Stresses electric polarizations from

Piezoelectric crystals are transducers that generate an oscillating electrical polarization when subjected to an external oscillating mechanical stress, and vice versa. The brothers Paul-Jacques and Pierre Curie discovered the piezoelectric effect in 1880 when they compressed certain crystals along certain axes (Curie and Curie, 1880). The reciprocal behavior was deduced from thermodynamic principles a year later by Gabriel Lippman... 

From Eq. 8.66, each component of the electric polarization is given in terms of all six components of the applied stress. For example, Pj is ... 

Vice versa, mechanical stress changes the dimensions of the material. This does not usually result in an electrical polarization of the material because most materials have a so-called center of symmetry canceling opposite charge displacements. However, crystals lack such a center of symmetry, and they generate an internal polarization, P, when mechanically deformed. These materials are called piezoelectric, with a direct conversion from mechanical to electrical energy. 

Piezoelectricity. When subjected to mechanical stress, some materials develop a voltage difference, called electrical polarization. The converse is also observed the same materials exposed to an electric field are lengthened or shortened according to the direction of the field and in proportion to its strength. These behaviors are called piezoelectric effects from the Greek piezein, meaning to press or to squeeze. 

Piezoelectric Effects Electric Polarizations Arising from Stress Tensors of Third Rank... 

Certainly the most prominent feature of the breakdown process is its dependence on the polarity of the electric field relative to the shock-velocity vector. This effect is manifest in current pulse anomalies from minus-x orientation samples or positively oriented samples subjected to short-pulse loading (see Fig. 4.8). The individual effects of stress and electric field may be delineated with short-pulse loadings in which fields can be varied by utilizing stress pulses of various durations [72G03]. 

From the electrical viewpoint, stress-induced changes in remanent polar-... 

Fig. 16. Spectra of the Si B—H stretching mode under stress taken with polarized light near 15 K. F is the applied force and E the electric vector of the light. [Reprinted with permission from The American Physical Society, Bergman, K., Stavola, M., Pearton, S.J., Hayes, T. (1988). Phys. Rev. B 38, 9643 ]...

Fig. Id represents the ionic changes and reversal of polarity of the membrane when the nerve is stimulated. Na+ ions enter the membrane ahead of the electrical charge and K+ ions pass out at the peak of the potential reversal.1 Fig. le shows how the ionic interchange is related to the action potential (or magnitude of polarity change). It must be stressed that the actual percentage changes of concentration are very small indeed. The exact nature of the restoration of the original concentration of ions is not completely known. Obviously a source of energy is required, and this is considered to be derived from the metabolism of the cell.

Some piezoelectric crystals are electrically polarised in the absence of mechanical stress one example is gem-quality tourmaline crystals. Normally, this effect is unnoticed because the crystal does not act as the source of an electric field. Although there should be a surface charge, this is rapidly neutralised by charged particles from the environment and from the crystal itself. However, the polarisation decreases with increasing temperature and this can be used to reveal the polar nature of the crystal. If tourmaline is heated its polarisation decreases and it loses some of its surface charges. On rapid cooling it has a net polarisation and will attract small electrically charged particles such as ash. Such crystals are known as pyroelectric, and ferroelectric crystals are a special subclass of pyroelectric crystals. 

Ionomers are superior to ldpe in resistance to environmental stress cracking, but, because ionomers are more polar, they are not as useful as electric insulators. Blow-molded containers made from ionomers, unlike those made from lope or hope do not store static electricity and hence do not attract dust. 

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