(1) Powder preparation
The current methods for producing high-purity alumina mainly include multiple crystallization methods (including ammonium aluminum sulfate pyrolysis method and ammonium aluminum carbonate pyrolysis method), alkoxide hydrolysis method, direct hydrolysis method (choline method) and modified Bayer method. Only a few domestic companies have improved Bayer process, aluminum alkoxide hydrolysis process and hydrothermal synthesis process. Japanese companies master the most advanced production technology of high-purity alumina, and can produce high-purity alumina and high-purity ultra-fine alumina with a purity of more than 99.995%.
(2) Sintering process
High-purity alumina ceramics usually need to be sintered and compacted at a temperature higher than 1600°C. The sintering temperature of high-purity alumina ceramics can usually be reduced by adding appropriate additives. At present, the sintering temperature of 99.99% alumina powder in Japanese companies only needs to be 1300°C. And domestic needs to be above 1600°C. The preparation of alumina ceramic balls for bearings mostly adopts hot isostatic sintering technology.
(3) Finishing
Alumina ceramic materials have high hardness, so harder materials such as SiC, C or diamond is usually used for grinding and polishing. Generally, Al2O3 powder or diamond smaller than 1μm can be used for grinding and polishing, as well as grinding and polishing methods such as laser processing and ultrasonic processing.
