Your gas lighter, your watch and a hospital ultrasound all run on one trick: a crystal that turns a squeeze into a voltage

A gas lighter, a quartz wristwatch, the ultrasound probe used for a pregnancy scan, an industrial inkjet printhead, the radio filters inside every mobile phone, and the scanner that moves the tip of an atomic force microscope all contain the same physical effect.

Squeeze certain crystals and charge appears on their faces. Apply a voltage across them and they physically change shape. One phenomenon, running in both directions, and the fact that it is reversible is what makes it so useful: the same component can be a sensor and an actuator, sometimes within microseconds of each other.

What makes it strange is the scale. A piezoelectric element expands by a fraction of a per cent at most, and often far less than that. It is one of the least impressive movements in engineering, and one of the most precisely controllable — and nearly every application follows from that contradiction.

Why some crystals do this and most do not

Inside a crystal, positive and negative charges sit in a repeating arrangement. In most materials that arrangement has a centre of symmetry: for every atom there is an equivalent one directly opposite through the centre of the unit cell. Squash such a crystal and the positive and negative charge centres move, but they move together, so nothing electrical happens.

Remove that symmetry — and a significant minority of crystal structures have no centre of symmetry — and the picture changes. Deforming the lattice now displaces the positive charge centre relative to the negative one, creating a net electric dipole in every unit cell. Multiply by the whole crystal and charge appears on the outer faces: a voltage.

Because it is a structural effect rather than a chemical one, it runs backwards just as well. Apply an electric field and each unit cell distorts to accommodate it, and the crystal changes dimension. The direct effect gives you a sensor; the converse effect gives you an actuator.

The workhorse material is lead zirconate titanate, universally shortened to PZT — a metal oxide ceramic from the same family as much of this catalogue, and by a wide margin the strongest performer in common use. Quartz is the other classic, far weaker but exceptionally stable. Aluminium nitride and zinc oxide dominate thin-film applications, and there are piezoelectric polymers too, notably PVDF, which is flexible and useful where a ceramic would crack.

Tiny motion, extraordinary precision

Here are the numbers that define what piezoelectric devices are for.

A good PZT actuator strains by roughly 0.1 per cent at full drive. A centimetre-long stack moves perhaps ten micrometres. That is a hopeless way to move anything anywhere.

But within that range, the position follows the applied voltage continuously, with sub-nanometre resolution, enormous stiffness, large blocking force, and a response time measured in microseconds. There is no backlash, because nothing slides or rotates — the material itself is deforming.

So piezoelectric actuators are terrible at travel and unbeatable at fine control, and every serious use plays to that. This is what moves the tip in a scanning tunnelling or atomic force microscope, where the whole point is to position something with atomic precision — which is the necessary partner to the resolution problem discussed in our article on measuring nanoparticles. Electron optics gave the field the ability to see at the nanoscale; piezoelectric scanners gave it the ability to move at the nanoscale, and scanning probe microscopy needed both.

The lighter, and why it makes thousands of volts

The everyday demonstration sits in most kitchens. Press the button on a piezo gas lighter and a spring-loaded hammer strikes a small PZT element hard and fast. The sudden strain generates a charge, and because the element has very low capacitance, that charge corresponds to a very high voltage — thousands of volts, enough to jump the air gap as a visible spark.

The click is the hammer; the spark is the crystal. And it is safe in the hand because while the voltage is high, the total energy is minute, which is exactly the combination that also makes piezo igniters suitable for gas appliances and barbecue burners.

Why every quartz watch runs at 32,768 Hz

Open the specification of almost any quartz timepiece and the crystal frequency is 32,768 hertz, which looks arbitrary and is not: it is 2¹⁵. A chain of fifteen binary divide-by-two stages turns it into exactly one pulse per second, using the simplest digital circuit imaginable.

The crystal itself is a tiny quartz tuning fork, and piezoelectricity does both jobs in the circuit: the drive electronics apply a voltage that flexes the fork, and the fork's own flexing generates the voltage that tells the circuit where it is in its cycle. The oscillator locks onto the mechanical resonance.

