Aluminium oxide

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Aluminium oxide
General
Other names	Alumina, Aluminium(III) Oxide
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	Octahedron.
Crystal structure	Cubic.
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	Not listed.
NFPA 704	NFPA 704 safety square
Flash point	Non-flammable.
Supplementary data page
Structure and properties	n, εr = 9.5, etc. Refractive index at different wavelengths
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
Related compounds	Aluminium hydroxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Aluminium oxide is a chemical compound of aluminium and oxygen with the chemical formula Template:Aluminium₂Template:Oxygen₃. It is also commonly referred to as alumina in the mining, ceramic and materials science communities.

Properties

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.

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

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

Natural occurrence

Corundum is the naturally occurring mineral form of aluminium oxide. Rubies and sapphires are gem-quality forms of corundum with their characteristic colors due to trace impurities in the corundum structure.

Industrial Fabrication Process

Aluminium oxide, also known as alumina, is the main component of bauxite, the principal ore of aluminium. A company producing a wide range of Aluminium Oxide qualities is Almatis. The bauxite ore is made up of impure Al₂O₃, Fe₂O₃, and SiO₂. These are then purified by the Bayer Process:

Al₂O₃ + 3H₂O + 2NaOH + heat → 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:

2Al(OH)₃ + heat → Al₂O₃ + 3H₂O

The formed Al₂O₃ is alumina.

In 1961, GE developed "Lucalox", a transparent alumina used in sodium vapor lamps.

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

Uses

Powdered aluminium oxide is frequently used as a medium for chromatography. In these applications alumina is available in the so-called Brockmann types neutral, basic (pH 9.5) and acidic (pH 4.5 when in water).

External links

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

Suppliers

• alumina grades commercial supplier