Piezoelectric Energy | Research & Encyclopedia Articles

Piezoelectric energy is a form of electric energy produced by certain solid materials when they are deformed. (The word piezohas its roots in the Greek word piezein meaning "to press.") Discovery of the piezoelectric effect is credited to Pierre and Jacques Curie who observed in 1880 that certain quartz crystals produced electricity when put under pressure.

Quartz is a crystalline piezoelectric material composed of silicon and oxygen atoms and is illustrative of a piezoelectric material. If no forces are applied to a quartz crystal the distribution of charges is symmetric at the sites of the atoms and there is no net internal electric field. Squeezing or stretching a quartz crystal displaces atoms from their normal positions producing a separation of positive charges and negative charges (electrons) that give rise to a net internal electric field. A voltage develops across the whole crystal making it, in a sense, a battery. As with a battery, an electric current is produced by the voltage when something is connected to the crystal.

The piezoelectric effect will not be used to produce energy to energize a light bulb, for example, yet there are many applications. One common usage is as an igniter in an outdoor gas-fired grill used for cooking. A spring-loaded plunger released by a button on the front of the grill gives a sharp blow to a piezoelectric crystal. The voltage produced generates a spark between two separated metal contacts near the incoming gas, causing the gas to ignite.

Another important but little-known piezoelectric effect is found in some electronic systems. Speaking produces pressure variations that propagate through the air. Forces are produced on anything in contact with this vibrating air so that when contact is with a piezoelectric crystal, tiny voltage variations are produced. A crystal microphone is designed to make use of this piezoelectric effect.

Pressure gauges used to monitor pressures in the fluids pumped throughout an automobile engine are also often based on the piezoelectric effect. An experimenter in a laboratory can apply a known pressure to a piezoelectric crystal and measure the voltage produced. Once the relationship between voltage and pressure is known, a pressure can be inferred from a measure of the voltage. In this manner, the piezoelectric material functions as a pressure gauge. A variety of gases and oils that rely on piezoelectric gauges are pumped throughout the working components of an automobile engine.

Interestingly, a reverse piezoelectric effect is the basis for a number of applications. In this reverse process, a piezoelectric crystal vibrates when an electric current is provided from an external source such as a battery. The precise frequency of these vibrations is used to great advantage in watches and other timepieces. The most common piezoelectric crystal used is quartz. Quartz occurs naturally, but quartz crystals can also be grown in the laboratory. The most common crystals in timepieces are tiny U-shaped tuning forks that vibrate 32,768 times per second. An electronic circuit in the timepiece records time by counting the vibrations. If you peek inside a "quartz" watch you will not see wheels and a spring as in a mechanical watch. Rather, you will see a battery that provides electric current for the crystal and an electronic circuit. You might also see the U-shaped tuning fork.

Since discovering and making use of the piezoelectric effect in naturally occurring crystals such as quartz and Rochelle salts, scientists have produced a wide range of piezoelectric materials in the laboratory. An early example is barium titanate, used in an electrical component called a capacitor. Currently, most piezoelectric materials are oxide materials based on lead oxide, zirconate oxide, and titanium. These very hard piezoelectric materials are termed piezoceramics.

Piezoelectricity is also a natural by occurring phenomenon in the human body. Studies have shown that piezoelectricity causes electric potentials in dry bones of humans and animals. Whether or not this is the cause of the electric potentials that occur in wet, living bone is debatable. More than likely, any piezoelectric effect occurring in moist bone is overshadowed by other sources of electric potentials.

Capacitors and Ultracapacitors.

Ikeda, T. (1990). Fundamentals of Piezoelectricity. New York: Oxford University Press.

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