Quartz is used rather than the much stronger PZT for two reasons. Its mechanical losses are extraordinarily low, so the resonance is very sharp and therefore very stable. And the crystal can be cut at a specific orientation relative to its axes such that the frequency's dependence on temperature is nearly flat around normal operating temperature — the turnover point of that curve is chosen deliberately. A watch worn on the wrist sits near that point; a watch left somewhere cold drifts measurably, and that is the main reason quartz watches keep worse time in a drawer in winter than on an arm.

Ultrasound, and why they always use gel

An ultrasound probe is an array of piezoelectric elements, usually PZT or a related single crystal. Each one receives a short electrical pulse, converts it to a pressure wave that travels into the body, then falls silent and listens — converting the faint returning echoes back into voltage. Transmit and receive on the same element, milliseconds apart, using the effect in both directions.

The gel is not lubricant or hygiene. It is an acoustic impedance match. Sound reflects at a boundary between materials with very different acoustic impedance, and the mismatch between a solid transducer, air, and tissue is so severe that an air gap of any thickness reflects essentially the entire pulse straight back. The gel displaces air and provides an intermediate medium, and without it the image is not poor — it is absent. The same principle explains why an ultrasound cannot image through bowel gas or lung tissue.

The same physics, in a different package, is a piezo inkjet printhead: a voltage deforms a chamber wall and ejects a precisely sized drop. Unlike thermal inkjet, which boils the ink to form a bubble, a piezo head never heats the fluid — which is why it dominates industrial and functional printing, including the deposition of nanoparticle and conductive inks that would be destroyed by boiling.

And there is one almost invisible mass application: acoustic RF filters. Every mobile phone contains dozens of SAW and BAW filters, which work by converting a radio signal into an acoustic wave travelling through a piezoelectric film and back again. Sound travels about a hundred thousand times slower than light, so an acoustic resonator is drastically smaller than an electromagnetic one at the same frequency — which is the only reason a phone can carry filters for dozens of frequency bands in a few square millimetres.

How a piezo element dies without showing it

PZT is ferroelectric, which means it contains domains — regions of aligned dipoles — that point in random directions when the ceramic is made. A freshly fired PZT component is not piezoelectric at all in bulk, because the domains cancel out.

It is made piezoelectric by poling: applying a strong electric field at elevated temperature so the domains align, then cooling under field so the alignment is locked in. Everything the component does afterwards depends on that alignment surviving.

Three things can destroy it. Heating past the Curie temperature randomises the domains and the material depoles permanently — and a partial loss begins well below that point, which is why soldering a piezo element carelessly, autoclaving one, or leaving a device on a dashboard can quietly ruin it. Applying a strong field in the wrong direction does the same. So does extreme mechanical stress.

The awkward part is that a depoled element looks, measures and feels entirely normal. It has the same colour, the same dimensions, the same resistance. It has simply stopped working, and the most common cause is a soldering iron applied for a few seconds too long by someone who did not know the part had a thermal limit that mattered. This is the same structural story as superparamagnetic particles losing their direction to heat — ordered domains, a barrier, and thermal energy winning.

The lead problem, stated honestly

PZT contains lead, and there is no comfortable way around that. Electronics regulations such as RoHS restrict lead heavily, and piezoelectric ceramics have carried an exemption for years — not because anybody is happy about it, but because no lead-free alternative yet matches PZT's combination of coupling strength, temperature range and cost across the full spread of applications.

The lead-free research effort is substantial and has produced real materials — potassium sodium niobate and bismuth sodium titanate families being the most developed — that are genuinely competitive in specific niches and still short of general replacement. This is a good example of a materials problem where the science, the regulation and the industrial economics are moving at different speeds, and where "why is lead still used?" has a technical answer rather than an evasive one.

Why piezoelectric floor tiles keep not happening

Energy harvesting is the application that recurs in the press every few years: tiles that power a station from footsteps, road surfaces that generate from traffic, devices that charge from vibration.

