Pyroelectric material

The discussion draws on the review articles by R.W. Whatmore [1] and by R.W. Whatmore and R. Watton [2], The now classical text by R.A. Smith, F.E. Jones and R.P. Chasmar [3] is recommended for supplementary reading. 

When an electric field E is applied to a polar material the total electric displacement D is given by 

Electroceramics Materials, Properties, Applications. 2nd Edition. Edited by A. J. Moulson and J. M. Herbert. 2003 John Wiley Sons, Ltd ISBN 0 471 49747 9 (hardback) 0 471 49748 7 (paperback) 

Therefore the pyroelectric coefficient is a vector but, because in practical applications the electrodes that collect the pyrocharges are positioned normal to the polar axis, the quantities are usually treated as scalars, and this is done in the following discussion. 

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. 

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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. 

Thus, pie is also a significant parameter for evaluating pyroelectric materials. 

Pyrocerams, 21 381 Pyro-Chek 68PB, 11 470-174 Pyrochemical processes, in plutonium metal preparation, 19 676 Pyrochlore, 17 133-134, 140, 141 colorants for ceramics, 7 347t Pyroelectricity, 11 95, 100, 106, 107 Pyroelectric materials, smart, 22 708t, 709 Pyroform process, 25 171 Pyrogallol... 

Various crystalline materials with desired properties have been synthesized, and this has driven the utilization of single crystals in the production of semiconductor, opto-electronic, piezoelectric, and pyroelectric materials. 

A wide array of ferroelectric, piezoelectric and pyroelectric materials have titanium, zirconium and zinc metal cations as part of their elemental composition Many electrical materials based on titanium oxide (titanates) and zirconium oxide (zirconates) are known to have structures based on perovskite-type oxide lattices Barium titanate, BaTiOs and a diverse compositional range of PZT materials (lead zirconate titanates, Pb Zr Tij-yOs) and PLZT materials (lead lanthanum zirconate titanates, PbxLai-xZryTii-yOs) are among these perovskite-type electrical materials. 

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 time dependence of the temperature of the pyroelectric material can be related to the spatial dependence of the temperature by means of Fourier s equation for heat... 

This only contains parameters describing properties of the pyroelectric material, and is therefore a figure-of-merit, which can be used to compare different materials for their potential voltage responsivities. If CA Jp CE the voltage response is proportional to ... 

All pyroelectric materials are also piezoelectric (i.e. they generate charge in response to mechanical strain). This has a number of important consequences ... 

The following discussion separates pyroelectric materials into 3 groups intrinsic pyroelectrics which are operated well below Tc, dielectric bolometer materials which are operated close to Tc, but with an electrical bias applied and ferroelectric thin films. 

There are many different types of pyroelectric, including single crystals, polymers, ceramics and thin films and several reviews [2,3,28,29] have considered the properties of many pyroelectric materials in detail, so the discussion here will be confined to a brief review of pyroelectric ceramics and thin films. 

R. W. Whatmore and R. Watton, Pyroelectric Materials and Devices, Published in Infrared Detectors and Emitters Materials and Devices, P. Capper and C. T. Elliott ed., Chapman and Hall, London, 99, 2000. 

Eleven acentric crystal classes are chiral, i.e., they exist in enantiomorphic forms, whereas ten are polar, i.e., they exhibit a dipole moment. Only five (1,2, 3, 4, and 6) have both chiral and polar symmetry. All acentric crystal classes except 432 possess the same symmetry requirements for materials to display piezoelectric and SHG properties. Both ferroelectricity and pyroelectricity are related to polarity a ferroelectric material crystallizes in one of ten polar crystal classes (1, 2, 3,4, 6, m, mm2, 3m, 4mm, and 6mm) and possesses a permanent dipole moment that can be reversed by an applied voltage, but the spontaneous polarization (as a function of temperature) of a pyroelectric material is not. Thus all ferroelectric materials are pyroelectric, but the converse is not true. 

A limited number of pyroelectric materials have the additional property that the direction of the polarization can be changed by an applied electric field or mechanical stress. Where the change is primarily due to an electric field the material is said to be ferroelectric when it is primarily due to a stress it is said to be ferroelastic. These additional features of a pyroelectric material cannot be predicted from crystal structure and have to be established by experiment. 

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 materials are used mainly for the detection of infrared radiation. The elements for the detectors are typically thin slices of material (e.g. 1.0 x 1.0 x 0.1 mm) coated with conductive electrodes, one of which is a good absorber of the radiation. 

In this equation H = pcAh is the heat capacity of the element where c is the specific heat of the pyroelectric material and, in this context, p is its density. In what follows the product pc, the volume specific heat, is given the symbol c. ... 

To obtain a continuous response from a pyroelectric material the incident radiation is pulsed, and this situation is analysed by assuming that the energy varies sinusoidally with frequency co and amplitude W0. Equation (7.6) then becomes... 

Table 7.1 Properties of some pyroelectric materials. (Data should be regarded as approximate)...

Pyroelectric materials respond to changes in the intensity of incident radiation and not to a temporally uniform intensity. Thus humans or animals moving across the field of view of a detector will produce a response as a result of the movement of their warm bodies which emit infrared radiation (a 10 /rm). To obtain a response from stationary objects requires the radiation from them to be periodically interrupted. This is usually achieved by a sector disc rotating in front of the detector and acting as a radiation chopper. 

All pyroelectric materials are piezoelectric and therefore develop electric charges in response to external stresses that may interfere with the response to radiation. This can largely be compensated for by the provision of a duplicate of the detecting element that is protected from the radiation by reflecting electrodes or masking, but which is equally exposed to air and mounting vibrations. The principle is illustrated in Fig. 7.7. The duplicate is connected in series with the detector and with its polarity opposed so that the piezoelectric outputs cancel. This results in a small reduction in sensitivity (< 3 dB) but compensation is an... 

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