2005/06/30 | 压电效应[Piezoelectric Effect]
类别(Ω〖物理〗) | 评论(0) | 阅读(142) | 发表于 13:23
Certain crystals are called piezoelectric when they exhibit a relationship between mechanical strain (tension or compression) and voltage across their surfaces. Specifically, when compressed or pulled, a piezoelectric crystal will build up alternate charges on opposite faces, thus acting like a capacitor with an applied voltage. A current, called piezoelectricity, can then be generated between the faces. On the other hand, when subjected to an external voltage, the crystal will expand or contract accordingly.

This effect is put to use in several ways, the most common of which is in quartz crystal oscillators. When these are incorporated into the proper circuitry, they resonate at precise frequencies, depending on their size and on the way in which they are cut. Every computer has at least one clock frequency which is generated by a quartz crystal. Also, many modern accelerometers and pressure sensors use piezoelectric crystals. More exotic compounds are used in ultrasonic technology; different compounds of barium titanate are common. When used for sonar, where long-distance transmission of sound is required, the crystals become large to generate low frequencies, and are usually driven with high voltages to produce higher-amplitude pulses. The same crystal is connected to appropriate circuitry to receive the weaker return pulses.

Unlike ferroelectric materials, piezoelectric material do not store charge after the force is removed. There are 21 of the 32 crystal classes which have no center of symmetry. Of these, 20 have a polar axis with different crystal forms at opposite ends and are piezoelectric. Examples include quartz and tourmaline.
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