The physics works. The arithmetic is the problem. The energy available is only what the deformation itself contains, a piezoelectric element converts a modest fraction of it, and the strains involved are small — so the output per footstep is in the millijoule range. Covering a busy concourse yields enough to run lighting for a display about the harvesting, which is very often exactly what it is used for.

Where harvesting does make sense is where the alternative is a battery that somebody has to reach. Self-powered light switches and doorbells, tyre pressure monitors, and wireless sensors on rotating or inaccessible machinery are all real products, and the case for them is maintenance cost rather than energy generation. That is a perfectly good reason, and it is a different claim from powering a building.

Thin-film piezoelectrics are where the growth actually is — aluminium nitride and PZT films integrated into MEMS devices, giving ultrasonic fingerprint sensors that image through glass, miniature ultrasonic transducer arrays, and the acoustic filters mentioned above. The nanoscale contribution here is film quality and crystallographic orientation: a piezoelectric film whose grains are not aligned is a film whose contributions partly cancel, which makes deposition control the whole game.

Why it matters for students and researchers

Piezoelectricity is the cleanest example in applied physics of reciprocity — the same device, unchanged, is a sensor when you read it and an actuator when you drive it. Students who internalise that stop thinking of sensing and actuation as separate disciplines, which is exactly the habit that MEMS, ultrasound and haptics all require.

It is also a good lesson in matching a technology to a regime rather than asking which is best. A piezo actuator is useless for moving something a centimetre and unmatched for moving it a nanometre; an electric motor is the reverse. Neither is superior, and engineering maturity is largely the ability to say which regime you are in before choosing.

The open problems are well defined. Lead-free ceramics that match PZT across temperature and coupling remain the field's headline goal, with real progress and no general answer. Thin-film integration with silicon processing — deposition temperature, orientation control, stress management — governs how far piezoMEMS can go. Depoling and fatigue under long-term cycling are still poorly predicted, which matters for anything expected to last a decade. And flexible and biocompatible piezoelectrics for implantable sensing are an active area where the materials and the regulatory path are both unsettled.

Frequently asked questions

Why does a gas lighter spark when I press it?

A spring-loaded hammer strikes a piezoelectric ceramic, and the sudden compression generates thousands of volts across it — enough to ionise the air gap and produce a spark. The click you hear is the mechanism releasing, not the spark itself. It is safe because although the voltage is high, the total energy stored is tiny.

Why does an ultrasound scan need gel?

Because sound reflects almost completely at an air boundary. The acoustic impedance of the transducer and of tissue are far closer to each other than either is to air, so even a very thin air gap sends essentially the whole pulse back before it reaches the body. The gel displaces the air and couples the probe to the skin. It is the same reason ultrasound cannot see through gas in the bowel or through the lungs.

Can piezoelectric floors really power a building?

No, and the limit is arithmetic rather than engineering. Each footstep offers a small amount of deformation energy, only a fraction is converted, and the totals land in the millijoule range per step. Harvesting is genuinely useful where replacing a battery is expensive or impossible — remote sensors, tyre monitors, self-powered switches — and that is a maintenance argument, not a generation one.

Why did my piezo buzzer or sensor stop working after I soldered it?

Most likely you depoled it. The aligned domains that make the ceramic piezoelectric are lost when it is heated towards its Curie temperature, and the loss is permanent and partly cumulative well below that point. The component will look and measure completely normal afterwards, which is what makes this failure so confusing. Piezo parts have specified maximum soldering temperatures and times, and they are not conservative padding.

Is quartz better than PZT?

They are for different jobs. PZT produces far larger strains and charges and is what you want for actuators, ultrasound and igniters. Quartz produces much less but has exceptionally low mechanical loss and excellent temperature stability, which makes it the right choice for frequency references and precision sensors. Choosing PZT for a watch crystal or quartz for an ultrasound probe would be a mistake in both directions.