Alumina ceramic

Claim 1

A process for producing an optically translucent polycrystalline alumina sintered body which consists essentially of providing at least a substantially homogeneous dispersion consisting essentially of alumina, MgO or precursor therefor, and an additive selected from the group consisting of ZrO 2 , HfO 2 , and a mixture thereof or precursors therefor, said alumina ranging in composition from α-Al 2 O 3 to at least about 80% by weight α-Al 2 O 3 with the remaining alumina being of polymorphic form other than α, said MgO being present in an amount ranging from about 0.03% by weight to less than about 0.15% by weight of said alumina, said ZrO 2 being present in an amount ranging from higher than 0.002% by weight up to about 0.07% by weight of said alumina, said HfO 2 being present in an amount ranging from higher than 0.003% by weight up to about 0.12% by weight of said alumina, said mixture of ZrO 2 and HfO 2 being composed of all ratios of ZrO 2 and HfO.sub. 2 and being present in a total amount ranging from higher than 0.002% by weight up to about 0.12% by weight of said alumina, said dispersion having an average crystalline size ranging from about 0.05 micron to less than one micron, forming said dispersion into a green body having a density of at least about 30% of the theoretical density of 3.98 g/cc for alumina, and sintering said body in an atmospheric of hydrogen having a dew point higher than +30° C. at a sintering temperature ranging from about 1750° C. to about 1950° C. producing a sintered body of theoretical density based on the density of 3.98 g/cc for alumina, said precursor decomposing completely below said sintering temperature to form said oxide and by-product gas.