Aluminum oxide, corundum, ruby, and sapphire have different names, their morphology, hardness, nature, and applications are not the same, and their values are far apart, however, their chemical composition is exactly the same, all are aluminum oxide.
The pure aluminum oxide is white amorphous powder, commonly known as bauxite, density 3.9-4.0g/cm3, melting point 2050°C, boiling point 2980°C, insoluble in water. alumina mainly has two variants: α-type and γ-type.
The naturally occurring α-alumina crystals are called corundum and often present different colors because they contain different impurities. Corundum is generally gray with blue or yellow, having a glass or diamond luster, the density of 3.9-4.1g/cm3, hardness 8.8, second only to diamond and silicon carbide, high-temperature resistance. The corundum containing oxides of Ferrum is called corundum normally dark gray or dark black. It is often used as an abrasive material for making various abrasive papers, grinding wheels, and grinding stones. It is also used for processing optical instruments and certain metal products.
Because the production of natural corundum is in short supply, the industry usually sinters pure α-alumina powder in a high-temperature electric furnace to produce artificial corundum, also called electrofused corundum. It can withstand temperatures above 1800°C. It is a raw material for the manufacture of high-grade special refractory materials and great solidness at high temperatures, good thermal shock resistance, strong erosion resistance, low coefficient of thermal expansion, etc., used in rocket engine combustion chamber linings, nozzles, radar antenna shields, atomic reactor materials, advanced high-frequency insulating ceramics, smelting pure metal And alloy crucibles, high temperature heating elements, thermocouple protection tubes, furnace lining of various high temperature furnaces, etc.