Piezoelectricity
Last updated: July 14, 2009.
You've probably used piezoelectricity (pronounced "pee-ay-zo-electricity")
quite a few times today. If you've got a quartz watch,
piezoelectricity is what helps it keep regular time. If you've been
writing a letter or an essay on your computer with the help of
voice recognition software, the
microphone you spoke into probably used
piezoelectricity to turn the sound energy in your voice into
electrical signals your computer could interpret. If you're a bit of
an audiophile and like listening to music on vinyl, your gramophone
would have been using piezoelectricity to "read" the sounds from
your LP records. Piezoelectricity (literally, "pressing electricity")
is much simpler than it sounds: it just means using crystals to convert mechanical energy into
electricity or vice-versa. Let's take a closer look at how it works
and why it's so useful!
Photo: A piezoelectric actuator used by NASA for various kinds of testing. Photo by courtesy of NASA Langley Research Center (NASA-LaRC).
What is piezoelectricity?
Squeeze certain crystals (such as quartz) and you can make
electricity flow through
them. The reverse is usually true as well: if you pass electricity
through the same crystals, they "squeeze themselves" by vibrating
back and forth. That's pretty much piezoelectricity in a nutshell
but, for the sake of science, let's have a formal definition:
Piezoelectricity (also called the piezoelectric effect) is the
appearance of an electrical potential (a voltage, in other words)
across the sides of a crystal when you subject it to mechanical
stress (by squeezing it).
In practice, the crystal becomes a kind of
tiny
battery with a positive charge on one face and a negative charge
on the opposite face; current flows if we connect the two faces
together to make a circuit. In the reverse piezoelectric electric, a
crystal becomes mechanically stressed (deformed in shape) when a
voltage is applied across its opposite faces.
What causes piezoelectricity?
Think of a crystal and you probably picture balls (atoms) mounted on bars (the bonds that
hold them together), a bit like a climbing frame. Now, by crystals,
scientists don't necessarily mean intriguing bits of rock you find
in gift shops: a crystal is the scientific name for any solid whose
atoms or molecules are arranged in a very orderly way based on
endless repetitions of the same basic atomic building block
(called the unit cell). So a lump of
iron is just as much of a crystal as a piece of quartz.
In a crystal, what we have is actually less like a climbing frame
(which doesn't necessarily have an orderly, repeating structure) and
more like three-dimensional, patterned wallpaper.
Photo: Quartz—probably the best known piezoelectric material. Photo by courtesy of US Geological Survey.
In most crystals (such as metals), the unit cell (the basic repeating unit) is symmetrical;
in piezoelectric crystals, it isn't. Normally, piezoelectric crystals
are electrically neutral: the atoms inside them may not be
symmetrically arranged, but their electrical charges are perfectly
balanced: a positive charge in one place cancels out a negative
charge nearby. However, if you squeeze or stretch a piezoelectric
crystal, you deform the structure, pushing some of the atoms closer
together or further apart, upsetting the balance of positive and
negative, and causing net electrical charges to appear. This effect
carries through the whole structure so net positive and negative
charges appear on opposite, outer faces of the crystal.
The reverse-piezoelectric effect occurs in the opposite way. Put a
voltage across a piezoelectric crystal and you're subjecting the
atoms inside it to "electrical pressure." They have to move
to rebalance themselves—and that's what causes piezoelectric
crystals to deform (slightly change shape) when you put a voltage
across them.
What is piezoelectricity used for?
There are all kinds of situations where we need to convert mechanical energy (pressure or
movement of some kind) into electrical signals or vice-versa. Often we can
do that with a piezoelectric transducer. A transducer is simply
a device that converts small amounts of energy from one kind into another (for example,
converting light, sound, or mechanical pressure into electrical signals).
Photo: A typical piezoelectric transducer.
In ultrasound equipment, a piezoelectric transducer converts electrical energy into extremely rapid mechanical vibrations—so fast, in fact, that it makes sounds,
but ones too high-pitched for our ears to hear. These ultrasound vibrations can be used for
scanning, cleaning, and all kinds of other things.
In a microphone, we need to convert
sound energy (waves of pressure travelling through
the air) into electrical energy—and that's something piezoelectric
crystals can help us with. Simply stick the vibrating part of the
microphone to a crystal and, as pressure waves from your voice
arrive, they'll make the crystal move back and forth, generating
corresponding electrical signals. The "needle" in a gramophone
(sometimes called a record player) works in the opposite way. As the
diamond-tipped needle rides along the spiral groove in your LP, it
bumps up and down. These vibrations push and pull on a lightweight
piezoelectric crystal, producing electrical signals that your stereo
then converts back into audible sounds.
In a quartz clock or watch, the reverse-piezoelectric effect
is used to keep time very precisely. Electrical energy from a battery is fed into a crystal to
make it oscillate thousands of times a second. The watch then uses
an electronic circuit to turn that into slower, once-per-second beats
that a tiny motor and some precision
gears use to drive the second, minute, and hour hands around the clockface.
Piezoelectricity is also used, much more crudely, in spark lighters for gas stoves and
barbecues. Press a lighter switch and you'll hear a clicking sound
and see sparks appear. What you're doing, when you press the switch,
is squeezing a piezoelectric crystal, generating a voltage, and
making a spark fly across a small gap.
Who discovered piezoelectricity?
The piezoelectric effect was discovered in 1880 by two French physicists, brothers
Pierre and Paul-Jacques Curie, in crystals of quartz, tourmaline, and
Rochelle salt (potassium sodium tartrate). They took the name from
the Greek work piezein, which means "to press."
Further reading
