Theoretical aspects of solid state reactions in a mixed particulate ensemble and kinetics of lead zirconate formation
Description: A theoretical analysis was carried out to estimate the quasi-steady-state external mass transport by lattice, surface and gaseous diffusion in terms of the stereologically measurable microstructural parameters of a mixed powder compact. It was shown that the gaseous transport can be described by a single dimensionless quantity termed sublimation transport modulus. Using these equations, the relative importance of the alternate external transport modes can be evaluated. Experimental work determined the reaction isotherms for the formation of lead zirconate from yellow PbO and monoclinic zirconia between 710/sup 0/C to 810/sup 0/C for two zirconia size distributions. The larger zirconia showed diffusion controlled shrinking core kinetics up to about 90 percent reaction while the smaller zirconia indicated a diffusion controlled spherical growth of up to 85 percent reaction after an instantaneous nucleation at a limited number of sites. The difference was attributed to the differences in the mixing time and not to the particulate sizes. It was observed that for the same size range, a longer mixing operation gave a better dispersion of reactants which resulted in a higher nucleation site density required for a shrinking core type of product morphology and faster kinetics. A microprobe profile analysis of partially reacted particles confirmed that for the shorter mixing period, reaction resultedin a nucleation-growth-impingement type of morphology. The activation energy as calculated from the nucleation-growth model fit to the data was 138 Kcal/mole which is close to 131 Kcal/mole reported for Pb/sup 2 +/ diffusion in PbZrO/sub 3/. Approximate calculations show that the gaseous external mass flux of PbO was negligible compared to the lattice or surface diffusive flux.
Date: September 1, 1979
Creator: Chandratreya, S.S.
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