Aug 1 2003
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. It 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.
Applications
As mentioned above, alumina is used for applications including ball valves, piston pumps and deep drawing tools.
Typical Properties
The typical properties of alumina are tabulated below.
Table 1. Key properties of alumina
| 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 |
About Goodfellow
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