Aluminium oxide

Aluminium oxide (also aluminium(III) oxide), commonly called alumina, is an inorganic compound of aluminium and oxygen with the chemical formula Al2O3. At ambient conditions, its thermodynamically stable crystalline form is α-Al2O3, which has the corundum structure. Gem-quality corundum forms rubies and sapphires. Aluminium oxide also occurs in several metastable crystalline forms, commonly called transition aluminas. Corundum is hard, alumina ceramics are electrical insulators, and aluminium oxide is amphoteric, reacting with both acids and bases. Industrial alumina is extracted from bauxite by the Bayer process, and most alumina is used as feedstock for aluminium production in the Hall–Héroult process. Other important uses include refractory and engineering ceramics, abrasives and polishing materials, catalysts and catalyst supports, adsorbents, electronic and optical materials, protective coatings and effect pigments, and components of glasses and polymer composites. Different applications use different forms of aluminium oxide, ranging from dense α-alumina ceramics and single-crystal sapphire to high-surface-area transition aluminas and thin aluminium-oxide films.