Effect of the pre-sintering atmosphere on the microstructures and optical qualities of transparent spinel ceramics

The densification process, microstructure evolution, and final optical quality of the transparent MgO center dot nAl(2)O(3) (n = 1.1 and 1.5) ceramics pre-sintered in air and under vacuum were investigated for the effects of pre-sintering atmosphere. The samples pre-sintered in air exhibited uniform microstructure, which was easier to obtain high optical quality at lower HIP temperatures. However, non-cubic precipitates appeared in the samples with a high Al2O3/MgO ratio after HIP treatment, resulting in the deterioration of transmittance. For the samples pre-sintered under vacuum, a large-grain layer was formed on the sample surface due to accelerated MgO volatili-zation. However, non-cubic precipitates did not appear due to the generation of oxygen vacancies. The mech-anisms of formation of the large-grain layer, non-cubic precipitates were discussed from the viewpoint of MgO vaporization, Al2O3/MgO molar ratio, and oxygen vacancy.