Alumina is a white, transparent ceramic material. Its chemical composition is Al2
. This means it
has the same chemical composition as sapphire
. Unlike these precious gem
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.
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
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
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.
The trusty Handbook of Chemistry and Physics