Aluminium oxide

Aluminium oxide
Aluminium oxide
CAS number 1344-28-1 YesY
PubChem 9989226
ChemSpider 8164808 YesY
UNII LMI26O6933 YesY
RTECS number BD120000
Jmol-3D images Image 1
Image 2
Molecular formula Al2O3
Molar mass 101.96 g mol−1
Appearance white solid
very hygroscopic
Odor odorless
Density 3.95–4.1 g/cm3
Melting point

2072 °C [1]

Boiling point

2977 °C [2]

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
Birefringence 0.008
Crystal structure Trigonal, hR30, SpaceGroup = R-3c, No. 167
Std enthalpy of
−1675.7 kJ·mol−1
Standard molar
50.92 J·mol−1·K−1
MSDS External MSDS
EU classification Not listed.
NFPA 704
NFPA 704.svg
Flash point non-flammable
Related compounds
Other anions aluminium hydroxide
Other cations boron trioxide
gallium oxide
indium oxide
thallium oxide
Supplementary data page
Structure and
n, εr, etc.
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
 YesY oxide (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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.[3]


Natural occurrence

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 in its powdered form.
Aluminium oxide in its powdered form.

Aluminium oxide is an electrical insulator but has a relatively high thermal conductivity (30 Wm−1K−1[4]) 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.[3]

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.[5] 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.[6]


Corundum from Brazil, size about 2x3 cm.

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.[7] Each has a unique crystal structure and properties. The so-called β-alumina proved to be NaAl11O17.[8]


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.[9] 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:[3]

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.[7] 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.[7]

Known as alundum (in fused form) or aloxite[10] 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.[3] 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 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 described below.[11]

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.

Alumina output in 2005

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

Laboratory furnace with insulation manufactured from aluminum oxide

In lighting, transparent alumina is used in some sodium vapor lamps.[12] 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.[3]

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 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.

Using a plasma spray process and mixed with titania, it is coated onto the braking surface of some aluminum bicycle rims to provide abrasion and wear resistance.

See also


  1. ^ P. Patnaik (2002). Handbook of Inorganic Chemicals. McGraw-Hill. ISBN 0-07-049439-8. 
  2. ^ Roew, Raymond (2009). "Adipic Acid". Handbook of Pharmaceutical Excipients. pp. 11–12 
  3. ^ a b c d e "Alumina (Aluminium Oxide) – The Different Types of Commercially Available Grades". The A to Z of Materials. Retrieved 2007-10-27. 
  4. ^ Material Properties Data: Alumina (Aluminum Oxide)
  5. ^ 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. 
  6. ^ "EPCRA Section 313 Chemical List For Reporting Year 2006" (PDF). US EPA. Archived from the original on 2008-07-16. Retrieved 2008-09-30. 
  7. ^ a b c G. Paglia (2004). "Determination of the Structure of γ-Alumina using Empirical and First Principles Calculations Combined with Supporting Experiments" (free download). Curtin University of Technology, Perth. Retrieved 2009-05-05. 
  8. ^ E. Wiberg and A. F. Holleman (2001). Inorganic Chemistry. Elsevier. ISBN 0-12-352651-5. 
  9. ^ "Bauxite and Alumina Statistics and Information". USGS. Retrieved 2009-05-05. 
  10. ^ "Aloxite". database. Retrieved 24 February 2007. 
  11. ^ L. Keith Hudson, Chanakya Misra, Anthony J. Perrotta, Karl Wefers, F. S. Williams “Aluminum Oxide” in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a01_557.
  12. ^ "GE Innovation Timeline 1957–1970". Retrieved 2009-01-12. 

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