The Theory of Piezoelectricity
Piezoelectricity is created when certain materials such as crystals are subjected to a mechanical force, thus causing them to generate a voltage. Piezoelectronics have important roles as transducers in several industries. They are significant in testing, measuring, monitoring, cleaning and detection.
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Background
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Brothers Pierre and Jacques Curie discovered the piezoelectric effect in the late1800s, when they were only in their early 20s. They found that certain crystals such as quartz, topaz and cane sugar that become polarized will create a voltage. Polarization has the effect that transverse waves, which are perpendicular, will vibrate in a plane. Examples are vertically polarized and horizontally polarized waves.
Pierre later went on to marry Marie Curie, and they jointly won the Nobel Prize for pioneering work in radiation.
The Piezoelectric Effect
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A mechanical force such as twisting, squeezing or stretching is applied to the material, and this causes it to vibrate. The crystals become polarized and produce an emf or voltage that is proportional to the amount of force. They also function the opposite way; when an AC (alternating current) voltage is applied to them, they begin vibrating and will deform mechanically, such as by shrinking or stretching. Their vibrations occur at the frequency of the applied voltage. For example, if the frequency of the voltage is 30 hertz or 30 cycles per second, this is also the frequency at which the crystal will vibrate.
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Features
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The piezoelectric effect occurs in certain crystals. Those that are affected include quartz, Rochelle salts and tourmaline.
Quartz crystals are the most abundant type in nature. This is why they are so inexpensive and are the also most popular. Rochelle salts are the most fragile, being prone to easy breakage. They are also the most sensitive to AC voltage. Tourmaline is the opposite; it is the least sensitive of the three to AC voltage, but is also the strongest.
Ceramics that are also piezoelectric are barium titanate, lead zirconate and lead titanate. The effect has also been produced in man-made crystals and ceramics.
Uses
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The piezoelectric effect is useful in pressure transducers. In optics, piezoelectric tranducers are used to help create small motions that are controlled.
Quartz crystals are found in watches, filters and radio frequency or RF oscillators. Rochelle salts, because of their sensitivity, are favored for audio applications such as loudspeakers, headphones and microphones. Ceramics are also used in microphones and ultrasonic transducers. An additional use of ceramics is in scanning tunneling microscopes. For very high frequency activities of 100 MHz (megahertz) or higher where vibrational breakage is a threat, tourmaline is favored.
Other Piezolectric Materials
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The piezoelectric effect is not limited to crystals. It occurs in biologic matter. It has been found in wood and silk. Inside bodies, it occurs in skin, tendons, bone, aortas, tracheas and intestines. Dentin, which helps form the teeth and tusks in mammals and is found in some plants, is also piezoelectric. The nucleic acids DNA and RNA have piezoelectric properties.
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References
Resources
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