Ceramic components containing up to almost 100% alumina are made by firing a preprepared and shaped “green” part at temperatures in excess of 1500°C. By carefully controlling the intrinsic shrinkage that takes place, the highly densified product offers high mechanical strength and hardness with resistance to wear and chemical corrosion. Additionally, alumina ceramics exhibit very favourable thermal and electrical resistance properties. The applications that benefit from these characteristics include:
- Wear resistance (see Grinding media application)
- Chemical resistance
- Semi-conductor substrates
- Solar panels
- Medical & food ceramics
- Shock resistance
- Ballistics protection
- Voltage insulation
- Porcelain based high voltage insulators
- Spark plugs
- Dielectric engineering ceramics
- Electronic substrates
- Thermal insulation
- Substrates for high-frequency chips
- Kiln furniture such as rollers
Aluminas with intermediate to relatively high specific surface areas (BET 3- 75m²/g) with alpha alumina contents varying between 20% and 95%.These are produced by controlled rotary calcination of aluminium trihydroxide.
Ground calcined alumina
These aluminas are ground in continuous ball-mills or air-jet mills to median particle sizes (d50) 3, 4, 5 or 6 µm.
Hard calcined alumina
These aluminas have low specific surface area (BET < 1m²/g) and are almost entirely alpha alumina. Standard soda content is 3000-4000 ppm and intermediate soda content is 2000-3000 ppm. Hard calcined aluminas are produced in rotary kilns.
Low soda alumina
This alumina has very low soda content: between 300 and 500ppm. Carefully controlling calcination using the proprietary Reynolds process also allows a wide range of primary crystal size to be offered.
Superground low soda alumina
These low soda aluminas are ball-milled to their primary crystal size, which ranges from 0.4 to 2.5 µm. Soda contents are between 300 and 500ppm.