Background

Minerals containing Alumina represent some 15% of the earth’s crust. It is therefore an abundant material and virtually inexhaustable, unlike raw materials for many alloys developed for special applications.
Properties

The combination of high thermal conductivity, low thermal expansion and high compressive strength leads to good thermal shock resistance, so Alumina is suited to furnace use as crucibles, tubes and thermocouple sheaths. High purity Alumina can be used up to 1700°C and is gas tight up to 1300°C. Few chemicals attack Alumina. Alumina also shows good electrical insulation at high temperatures, good wear resistance and high hardness, making it suitable for components such as ball valves, piston pumps and deep drawing tools. Diamond tools are needed to machine or grind Alumina.
Alumina Fibres

Remarkably, continuous filament yarns have been made from alumina with reasonable but not complete success and two, similar, versions are available from Goodfellow. They are both much less flexible than normal continuous filament yarns and are rather “hairy” i.e. have broken filaments - especially the FP version.
Typical Properties

Chemical Resistance










Acids - concentrated


Good

Acids - dilute


Good

Alkalis


Good

Halogens


Good

Metals


Good

Electrical Properties







Dielectric constant


9.0-10.1

Dielectric strength ( kV.mm-1 )


10-35

Volume resistivity ( Ohm.cm )


>1014 @ 25°C

Mechanical Properties










Compressive strength ( MPa )


2200-2600

Hardness - Knoop ( kgf.mm-2 )


2100

Hardness - Vickers ( kgf.mm-2 )


1500-1650

Shear strength ( MPa )


330

Tensile modulus ( GPa )


300-400

Tensile strength ( MPa )


260-300

Physical Properties










Apparent porosity ( % )


0

Density ( g.cm-3 )


3.9

Water absorption - saturation ( % )


0

Thermal Properties










Coefficient of thermal expansion ( x10-6 K-1 )


8.0 @ 20-1000°C

Melting point (°C )


2100

Specific heat ( J.K-1.kg-1 )


850-900 @ 25°C

Thermal conductivity ( W.m-1.K-1 )


26-35 @ 20°C

Upper continuous use temperature ( °C )


1700

Properties Alumina Fibre




Property








Value

Coefficient of thermal expansion - Longitudinal


x10-6 K-1





5.7

Density


g.cm-3





3.9

Modulus


GPa





380

Tenacity


GPa





1.4

Applications

As mentioned above, alumina is used for applications including ball valves, piston pumps and deep drawing tools.