Alumina is a white, transparent ceramic material. Its chemical composition is Al
2O
3. This means it
has the same chemical composition as
sapphire or
ruby. Unlike these precious
gems, however,
alumina is colorless, not having the metal impurities that give the gems their color. The natural form is called
corundum. The name alumina usually means the synthetic form.
Alumina appears
in both a polycrystalline and a monocrystalline form, usually hexagonal. This form is called
alpha-alumina. The other from, cubic gamma-alumina is less stable. Amorphous alumina does not
exist.
Alpha-alumina is an extremely hard material, having a hardness of 9 on the Mohs scale. This
makes it well suited for many technical applications, as it's a lot cheaper than other hard
materials like diamond.
Furthermore, alpha-alumina has excellent thermal properties. There are many ceramics with high
melting points, and alpha-alumina's melting point of 2300 K is not really special. However, it
also has a decent thermal conductivity of 30 Watt per meter per Kelvin at room temperature.
Compare this to a measly 0.8 W/m K for quartz and you see why alpha-alumina is popular for making
things that must withstand extreme heat. Its ability to withstand temperature changes is further
augmented by a decent specific heat of 77 Joule per kilogram per Kelvin. These properties are
employed for instance in high-powered plasma lamps and ore smelters.
Polycrystalline gamma-alumina can be made in a form that has many holes in it. These holes greatly
enhance the surface of the material, making gamma-alumina a popular material to use in chemical
catalysts.
Alumina is not known to be very hazardous, although inhaling fine alumina dust is probably not a
good idea. This makes it a far more desirable material to use than the related beryllia, which
has better thermal properties, but is extremely toxic.
Sources:
The trusty Handbook of Chemistry and Physics