron (HRTEM) micrograph showing disordered domains in a particle of the PSTmechanically activated for 20 hrs.

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Conclusions

Nanocrystalline perovskite PST has been synthesized from the mixed oxides via a one-step mechanicalactivation route. XRD studies indicated that the as-formed PST has a long-range disordered structure.Nanocrystalline domains of;8nm size were detected within PST particles. Sintered PST retained thedisordered perovskite structure up to 1200°C, exhibiting a maximum dielectric constant of;23000.Sintering temperatures above 1200°C led to the formation of undesirable pyrochlore phases anddegradation in dielectric properties.

References

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10. O. Abe and Y. Suzuki, Mater. Sci. Forum. 225, 563 (1996).11. J. Wang, D. M. Wan, J. M. Xue, and W. B. Ng, Singapore Patent No. 9801566–2 (1998).12. J. Wang, D. M. Wan, J. M. Xue, and W. B. Ng, J. Am. Ceram. Soc. 82(2), 477 (1999).

Figure 3. Dielectric properties of PST sintered at 1200°C for 6 hrs as a function of temperature.

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