Alumina ceramics are classified by the purity of alumina. Alumina ceramics whose purity is more than 99% are called corundum ceramics. Alumina ceramics whose purity is about 99%, 95% and 90% are called 99, 95 and 90 ceramics. Alumina ceramics whose content is more than 85% are generally called high alumina ceramics. The bulk density of 99.5% alumina ceramics is 3.95g/cm3, the flexural strength is 395 MPa, the linear expansion coefficient is 8.1*10-6, the thermal conductivity is 32 W/(m.K), and the insulation strength is 18 KV/mm.
Alumina has many homogeneous and heterogeneous crystals, such as alpha-Al2o3, beta-Al2o3, gamma-Al2o3, etc. Among them, alpha-Al2o3 has high stability, its crystal structure is compact, its physical and chemical properties are stable, and it has the advantages of high density and mechanical strength. It is also widely used in industry.