Piezoelectricity

Piezoelectricity is a two-way coupling between mechanical strain and electric polarization that occurs in crystals lacking a centre of symmetry. Squeeze a piezoelectric crystal along the right axis, and a voltage appears across it. Apply a voltage across it, and the crystal deforms by a tiny but extremely repeatable amount (typically picometres per volt). Quartz, tourmaline, Rochelle salt, lead zirconate titanate (PZT), and barium titanate are all piezoelectric.

The microscopic mechanism is geometric. In a non-centrosymmetric crystal, the positive and negative ions sit in lattice positions that, on average, produce no net dipole moment. Squeeze the lattice along a polar axis, however, and the centroids of positive and negative charge shift relative to each other — a polarization appears, and surface charges accumulate on the deformed faces. The same mechanism, run in reverse, lets an applied field push the ions apart or together, deforming the lattice. The two effects are linked by a single tensor, the piezoelectric coefficient, that pins down the conversion factor between strain and polarization.

Modern technology runs on piezoelectricity. Every quartz watch keeps time by counting the vibrations of a tiny tuning-fork-shaped piece of quartz driven at 32,768 Hz. Every ultrasound transducer is a sandwich of PZT ceramic discs that pulse and listen. Sonar, fuel injectors, microbalances, scanning-tunnelling microscopes, ink-jet printers, butane lighters — they all exist because someone needed to convert tiny voltages into tiny motions or vice versa, and a piezoelectric crystal does it cleanly. The global market for piezoelectric devices is in the tens of billions of dollars per year.

Pierre and Jacques Curie discovered piezoelectricity in 1880 at the Sorbonne, working on the symmetry properties of crystals. They predicted the converse effect — that an applied field should deform the crystal — from thermodynamic considerations the following year, and Gabriel Lippmann verified it experimentally within months. The technology lay mostly dormant until World War I, when Paul Langevin built the first practical sonar transducer from a sandwich of quartz crystals to detect German submarines. Quartz oscillators followed in the 1920s, ceramic piezoelectrics (barium titanate, then PZT) in the 1940s, and the field has expanded ever since.