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The world of alumina >
Aluminas
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Alumina in all its forms
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Aluminium oxide or alumina exists in several physical and chemical forms.
Three main families of alumina can be defined; they are described below.
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• HYDRATES
There are two different types: trihydroxides Al(OH)3 and oxyhydroxides AlOOH, and each type exists in several crystallographic forms. There are numerous ways of synthesizing hydrates, going from the gibbsite obtained through the Bayer process to precipitated amorphous gel.
• TRANSITION ALUMINAS
During the thermal decomposition of hydrates, the progressive loss of water leads to the formation of transition aluminas. These are aluminas with an intermediary crystallographic structure between hydrate and alpha alumina. The different transition aluminas generally coexist in the form of a mixture the proportions of which depend on the precursor hydrate and on the decomposition conditions.
• ALPHA ALUMINA
This concerns the ultimate stage in thermal decomposition. Different types can be distinguished through different manufacturing processes even though the same crystalline structure is involved:
Above 1100°C, transition aluminas are transformed into calcined or ultra pure aluminas depending on the precursor.
From 1600°C, calcined alumina is compacted into tabular alumina, (sintering).
At 2050°C, alumina melts, from which corundum or sapphire can be made.
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