Aluminium oxide is an amphoteric oxide with the chemical formula Al2O3. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry. Its most significant use is in the production of aluminium metal, although it is also used as an abrasive due to its hardness and as a refractory material due to its high melting point.
Corundum is the most common naturally occurring crystalline form of aluminium oxide. Rubies and sapphires are gem-quality forms of corundum, which owe their characteristic colors to trace impurities. Rubies are given their characteristic deep red color and their laser qualities by traces of chromium. Sapphires come in different colors given by various other impurities, such as iron and titanium.
Aluminium oxide is an electrical insulator but has a relatively high thermal conductivity (30 Wm−1K−1) for a ceramic material. In its most commonly occurring crystalline form, called corundum or α-aluminium oxide, its hardness makes it suitable for use as an abrasive and as a component in cutting tools.
Aluminium oxide is responsible for resistance of metallic aluminium to weathering. Metallic aluminum is very reactive with atmospheric oxygen, and a thin passivation layer of alumina (4 nm thickness) forms in about 100 picoseconds on any exposed aluminium surface. This layer protects the metal from further oxidation. The thickness and properties of this oxide layer can be enhanced using a process called anodising. A number of alloys, such as aluminium bronzes, exploit this property by including a proportion of aluminium in the alloy to enhance corrosion resistance. The alumina generated by anodising is typically amorphous, but discharge assisted oxidation processes such as plasma electrolytic oxidation result in a significant proportion of crystalline alumina in the coating, enhancing its hardness.
Aluminium oxide was taken off the United States Environmental Protection Agency's chemicals lists in 1988. Aluminium oxide is on EPA's TRI list if it is a fibrous form.
The most common form of crystalline alumina is known as corundum. The oxygen ions nearly form a hexagonal close-packed structure with aluminium ions filling two-thirds of the octahedral interstices. Each Al3+ center is octahedral. In terms of its crystallography, corundum adopts a trigonal Bravais lattice with a space group of R-3c (number 167 in the International Tables). The primitive cell contains two formula units of aluminium oxide.
Alumina also exists in other phases, namely γ-, δ-, η-, θ-, and χ-aluminas. Each has a unique crystal structure and properties. The so-called β-alumina proved to be NaAl11O17.
Aluminium hydroxide minerals are the main component of bauxite, the principal ore of aluminium. A mixture of the minerals comprise bauxite ore, including gibbsite (Al(OH)3), boehmite (γ-AlO(OH)), and diaspore (α-AlO(OH)), along with impurities of iron oxides and hydroxides, quartz and clay minerals. Bauxites are found in laterites. Bauxite is purified by the Bayer process:
Al2O3 + 3 H2O → 2 Al(OH)3
Except for SiO2, the other components of bauxite do not dissolve in base. Upon filtering the basic mixture, Fe2O3 is removed. When the Bayer liquor is cooled, Al(OH)3 precipitates, leaving the silicates in solution. The solid is then calcined (heated strongly) to give aluminium oxide:
2 Al(OH)3 → Al2O3 + 3 H2O
The product alumina tends to be multi-phase, i.e., consisting of several phases of alumina rather than solely corundum. The production process can therefore be optimized to produce a tailored product. The type of phases present affects, for example, the solubility and pore structure of the alumina product which, in turn, affects the cost of aluminium production and pollution control.
Known as alundum (in fused form) or aloxite in the mining, ceramic, and materials science communities, alumina finds wide use. Annual world production of alumina is approximately 45 million metric tons, over 90% of which is used in the manufacture of aluminium metal. The major uses of specialty aluminium oxides are in refractories, ceramics, and polishing and abrasive applications. Large tonnages are also used in the manufacture of zeolites, coating titania pigments, and as a fire retardant/smoke suppressant.
The great majority of alumina is consumed for the production of aluminium, usually by the Hall process.
As a filler
Being fairly chemically inert, relatively non-toxic, and white, alumina is a favored filler for plastics. Alumina is a common ingredient in sunscreen.
As a catalyst and catalyst support
Alumina catalyses a variety of reactions that are useful industrially. In its largest scale application, alumina is the catalyst in the Claus process for converting hydrogen sulfide waste gases into elemental sulfur in refineries. It is also useful for dehydration of alcohols to alkenes.
Alumina serves as a catalyst support for many industrial catalysts, such as those used in hydrodesulfurization and some Ziegler-Natta polymerizations. Zeolites are produced from alumina.
Gas purification and related absorption applications
Alumina is widely used to remove water from gas streams. Other major applications are below.
As an abrasive
Aluminium oxide is used for its hardness and strength. It is widely used as a coarse or fine abrasive, including as a much less expensive substitute for industrial diamond. Many types of sandpaper use aluminium oxide crystals. In addition, its low heat retention and low specific heat make it widely used in grinding operations, particularly cutoff tools. As the powdery abrasive mineral aloxite, it is a major component, along with silica, of the cue tip "chalk" used in billiards. Aluminium oxide powder is used in some CD/DVD polishing and scratch-repair kits. Its polishing qualities are also behind its use in toothpaste. Alumina can be grown as a coating on aluminium by anodising or by plasma electrolytic oxidation (see the "Properties" section, above). Both its strength and abrasive characteristics are due to aluminium oxide's great hardness (position 9 on the Mohs scale of mineral hardness). Most pre-finished wood flooring now uses aluminium oxide as a hard protective coating.
In dentistry, it is used as a polishing agent to remove stains. It is an alternative to sodium bicarbonate, for patients that have high blood pressure.
Niche applications and research themes
In lighting, transparent alumina is used in some sodium vapor lamps. Aluminium oxide is also used in preparation of coating suspensions in compact fluorescent lamps.
In chemistry laboratories, alumina is a medium for chromatography, available in basic (pH 9.5), acidic (pH 4.5 when in water) and neutral formulations.
Health and medical applications include it as a material in hip replacements.
As well, it is used as a dosimeter for radiation protection and therapy applications for its optically stimulated luminescence properties.
Aluminium oxide is widely used in the fabrication of superconducting devices, particularly single electron transistors and superconducting quantum interference devices (SQUID), where it is used to form highly resistive quantum tunneling barriers.
Insulation for high temperature furnaces is often manufactured from aluminum oxide. Sometimes the insulation has varying percentages of silica depending on the temperature rating of the material. The insulation can be made in blanket, board, brick and loose fiber forms for various application requirements.