### 3.6.3 Summary to: Special Dielectrics

| Polarization P of a dielectric material can also be induced by mechanical deformation e or by other means. |  |
P = const. · e
|  | Piezo electric materials are anisotropic crystals meeting certain symmetry conditions like crystalline quartz (SiO2): the effect is linear. |
|  | The effect also works in reverse: Electrical fields induce mechanical deformation |
|  | Piezo electric materials have many uses, most prominent are quartz oscillators and, recently, fuel injectors for Diesel engines. |
| Electrostriction also couples polarization and mechanical deformation, but in a quadratic way and only in the direction "electrical fields induce (very small) deformations". |  |
e | = | D l l | = const · E2
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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. |
| Ferro electric materials posses a permanent dipole moment in any elementary cell that, moreover, are all aligned (below a critical temperature). |  |
BaTiO3 unit cell
|  | There are strong parallels to ferromagnetic materials (hence the strange name). |
|  | 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) |
| 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. |  |
Questionaire
Multiple Choice questions to all of 3.6

© H. Föll (Advanced Materials B, part 1 - script)