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Crystal structure
Structure of α-alumina
Corundum structure
__ Al 3+      __ O 2−
Surname Alumina
other names
  • Corundum (mineral)
  • Clay
  • Aluminum (III) oxide
  • Aluminum sesquioxide
Ratio formula Al 2 O 3
Brief description

white, odorless solid

External identifiers / databases
CAS number 1344-28-1
EC number 215-691-6
ECHA InfoCard 100.014.265
PubChem 9989226
ChemSpider 8164808
DrugBank DB11342
Wikidata Q177342
Molar mass 101.96 g · mol -1
Physical state



3.94 g cm −3

Melting point

2050 ° C

boiling point

2980 ± 60 ° C


almost insoluble in water, hardly soluble in acids and bases

Refractive index

1.77 (sapphire @ 500 nm)

safety instructions
GHS labeling of hazardous substances
no GHS pictograms
H and P phrases H: no H-phrases
P: no P-phrases
Toxicological data

> 5000 mg kg −1 ( LD 50ratoral )

Thermodynamic properties
ΔH f 0

−1676 kJ mol −1

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C

Aluminum oxide on a watch glass bowl

Aluminum oxide is the oxygen compound of the chemical element aluminum . In the technical field, aluminum oxide is known as electrical corundum ( ELK ).

Extraction and presentation

From bauxite is aluminum hydroxide by digesting in sodium hydroxide solution recovered ( Bayer process ). By removing the water, for example by burning, sintering or calcining, aluminum oxide is obtained.

Aluminum oxide can also be prepared by carefully dehydrating gibbsite (hydrargillite) or boehmite .

Aluminum oxide is also formed when aluminum powder is burned with ammonium perchlorate in solid fuel rockets .

The pure metal aluminum shows a thin spontaneous aluminum oxide layer (self-passivation) after storage in air, which protects it from corrosion. By means of an electrolytically applied aluminum oxide layer, the aluminum surfaces of everyday objects are provided with an extremely hard ( e.g. Mohs hardness 9) and corrosion-resistant protective layer by anodizing . These anodized aluminum types are used to manufacture electrolytic capacitors .

The global production of aluminum oxide rose from 108 million t in 2014 to an estimated 118 million t in 2015. The three largest producers of aluminum oxide in 2014 were China (47.8 million t), Australia (20.5 million t ) and Brazil (10.6 million t). The USGS states the average US import price for aluminum oxide at USD 410 per ton in 2015, while import prices for bauxite in the same year were USD 28 per ton.

The waste product of the extraction of aluminum oxide is red mud , which contains caustic soda and poisonous heavy metals and is partly stored in open landfills or discharged into rivers around the world.



The most important modifications of the aluminum oxide are:

  • the cubic γ-Al 2 O 3 (alumina, raw material for ceramic and aluminum production)
  • the rhombohedral (trigonal) α-Al 2 O 3 (known as mineral corundum , sapphire or - with chromium doping - ruby , as an abrasive and aluminum oxide ceramic)
  • The β-alumina (β-Al 2 O 3 ) is also known, this is a historical error. This is the connection of Na 2 O and Al 2 O 3 to Na 2 Al 22 O 34 (Na 2 O · 11Al 2 O 3 ), also known under the mineral name diaoyudaoite

Electrical Properties

Al 2 O 3 is a very good insulator and has a very high dielectric strength of 35 kV / mm. The specific resistance is 10 12  Ω m at 20 ° C , and drops to 10 7  Ω m at 1000 ° C. The relative permittivity is 9–10 at 100 MHz, the loss factor about 10 −4 .

Thermal properties

Under standard conditions , the thermal conductivity due to the phonon resonance is a value of 35.6–39 W · m −1 · K −1, which is comparatively very high for ceramic materials (single-crystal corundum: 40 W · m −1 · K −1 , dense ceramic with 96 % Al 2 O 3 approx. 25 W m −1 K −1 ), which increases sharply with decreasing temperature and decreases with increasing temperature at 1000 ° C to approx. 5 W m −1 K −1 .

The expansion coefficient is in the range 6.5–8.9 · 10 −6  K −1 .

