Fun Facts About Piezoelectrics

In 1880, Pierre and Jacque Curie published a paper describing a "piezoelectric effect" in certain naturally-occurring crystals: When you squeezed a piezoelectric crystal (like quartz), charge developed on the surface. ("piezo" comes from the Greek "to squeeze")

Now, you would have been immediately suspicious that if these materials could transduce energy from a mechanical domain to an electrical one, it would be reasonable to hypothesize they might also convert energy in the opposite direction: that applying a charge (voltage) could produce a stress (displacement).

Unfortunately, Pierre and Jacque did not have the benefit of an MIT undergraduate education in mechanical engineering with a strong emphasis on balancing energy and force. But the prediction of this reverse effect (from electrical to mechanical domain) did come shortly afterward, nevertheless [based on thermodynamic convervation principles] - thanks to a clever French physicist [and future Nobel prize winner] named Lippman. The Curie brothers soon confirmed that piezoelectric crystals did in fact exhibit this reciprical property.

Because of the reciprocity, piezos are useful both as

Sensors, and
Actuators

. These materials are both:

Naturally-occurring, (most notably quartz), and
Man-made (in particular those in the 'lead (Pb) zirconate (Zr) titanate (Ti)' family, like'PZT-4, PZT-5H, and PZT-5A.

The man-made materials are designed for good (high) piezoelectric coupling, while the natural crystals (like quartz) tend to have much better aging and thermal stability. (PZT's are typically used in controlling and/or sensing 'active' structures (like silent, killer torpedoes) or 'passive' damping (as in these expensive skis).

A Quartz Tuning Fork

Personally, I think one of the niftier applications of the piezoelectric effect is the simple, modest (and very cheap!) quartz oscillator used for a watch.

If you played a musical instrument in high school (and even if you didn't), you've probably seen a tuning fork at some point.
The 'crystal oscillator' used to regulate many watches is actually a tiny, quartz tuning fork! The tuning fork geometry increases the quality of the lowest mode of vibration, so the two cantilevers vibrate in opposition (both in, then both out, then both in, etc). The tuning fork is plated with electrodes along the sides. Circuitry in the watch takes advantage of the strong and regular oscillation, providing the extra energy needed to prevent the oscillations from dying out but relying on the natural mechanical vibration of the structure to ensure periodicity (as a pendulum does in a grandfather clock).

The only reason I ever learned about quartz tuning forks is because they are now commonly used to actuate and/or sense vibration of the finely-tipped probes used in tapping-mode scanned probe microscopy.

One last comment : Every manufacturer of quartz tuning forks (of which I'm aware) produces some model(s) of oscillator which have a natural frequency of 32,768 Hz. Can you guess why such a [seemingly] peculiar frequency is so popular?
(Answer at top of next page...)

This great picture of a quartz tuning fork [above] was snarfed from this website.
The fork comes encapsulated in a tiny can, which you can pry off -
with a little delicacy [as was done here, to expose the innards].

gonzo@mit.edu

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2.010 Tutorial #7, 5-Nov-00