Why Does Squeezing a Crystal Make a Spark? The Charges Move Sideways

In most crystals the positive and negative charges are arranged so symmetrically that their centres coincide, and deforming the crystal moves them together, so nothing changes electrically. In a crystal lacking a particular kind of symmetry, squeezing shifts the positive and negative centres apart, which leaves one face of the crystal positively charged and the opposite face negatively charged. That separation is a voltage, and it can be large. The requirement is precisely the absence of a centre of symmetry in the crystal structure, which is why only certain materials do it and why heating past a critical temperature destroys the effect by changing that structure.

Applying a voltage does the reverse, which doubles the usefulness:

• •A voltage across the crystal makes it change shape slightly
• •An alternating voltage makes it vibrate at that frequency
• •So the same material can be a sensor and an actuator
• •Movements are tiny, measured in fractions of a micrometre
• •Forces generated are very large for that movement
• •Response is extremely fast, which suits high frequencies

The applications are more numerous than most people realise and cover both directions of the effect. A gas lighter or a barbecue igniter strikes a spring-loaded hammer against a crystal, generating several thousand volts and a spark. A quartz watch uses a tiny tuning fork of quartz driven electrically and vibrating at a precise frequency, which the circuit counts. Ultrasound scanners use a crystal to generate pulses and the same crystal to detect the returning echoes. Microphones, guitar pickups, inkjet print heads, sonar transducers, vehicle knock sensors and precision positioning stages in microscopes all use one direction or the other.

A closely connected property appears in a subset of the same materials and is worth separating. Certain crystals generate a voltage when their temperature changes rather than when they are squeezed, because heating expands the structure and shifts the charge centres in the same way that pressure does. That responds to change rather than to absolute temperature, so it detects something warm moving into view and ignores a steady background, which is exactly what a motion sensor needs. Those sensors sit in security lights and alarms everywhere and work by detecting the infrared from a body crossing the field, using a material that is also piezoelectric.

Jacques and Pierre Curie demonstrated the effect in 1880, predicting it from crystal symmetry and confirming it with quartz and several other crystals, and the reverse effect was predicted mathematically by Gabriel Lippmann the following year and confirmed by them immediately afterwards. It remained a laboratory curiosity until the First World War, when Paul Langevin used quartz to generate and detect underwater sound for submarine detection, which is the origin of sonar and the first substantial application. Synthetic ceramics developed from the 1940s produce far stronger effects than natural quartz and are what most modern devices use.

Squeezing a crystal that lacks a centre of symmetry separates its positive and negative charge centres, producing a voltage across it, and applying a voltage deforms it in return, so one material serves as both sensor and actuator. Gas lighters, quartz watches, ultrasound scanners and inkjet heads all use it. The Curies demonstrated it in 1880 and submarine detection in the First World War made it useful.