Silicate ceramics

Amongst the materials of silicate ceramics are:

• Technical porcelain
• Steatite
• Cordierite
• Mullite-ceramic

The natural starting materials of technical porcelain (Group C 100 alkaline aluminio silicate porcelain) are quartz, feldspar and kaolin.
The technical application of siliceous porcelain dates back to the last century.
In view of increasing demands ceramists have developed porcelain with improved electrical, mechanical and thermal properties. By changing the composition, aluminous porcelain (C 120 and C 130) which exhibits more than double the strength was created. Ceramists replaced the less expensive quartz (SiO2) by alumina (Al2O3). Thus the outstanding feature of aluminous porcelain is:

• Higher strength even under permanent thermal load and
• Favourable long-term behaviour in outdoor conditions.

The reasonably priced siliceous porcelain is important in

• Low voltage areas.
• Aluminous porcelain is mostly used for
• Overhead cable insulators.

These inorganic materials with their basis of natural raw materials (major component: soapstone, additives: clay and flux) form the group of magnesium silicates (C 200 group).
The type of flux influences the electrical characteristics and leads to the following differentiation: steatite for low-frequency (C 210), normal steatite (C 220), as well as special steatite with "low loss factor" (C 221)- sometimes kwon as high-frequency steatite.
Steatite possesses very high mechanical strength and interesting dielectrical propertie

The outstanding feature of special steatite is its very low loss factor.
Steatite has found many uses in electrical engineering, especially for high-frequency components and, because its good workability, is ideal for the production of Porous Steatite
Porous steatite (C 230) can also be used for producing samples because this material can be machined after sintering using standard tools.

These magnesium silicates (C 400 Group) occur during the sintering of soapstone with added clay, kaolin, Corundium and Mullite.
Cordierite has

• A low thermal expansion coefficient and a high resistance to thermal shock.

The porous types have in comparison to the denser materials a lower flexural strength, but possess a higher resistance to thermal shock. Cordierite is generally used

• In heat engineering and above all
• In electrical engineering

These materials differ because of their raw material composition i.e. their proportion of Al2O3 and SiO2, as well as additional glass phase forming components.
These determine the existing material-phase share of Mullite (3 Al2O3 2 SiO2) and Corundium (Al2O3) after firing, as well as additional glass phases.
Thus it is possible to differentiate between pure-dense-sintered and open-porous-Mullite ceramic.

Important properties of Mullite ceramic are:

• High strength,
• Low thermal expansion,
• High resistance to thermal shock and
• High resistance to leakage at high temperatures from glass phase free Mullite.

This is why the materials are used for

• Thermocouple protection tubes,
• Carrying rollers for high temperature roller kilns and
• Other uses in the high temperature engineering up to 1650 °C