At present, alumina ceramics are divided into two types: high purity type and common type. High purity alumina ceramics containing more than 99.8% Al_2O_3 are usually made into molten glass instead of platinum crucible because of their sintering temperature as high as 1650-1990 C and transmission wavelength as 1-6 um. They are used as sodium lamp tubes by their light transmittance and alkali-resistant corrosion resistance, and as IC substrates in the electronic industry. High frequency insulation material.