pyroelectricity
/ PY-ro-ee-lek-TRISS-it-ee /
A pyroelectric crystal senses a change in temperature by producing a burst of electric charge. This is the effect behind the passive infrared (PIR) sensor that flicks on a security light or a hallway lamp when you walk past: your body's warmth, sweeping across the sensor as you move, changes its temperature slightly, and that change alone generates the signal that says 'something warm just moved.'
The mechanism rests on a permanent, built-in polarization. A pyroelectric is a polar crystal that carries a spontaneous polarization even with no applied field — its surfaces hold a bound charge, normally quietly neutralised by stray free charges that have drifted in over time. The size of that spontaneous polarization depends on temperature. Change the temperature by dT and the polarization changes, releasing (or demanding) a surface charge density equal to p times dT, where p is the pyroelectric coefficient in coulombs per square metre per kelvin. Crucially the signal is a derivative: it responds to the rate of change of temperature, not to temperature itself, so a steady, unchanging warm object produces no output once the free charges have re-neutralised it. Every pyroelectric is also piezoelectric (a polar crystal must be non-centrosymmetric), and every ferroelectric is a pyroelectric whose polarization can additionally be switched.
Pyroelectric ceramics are the workhorses of uncooled infrared detection: motion sensors, thermal imagers, flame and gas sensors, and non-contact temperature detectors, using materials such as lithium tantalate, modified PZT, triglycine sulphate, and the polymer PVDF. The great advantage over other infrared detectors is that pyroelectrics need no cooling and work at room temperature. Their defining limitation is the flip side of the mechanism: because they respond only to a change in temperature, they cannot see a stationary hot object — a PIR sensor is blind to someone standing perfectly still, which is why practical thermal imagers place a rotating 'chopper' in front of the sensor to keep the incoming radiation modulating. Being piezoelectric too, they are also microphonic: vibration can masquerade as a thermal signal.
A hallway PIR light contains a small pyroelectric ceramic behind a segmented lens. As a warm person crosses, the lens sweeps their heat on and off the sensor; each swing changes the ceramic's temperature by a fraction of a degree, and the resulting charge pulses trip the light. Stand dead still and, after a moment, the light gives up — the sensor sees only change.
Pyroelectricity is the derivative of polarization with temperature: it reports change, never a steady state, which is both its gift and its blind spot.
A pyroelectric detector cannot see a stationary heat source. It signals only while the temperature is changing, so real imagers modulate the incoming radiation with a chopper to keep producing a reading.