Pyroelectric crystals

The most important materials among nonlinear dielectrics are ferroelectrics which can exhibit a spontaneous polarization PI in the absence of an external electric field and which can spHt into spontaneously polarized regions known as domains (5). It is evident that in the ferroelectric the domain states differ in orientation of spontaneous electric polarization, which are in equiUbrium thermodynamically, and that the ferroelectric character is estabUshed when one domain state can be transformed to another by a suitably directed external electric field (6). It is the reorientabiUty of the domain state polarizations that distinguishes ferroelectrics as a subgroup of materials from the 10-polar-point symmetry group of pyroelectric crystals (7—9). 

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. 

Ferroelectrics. Among the 32 crystal classes, 11 possess a centre of symmetry and are centrosymmetric and therefore do not possess polar properties. Of the 21 noncentrosymmetric classes, 20 of them exhibit electric polarity when subjected to a stress and are called piezoelectric one of the noncentrosymmetric classes (cubic 432) has other symmetry elements which combine to exclude piezoelectric character. Piezoelectric crystals obey a linear relationship P,- = gijFj between polarization P and force F, where is the piezoelectric coefficient. An inverse piezoelectric effect leads to mechanical deformation or strain under the influence of an electric field. Ten of the 20 piezoelectric classes possess a unique polar axis. In nonconducting crystals, a change in polarization can be observed by a change in temperature, and they are referred to as pyroelectric crystals. If the polarity of a pyroelectric crystal can be reversed by the application on an electric field, we call such a crystal a ferroelectric. A knowledge of the crystal class is therefore sufficient to establish the piezoelectric or the pyroelectric nature of a solid, but reversible polarization is a necessary condition for ferroelectricity. While all ferroelectric materials are also piezoelectric, the converse is not true for example, quartz is piezoelectric, but not ferroelectric. 

In solid state all the 10 pyroelectric crystal groups allow in principle for bistable switching behaviour. This is the proper ferroelectricity. Under certain conditions ferroelectricity (improper) can be realized in liquid crystals. This was shown by Meyer and coworkers115 in 1975. Since that time intense activities have been initiated, applying this property for flat-panel devices, switches, light modulators etc. In principle, three effects can be observed and used ... 

The piezoelectric crystals are those that become polarized or undergo a change in polarization when they are stressed conversely, when an electric field is applied they become strained. The 10 polar crystal types are pyroelectric as well as piezoelectric because of the polarization inherent in their structure. In a pyroelectric crystal a change in temperature produces a change in polarization. 

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. 

For portable instruments (Figure 12.4), miniature X-ray generators of low power (<500 mW) have been developed. The required electrons that produce X-rays in a target material (Cu) are obtained either by impact laser or from a pyroelectric crystal. 

The generator taken as an example (Figure 12.5) uses a pyroelectric crystal (tantalum) oriented in a fashion so that its top surface depolarizes reversibly when heated. In this way, when the temperature increases the uppermost face gets positively charged and attracts electrons from the ionized gas in the surroundings. White... 

Figure 4.11 Pyroelectric crystals (a) an electric dipole, p, represented by a vector pointing from the negative to the positive charge (b) schematic representation of a unit cell containing a permanent electric dipole (c) a pyroelectric crystal built of unit cells containing a permanent electric dipole...

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.]... 

Pyroelectric crystals are ones that are spontaneously polarizable (see below) and in which a change in temperature produces a change in that spontaneous polarization. A limited number of pyroelectric crystals have the additional property that the direction of spontaneous polarization can be reversed by application of an electric field, in which case they are known as ferroelectrics. Thus a ferroelectric is a spontaneously polarized material with reversible polarization. Before proceeding much further it is important to appreciate that not all crystal classes can exhibit polar effect. 

The neutral beam density in the interaction region can be determined from a measurement of the energy deposited by the fast neutral beam into a pyroelectric crystal whose response is first calibrated by a well-characterized ion beam, as discussed by Wetzel et al. (1987). As an alternative, the well-established Kr or Ar absolute ionization cross sections (known to better than 5%) can be used to calibrate the pyroelectric crystal. The calibrated detector is then used to determine the flux of the neutral target beam in absolute terms. This procedure avoids the frequent and prolonged exposure of the delicate pyroelectric crystal to fairly intense ion beams (Freund et al., 1990 Tamovsky and Becker, 1992). 

In the case of a pyroelectric solid a change of temperature induces a polarisation change. The change in polarisation found on heating is reversed on cooling. Pyroelectric crystals are a subset of piezoelectrics. All pyroelectric crystals are piezoelectrics, but not all piezoelectrics demonstrate pyroelectricity. A material that is a pyroelectric is found to possess a spontaneous polarisation, Ps. This means that a pyroelectric crystal shows a permanent polarisation that is present both in the absence of an electric field and in the absence of mechanical stress. 

What is the relationship between ferroelectric and pyroelectric crystals ... 

The crystal structure of pyroelectric crystals must contain ... 

There is a structural requirement for ferroelectricity. There are a total of 32 different symmetry point groups, 21 of which do not possess a center of symmetry. Ferro-electrics are part of a small subgroup of noncentrosym-metric crystals. Related properties are piezoelectricity and pyroelectricity. Dielectrics belonging to all but one of the groups of noncentrosymmetric crystals are piezoelectric. Pyroelectric crystals form a further subgroup of 10 types of crystal having especially low symmetry as shown in Table 31.5. 

The electric field developed across a pyroelectric crystal can be extremely large when it is subjected... 

A large number of pyroelectrics crystallize in the so-called layer perovskite phases. These phases are made of layers of perovskites separated by... 

Fig. 13.1 Examples of nrai-polar, piezoelectric and pyroelectric crystals calcite (a), ZnS (b) and tourmalme (c). An arrow shows the direction of the polar axis in tourmaline...

Pyroelectrics. In a crystal belonging to polar classes there is only one polar axis with a symmetry of the polar vector. These crystals are also piezoelectric, but, in addihon, manifest spontaneous polarization and all other polar properties. Such crystals are called pyroelectrics. An example is tourmaline having symmetry and shown in Fig. 13.1c. Pyroelectric crystals are also used in techniques as piezoelectrics and also as detectors of infrared light or a heat how. There are many organic pyroelectric crystals, e.g., p-nitroaniline, one of the best generators of the optical second harmonic. 

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