Alumina is the most widely used oxide ceramic material. Its applications are widespread, and include spark plugs, tap washers, pump seals, electronic substrates, grinding media, abrasion resistant tiles, cutting tools, bioceramics, (hip-joints), body armour, laboratory ware and wear parts for the textile and paper industries. Very large tonnages are also used in the manufacture of monolithic and brick refractories. It is also used mixed with other materials such as flake graphite where even more severe applications are envisaged, such as pouring spouts and sliding gate valves.
Key Properties

The characteristics which alumina has and which are important for these applications are shown below.

· High compression strength

· High hardness

· Resistant to abrasion

· Resistant to chemical attack by a wide range of chemicals even at elevated temperatures

· High thermal conductivity

· Resistant to thermal shock

· High degree of refractoriness

· High dielectric strength

· High electrical resistivity even at elevated temperatures

· Transparent to microwave radio frequencies

· Low neutron cross section capture area

· Raw material readily available and price not subject to violent fluctuation
Annual Production

Annual production of alumina is some 45 million tonnes, of which 90% is used in the manufacture of aluminium metal by electrolysis.
Where Does Alumina Come From?

Most of the aluminium oxide produced commercially is obtained by the calcination of aluminium hydroxide (frequently termed alumina trihydrate or ATH). The aluminium hydroxide is virtually all made by the Bayer Process. This involves the digestion of bauxite in caustic soda and the subsequent precipitation of aluminium hydroxide by the addition of fine seed crystals of aluminium hydroxide.
Phases of Alumina

Aluminium oxide exists in many forms, a, c, h, d, k, q, g, r; these arise during the heat treatment of aluminium hydroxide or aluminium oxy hydroxide. The most thermodynamically stable form is a-aluminium oxide.