Pyroelectric charge

Note that most, but not all, detectors are made so that the direction of it is normal to the element electrode plane, i.e. p = 7r. In the discussion which follows, it will be assumed that this is the case and the term pyroelectric coefficient will be applied to p.) The pyroelectric charges can be detected as a current ip, flowing in an external circuit such that ... 

Pyroelectric charges are generated in certain materials by heating or cooling them, or by subjecting them to pressure. The stresses set up... 

Fig. A.5-42 Li2GevOi5. Pg versus T. P was determined by pyroelectric-charge measurement...

The pyroelectric effect may be defined as the change in spontaneous polarisation, s, as a function of temperature. The symmetry requirements for pyroelectricity are far more restrictive compared with SHG and piezoelectricity. To exhibit a spontaneous polarisation, the material in question must crystallise in one of ten polar crystal classes (1, 2, 3, 4, 6, m, mm2, 3m, 4mm, or 6mm). Thus, polarity is required for pyroelectric behaviour. Determining the pyroelectric coefficient may be done two ways - either measuring the pyroelectric current or the pyroelectric charge. Both techniques will be described. 

The original method for measuring the pyroelectric charge was developed in 1915. This technique, known as the static method, determines the... 

Fig. 30.18. Spontaneous polarization as a function of temperature in -Gd2(Mo04)) single crystal. Above 75 K, P, is deduced from low-frequency (0.01 Hz) isothermal loops. Below 75 K, P, is measured from pyroelectric charge and discharge of single-domain samples (Sawaguchi and Cross, 1973).

Some electrical properties are shown in Table 3. Values of other parameters have been pubflshed (146). Polymorphism of the PVDF chains and the orientation of the two distinct dipole groups, —CF2— and —CH2—, rather than trapped space charges (147) contribute to the exceptional dielectric properties and the extraordinarily large piezoelectric and pyroelectric activity of the polymer (146,148,149). 

Pyroelectrics. Pyroelectric ceramics are materials that possess a uoique polar axis and are spontaneously polarized ia the abseace of an electric field. Pyroelectrics are also a subset of piezoelectric materials. Ten of the 20 crystal classes of materials that display the piezoelectric effect also possess a unique polar axis, and thus exhibit pyroelectricity. In addition to the iaduced charge resultiag from the direct pyroelectric effect, a change ia temperature also iaduces a surface charge (polarizatioa) from the piezoelectric aature of the material, and the strain resultiag from thermal expansioa. 

Theoretical estimations and experimental investigations tirmly established (J ) that large electron delocalization is a perequisite for large values of the nonlinear optical coefficients and this can be met with the ir-electrons in conjugated molecules and polymers where also charge asymmetry can be adequately introduced in order to obtain non-centrosymmetric structures. Since the electronic density distribution of these systems seems to be easily modified by their interaction with the molecular vibrations we anticipate that these materials may possess large piezoelectric, pyroelectric and photoacoustic coefficients. 

For practical use of pyroelectric elements as infrared sensors and so forth, the induced voltage V is an important quantity. From the relations concerning surface charge. 

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. 

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. 

The pyroelectric current is expected from the depolarization of spontaneous polarization (i.e., motion of polarization charge) as well as from the motion of true charge. The polarity of the pyroelectric current is, however, inverted between the two, as is shown schematically in Fig. 22. It is difficult to assume a polarization charge in non-polar or weakly polar polymers such as polyethylene (PE), polypropylene (PP), and... 

A close correlation between the polarities of piezoelectricity and pyroelectricity was found for PVC and poly (vinylidene fluoride) (PVDF) films (Nakamura and Wada, 1971). However, it must be emphasized that the polarity of piezoelectricity is determined not only by the polarity of the charge distribution but also by that of heterogeneous strain. The origin of heterogeneous strain in the elongation of film may derive from heterogeneity in the structure of the film. 

The pyroelectricity or depolarization current is closely related to the piezoelectricity of Groups (B) and (C), because the pyroelectric current is caused by the thermal activation of charges in the film. The application of polypeptide film to an acoustic transducer was proposed by Fukada, Tamura, and Yamamoto (1968). 

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