Pyroelectric crystal
Encyclopedia
Pyroelectric crystals are crystals that generate electricity when heated. It is similar to piezoelectricity
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s 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. Only 10 of the 32 point groups are polar.
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.
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.
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 construction 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 on 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 to drive the motion. By changing the voltage input to the tube on the AFM, we can modify the motion of the cantilever tip according to our specifications. More applications of the piezoelectric effect include unimorphs, bimorphs, and stacks.
Piezoelectricity
Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...
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Crystal symmetry
Crystal structureCrystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
s 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. Only 10 of the 32 point groups are polar.
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
Electret
Electret is a dielectric material that has a quasi-permanent electric charge or dipole polarisation. An electret generates internal and external electric fields, and is the electrostatic equivalent of a permanent magnet. Oliver Heaviside coined this term in 1885...
is the electrical equivalent of a permanent magnet.
Pyroelectricity
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 pyroelectricPyroelectricity
Pyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled. The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change...
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
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
The piezoelectric effect was discovered in the early 1880’s 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 construction 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 on 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 to drive the motion. By changing the voltage input to the tube on the AFM, we can modify the motion of the cantilever tip according to our specifications. More applications of the piezoelectric effect include unimorphs, bimorphs, and stacks.
External links
- Lithium Tantalate (LiTaO3)
- Lithium Tantalate (LiTaO3)
- laser detection with lithium tantalate
- Pyroelectricity: From Ancient Curiosity to Modern Imaging Tool
- Strontium Barium Niobate (SrBaNb2O6)
- Strontium Barium Niobate (SrBaNb2O6)
See also
- Lithium tantalateLithium tantalateLithium tantalate , is a crystalline solid which possesses unique optical, piezoelectric and pyroelectric properties which make it valuable for nonlinear optics, passive infrared sensors such as motion detectors, terahertz generation and detection, surface acoustic wave applications, cell phones...
- Zinc oxideZinc oxideZinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
- Pyroelectric fusionPyroelectric fusionPyroelectric fusion refers to the technique of using pyroelectric crystals to generate high strength electrostatic fields to accelerate deuterium ions into a metal hydride target also containing deuterium with sufficient kinetic energy to cause these ions to undergo nuclear fusion. It was...
- PyroelectricityPyroelectricityPyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled. The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change...