Vaporization of Lead Zirconate‐Lead Titanate Materials: II, Hot‐Pressed Compositions at Near Theoretical Density*

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Abstract
The vaporization of lead monoxide from lead zirconate‐lead titanate materials was studied by thermogravimetric techniques in vacuum with hot‐pressed pellets of near theoretical density. The initial step of the complex, two‐step vaporization observed in cold‐pressed pellets in an earlier investigation is absent. The rate‐determining step for both the porous, cold‐pressed, and pore‐free, hot‐pressed materials is the bulk diffusion of lead across a thickening lead‐depleted layer at the surface of the pellet and results in a parabolic time dependence. Although parabolic rate constants are slightly lower overall for the dense materials, the activation energies for the weight loss process remain approximately 38.5 and 35 kcal/mole for the pure and Bi‐doped Pb(Zr0.65Ti0.35)O3 compositions, respectively. Vaporization mechanisms and rates were unchanged under nitrogen or oxygen pressures up to 50 torr at 1000°C. Alumina Knudsen cells were used to study the equilibrium vapor above a powdered, hot‐pressed Pb(Zr0.65.Ti0.35)O3 composition. Mass spectrometric analysis of the vapor at 950°C showed that the vapor species and their relative abundances were the same as the equilibrium vapor species above pure lead monoxide. Continuous weight loss data from these cells gave an apparent vapor pressure that was 2% of the vapor pressure of pure lead monoxide at a given temperature, but gave the same second law enthalpy of vaporization. The overall vaporization process can be represented by the reversible equilibria: the diffusion of Pb2+ and O2‐ to the surface through a lead‐depleted region whose phases and compositions are determined by the subsolidus phase relations in the PbO‐ZrO2‐TiO2 ternary system, the reaction at or near the surface to form PbO, and the subsequent vaporization of PbO.

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