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PTC(Positive Temperature Coefficient) thermistors
There are four main types of PTC
Silicon PTC Thermistors display a nearly linear positive temperature coefficient - Silistor
About 4000ppm/deg K
Doped polycrystalline ceramic (containing barium titanate (BaTiO3) and other compounds). This is a ferroelectric(most ferroelectric materials do not contain iron!). The electric permittivity, P/E, is not constant as in dielectrics but is a function of the external electric field.
As a ferroelectric it has a nonlinear electric polarization with hysteresis that can be reversed by the application of an external electric field. the polarization is therefore dependent not only on the current electric field but also on its history, yielding a hysteresis loop.
Ferroelectricity only happens below a phase transition temperature, called the Curie temperature, Tc, and become paraelectric above this temperature: the spontaneous polarization vanishes, and the ferroelectric crystal transforms into the paraelectric state. ( Many ferroelectrics also lose their piezoelectric properties above Tc). The dielectric constant varies inversely with temperature as it crosses the Tc.
Ferroelectric Electric field vs polorization
Paraelectric Electric field vs polorization
Below the Curie point temperature, the device has a small negative temperature coefficient. At the Curie point temperature, the dielectric constant drops sufficiently to allow the formation of potential barriers at the grain boundaries, and the resistance increases sharply with temperature. At even higher temperatures, the material reverts to NTC behaviour.
The high dielectric constant below Tc prevents the formation of potential barriers between the crystal grains, thus low resistance. As we pass the Curie point temperature, the dielectric constant drops sufficiently to allow the formation of potential barriers at the grain boundaries, and the resistance increases sharply with temperature. At even higher temperatures, the material reverts to NTC behaviour.
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