The oxide of aluminum is Al2O3. The natural crystalline mineral is called corundum, but the synthetic crystals used for abrasives are designated usually as aluminum oxide or marketed under trade names. For other uses and as a powder it is generally called alumina. It is widely distributed in nature in combination with silica and other minerals, and is an important constituent of the clays for making porcelain, bricks, pottery, and refractories.

The crushed and graded crystals of alumina when pure are nearly colorless, but the fine powder is white. Off colors are due to impurities. American aluminum oxide used for abrasives is at least 99.5% pure, in nearly colorless crystals melting at 2050°C. The chief uses for alumina are for the production of aluminum metal and for abrasives, but it is also used for ceramics, refractories, pigments, catalyst carriers, and in chemicals.

Aluminum oxide crystals are normally hexagonal, and are minute in size. For abrasives, the grain sizes are usually from 100 to 600 mesh. The larger grain sizes are made up of many crystals, unlike the single-crystal large grains of SiC. The specific gravity is about 3.95, and the hardness is up to 2000 Knoop.

There are two kinds of ultrafine alumina abrasive powder. Type A is alpha alumina with hexagonal crystals with particle size of 0.3 |im, density 4.0, and hardness 9 Mohs, and Type B is gamma alumina with cubic crystals with particle size under 0.1 |im, specific gravity of 3.6, and a hardness 8. Type A cuts faster, but Type B gives a finer finish. At high temperatures gamma alumina transforms to the alpha crystal. The aluminum oxide most frequently used for refractories is the beta alumina in hexagonal crystals heat-stabilized with sodium.

Activated alumina is partly dehydrated alumina trihydrate, which has a strong affinity for moisture or gases and is used for dehydrating organic solvents, and hydrated alumina is alumina trihydrate.

Activated alumina F-1 is a porous form of alumina, Al2O3, used for drying gases or liquids and is also used as a catalyst for many chemical processes.

Alumina ceramics are the most widely used oxide-type ceramic, chiefly because Al2O3 is plentiful, relatively low in cost, and equal to or better than most oxides in mechanical properties. Density can be varied over a wide range, as can purity — down to about 90% Al2O3 — to meet specific application requirements.

Al2O3 ceramics are the hardest, strongest, and stiffest of the oxides. They are also outstanding in electrical resistivity and dielectric strength, are resistant to a wide variety of chemicals, and are unaffected by air, water vapor, and sulfu-rous atmospheres. However, with a melting point of only 2037°C, they are relatively low in refractoriness, and at 1371°C retain only about 10% of room-temperature strength. Besides wide use as electrical insulators and chemical and aerospace applications, the high hardness and close dimensional tolerance capability of alumina make this ceramic suitable for such abrasion-resistant parts as textile guides, pump plungers, chute linings, discharge orifices, dies, and bearings.

Alumina Al-200, which is used for high-frequency insulators, gives a molded product with a tensile strength of 172 MPa, compressive strength of 2000 MPa, and specific gravity of 3.36. The coefficient of thermal expansion is half that of steel, and the hardness about that of sapphire. Alumina AD-995 is a dense vacuum-tight ceramic for high-temperature electronic use. It is 99.5% Al2O3 with no SiO2. The hardness is Rockwell N80, and dielectric constant 9.27. The maximum working temperature is 1760°C, and at 1093°C it has a flexural strength of 200 MPa.

Other alumina products have found their way in the casting of hollow jet engine cores. These cores are then incorporated in molds into which eutectic superalloys are poured to form the turbine blades.

Alumina balls are available in sizes from 0.6 to 1.9 cm for reactor and catalytic beds. They are usually 99% alumina, with high resistance to heat and chemicals. Alumina fibers in the form of short linear crystals, called sapphire whiskers, have high strength up to 1375 MPa for use as a filler in plastics to increase heat resistance and dielectric properties. Continuous single-crystal sapphire (alumina filaments) have unusual physical properties: high tensile strength (over 2069 MPa) and modulus of elasticity of 448.2 to 482.7 GPa. The filaments are especially needed for use in metal composites at elevated temperatures and in highly corrosive environments. An unusual method for producing single-crystal fibers in lieu of a crystal growing machine is the floating zone fiber-drawing process. The fibers are produced directly from a molten ceramic without using a crucible.

FP, a polycrystalline alumina (Al2O3) fiber, has been developed. The material has greater than 99% purity, and a melting point of 2045°C, which makes it attractive for use with high-temperature metal-matrix composite (MMC) processing techniques. Thanks to a mechanism, currently not explainable by the developer of FP fibers (Du Pont), a silica coating results in an increase in the tensile strength of the filaments to 1896 MPa even though the coating is approximately 0.25 |im thick and the modulus does not change. Fiber FP has been demonstrated as a reinforcement in magnesium, aluminum, lead, copper, and zinc, with emphasis to date on aluminum and magnesium materials.

Fumed alumina powder of submicrometer size is made by flame reduction of aluminum chloride. It is used in coatings and for plastics reinforcement and in the production of ferrite ceramic magnets.

Aluminum oxide film, or alumina film, used as a supporting material in ionizing tubes, is a strong, transparent sheet made by oxidizing aluminum foil, rubbing off the oxide on one side, and dissolving the foil in an acid solution to leave the oxide film on the other side. It is transparent to electrons. Alumina bubble brick is a lightweight refractory brick for kiln lining, made by passing molten alumina in front of an air jet, producing small hollow bubbles which are then pressed into bricks and shapes.

The foam has a density of 448.5 kg/m3 and porosity of 85%. The thermal conductivity at 1093°C is 0.002 W/(cm2)(°C).

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