Effect of Barium Titanate Seed Particles on the Sintering and Lattice Parameters in PbMg 1/3 Nb 2/3 O 3 Ceramics
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A.V.C. Andrade Universidade Estadual de Ponta Grossa. Physics Department, Ponta Grossa (PR), Brazil, 84.030-000
A.A. Cavalheiro Chemistry Institute, Physics-Chemistry Department, UNESP PO Box 355, Araraquara (SP), Brazil, 14801-970
S.M. Tebcherani Universidade Estadual de Ponta Grossa. Chemistry Department, Ponta Grossa (PR), Brazil, 84.030-000
J.A. Varela Chemistry Institute, Physics-Chemistry Department, UNESP PO Box 355, Araraquara (SP), Brazil, 14801-970 (Received 4 May 2001; accepted 28 December 2001)
PbMg1/3Nb2/3O3 (PMN) powder was prepared by citrate organic solution, and barium titanate (BT) seed particles were added to encourage the perovskite phase formation. Sintering was followed using the constant heating rate mode of a dilatometer, and it was observed that the seed concentration affected the PMN shrinkage rate and crystal structure. The study of the lattice parameters of the samples after the sintering process indicates that the diffusion of the titanium and of the barium inside perovskite and pyrochlore PMN phases occurs. Moreover, this substitution provoked a decrease of the lattice parameters as showed by the Rietveld refinement.
I. INTRODUCTION
Lead-based ferroelectrics are increasingly being used as capacitors, tunable transducers, actuators, and memory devices.1 These materials are also used as micropositioners or actuators in dot-matrix printers and videotape heads because of their large electrostrictive strains. Most of these materials are lead magnesium niobate (PbMg1/3Nb2/3O3; PMN) dielectrics that have a high dielectric constant (k > 15,000) and low sintering temperature.1 Depending on the processing conditions, a second phase that has the pyrochlore structure may be present, which reduces the dielectric constant of the material.2 It is difficult to prepare PMN phase-pure perovskite (pyrochlore free) ferroelectrics via the conventional solid-state method. Methods such as coprecipitation, sol-gel,3,4 microencapsulation, molten salt, organic solution of citrates,5 and the two-step columbite method6,7 have been a)
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J. Mater. Res., Vol. 17, No. 3, Mar 2002 Downloaded: 16 Mar 2015
used for processing relaxors with reduced amounts of pyrochlore phase. The main problems in the processing of lead-based ferroelectrics are (i) formation of a lowdielectric phase (such as pyrochlore), (ii) PbO volatilization, and (iii) irreproducible dielectric property.1 It is known that ceramic materials are transformed via a nucleation and growth process, often requiring high temperatures to surmount the large energy barriers of the nucleation and growth the stable phase. Consequently, these energy barriers frequently determine the calcination conditions, and therefore the characteristics such as particle size, morphology, and degree of aggregation of the precursor powder. Therefore, controlled nucleation and growth processes can play a significant role in processing control of advanced ceramics.8
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