Two modifications are known of alumina. The α form forms crystals with high hardness. This modification is present in mineral corundum and is required for the production of abrasives. It is neither soluble in water nor in acids or alkalis. The hardness according to Mohs is 9 to 9.5.
The γ form forms a white powder that attracts water and is hygroscopic and dissolves in acids and alkalis. This form is also known as alumina, an important raw material for the production of ceramics and aluminium. The γ alumina has a very large surface area so that, for example, dyes can be applied. When heated strongly, the γ form changes into the α form.
The industry extracts aluminium oxide from bauxite, which is an intermediate product in the extraction of aluminium. Anodising aluminium produces a thin layer of aluminium oxide, which protects the metal from corrosion.
When aluminium powder is burnt in the air, aluminium nitride AlN is produced in addition to aluminium oxide through a reaction with nitrogen. For extraction in the laboratory, aluminium powder would have to be burnt in pure oxygen. However, this reaction is very violent. This is one of the reasons why it is not suitable for the production of aluminium oxide on an industrial scale: the production of aluminium is very complex and expensive.
Al2O3 is marketed under the designation electrocorundum (ELK) as white corundum, semi-precious corundum and normal corundum. It is produced in an electric furnace at about 2,000 °C. The resulting melt cake is crushed and screened according to the grain sizes specified in DIN.
α Alumina is used in the watch industry for the production of bearings. Due to its high hardness, it is an important abrasive and polishing agent. Due to its high refractoriness, it is required for furnace linings and laboratory equipment. In spark plugs it is used as an insulator. The use of aluminium oxide ceramics is also widespread: They are obtained by pressing and subsequent heating to over 1500 °C.
They are used, for example, in electronic components, in vehicle armouring, in cutting devices, in grinding balls for ball mills or in crucibles. γ alumina is needed for catalysts, in chromatography or as an adsorbent.
Recently, Al2O3 ceramics have also been used in vehicle armour. The ceramic tiles are bonded to an aramid or Dyneema fabric. This type of armouring achieves twice the protective effect of armour steel for the same weight per unit area. The ceramic fragments the bullet, the aramid fibres then catch the fragments.
In electrical engineering, alumina ceramic is used as a dielectric because of its low dielectric loss factor. The main area of application is the realization of strip lines and capacitors in high frequency technology. Alumina ceramic plates also serve as substrates for thick-film technology, thin-film technology and for platinum temperature measurement resistors (see PT100). The good metallizability of this ceramic also allows direct soldering of electronic components such as resistors or LEDs. The ceramic also functions as a heat sink. These ceramic electronic systems are just as effective as systems containing metallic heat sinks. Aluminium oxides are also used to manufacture fuse bodies.
The high dielectric strength and maximum operating temperature of up to 1900 °C make aluminium oxide the ideal insulator for spark plugs.
In plant and mechanical engineering, aluminium oxide ceramics are used in particular for wear and corrosion protection. For example, transport channels and chutes, drum mills and mixers are lined with tiles made of high-performance ceramics to increase the service life of the plants. The corrosion resistance of glass surfaces can be significantly increased by a coating of aluminum oxide.
Nozzles made of aluminium oxide have also proven to be effective in plasma welding. Due to their good tribological properties, components such as sealing and regulating discs, bearing bushes and shafts, thread guides in the textile industry as well as hip joint balls and cups in endoprosthetics have proven particularly successful. The use of ceramic nubs in the approach track of ski jumps is also innovative.
The latest sintering processes make it possible to use aluminium oxide to produce extremely solid nanoscale glass ceramics, e.g. for wristwatch crystals.