The grain growth of alumina at 1600°-1960°C in various atmospheres, such as vacuum, wet or dry hydrogen, carbon monoxide, argon and air was investigated.
The accelerating effect of hydrogen was observed in the surface layer of the specimens sintered at the temperatures higher than 1700°C. Dry hydrogen proved to be the most effective, while that loaded with moisture gave a little lower values.
The grain growth of alumina sintered in CO at higher temperatures than 1900°C was investigated with a result showing that the accelerating effect was next to hydrogen.
Little difference could be observed in the grain growth of the specimens sintered in vacuum and in air.
Argon accerelated the growth a little more than vacuum, although the effect was smaller than that of CO.
The authors concluded that the mechanism could be represented by
O2-(l)_??__??_o2-+1/2O2(g)+2_??_
This equation shows that the rate of diffusion of O2- may be accelerated with the increasing number of O2- vacancies in the crystal which is liable to be affected by the atmospheric conditions.