| MadSci Network: Physics |
OVERVIEW: The properties of ferroelectricity, piezoelectricity, and pyroelectricity are determined by the crystal structure of a material. Ferromagnetic materials possess a natural electric polarization. Piezoelectricity refers to property that the polarization (or electric field) can be changed by mechanical perturbation of the structure. Pyroelectricity refers to the change in polarization by changes to structure from thermal effects. All ferroelectric materials are piezoelectric, but not all piezoelectric materials are pyroelectric. CRYSTAL SYMMETRY: 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 of the 32 crystal classes are piezoelectric. All 20 piezoelectric classes lack a center of symmetry. 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. 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. PIEZOELECTRIC EFFECT: The piezoelectric effect is a linear, reversible electromechanical interaction occurring in materials possessing the proper symmetry properties. The direct piezoelectric effect is the production of an electric polarization by a strain; the converse piezoelectric effect is the production of a stress by an electric field. Piezoelectric materials have wide applications as transducers - transfering mechanical motion into electricity or electricity into mechanical motion. One of the most wide spread 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. 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 pyroelectric effect. All polar crystals are pyroelectric, so the 10 polar crystal classes are sometimes referred to as the pyroelectric classes. The property of pyroelectricity is the measured 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, and I have even read some talk of using them as TV tubes (small ones). Sincerely, Dr. T
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