aluminumoxide

Aluminum oxide, or aluminum oxide (Al2O3), is a widely occurring compound of aluminum and oxygen. It occurs naturally as corundum, the mineral family that yields the gemstones ruby (red) and sapphire (blue and other colors) when trace impurities are present. The compound also forms several hydrates, but the anhydrous oxide is the form most important in industry and materials science. The alpha polymorph (alpha-alumina) has a hexagonal close-packed structure and is the most thermodynamically stable form at high temperatures; gamma-alumina is a porous form used as a catalyst support; boehmite and gibbsite are hydroxide precursors.

Properties: Al2O3 is extremely hard (Mohs hardness 9), has a high melting point (about 2072°C), and a density of about 3.95 g/cm3. It is a high-temperature ceramic, chemically stable and an electrical insulator. It is amphoteric, dissolving in strong acids or bases under suitable conditions.

Production and applications: The primary source is bauxite, from which alumina is produced by the Bayer process. Alumina is then purified and can be reduced electrolytically to aluminum metal in the Hall-Héroult process. Industrial uses include refractories, abrasive powders, and polishing compounds; ceramic materials such as alumina ceramics; dielectric substrates and catalyst supports; and its use as a transparent ceramic (sapphire) in optics and jewelry.

Safety: Inhalation of fine Al2O3 powders can be hazardous; standard industrial hygiene controls apply.