The pyroelectric effect

The relationship between the change in spontaneous polarisation, APs and the change in temperature, A T, can be written as  

The pyroelectric effect that is normally observed in a crystal is, in fact, composed of two separate effects called the primary (or true) pyroelectric effect and the secondary pyroelectric effect. If a crystal is fixed so that its size is constant as the temperature changes, the primary effect is measured. Normally, though, a crystal is unconstrained. An additional pyroelectric effect will now be measured, the secondary pyroelectric effect, caused by strains in the crystal produced by the thermal change. In general, the secondary effect is much greater than the primary effect, but both are utilised in devices. 

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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. 

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. 

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... 

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. 

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. 

The difference between the two definitions rests with the fact that a pyroelectric crystal must possess an overall (observable) permanent electric dipole. Thus a pyroelectric crystal is built from unit cells, each of which must contain an overall electric dipole, (Figure 4.11b). The pyroelectric effect will only be observed, however, if all of these dipoles are aligned throughout the crystal, (Figure 4.11c). [Note that a ferroelectric crystal is defined in a similar way. The difference between a pyroelectric crystal and a ferroelectric crystal lies in the fact that the direction of each overall electric dipole in a ferroelectric crystal can be altered by an external electric field.]... 

A change in coordinate system transforms the polar vector p according to equation (4.5). It is easy to show that the pyroelectric effect can only appear in directions which are polar and unique, and hence only in the following groups ... 

The pyroelectric effect is due, above all, to the piezoelectric effect (Section 4.4.5). During heating the crystal deforms because of the thermal expansion. 

As in the case of piezoelectrics, the elementary dipoles will cancel out if the crystallographic unit cell has a centre of symmetry. However, another condition is also needed to produce a spontaneous polarisation, the presence of a unique polar axis, which is a direction in the crystal unrelated by symmetry to any other direction, not even the antiparallel direction. The dipoles lie parallel to the polar axis of the crystal (see Section 5.1.3). Of the 20 piezoelectric crystal classes, only 10 fulfil this criterion and give rise to the pyroelectric effect. The relationship between the appearance of piezoelectricity and pyroelectricity and the symmetry of the crystal is set out in Figure 11.12. 

The spontaneous polarization of a dielectric depends strongly on T, this is the pyroelectric effect that we use for infrared (IR) detection (e.g., intruder alarms and thermal imaging). 

The pyroelectric effect can also be employed as a sensor of an enzymic reaction. Dessy et al (57) placed two poly(vinylldene fluoride) films into contact, with one exposed surface coated with enzyme and contacting a flow injection sample stream. The potential resulting from the thermal bias across the films could be related to the enzyme substrate concentration. 

The pyroelectric effect is used in pyroelectric energy harvesting, and the electrocaloric effect is currently being explored for refrigeration. Perovskite aystals that show the pyroelectric effect contain a unique polar axis and a spontaneous electric polarisation, P, directed along this axis. As all ferroelectrics are also pyroelectrics (Chapter 6),... 

The importance of the pyroelectric effect is considered, with particular reference to the application of thermal imaging. A range of pyroelectric materials is reviewed, and the suitability of Langmuir-Blodgett (LB) films to diis application is highlighted. 

The pyroelectric effect is well documented, and has been reported for a wide range of inorganic and organic materials [1]. One of the most promising applications of pyroelectricity is in the area of infra-red or thermal imaging. Thermal information from a scene is derived by measuring the pyroelectric currents from elements in an array onto which the scene has been projected. In order to optimise the performance of thermal imaging devices, it is necessary to be able to characterise pyroelectric materials,both in terms of their intrinsic properties,and their performance in m el systems. 

The phase change in ferroelectric liquid crystals can give rise to the pyroelectric effect [3]. Values of p and p/Er are also respectable. However, all the materials reported to date have a large dielectric loss. In addition, it is not possible to obtain optimum thickness structures, whilst still maintaining acceptable thermal mass and conductivity. 

The pyroelectric effect of pyroelectric pyroelectrics usually exists below a certain transition temperature called the Curie point, T, in proper pyroelectrics and is more temperature dependent than that of the non-pyroelectric pyroelectrics. 

Thermodynamic analysis of the pyroelectric effect yields the expression... 

In the discussions which follow, attention will be directed toward two approaches which have found the greatest utility in infrared systems, namely, bolometers and the pyroelectric effect. Others will be discussed briefly. A list of thermal effects is included in Table 2.4. 

In addition to the various types of bolometers and the pyroelectric effect, several other thermal effects have been exploited as radiation detectors. They are described below. 

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