the PTC thermistor
/ P-T-C THERM-iss-ter /
A thermistor is a resistor whose resistance is deliberately made to depend strongly on temperature. A PTC thermistor is the kind whose resistance jumps upward — sharply, by orders of magnitude — once it gets hot enough. Cold, it conducts happily; heat it past a set threshold and it suddenly chokes off the current almost entirely. That self-limiting behaviour makes it act like a resettable fuse and a self-regulating heater rolled into one, and unlike a semiconductor version it is built from a ceramic.
The classic ceramic PTC is semiconducting barium titanate — the same ferroelectric perovskite, but donor-doped (for example with a little lanthanum or niobium) so that it conducts, and processed as a polycrystalline body. The action lives at the grain boundaries. In the Heywang model, thin acceptor-rich layers at each grain boundary form potential-energy barriers that impede electron flow. Below the Curie point the material is ferroelectric with a high permittivity, which electrically 'screens' those barriers and keeps them low, so the ceramic conducts. Heat it above the Curie point and it becomes paraelectric: the permittivity collapses following the Curie-Weiss law, the screening disappears, the grain-boundary barriers rise steeply, and the resistance leaps by a factor of a thousand to ten million within a few degrees. The switch temperature is exactly the barium-titanate Curie point, and it can be tuned by substituting strontium (to lower it) or lead (to raise it).
This makes PTC ceramics beautifully self-regulating heaters — they warm up, hit their switch temperature, throttle their own current, and hold a stable temperature without any control electronics, which is why they heat car door mirrors, diesel engine intakes, and hair dryers. They also serve as resettable overcurrent protectors and as the degaussing element in old CRT televisions. An honest caveat that is easy to miss: the PTC jump is a grain-boundary phenomenon, so it exists only in a properly doped, properly reoxidized polycrystalline microstructure — a single crystal of the same composition shows almost no PTC effect at all. The device is made by the boundaries, not just the bulk.
A PTC ceramic pellet behind a car's heated door mirror needs no thermostat: cold, it draws current and warms the glass; as it reaches its Curie-point switch temperature its resistance soars, its own current falls, and it settles at a safe, self-limited temperature — hotter days simply push it there faster.
The PTC switch temperature is the ferroelectric Curie point: below it the ceramic conducts, above it grain-boundary barriers slam the resistance up.
The PTC effect is not a bulk property but a grain-boundary one. The same barium-titanate composition grown as a single crystal barely shows it — the giant resistance jump depends on the doped, reoxidized polycrystalline microstructure.