Basic Science

How to squeeze electricity out of crystals

This is a crystal of sugar. If you press on it, it will actually generate its own electricity. How can this simple crystal act like a tiny power source? Because sugar is piezoelectric. Piezoelectric materials turn mechanical stress, like pressure, sound waves, and other vibrations into electricity and vice versa. This odd phenomenon was first discovered by the physicist Pierre Curie and his brother Jacques in 1880.

They discovered that if they compressed thin slices of certain crystals, positive and negative charges would appear on opposite faces. This difference in charge, or voltage, meant that the compressed crystal could drive current through a circuit, like a battery. And it worked the other way around, too. Running electricity through these crystals made them change shape. Both of these results, turning mechanical energy into electrical, and electrical energy into mechanical, were remarkable.

But the discovery went uncelebrated for several decades. The first practical application was in sonar instruments used to detect German submarines during World War I. Piezoelectric quartz crystals in the sonar’s transmitter vibrated when they were subjected to alternating voltage. That sent ultrasound waves through the water. Measuring how long it took these waves to bounce back from an object revealed how far away it was. For the opposite transformation, converting mechanical energy to electrical, consider the lights that turn on when you clap. Clapping your hands send sound vibrations through the air and causes the piezo element to bend back and forth. This creates a voltage that can drive enough current to light up the LEDs, though it’s conventional sources of electricity that keep them on.

So what makes a material piezoelectric? The answer depends on two factors: the materials atomic structure, and how electric charge is distributed within it. Many materials are crystalline, meaning they’re made of atoms or ions arranged in an orderly three-dimensional pattern. That pattern has a building block called a unit cell that repeats over and over. In most non-piezoelectric crystalline materials, the atoms in their unit cells are distributed symmetrically around a central point. But some crystalline materials don’t possess a center of symmetry making them candidates for piezoelectricity.

Let’s look at quartz, a piezoelectric material made of silicon and oxygen. The oxygens have a slight negative charge and silicons have a slight positive, creating a separation of charge, or a dipole along each bond. Normally, these dipoles cancel each other out, so there’s no net separation of charge in the unit cell. But if a quartz crystal is squeezed along a certain direction, the atoms shift. Because of the resulting asymmetry in charge distribution, the dipoles no longer cancel each other out.

The stretched cell ends up with a net negative charge on one side and a net positive on the other. This charge imbalance is repeated all the way through the material, and opposite charges collect on opposite faces of the crystal. This results in a voltage that can drive electricity through a circuit. Piezoelectric materials can have different structures. But what they all have in common is unit cells which lack a center of symmetry. And the stronger the compression on piezoelectric materials, the larger the voltage generated. Stretch the crystal, instead, and the voltage will switch, making current flow the other way.

More materials are piezoelectric than you might think. DNA, bone, and silk all have this ability to turn mechanical energy into electrical. Scientists have created a variety of synthetic piezoelectric materials and found applications for them in everything from medical imaging to ink jet printers. Piezoelectricity is responsible for the rhythmic oscillations of the quartz crystals that keep watches running on time, the speakers of musical birthday cards, and the spark that ignites the gas in some barbecue grill lighters when you flick the switch.

And piezoelectric devices may become even more common since electricity is in high demand and mechanical energy is abundant. There are already train stations that use passengers’ footsteps to power the ticket gates and displays and a dance club where piezoelectricity helps power the lights. Could basketball players running back and forth power the scoreboard? Or might walking down the street charge your electronic devices? What’s next for piezoelectricity?

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