Pyroelectricity (from the Greek pyr , fire, and electric) is the nature of certain crystals that are naturally electrically polarized and as a result contain a large electric field. The most important example is the gallium nitride semiconductor. The large electric field in this material is not desirable for light-emitting diode (LED), but it is helpful for making power transistors. Alternatively, the pyroelectric is interpreted as the ability of certain materials to produce a transient voltage when heated or cooled. The temperature change changes the atomic position slightly inside the crystal structure, so that the polarization of matter changes. This polarization change creates a voltage across the crystal. If the temperature remains constant at its new value, the pyroelectric voltage gradually disappears because of a leakage current (leakage can be due to electrons moving through the crystal, ions moving through the air, leaking current through a crystal-mounted voltmeter, etc.).
Pyroelectricity should not be equated with thermoelectricity, different thermal effects with different mechanisms.
Video Pyroelectricity
Description
Pyroelectricity can be visualized as one side of the triangle, where each angle represents the energy state in the crystal: kinetic, electrical and thermal energy. The sides between the electric and heat angles represent the pyroelectric effect and do not produce kinetic energy. The sides between the kinetic and electric angles symbolize the piezoelectric effect and do not generate heat.
The pyroelectric charge in the mineral develops on the opposite face of asymmetric crystals. The direction in which the charge propagation tends toward is normally constant across the pyroelectric material, but, in some materials, this direction can be changed by the nearest electric field. These materials are said to indicate ferroelectricity. All pyroelectric materials are also piezoelectric, two closely related properties. Note, however, that some piezoelectric materials have crystal symmetry that does not allow pyroelectricity.
The nature of pyroelectricity is the change measured in net polarization (vector) that is proportional to the temperature change. The total pyroelectric coefficient measured at a constant voltage is the sum of the pyroelectric coefficients at constant voltage (primary pyroelectric effect) and the piezoelectric contribution of thermal expansion (secondary pyroelectric effect). Under normal circumstances, even polar materials do not display the moment of the net dipole. As a result there is no electric dipole equivalent of a bar magnet because the intrinsic dipole moment is neutralized by the "free" electrical charge accumulated on the surface by the internal conduction or from the ambient atmosphere. Polar crystals only reveal their properties when disturbed in some modes that momentarily disrupt the balance with a compensating surface charge.
Spontaneous polarization depends on temperature, so a good perturbation probe is a temperature change that causes the flow of charge to and from the surface. This is a pyroelectric effect. All polar crystals are pyroelectric, so 10 classes of crystalline poles are sometimes referred to as pyroelectric classes. The pyroelectric crystal property is to measure the change in net polarization (vector) proportional to the temperature change. The total pyroelectric coefficient measured at a constant voltage is the sum of the pyroelectric coefficients at constant voltage (primary pyroelectric effect) and the piezoelectric contribution of thermal expansion (secondary pyroelectric effect). Pyroelectric materials can be used as infrared and millimeter wavelength detectors.
Electret is electric equivalent to a permanent magnet.
Math description
Koefisien piroelektrik dapat digambarkan sebagai perubahan dalam vektor polarisasi spontan denan suhu:
where p i (Cm -2 K -1 ) is a vector for pyroelectric coefficients.
Maps Pyroelectricity
History
The first reference to the pyroelectric effect in writings by Theophrastus in 314 BC, which notes that lyngourion, tourmaline, can attract sawdust or pieces of straw when heated. The Tourmaline property was rediscovered in 1707 by Johann Georg Schmidt, who notes that the stone only attracts hot ashes, not the cold ones. In 1717 Louis Lemery noticed, as Schmidt put it, that small pieces of unused material were first attracted to tourmaline, but were subsequently rejected by them as soon as they contacted the stone. In 1747 Linnaeus first connected the phenomenon with electricity (he called tourmaline Lapidem Electricum, "electric rock"), though this was not proven until 1756 by Franz Ulrich Theodor Aepinus.
Research in pyroelectricity became more sophisticated in the 19th century. In 1824, Sir David Brewster had an influence on the name he had today. Both William Thomson in 1878 and Woldemar Voigt in 1897 helped develop the theory for the process behind pyroelectricity. Pierre Curie and his brother, Jacques Curie, studied pyroelectricity in the 1880s, leading to their discovery of some of the mechanisms behind piezoelectrics.
Crystal class
All crystal structures can be divided into 32 crystal classes, corresponding to the number of rotation axes and reflection plane they show which leave the crystal structure unchanged. Of the thirty-two crystal classes, twenty-one is non-centrosymmetric (has no central symmetry). Of the twenty-one, twenty direct piezoelectric exhibits, the remaining is the cubic class of 432. Ten of these twenty piezoelectric classes are polar, that is, they have spontaneous polarization, have dipoles in their unit cells, and show pyroelectrics. If this dipole can be reversed by the application of an electric field, the material is said to be ferroelectric. Each dielectric material develops a dielectric (electrostatic) polarization when an electric field is applied, but a substance that has a natural charge separation even without a field is called polar. Whether a material is a pole is determined solely by its crystal structure. Only 10 of 32 groups of polar points. All polar crystals are pyroelectric, so ten classes of polar crystals are sometimes referred to as pyroelectric classes.
