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Polarization P
of a dielectric material can also be induced by mechanical deformation
e or by other means. |
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Piezo electric
materials are anisotropic crystals meeting certain symmetry
conditions like crystalline quartz (SiO2): the effect is
linear. |
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The effect also works in reverse: Electrical
fields induce mechanical deformation |
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Piezo electric materials have many uses, most
prominent are quartz oscillators and, recently, fuel injectors for Diesel
engines. |
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Electrostriction also couples polarization and
mechanical deformation, but in a quadratic way and only in the direction
"electrical fields induce (very small) deformations". |
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The effect has little uses so far; it can be used
to control very small movements, e.g. for manipulations in the nm
region. Since it is coupled to electronic polarization, many materials show
this effect. |
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Ferro
electric materials posses a permanent dipole moment in any
elementary cell that, moreover, are all aligned (below a critical
temperature). |
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| BaTiO3 unit cell |
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There are strong parallels to ferromagnetic
materials (hence the strange name). |
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Ferroelectric materials have large or even very
large (er > 1.000) dielectric
constants and thus are to be found inside capacitors with high capacities (but
not-so-good high frequency performance) |
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Pyro
electricity couples polarization to temperature changes; electrets are materials with permanent polarization,
.... There are more "curiosities" along these lines, some of which
have been made useful recently, or might be made useful - as material science
and engineering progresses. |
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© H. Föll
