The raw material of corundum is abundant, accounting for about 25% of the earth’s crust. It is cheap and has many excellent properties. There are many allomorphs of Al2O3, and there are more than ten kinds of variants reported, but there are three main ones, namely α-Al2O3, β-Al2O3, and γ-Al2O3.
Tabular alumina
γ-Al2O3 belongs to the spinel structure and is unstable at high temperature, so it is rarely used alone as a material. β-Al2O3 is essentially an aluminate containing alkali metal or alkaline earth metal. The chemical composition can be approximated by RO·6Al2O3 and R2O·11Al2O3, hexagonal lattice, density 3.30~3.63g/cm3, 1400~1500 °C begins to decompose, and turns into α-Al2O3 at 1600 °C. α-Al2O3 is a high-temperature form with a stable temperature as high as the melting point. The density is 3.96~4.01g/cm3, which is related to the impurity content. The unit cell is a sharp facet, which exists in nature in the form of natural corundum, ruby and sapphire. α-Al2O3 has compact structure, low activity, good electrical properties and excellent mechanical properties. Mohs hardness is 9. α-Al2O3 belongs to hexagonal system, corundum structure, a=4.76, c=12.99.
The mechanical strength of Al2O3 is high, the purer the composition of Al2O3, the higher the strength, and the mechanical strength can be used to make device porcelain and other mechanical components. Al2O3 has high resistivity and good electrical insulation performance. The resistivity at room temperature is 1015Ω·cm and the dielectric strength is 15kV/mm. Utilizing its insulation and strength, it can be made into substrates, tube sockets, spark plugs, circuit casings, etc. Al2O3 has high hardness, Mohs hardness of 9, and excellent wear resistance, so it is widely used in the manufacture of knives, grinding wheels, abrasives, wire drawing dies, bearings, bearing bushes and artificial gemstones. Al2O3 has a high melting point and corrosion resistance. The melting point is 2050°C. It can better resist the erosion of molten metals such as Be, Sr, Ni, Al, V, Ti, Mn, Fe, and CO, and the erosion of sodium hydroxide, glass, and slag. It also has high resistance; it does not interact with Si, P, Sb, and Bi in an inert atmosphere, so it can be used as refractory materials, furnace tubes, glass drawing crucibles, hollow spheres, fibers, and thermocouple protective sleeves.
Al2O3 has excellent chemical stability, and many compound sulfides, phosphides, arsenides, chlorides, nitrides, bromides, iodides, dry fluorides, sulfuric acid, hydrochloric acid, nitric acid, and hydrofluoric acid do not interact with Al2O3. Therefore, it can be made into crucibles for pure metal and single crystal growth, human joints, artificial bones, etc. Al2O3 has unique optical properties and can be made into light-transmitting materials to make Na vapor lamps, microwave fairings, infrared windows and laser oscillation components, etc. The ionic conductivity of Al2O3 is used as solar cell material and storage battery material. Al2O3 is also commonly used in ceramic surface metallization technology.
The main crystal phase of bauxite-based fused corundum is a corundum phase with a size of 1.0-1.5mm and interlaced crystals, and the rest are trace amounts of rutile, alumina and aluminum titanate, and are located inside the corundum phase or between the crystal phases. a small amount of glassy phase. In China, after more than ten years of unremitting efforts, the smelting process of bauxite-based fused corundum has made great progress, with an annual production capacity of more than 110,000 tons. Alumina-based fused corundum has been successfully used as a raw material for various fired bricks and monolithic refractory materials, such as partially replacing dense corundum in castables for blast furnace iron troughs, as matrix materials and pellets for the manufacture of low-creep High alumina bricks are used to replace white corundum in other Al2O3-SiO2 refractory materials to prepare high-performance products.