Piezoelectric crystal class: 1, 2, m, 222, mm2, 4, -4, 422, 4mm, -42m, 3, 32, 3m, 6, -6,622,6mm; -62m, 23, -43m
Piroelektrik: 1, 2, m, mm2, 3, 3m, 4, 4mm, 6, 6mm
Efek terkait
Two effects closely related to pyroelectricity are ferroelectric and piezoelectric. Usually the material is very electrically neutral at the macroscopic level. However, the positive and negative charges that make up the material do not have to be distributed symmetrically. If the total load time for all elements of a base cell is not equal to 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. If the dipole moment is changed by the effect of the applied temperature change, the applied electric field, or applied pressure, the material is pyroelectric, ferroelectric or piezoelectric.
The ferroelectric effect is shown by a material having electrical polarization in the absence of an electric field applied externally so that the polarization may reverse if the electric field is reversed. Since all ferroelectric materials show spontaneous polarization, all ferroelectric materials are also pyroelectric (but not all pyroelectric materials are ferroelectric).
The piezoelectric effect is shown by crystals (such as quartz or ceramic) in which the electrical voltage across the material appears when pressure is applied. Similar to the pyroelectric effect, this phenomenon is due to the asymmetrical structure of the crystal that allows the ion to move more easily along one axis than the other. When pressure is applied, each side of the crystal takes the opposite charge, resulting in a voltage drop across the crystal.
Pyroelectricity should not be equated with thermoelectricity: In a typical demonstration of pyroelectricity, all crystals are converted from one temperature to another, and the result is a transient voltage across the crystal. In typical thermoelectricity demonstrations, one part of the device is stored at one temperature and another at different temperatures, and the result is a permanent voltage across the device as long as there is a difference temperature. Both effects change the temperature change into an electric potential, but the pyroelectric effect changes the temperature change during time into electrical potential, while the thermoelectric effect changes the temperature change with position into electrical potential.
Pyroelectric materials
Although artificial pyroelectric materials have been engineered, the effect is first found in minerals such as tourmaline. The pyroelectric effect also exists in the bones and tendons.
Advances have been made in the creation of artificial pyroelectric materials, usually in the form of thin films, out of gallium nitride (GaN), cesium nitrate (CsNO 3 ), polyvinyl fluoride, derivatives of phenylpyridine, and cobalt phthalocyanine. Lithium tantalate (LiTaO 3 ) is a crystal that exhibits piezoelectric and pyroelectric properties, which have been used to create small-scale nuclear fusion ("pyroelectric fusion").
Apps
Heat sensor
Very small temperature changes can produce electrical potential, due to the material pyroelectricity. Passive infrared sensors are often designed around the pyroelectric material, because human or animal heat from a few feet is enough to produce a difference in load.
Power plant
A pyroelectric can be heated repeatedly and cooled (such as a heat engine) to produce usable electrical power. One group calculated that the pyroelectric in the Ericsson cycle could reach 50% of Carnot's efficiency, while different studies found materials that could, in theory, achieve 84-92% of Carnot's efficiency (this efficiency value for the pyroelectric itself, ignoring the losses from heating and substrate cooling, other heat transfer losses, and all other losses elsewhere in the system). Possible gains from pyroelectric generators to generate electricity (compared to conventional heat engines plus electric generators) include: lower possible operating temperatures, larger equipment, and fewer moving parts. Although several patents have been filed for such devices, there seems to be no place close to commercialization.
Nuclear Fusion
Pyroelectric materials have been used to produce the large electric fields required to direct deuterium ions in nuclear fusion processes. This is known as pyroelectric fusion.
See also
- Electrocatalytic effect, opposite effect of pyroelectricity
- Thermoelectric
- Kelvin probe force microscope
- Lithium tantalate
- Zinc oxide
References
- Gautschi, Gustav, 2002, Piezoelectric Sensorics , Springer, ISBN 3-540-42259-5 [1]
External links
- An important explanation of pyroelectric detector operations
- Infrared Piroelectric Infrared Infrared Detector
- DoITPOMS Teaching and Learning Packages - "Pyroelectric Materials"
- Lithium Tantalate (LiTaO3)
- Lithium Tantalate (LiTaO3)
- laser detection with lithium tantalate
- Strontium Barium Niobate (SrBaNb2O6)
- Strontium Barium Niobate (SrBaNb2O6)
Source of the article : Wikipedia
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