3.4.2.7 Barium
Titanate
Barium titanates are amongst
the materials used as functional ceramics.
They possess extremely high permittivities, therefore finding
application as capacitor dielectrics. They are also used as
piezoelectric ceramic materials.
Barium carbonate, titanium dioxide and other raw materials
for doping purposes are sintered at between 1,200°C and
1,400°C to form polycrystalline barium titanate. This
exhibits semiconducting properties together with a positive
temperature coefficient of the ohmic resistance, for which
reason it is used as a positive temperature coefficient resistor
(PTC). This effect is characterised by a very sharp rise of
electrical resistance – several powers of ten –
starting at a reference temperature (Tb).
Figure 19: Resistance curves of PTC ceramic
PTC ceramics are used as temperature sensors in instrumentation
and control technology, and as limit sensors for motor and
machine protection. The material is also applied for self-regulating
heating elements operating from low and mains voltage, as
switching delay elements (for electric motor starting and
de-magnetisation), and for overload protection.
Figure 20: Components manufactured from PTC
ceramics
|
