Aluminium oxide

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Aluminium oxide
Image:Aluminium oxide.jpg
General
Other names	Alumina
Molecular formula	Al2O3
Molar mass	101.96 g/mol
CAS number	[1344-28-1]
Properties
Density and phase	3.97 g/cm3, solid
Solubility in water	insoluble
Melting point	2054 °C
Boiling point	~3000 °C
Thermal Conductivity	18 W/m·K
Structure
Coordination geometry	octahedral
Crystal structure	?
Thermodynamic data
Standard enthalpy of formation ΔfHosolid	-1675.7 kJ/mol
Standard molar entropy Sosolid	50.92 J/(mol K)
Heat capacity Cp	79.04 J/(mol K)
Hazards
MSDS	External MSDS
EU classification
NFPA 704	Image:Nfpa h0.png Image:Nfpa f0.png Image:Nfpa r0.png
R-phrases
S-phrases
Flash point	non-flammable
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Aluminium hydroxide
Other cations	Boron trioxide Gallium oxide Indium oxide Thallium oxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Aluminium oxide or aluminum oxide is a chemical compound of aluminium and oxygen with the chemical formula Al₂O₃. It is also commonly referred to as alumina in the mining, ceramic, and materials science communities.

Aluminium oxide is the main component of bauxite, the principal ore of aluminium. Industrially, bauxite is purified to aluminium oxide via the Bayer process, and then converted to aluminium metal in the Hall-Heroult process.

The bauxite ore is made up of impure Al₂O₃ + Fe₂O₃ + SiO₂. This is then purified by the Bayer Process: Al₂O₃ + 3H₂O + 2NaOH --(heated)--> 2NaAl(OH)₄. The Fe₂O₃ does not dissolve in the base. The SiO₂ dissolves as silicate Si(OH)₆^-6. Upon filtering, Fe₂O₃ is removed. With the addition of an acid, Al(OH)₃ precipitates. The silicate remains in solution. Then, Al(OH)₃ --(heated)--> Al₂O₃ + 3H₂O. The Al₂O₃ is of course, alumina.

The gems ruby and sapphire are mostly aluminium oxide, given their characteristic colors by trace impurities.

Aluminium oxide is responsible for metallic aluminium's resistance to weathering. Metallic aluminium is very reactive with atmospheric oxygen, and a thin layer of aluminium oxide quickly forms 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 aluminum bronzes, exploit this property by including a proportion of aluminum in the alloy to enhance corrosion resistance.

Aluminium oxide is an excellent thermal and electrical insulator. In its crystalline form, called corundum, its hardness makes it suitable for use as an abrasive and as a component in cutting tools.

Powdered aluminium oxide is frequently used as a medium for chromatography.

In August 2004, scientists in the United States working for 3M developed a technique for making an alloy of alumina and rare earth elements to produce a strong glass called transparent alumina.

Aluminium oxide was taken off the EPA's chemicals lists in 1988.

External links

• International Chemical Safety Card 0351
• PhysicsWeb article on Transparent alumina