Converse electrostrictive effect

The converse electrostrictive effect—the stress dependence of the permittivity—is also used in stress sensors [19]. A himorph structure provides superior stress sensitivity and temperature stability. A measuring system with a himorph structure, which subtracts the static capacitances of two dielectric ceramic plates, has been proposed [ 19]. The capacitance changes of the top and bottom plates have opposite signs for uniaxial stress and the same sign for temperature deviation. The response speed is limited by the capacitance measuring frequency to about 1 kHz. Unlike piezoelectric sensors, electrostrictive sensors are effective in the low-frequency range, especially DC. 

All materials undergo a small change in dimensions when subjected to an electric field. If the resultant strain is proportional to the square of the field it is known as the electrostrictive effect. Some materials show the reverse effect - the development of electric polarization when they are strained through an applied stress. These are said to be piezoelectric (pronounced pie-ease-oh ). To a first approximation the polarization is proportional to the stress and the effect is said to be direct . Piezoelectric materials also show a converse effect, i.e. the development of a strain x directly proportional to an applied field. 

Electrostriction is related to the converse piezoelectric effect. At modest electric field strengths, the piezoelectric equations given previously are adequate and there is a linear relationship between strain and electric field. However, at higher electric field strengths, these equations need to be extended to include a further term quadratic with respect to the electric field. The strain is now given by... 

The most direct experimental access to this effect is the direct measurement of deformation (changes of specimen thickness, e.g.) under the action of an electric field. In piezoelectric ciystals, however, this effect is masked to a large extent by the superimposed converse piezoelectric effect, hnear in the electric field. Nevertheless, Uchino etal (1982) andLnymes (1983) have constmcted measuring apparatus based on an interferometric techniqne. By compensating the linear piezoelectric response Luymes (1983) measured an electrostriction constant of a-quartz. 

The information content of AV and AS is similar. Values of AV are usually more precise than those of AS, although they require specialized apparatus for their measurement. If ions are being formed in the activation step, AV may be -20 cm3 mol-1. This effect reflects the electrostriction of the solvent. If the transition state features bond breaking, as in an SnI reaction, AV 10 cm3 mol-1. Conversely, AV -10 cm3 mol-1 is characteristic of bond making. 

A piezoelectric solid (e.g., quartz) acquires an electrical dipole moment upon mechanical deformation and, conversely, if it is subjected to an electric field E it becomes distorted by an amount proportional to the field strength E. The dipole moment disappears without the mechanical force. Piezoelectricity is only possible in lattices that do not have an inversion center. Electrostriction is also mechanical distortion in an electric field (strain proportional to E ) but ionic lattices that have a center of symmetry also show this effect. Figure 4.25 is a schematic representation of the source of these effects using the interatomic potential curve. A ferroelectric material is not only piezoelectric but its lattice has a permanent electric dipole moment (below its Curie temperature), which most other piezoelectric materials (such as quartz) do not have. 

Unfortunately, even today, inverse (or converse) piezoelectricity is still sometimes called electrostriction because the name electrostriction suggests the electromechanical direction (electrical stimulus leads to mechanical response), while piezoelectricity seems to refer only to the opposite mechano-electrical direction. In order to avoid the misleading use of the term, it should be kept in mind that our modem terminology is based on phenomenological thermodynamical relations so that the linear effects of direct and inverse piezoelectricity must be identical due to the mathematically required reciprocity. 

Figure 55. Analogy between the piezoelectric and the electroclinic effect. A translational distortion in the former corresponds to an angular distortion in the latter. If the sign of the applied field E is reversed, the sign of P and 9 is also reversed. But there is no converse effect in the electroclinic case. The mechanical deformation is electrostrictive, i.e. proportional to E. In the smectic C case there is also a component proportional to E.

---