Particle and grain size effects on the dielectric behavior of the relaxor ferroelectric Pb(Mg 1/3 Nb 2/3 )O 3
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The role of particle and grain size on the dielectric behavior of the perovskite relaxor ferroelectric Pb(Mgi/3Nb2/3)O3 [PMN] was investigated. Ultrafine powders of PMN were prepared using a reactive calcination process. Reactive calcination, the process by which morphological changes take place upon reaction of the component powders, produced particle agglomerates less than 0.5 jam. Through milling, these structures were readily broken down to —70 nanometer-sized particulates. The highly reactive powders allowed densification as low as 900 °C, but with corresponding grain growth in the micron range. Such grain growth was associated with liquid phase sintering as a result of PbO-Nb2O5 second phase(s) pyrochlore. Sintering, assisted by hot uniaxial pressing, below the temperature of liquid formation of 835 °C, allowed the fabrication of highly dense materials with a grain size less than 0.3 fj.m. The dielectric and related properties were determined for samples having grain sizes in the range of 0.3 jum to 6 ju,m. Characteristic of relaxors, frequency dependence {K and loss) and point of Tmix were found to be related to grain and/or particle size and secondarily to the processing conditions. Modeling of particle size/dielectric behavior was performed using various dielectric properties of 0-3 composites comprised of varying size powder in a polymer matrix. An intrinsic-microdomain perturbation concept was proposed to interpret observed scaling effects of the relaxor dielectric behavior in contrast to normally accepted extrinsic grain boundary models. I. INTRODUCTION
Lead magnesium niobate, Pb(Mgi/3Nb2/3)O3, belongs to a family of complex Pb(B'B")O3 perovskites which are known as relaxor ferroelectrics. First discovered by Smolenskii,1'2 these materials exhibit broad and anomalously large dielectric maximas which make them ideal candidates for multilayer capacitors (MLCs) and electrostriction actuators, electro-optics.3'4 The following materials are distinguished from "normal" ferroelectrics by the presence of a broad diffuse and dispersive phase transition. The dielectric constant (K) peaks at Tm, but because of the dispersion in the Curie temperature can be defined only in reference to the frequency at which the measurements are made. The spontaneous polarization, Ps, is not lost at Tm but gradually decays to zero with increasing temperature above Tm. Relaxor materials show no evidence of optical anisotropy or x-ray line splitting (pseudo-cubic structural changes), even well below Tm. The most widely accepted models for the understanding of relaxor ferroelectric phenomena come from Smolenskii5 and Cross.6 The generally accepted inhomogeneous microregion model postulated by Smolenskii et al. bases the origin of the diffuse phase transition on local compositional fluctuations associated with B-site cation disorder, resulting in a distribution of Curie temperatures. Cross further enhanced the understanding of 2902
J. Mater. Res., Vol. 5, No. 12, Dec 1990
relaxor dielectric behavior, suggesting that localized polar
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