Relative permittivity, piezoelectric

The isovalent A-site substituents barium, strontium and calcium lower the Curie point and have a small influence on the morphotropic composition. At the 5-10 mol.% level they enhance the permittivity and piezoelectric properties. Pb0 94Sr0 06Ti0 47Zr0 53 for instance has a relative permittivity of 1300, a kp of 0.58... 

Some crystals, however, exhibit relative permittivity values many orders of magnitude higher than found in normal dielectrics. By analogy with magnetic behaviour, this behaviour is called ferroelec-tricity, and the materials are called ferroelectrics. Ferroelectrics also possess a spontaneous polarisation, Ps, in the absence of an electric field and a mechanical distortion. They are, therefore, a subset of pyroelectrics and, as such, all ferroelectrics are also pyroelectrics and piezoelectrics. The feature that distinguishes ferroelectrics from pyroelectrics is that the direction of the spontaneous polarisation, Ps, can be switched (changed) in an applied electric field. 

It is clear that a sharp Curie tenperature cannot be defined for relaxor materials in the same way as for simple materials such as BaTiOj. To take account of this, a depolarisation temperature, is taken as the temperature at which a partial or complete loss of polarisation, and a consequent degradation in piezoelectric performance, occurs. In materials with relative permittivity behaviour similar to that in Figure 6.18b,... 

Hard PZT ceramics are characterized by low piezoelectric coefficients, a smaller relative permittivity, minor dielectric losses, lower insulating resistance, high mechanical quality factor and high coercive field strength. Corresponding applications fields are ultrasonic generators with the highest required output powers such as ultrasonic cleaners or transducers for sonar applications. 

Although piezoelectric properties such as relative permittivity and piezoelectric coefficients change with the temperature, the total effect for the calibration of PZT sensors far below the Curie temperature is very small. However, PVDF foils show a significant temperature dependence of the pyroelectric properties in addition to their temperature depending piezoelectric features. Thus PVDF sensors are quite temperature sensitive, and in general, an appropriate temperature compensation is necessary. This behaviour, however, can be improved by special forming and spatial distribution of the PVDF sensors [100]. 

A ceramic material is heterogenous it is polycrystalline and contains pores it is often multiphase, the phases being distributed at grain boundaries, as a result of a liquid phase sintering, or forming inclusions. We may ask ourselves in what measure it is still possible to speak of electrical conductivity, relative dielectric permittivity, piezoelectric constant, etc. of the material. Or, to put it plainly, does the notion of material have meaning The answer is experimental, as we will see in section 11.5.1. 

Crystals with one of the ten polar point-group symmetries (Ci, C2, Cs, C2V, C4, C4V, C3, C3v, C(, Cgv) are called polar crystals. They display spontaneous polarization and form a family of ferroelectric materials. The main properties of ferroelectric materials include relatively high dielectric permittivity, ferroelectric-paraelectric phase transition that occurs at a certain temperature called the Curie temperature, piezoelectric effect, pyroelectric effect, nonlinear optic property - the ability to multiply frequencies, ferroelectric hysteresis loop, and electrostrictive, electro-optic and other properties [16, 388],... 

Technically useful properties of such perovskite ceramics are their high permittivities (relative dielectric constants), the semiconductor properties of certain chemical compositions and their piezoelectric properties. 

Table 8.8 Piezoelectric coefficients in p(As)/N, relative dielectric permittivities, and effective electromechanical coupling coefficients in percent of LGS, LGN, LGT (Bohm et a ., 2000), STGS (Takeda et al., 2000 Heimann and Hengst, 2004) and SNGS (Heimann and Hengst, 2004 Chilla et al., 2003). Quartz is given for comparison. The signs of dij conform to the IEEE standard.

As is seen from the above date, the maximal piezoelectric constants of 0-3 polymer-ceramic composiles are relatively low. For exanqile, the maximum dyx is about 1/10 that of PZT, even though 60% volume fraction in composite is achieved (Fig. 48). This is due primarfly to the substantial differences in permittivities of PZT and PVDF (122). 

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