Piezoelectricity

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Piezoelectricity is the phenomenon in which certain crystals generate an electric potential when subjected to mechanical compression or tension. There is also the reverse effect: when voltage is applied, these crystals change their shape. Thus, the phenomenon is reversible.

The magnitude of the potential produced by piezoelectric means is directly proportional to the magnitude of the applied force.

Source: http://upload.wikimedia.org/wikipedia/commons/c/c4/SchemaPiezo.gif, accessed: 12/10/2011

In crystals, piezoelectric properties appear under two basic conditions:

The crystal must contain numerous ionic bonds,
These bonds must be anisotropic.

Piezoelectric properties are utilized in many ways:

Sound generation (e.g., speakers based on piezoelectric elements)
Producing an electric spark (e.g., in lighters)
Detection of small vibrations (e.g., in seismic sensors, car alarms, etc.)
Sound recording (microphones based on piezoelectric elements)
Piezoelectric plates cut from quartz are used to stabilize oscillation frequencies as quartz resonators

What’s Next?

If we want to get acquainted with the phenomenon of piezoelectricity in more detail, we need to obtain an element that exhibits these properties. The simplest way is to acquire a miniature piezoelectric speaker from a musical greeting card. Piezo plates (so-called “piezo discs”) can also be bought in electronics stores. The element looks like this:

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This is a round metal plate onto which a disk made of synthetic piezocrystal is placed. A thin layer of metal is deposited on its top surface, forming one electrode. The metal plate itself serves as the second electrode. When an alternating electrical signal is applied to these electrodes, the piezoelectric material deforms in step with the signal, which produces sound. This is the basic role of the element.

Remember, the piezoelectric phenomenon is completely reversible. By bending the plate, we should observe the generation of a voltage at the electrodes. Let’s connect the piezo disc to a voltmeter. We can set the range to 10 V, as the voltages obtained are by no means negligible. You can see the effect below.

When bending the plate, a voltage of several volts in amplitude indeed appears! Notice that the direction of current flow depends on which way the plate is bent: if we bend the plate alternately in both directions, the current also reverses its direction.

Enjoy your experiments!