piezoelectricity
/ pee-AY-zo-ee-lek-TRISS-it-ee /
Press the button on a spark lighter or a gas-barbecue igniter and it clicks and sparks — no battery involved. Inside is a small ceramic that, when sharply squeezed, produces a voltage high enough to jump a spark gap. That is piezoelectricity: certain crystals turn mechanical force into electric charge, and, run in reverse, turn an applied voltage into a change of shape. It is a genuine two-way street between the mechanical and electrical worlds, discovered in 1880 by the brothers Pierre and Jacques Curie.
The effect needs a crystal whose atomic arrangement has no centre of symmetry. In such a lattice, squeezing it displaces the positive and negative ions by different amounts, so the little internal dipoles no longer cancel and a net charge appears on the surfaces — this is the direct effect. Conversely, applying a field displaces those same ions and changes the crystal's dimensions — the converse effect. The two are linked by the piezoelectric coefficient d: as a charge coefficient it gives charge per unit force (in picocoulombs per newton), and as a strain coefficient the identical number gives strain per unit field (in picometres per volt). A centrosymmetric crystal such as ordinary rock salt shows none of this, because squeezing it keeps the charge centres coincident.
Piezoelectric ceramics are the muscles and nerves of modern devices: sensors (accelerometers, microphones, pressure gauges, sonar receivers), actuators (fuel injectors, inkjet heads, nanopositioners), transducers (medical ultrasound, sonar, ultrasonic cleaners), resonators (the quartz crystal in every clock, RF filters), and igniters. PZT dominates the ceramic side. An important honesty: a piezoelectric ceramic is polycrystalline and starts out inactive, because its ferroelectric domains point at random and cancel — it must first be poled to break that symmetry. And note the family relations: quartz is piezoelectric but not ferroelectric (it cannot be poled or switched), while every ferroelectric is automatically piezoelectric; a poled piezoceramic also loses its effect if heated above its Curie point.
A medical ultrasound probe is a slab of poled PZT run both ways at once: a voltage pulse makes it flex and launch a sound wave into the body (converse effect), and the faint echoes bouncing back press on it and generate tiny voltages (direct effect) that the machine turns into an image.
Piezoelectricity is reversible: the same ceramic can act as a loudspeaker and a microphone, a pusher and a sensor, depending on which way you drive it.
Piezoelectric is not the same as ferroelectric. All ferroelectrics are piezoelectric, but many piezoelectrics (quartz, for one) are not ferroelectric — they have a fixed, unswitchable polarity and need no poling.