3.4.1.3 Cordierite
Cordierite materials and
others based on it are magnesium aluminosilicates, fabricated
by sintering soapstone or talcum with the addition of clay,
kaolin, fireclay, corundum and mullite. A simplified approximation
to the composition of pure ceramic cordierite is approx. 14
% MgO, 35 % Al2O3 and 51 % SiO2.
Figure 5: Microstructure of a porous cordierite
(C 520)
Cordierite materials have a low coefficient
of thermal expansion. This is the reason for their outstanding
thermal shock resistance combined with good mechanical strength.
Two types of cordierite, porous and dense, are distinguished.
The porous type has a low bending strength but even higher
thermal shock resistance than the dense type, since it can
absorb mechanical stresses in its porous structure.
Cordierite materials, moreover, behave inertly towards heating
element alloys at temperatures up to 1,000°C or even 1,200
°C (there is no reaction between the heating element and
the ceramic).
For this reason, cordierites are often found
in electric heating applications, and in heat engineering
in general.
Applications include, for example, insulators for continuous
flow electric water heaters, heating element pipes, heating
element supports in furnaces, link heaters, heating cartridges
for soldering irons, gas heater inserts, spark protectors
and catalyst carriers in automobiles.
For special purposes a material is available
containing cordierite and mullite and having a high aluminium
oxide content (C 530) which, for instance, permits higher
application temperatures.
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