Pyroelectric crystal

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Pyroelectric crystals are crystals that generate electricity when heated. It is similar to piezoelectricity.

Crystal symmetry[edit]

Crystal structures can be divided into 32 classes, or point groups, according to the number of rotational axes and reflection planes they exhibit that leave the crystal structure unchanged. Twenty-one of the 32 crystal classes lack a center of symmetry, and of these, 20 are piezoelectric. Of these 20 piezoelectric crystal classes, 10 of them are pyroelectric (polar). Any material develops a dielectric polarization when an electric field is applied, but a substance which has such a natural charge separation even in the absence of a field is called a polar material. Whether or not a material is polar is determined solely by its crystal structure.

Under normal circumstances, even polar materials do not display a net dipole moment. As a consequence there are no electric dipole equivalents of bar magnets because the intrinsic dipole moment is neutralized by "free" electric charge that builds up on the surface by internal conduction or from the ambient atmosphere. Polar crystals only reveal their nature when perturbed in some fashion that momentarily upsets the balance with the compensating surface charge.

Electret is the electrical equivalent of a permanent magnet.

Pyroelectricity[edit]

Spontaneous polarization is temperature dependent, so a good perturbation probe is a change in temperature which induces a flow of charge to and from the surfaces. This is the pyroelectric effect. All polar crystals are pyroelectric, so the 10 polar crystal classes are sometimes referred to as the pyroelectric classes. The property of pyroelectric crystal is to measure change in net polarization (a vector) proportional to a change in temperature. The total pyroelectric coefficient measured at constant stress is the sum of the pyroelectric coefficients at constant strain (primary pyroelectric effect) and the piezoelectric contribution from thermal expansion (secondary pyroelectric effect). Pyroelectric materials can be used as infrared and millimeter wavelength detectors.

Ferroelectricity[edit]

Ferroelectrics are materials which possess an electric polarization in the absence of an externally applied electric field such that the polarization can be reversed if the electric field is reversed. Normally materials are very nearly electrically neutral on the macroscopic level. However, the positive and negative charges which make up the material are not necessarily distributed in a symmetric manner. If the sum of charge times distance for all elements of the basic cell does not equal zero the cell will have an electric dipole moment which is a vector quantity. The dipole moment per unit volume is defined as the dielectric polarization. Since all ferroelectric materials exhibit a spontaneous polarization, all ferroelectric materials are also pyroelectric (but not all pyroelectric materials are ferroelectric).

Piezoelectric effect[edit]

Main article: Piezoelectricity

The piezoelectric effect was discovered in the early 1880s by Pierre and Jacques Curie. They found that when pressure is applied to certain crystals (such as quartz or ceramic), an electric voltage across the material appears. The word piezo- comes from the Greek word piezein meaning to press tight, to squeeze. The phenomenon is due to the asymmetric structure of the crystals, that allows ions to move more easily along one axis than the others. As pressure is applied, each side of the crystal takes on an opposite charge, resulting in a voltage drop across the crystal. This effect is linear, and disappears when the pressure is completely taken away.

Piezoelectric materials have wide applications as transducers - transferring mechanical motion into electricity or electricity into mechanical motion. One of the most widespread examples is a quartz resonator. The quartz resonator converts the electrical potential energy of a battery into a steady beat that becomes the oscillator (counter) of a watch. Other common examples include cigarette and gas burner lighters which produce a spark, buzzers found in microwave ovens and phones, tiny microphones and earphones, and inkjet printers (specifically the Epson brand).

Today, examples of the inverse piezoelectric effect can be seen more readily. The inverse effect uses a voltage applied to a piezoelectric crystal to bend it in a desired direction. By constructing a tube with three piezoelectric crystals, motion can be achieved in all three dimensions. Because of their high precision (on the nanometer scale), these piezoelectric tubes are used in cases where small controlled motion is necessary.

One use of a piezoelectric tube is in an Atomic Force Microscope (AFM). This instrument can create images of objects on the micrometer scale by moving a cantilever tip with respect to a sample surface using a piezoelectric tube. By changing the voltage input to the tube in the AFM, one can control the position of the cantilever tip. More applications of the piezoelectric effect include unimorphs, bimorphs, and stacks.

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