Crystallised Aluminium Oxide

crystallised aluminium oxide (a-Al2O3) is a mineral that crystallises in the crystalline corundum structure. In this structure trivalent aluminum cations are arranged in octahedral close packing and oxygen anions form two regular triangles on both sides, twisted by 180deg. This gives the surface a layer of oxygen anions with the capacity to chemisorb water molecules and proteins.

The corundum is a refractory material and has high hardness. This is mainly due to the high strength of the oxide crystals. It is refractory because it forms a coating on aluminium that prevents its rusting in air.

Various methods are used for precipitation from basic solutions such as sodium aluminate solution and sulphuric acid solution. In some techniques there are cold and hot precipitation stages, in others a constant pH is used or both modifications of the oxide are mixed together, for example in a sulphuric acid solution at 20-25 degC and a sodium aluminate solution at 90-95 degC, etc.

Gamma aluminium oxide powder can be calcined to produce the a-Al2O3 phase which is often the most common type of aluminium oxide ceramic. This is a major commercially important phase and can be found in applications such as catalytic substrates for automotive and petroleum sectors, composite materials for spacecraft, and abrasive and thermal wear coatings.

A-Al2O3 has a relatively low electrical conductivity, but is often doped with Mg2+ or Li+. This is done to compensate for the charge compensation that takes place in the crystal structure. This has the effect of lowering the resistivity and producing a more practical electrolyte.

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