Optimum sintering conditions for optical properties of translucent aluminum nitride ceramics

Abstract

High translucency is one of the excellent properties of AlN ceramics because of its wide optical band gap energy of 6.2 eV. We have achieved success in producing AlN ceramic tube of 98% total visible light transmittance at 0.6 mm thick tube wall by applying an improved sintering technique. This AlN ceramic was produced by sintering at 1880°C using Ca3Al2O6 as a sintering additive and in reduction atmosphere to remove the sintering additive from the final sintered material. After the sintering, the annealing with Al2O3 was carried out to improve the transmittance of AlN ceramics. The amount of metal impurities and carbon were 27ppm and 100ppm, respectively. It is noteworthy that the amount of oxygen in AlN ceramics after the annealing was 0.36wt% which was higher than the 0.07wt% of the as-sintered AlN ceramics. Therefore, total transmittance was improved by the oxygen doping. On the other hand, thermal conductivity of AlN after the annealing was lowered by 35% compared with that of as-sintered AlN. Positron Annihilation Lifetime (PAL) measurement was applied to investigation of a relationship between the total transmittance and lattice defects. The present PAL results confirms the presence of negatively charged atomic-size vacancies that is aluminum site vacancies and also suggests the increase in the concentration of the vacancies after the annealing compared with that of the as-sintered material. However, it can be concluded that there is no relation between aluminum site vacancies and the light transmittance of AlN ceramics.