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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
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