Vaporization of zinc oxide from defective spinel powders in vacuum.

The mechanism for the vaporization of ZnO from powders of (Zn0.2Co0.8)O⋅1.1Al2O3 and (Zn0.2Ni0.8)O⋅1.1Al2O3 spinels was studied at 1335° to 1500°C in vacuum of 3×10-5 to 10-4 Torr. The vaporization of ZnO occurred in two stages-vaporization of constant rate at the first stage and of decreasing rate at the second stage. The observed fractions of ZnO vaporized deviated from the calculated values at higher temperatures. The results were interpreted with the assumption that the exsoluted corundum layer at the powder surface sintered more densely at higher temperatures causing the decrease in vaporization surface area. The ratio of the concentration of ZnO at the surface to the initial concentration at the time the vaporization mechanism changed from the first to the second stage was approximately 0.9 and independent of temperature.