Pyroelectric effect spontaneous electric polarization

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. 

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),... 

Piezoelectrics. In the neutral-polar classes there are polar directions (not axes), which can be described by several vectors with their vector sum equal to zero. Such crystals do not possess spontaneous polarization and do not manifest polar properties (such as pyroelectric, photogalvanic or linear electrooptical effects) however, the polarization can be induced not only by an electric held but also by a pure mechanical stress. These crystals are called piezoelectrics. Examples are crystals of quartz or ZnS having cubic symmetry with four polar direction but no polar axis. Fig. 13.1b. Such crystals are used in technics as microphones, mechanical micro-motors and sensors, etc. 

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 effect of the film orientation on the electrical properties is shown in Table 27.6, where the spontaneous pyroelectric coefficient of perovskite (Ca, Pb) TiOs thin films is indicated. It is observed that the pyroelectric response increases with a preferred orientation along the polar c-axis of the perovskite. 

Of the 32 ciystal classes, 21 are non-centrosymmetric (not having a center of synunetiy), and of these, 20 exhibit direct piezoelectricity (the 21 is the cubic class 432). Ten of these are polar (i.e. they spontaneously polarize), having a dipole in their unit cell, and exhibit pyroelectricity. If this dipole can be reversed by the application of an electric field, the material is said to be ferroelectric (see Table 11.8). Then, the ferroelectric materials are not the only ones to exhibit this effect, as shown by quartz (synunetiy group 32), zinc oxide (6 nun) or aluminum nitride (6 mm). 

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