Pyroelectric constants

At high temperatures, the module of the pyroelectric constants of both compounds increases more significantly and reaches an extremum at about 480K. Fig. 113 shows the temperature dependence of the pyroelectric coefficients. This phenomenon could be related to the change in dilatation mechanism that was observed while investigating the temperature dependence of the lattice parameters (see Fig. 102). 

When the film is short-circuited and heated to high temperatures at which the molecules attain a sufficiently high mobility, a current is observed in the external circuit. This phenomenon is called pyroelectric effect, thermally stimulated current, or, when the film has been polarized by a static field prior to measurement, depolarization current. The conventional definition of pyroelectricity is the temperature dependence of spontaneous polarization Ps, and the pyroelectric constant is defined as dPJdd (6 = temperature). In this review, however, the term will be used in a broader definition than usual. The pyroelectric current results from the motion of true charge and/or polarization charge in the film. Since the piezoelectricity of a polymer film is in some cases caused by these charges, the relation between piezoelectricity and pyroelectricity is an important clue to the origin of piezoelectricity. 

Among the 20 crystal classes lacking a center of symmetry ten of them contain a unique polar axis and exhibit pyroelectricity in addition to piezoelectricity, i.e. in the unstrained dipolar network of these crystals the dipole moment components remain and add to a resultant polar-axis moment. The term pyroelectricity is assigned because thermal expansion will expand or contract the dipole. The pyroelectric constant is defined by ... 

Table 14.14. Piezoelectric and Pyroelectric Constants of PVDF Filmi ...

The pyroelectric effect of PVDF form I sample was also interpreted in a similar way with tlut discussed in the above section. For example. Broadhurst, et al. presented the equation of pyroelectric constant by exchanging the effect of pressure used in the evaluation of d, with that of temperature T (253) ... 

Nylon-5,7 has only y form, which can be produced by thermal or mechanical treatments. Nyk)n-S,7 has a net dipole moment, and both poled os well os unpoled samples exhibit pyroelectricity. Litt and Lin [26] reported a pyroelectric constant of 100 pC/em K. The effea of water on piezoelectricity of odd nylons has been studied by Kim [27]. The piezoelectric constant increases with increasing water content, probably due to the crystalline change. Lin et al. [10] reported a pyroelectric coefficient of loi-100 Ocm K for nylon-5,7. 

Pyroelectric effect of bone and tendon was first reported by Lang in 1966 [47). The magnitude of the pyroelectric constant is as small as 3 X 10 C/m , It is rnostly the... 

The thinner the ferroelectric layer is. the better il can fulfill the thermal diffusion. If it becomes thirmer than 0.1 pm in order, the hysleresia characteristic will worsen (9). The polarizatioo reverses by applying reverse electric field over the coercive electric field. Figure 9 shows the temperature dependence of the pyroelectric constant P, of 52/ 48 copolynur under a bias field (10). The decrease of the coetdve field with increasing... 

This is quantified by the pyroelectric constant p with temperature T ... 

Equations (5.30) and (5.31) clearly show that materials with a high pyroelectric constant and small specific heat capacity are required. However, the final choice of material depends on the capacitance of the amplifier to be used and the size of the detector element. 

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