the converse piezoelectric effect

The converse piezoelectric effect is the electrical-to-mechanical half of piezoelectricity: apply a voltage across the crystal and it changes shape — stretching or shrinking by a tiny, controllable amount. This is the effect at work whenever a piezoelectric acts as an actuator or a motion source, and it is the transmit side of every ultrasound and sonar system, the counterpart of the direct (sensor) effect.

Apply an electric field E across a poled piezoceramic and it develops a strain (a fractional change in length) proportional to the field: S = d times E, where d is the very same piezoelectric coefficient that governs the direct effect — thermodynamics forces the two coefficients to be identical. Physically the field pushes the charged ions to new positions, and the whole lattice changes dimension. The strains are small, typically around 0.1 percent, so a centimetre of ceramic moves only about ten microns; but the motion is fast, extremely precise (down to sub-nanometre resolution), and can push with great force. To get useful travel at a modest voltage, makers cofire hundreds of thin layers into a multilayer actuator, so the small strain of each layer adds up.

The converse effect drives diesel and gasoline fuel injectors, inkjet printheads, camera autofocus and optical image stabilisation, atomic-force-microscope and nanopositioning stages, ultrasonic cleaners and welders, buzzers and piezo speakers, and the transmit pulse of medical ultrasound. It is also, less welcomely, why a Class II ceramic capacitor 'sings' — AC voltage makes it vibrate audibly. An honest caveat: the displacement is small and not perfectly linear — it shows hysteresis and creep, so precision positioning needs closed-loop feedback — and if the field is pushed too high the domains switch or the ceramic depoles or cracks, so actuators are driven within a safe field and, ideally, kept in compression.