The melting temperature is 2072 ° C, so the application temperature of high-purity aluminum oxide ceramics should be below 1900 ° C.

Chemical properties

Al 2 O 3 is an amphoteric salt , which means that it can react as an acid (in connection with a base) or as a base (in connection with an acid).

The γ-Al 2 O 3 is a hygroscopic , white, loose powder that is not soluble in water, but in strong acids and bases . Already from 800 ° C the γ-Al 2 O 3 changes into the α-Al 2 O 3, which is generally insoluble in acids and bases .

γ-Al 2 O 3 is a porous material, the surface structure of which can be strongly influenced by the manufacturing process and its temperature. It is used as a stationary phase in chromatography .

Aluminum oxide forms aluminates with various metal oxides .

Mechanical properties

The mechanical properties of the aluminum oxide ceramic depend on the purity and structure of the ceramic produced. The purer the variety, the better the properties, but also the more complex the entire manufacturing process. In addition to the properties listed in the table below, aluminum oxide ceramics are also characterized by very good tribological properties and very good friction and wear behavior:

Mechanical properties of alumina by purity
property 96% 99.8%
density 3.75 g / cm³ 3.96 g / cm³
Flexural strength 310 MPa 630 MPa
Weibull module 13 15th
Compressive strength 2500 MPa 4000 MPa
Fracture toughness 4.0 MPam½ 4.3 MPam½
Modulus of elasticity 350 GPa 406 GPa
Vickers hardness HV1 1620 MPa 2000 MPa


Over 70% of the global annual production of around 120 million tons of aluminum oxide in 2016 went into the extraction of metallic aluminum ( Hall-Héroult process ).

Hybrid circuit on an alumina ceramic substrate
High-pressure sodium vapor lamps with discharge tubes (these are the matt rods inside) made of transparent aluminum oxide ceramic

The α-Al 2 O 3 has a Mohs hardness of 9 to 9.5 and is used, among other things, to make bearing stones for measuring instruments and watches, as well as abrasives . The basis for this is often the by-product of the aluminothermy resulting Alundum .

Calcined aluminum oxides are used in ceramics (e.g. in wash basins, hotel dishes, bulletproof clothing) or in the broadest sense as a polishing agent (e.g. in glass ceramic cleaners, car care products, brake pads, toothpastes). Furthermore, sintered α-Al 2 O 3 (sintered corundum) serves as a refractory material in furnace linings or laboratory equipment.

When contaminated with small amounts of Cr 2 O 3 or TiO 2 , the corundum forms the precious stones ruby (watch stones, drawing dies , ruby lasers ) and sapphire .

Ti 2 O 3 doped Al 2 O 3 - single crystals constituting the core of the titanium: sapphire laser .

γ-Al 2 O 3 serves as an adsorbent and as a catalyst carrier, as well as the catalyst itself.

In electrical engineering , aluminum oxide ceramics are used as a dielectric due to their low dielectric loss factor . The main area of application is the implementation of striplines and capacitors in high-frequency technology . Aluminum oxide ceramic plates also serve as a substrate for thick-film technology , thin-film technology and for platinum temperature measuring resistors (see PT100 ). The fact that this ceramic can be easily metallized also enables electronic components such as resistors or LEDs to be soldered directly. The ceramic also functions as a heat sink . These ceramic electronic systems are just as effective as systems that contain metallic heat sinks. Aluminum oxides are also used to manufacture fuse bodies.

The high dielectric strength and maximum operating temperature of up to 1900 ° C make aluminum oxide the ideal insulator for spark plugs .

In plant and mechanical engineering, aluminum oxide ceramics are used in particular for wear and corrosion protection. For example, transport troughs and chutes, drum mills and mixers are lined with tiles made of high-performance ceramics in order to increase the service life of the systems. The corrosion resistance of glass surfaces can be significantly increased by a coating of aluminum oxide. Nozzles made of aluminum oxide have also proven themselves in plasma welding. Due to their good tribological properties, components such as sealing and regulating disks, bearing bushings and shafts, thread guides in the textile industry, and hip joint balls and sockets in endoprosthetics have proven themselves. The use of ceramic knobs in the inrun of ski jumps is also innovative.

