- Aluminium oxide
Aluminium oxide Identifiers CAS number PubChem ChemSpider UNII RTECS number BD120000 Jmol-3D images Image 1
Properties Molecular formula Al2O3 Molar mass 101.96 g mol−1 Appearance white solid
Odor odorless Density 3.95–4.1 g/cm3 Melting point
2072 °C 
2977 °C 
Solubility in water insoluble Solubility insoluble in diethyl ether
practically insoluble in ethanol
Refractive index (nD) nω=1.768 – 1.772
nε=1.760 – 1.763
Structure Crystal structure Trigonal, hR30, SpaceGroup = R-3c, No. 167 Coordination
octahedral Thermochemistry Std enthalpy of
o298 −1675.7 kJ·mol−1 Standard molar
o298 50.92 J·mol−1·K−1 Hazards MSDS External MSDS EU classification Not listed. NFPA 704 Flash point non-flammable Related compounds Other anions aluminium hydroxide Other cations boron trioxide
Supplementary data page Structure and
n, εr, etc. Thermodynamic
Solid, liquid, gas
Spectral data UV, IR, NMR, MS (what is: /?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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 owing to its hardness and as a refractory material owing to its high melting point.
- 1 Natural occurrence
- 2 Properties
- 3 Structure
- 4 Production
- 5 Applications
- 6 See also
- 7 References
- 8 External links
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 aluminium 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.
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.
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 tonnes, 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 and white, alumina is a favored filler for plastics. Alumina is a common ingredient in sunscreen and is sometimes present in cosmetics such as blush, lipstick, and nail polish.
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 is widely used to remove water from gas streams. Other major applications are described 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" above). Both its strength and abrasive characteristics originate from the high hardness (9 on the Mohs scale of mineral hardness) of aluminium oxide. 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.
As an effect pigment
Aluminium oxide flakes are base material for effect pigments. These pigments are widely used for decorative applications e.g. in the automotive or cosmetic industry. See main article Alumina effect pigment.
Niche applications and research themes
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 aluminium 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.
Small pieces of alumina are often used as boiling chips in chemistry.
It is also used to make spark plug insulators.
- ^ P. Patnaik (2002). Handbook of Inorganic Chemicals. McGraw-Hill. ISBN 0-07-049439-8.
- ^ Roew, Raymond (2009). "Adipic Acid". Handbook of Pharmaceutical Excipients. pp. 11–12
- ^ a b c d e "Alumina (Aluminium Oxide) – The Different Types of Commercially Available Grades". The A to Z of Materials. http://www.azom.com/details.asp?ArticleID=1389. Retrieved 2007-10-27.
- ^ Material Properties Data: Alumina (Aluminum Oxide)
- ^ Campbell, Timothy; Kalia, Rajiv; Nakano, Aiichiro; Vashishta, Priya; Ogata, Shuji; Rodgers, Stephen (1999). "Dynamics of Oxidation of Aluminium Nanoclusters using Variable Charge Molecular-Dynamics Simulations on Parallel Computers". Physical Review Letters 82 (24): 4866. Bibcode 1999PhRvL..82.4866C. doi:10.1103/PhysRevLett.82.4866. http://cacs.usc.edu/papers/Campbell-nAloxid-PRL99.pdf.
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