alphaalumina

Alpha-alumina, or alpha-Al2O3, is the most thermodynamically stable phase of aluminum oxide. In mineral form it is known as corundum; when trace impurities confer color, it appears as gemstone varieties such as ruby (chromium-doped) and sapphire (various dopants).

Structure and properties: Alpha-alumina crystallizes in a hexagonal close-packed lattice of oxide ions with aluminum ions occupying two-thirds of the octahedral interstices, yielding the corundum structure. It is extremely hard (Mohs hardness about 9), has a high melting point near 2072°C, and exhibits excellent thermal stability and chemical inertness. Pure alpha-alumina is transparent and an electrical insulator, and it offers good wear resistance.

Production and forms: Naturally occurring as the mineral corundum, alpha-alumina is widely synthesized for industrial use. Commercial production begins from purified alumina hydrate derived from alumina refining and proceeds via high-temperature processes such as the Verneuil (mercury) method, Czochralski growth, or flame fusion to yield single crystals or dense polycrystalline ceramics. Alpha-alumina is also used as a high-grade abrasive (synthetic corundum) and as a refractory material. Transparent single crystals are used as sapphire windows and as substrates for optical and electronic devices, including LEDs and high-power electronics.

Applications: Its main uses include abrasive grains for grinding and polishing, refractory ceramics and crucibles, cutting tools, coatings, and optoelectronic applications where optical transparency and chemical inertness are required.

Phase behavior: At high temperatures, other alumina polymorphs (gamma, delta, theta) transform to alpha-alumina. Transformation depends on particle size, impurities, and heating conditions.