Highly pure, large crystalline and therefore transparent aluminum oxide ceramics are used to manufacture burner tubes for high pressure gas discharge lamps ( sodium vapor lamps , metal halide lamps ). In the past it was also used as an ultraviolet- transparent window material for EPROMs .

The latest sintering processes make it possible to use aluminum oxide for the production of extremely solid nanoscale glass ceramics , e.g. B. in wristwatch glasses .

Recently, Al 2 O 3 ceramics have also been used in vehicle armor. The ceramic tiles are glued to an aramid or Dyneema fabric. This type of armor achieves twice the protective effect of armor steel with the same weight per unit area. The ceramic fragments the bullet, the aramid fibers then catch the fragments.

Al 2 O 3 is sold under the name Elektrokorund (ELK) as fine corundum, semi-precious corundum and normal corundum. It is made in an electric furnace at around 2,000 ° C. The resulting melt cake is broken up and sieved according to the grain size specified in DIN. Corundum is used in technology as an abrasive in the manufacture of grinding wheels . It is also used as an abrasive and as a polishing powder .

More aluminum oxides

In addition to the trivalent aluminum oxide, two other aluminum oxides in lower oxidation states, aluminum (I) oxide and aluminum (II) oxide are known. However, these are only stable in the gas phase at high temperatures.

See also

Web links

Commons : alumina  - collection of pictures, videos and audio files

Individual evidence

  1. Entry on ALUMINA in the CosIng database of the EU Commission, accessed on February 12, 2020.
  2. a b c d e f data sheet aluminum oxide (PDF) from Merck , accessed on February 20, 2007.
  3. Refractive index of Al2O3 (Aluminum oxide)
  4. a b c Entry on aluminum oxide in the GESTIS substance database of the IFA , accessed on December 19, 2019(JavaScript required) .
  5. Swiss Accident Insurance Fund (Suva): Limits - Current MAK and BAT values (search for 1344-28-1 or aluminum oxide ), accessed on November 2, 2015.
  6. PAETEC Formula Collection Edition 2003, p. 116.
  7. Klaus D Linsmeier: Technical ceramics - material for the highest demands. (= The Library of Technology. Volume 208). 2010, ISBN 978-3-937889-97-9 , pp. 13-15.
  8. BAUXITE AND ALUMINA. (PDF 29.2 kB p. 2) USGS , accessed on April 16, 2016 (English).
  9. Diaoyudaoite Mineral Data
  10. Aluminum oxide ceramic DIN EN 60 672, type C799. ( Memento of October 11, 2012 in the Internet Archive ) (PDF; 55 kB). Metoxit AG, January 2007, accessed on August 19, 2010.
  11. a b W. Martienssen, Hans Warlimont: Springer handbook of condensed matter and materials data . Springer, 2005, ISBN 3-540-44376-2 , pp. 438-439, 445-446 .
  12. U. Seifert: Ceramic modification using laser. Dissertation. University of Engineering Mittweida, 1989.
  13. a b CeramTec: "Material data" data sheet. ( Memento of October 29, 2013 in the Internet Archive ) (PDF; 314 kB)
  14. BAUXITE AND ALUMINA. (PDF 28.6 kB p. 2) USGS , accessed on November 19, 2018 (English).
  15. BAUXITE AND ALUMINA. (PDF 25.7 kB p. 2) USGS , accessed on November 19, 2018 (English).
  16. St. Kuhn, R. Linke, Th. Hädrich: Modification of hot glass surface with alumina by combustion CVD. In: Surface & Coatings Technology. 205, 2010, pp. 2091-2096.
  17. A. Rosenflanz, M. Frey, B. Endres, T. Anderson, E. Richards, C. Schardt: Bulk glasses and ultrahard nanoceramics based on alumina and rare-earth oxides. In: Nature. Volume 430, No. 7001, 2004, pp. 761-764, doi: 10.1038 / nature02729 .
  18. Klaus D Linsmeier: Technical ceramics - material for the highest demands. (= The Library of Technology. Volume 208). 2010, ISBN 978-3-937889-97-9 , pp. 65ff.
  19. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1156.