Pyroelectric effects

Practical application of matrix notation in the component transformation could be found in tables (Hearmon 1957 Petrzilka et al. 1960, p. 99). Transformation matrix is substantially simplified in case of the rotation around crystallographic axis. It results therefore also in simpler transformation of the piezoelectric coefficients. As an example, the transformation of the piezoelectric coefficients di/j, and ej j, for a-quartz crystal 

Every pyroelectric material is necessarily also piezoelectric. On the contrary, there are materials from polar-neutral symmetry classes, where no pyroelectricity is allowed by the symmetry. a-Quartz (symmetry class 32) is a good example of it. 

The thermodynamics of the pyroelectric effect has been already described in Table 4.1 in general. We try to explain this phenomenon more in details in following text. Let ITS choose the independent variables - temperature , electric field Ek and mechanical stress T. Bormdary conditions of the mechanically free sample with shortened electrodes are assumed for constant electric field and mechanical stress. The equations of state for the electric displacement reduce to 

Similar relationship between electric displacement A and the temperature change A could be derived through the detour demonstrated by the dashed arrows and A in Fig. 5.3. The temperature change results in mechanical strain due to the thermal expansiom Mechanical strain is further related to the electric displacement D, because of piezoelectric effect. For the independent variables A and at constant electric field Ek, we can get the electric displacement 

Similarly, the mechanical strain at constant electric field is 

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Success of depositing compounds where an 18-carbon chain was attached to one end of an azobenzene group and various different hydrophilic groups attached to the other end has been reported in X and Z mode [52] and piezo-and pyroelectric effects were demonstrated. 

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. 

Ceramics that display the pyroelectric effect also exhibit a variatioa ia polarizatioa with temperature, as showa ia Figure 2. The aature of the temperature variatioa is depeadeat oa the type of crystallographic transformatioa that the material displays at the Curie poiat ie, whether the transitioa is first or secoad order. 

The most commercially important application that takes advantage of the pyroelectric effect ia polycrystalline ceramics is iafrared detection, especially for wavelengths ia excess of 2.5 p.m. AppHcations range from radiometry and surveillance to thermal imaging, and pyroelectric materials work under ambient conditions, unlike photon detectors, which require cooling. 

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],... 

The above conclusions introduce intrinsic limitations to the use of the ID conjugated systems in nonlinear optical devices. Although these may benefit (38) from the high nonlinearities,their response speed will be limited by the motion of such defects. These may also be formed by other means than light and this will clearly have implications on photoelastic, pyroelectric and piezoelectric effects as well. We point out that materials like polydiacetylenes may show appreciable quadrupolar pyroelectric effect (39). 

In fact, as also indicated in Figure 8.12, an achiral SmC phase possesses antiparallel polarized sheets, in this case with a pitch of half the layer spacing. Photinos and Samulski have made much of this polar symmetry of the SmC phase,28 but neither the SmCA nor the SmC phases have net macroscopic polar symmetry (the SmCA is Di/, while the SmC is C21, as mentioned above), and thus neither shows properties associated with polar materials (e.g., a pyroelectric effect). 

If we consider the mechanism of the pyroelectric effect in the microscopic level, the spontaneous polarization P is given by [17]... 

Recently, Curtin and Paul (114) have used the pyroelectric effect for the direct assignment of the absolute structure of crystals of p-bromobenzoic anhydride. 

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. 

Lang,S.B. Pyroelectric effect in bone and tendon. Nature 212,704 (1966). 

Murayama.N. Piezoelectric and pyroelectric effects of polymer electrets. Microsymposium on Electrical Properties of Polymers, Tokyo (Jan. 1972). 

Dt, electric displacement Pi AT, pyroelectric effect Ky Ej, permittivity Ky Hj, magnetoelectric polarizability dyic rSjk, piezoelectric effect... 

This expression is the basic description for the use of the pyroelectric effect in a host of sensor applications including the well known optical detection devices (82,83). A particularly useful way of describing this type of system is with an equivalent circuit where the pyroelectric current generator drives the pyroelectric impedance and the measuring amplifier circuit as shown in Figure 11. 

The origin of the pyroelectric effect, particularly in crystalline materials, is due to the relative motions of oppositely charged ions in the unit cell of the crystal as the temperature is varied. The phase transformation of the crystal from a ferroelectric state to a paraelectrlc state involves what is called a "soft phonon" mode (9 1). In effect, the excursions of the ions in the unit cell increase as the temperature of the material approaches the phase transition temperature or Curie temperature, T. The Curie temperature for the material used here, LiTaO, is 618 C (95). The properties of a large number of different pyroelectric materials is available through reference 87. For the types of studies envisaged here, it is preferable to use a pyroelectric material whose pyroelectric coefficient, p(T), is as weakly temperature dependent as possible. The reason for this is that if p(T) is independent of temperature, then the induced current in the associated electronic circuit will be independent of ambient temperature and will be a function only of the time rate of change of the pyroelectric element temperature. To see this, suppose p(T) is replaced by pQ. Then Equation U becomes... 

The Curie brothers were drawn to the subject of piezoelectricity because of their familiarity with a phenomenon known for many centuries, that of pyroelectricity. Pyroelectricity refers to the tendency of certain materials to generate an electric current when they are heated. The phenomenon was first described in 314 b.c.e. by the Greek philosopher Theophrastus (ca. 370-ca. 285 b.c.e.), who observed the effect with the mineral tourmaline. Little research was done on pyroelectricity until the early 1800s, when the effect was rediscovered and studied in detail by the Scottish physicist Sir David Brewster (1781-1868). Then in 1878, William Thomson, Lord Kelvin (1824-1907), offered an explanation of the atomic changes that take place when pyroelectric effects occur. These developments in the understanding of pyroelectricity led the Curie brothers to study the possibility of producing electricity from crystals by physical means other than heating. 

Following Maxwell s equations, the spontaneous polarization is connected with surface charges Ps = cr. The surface charges in general are compensated by charged defects. A temperature change changes the spontaneous polarization. This effect is called the pyroelectric effect. 

The contribution E(ds/dT) (Eq. (7.3)) can be made by all dielectrics, whether polar or not, but since the temperature coefficients of permittivity of ferroelectric materials are high, in their case the effect can be comparable in magnitude with the true pyroelectric effect. This is also the case above the Curie point and where, because of the absence of domains, the dielectric losses of ferroelectrics are reduced, which is important in some applications. However, the provision of a very stable biasing field is not always convenient. 

Since pyroelectric materials are polar, they are also piezoelectric, and the strain resulting from thermal expansion will result in the development of a surface charge. However, this is a small effect that seldom exceeds 10% of the primary pyroelectric effect. 

Pyroelectric Separator—The pyroelectric effect is used mainly for the separation of quartz from feldspar. The mixture is heated in the hopper feeder by means of steam. On passing to a cold rotating cylinder below, the material causes pyroelectric polarization to appear on the quartz. This mineral adheres to the cylinder, while the feldspar is not affected. 

The pyroelectric effect results from the electric charge separation resulting from the stress caused by the temperature change. A small potential difference, sometimes too small to measure, develops across... 

FIGURE 5,18. The pyroelectric effect. Charges develop on opposite faces on heating a pyroelectric crystal. These changes can be located by suitable powders, as shown. 

Pyroelectricity The observation of a pyroelectric effect implies a noncentrosymmetric space group. It can only exist if there is a unique polar axis in the point group of the crystal. If no effect is observed, it is presumed (but not certain ) that the crystal possesses a center of symmetry. 

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