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A. D. Hilton and D. J. Barber Department of Physics, University of Essex, Wivenhoe Park, Colchester, Essex, United Kingdom

T. R. Shrout Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 16 July 1990; accepted 24 November 1992)

This paper addresses the observed grain size with dependence of the dielectric behavior for Pb(Mgi/3Nb2/3)O3 :PbTiO 3 ceramics grain sizes 2=1.0 /um. A combined transmission electron microscopy (TEM) analysis and dielectric characterization are modeled with a modified brick wall approach. From this model, it is possible to extrapolate information such as single crystal values of dielectric maximum, Km?[X, the diffuseness coefficient, S, and the average intergranular thickness for relaxor ceramics. The calculated intergranular thickness agrees well with TEM observations, —2.0 nm. This semi-empirical method may be potentially useful in development work of relaxor ceramics to predict the optimized dielectric properties obtainable within microstructural restrictions.

I. INTRODUCTION Complex lead-based perovskites, with the general formula Pb(B!B2)O3, exhibiting diffuse frequency dependent dielectric permittivities are commonly referred to as relaxor ferroelectrics. A typical example of a relaxor from this complex lead perovskite family is the Pb(Mg1/3Nb2/3)O3 (PMN) compound. Solid solutions of relaxor PMN and the first order ferroelectric, PbTiO3 (PT), exhibit many attractive properties for dielectric and electrostrictive applications.1 The high dielectric constants, over broad temperature ranges, close to room temperature, make systems like (1 — x)PMN: xPT (x = 0.07) very attractive for multilayer capacitor (MLC) and actuator applications. In several previous investigations for the dielectric properties, including Kmax, diffuseness (the effective width of the maxima) and aging, have been shown to be dependent on processing variations, in particular the role of grain size on these properties. These earlier investigations did not lead to simple interpretation, partly because of the presence of second phase pyrochlore.1 The present paper successfully interprets the structure-property relations in (1 — x)PMN: xPT(x = 0.07) ceramics, using good processing control, dielectric characterization, and transmission electron microscopy techniques.

processing the columbite precursor, the poor dispersion characteristics associated with magnesium carbonate powders were addressed using both steric hindrance (polyelectrolyte dispersant) and electrostatic repulsion (pH adjustment by ammonia) in conjunction with high energy milling. Upon calcination the appropriate amount of PbO was added followed by mixing in a dispersant and adjusting the pH to obtain minimum lead dissolution. Calcination at 700 °C for 4 h produces the desired single phase perovskite powder. Uniformity and reactivity of PMN:PT powder were further enhanced by milling. Grain size variations in the PMN:PT ceramics were achieved by firing samples at different temperatures and times, a