Pyroelectric applications

Pyroelectric devices convert changing incident thermal radiation to an electrical output, and are now much used in intruder detectors, thermal imaging systems etc. Conventionally, ceramics have been used in such applications however, considering the desirable properties of large pyroelectric coefficient, high volume resistivity, low dielectric constant and loss, and low specific heat, it can be seen that, apart from the rather low pyroelectric coefficient, polymeric materials are superior to ceramics in several respects. 

A number of experimental configurations have been described in the literature [61-64], and one or two simple devices based on the use of 

PVDF are commercially available (Siemens, Microwatt Applications). The relatively low pyroelectric coefficient of PVDF and, to some extent, the difficulty of handling small pieces of the thinnest films seem to have inhibited its more widespread adoption in practical devices. The availability of large active areas does not of itself seem to have provided sufficient advantage to device engineers. 

This chapter has provided the reader with an introduction to a unique class of polymers. From the discussions of properties and applications, it can be concluded that ferroelectric polymers offer advantages over existing ferroelectric materials, including flexibility, robustness and availability in large areas, thin sheets or unusual geometries. 

However, it is widely believed that these materials will become established in applications that exploit their unique features, rather than through the substitution of existing transducer materials. This requires an innovative approach, which, on examination of recent literature, appears to have already begun and looks set to continue. 

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The maj or pyroelectric applications are those related to infrared sensing such as cooking controls, fire or heat alarms, door openers, etc. Typical materials are LiTa03, PbTi03, andPb(Zr.Ti)03. 

FLCPs can be used for many different applications, especially for electro-optic, thermo-optic, nonlinear optic, pieziolectric, or pyroelectric applications. In contrast to low-molar-mass FLCs, polymers have a much higher viscosity. This can be an advantage or a disadvantage for possible applications. Due to the high flow viscosity of the polymers, very simple flexible di.splays can be produced. The optical... 

Nualpralaksana, S., Heimann, R.B., and Phanichphant, S. (2004) Characterization of hydrothermally synthesized PLZT for pyroelectric applications. J. Electroceram., 13, 209-214. 

The ferroelectricity, combined with excellent mechanical properties and the ease of fabrication, have rendered PVF2 a versatile material for piezoelectric and pyroelectric applications (transducer, stems, ultrasonics, loudspeakers, microphones, finger-press switches, ultrared detectors). Furthermore, PVF2 can be found in semiconductor applications and in the electrical-electronic market (plenum cables, aircraft wiring, computer... 

The heat capacity of a material is defined as the amount of heat required to raise its temperature by 1 K. In pyroelectric applications it is more... 

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. 

The compounds K5Nb3OFi8 and Rb5Nb3OFi8 display promising properties for their application in electronics and optics. The compounds can be used as piezoelectric and pyroelectric elements due to sufficient piezo- and pyroelectric coefficients coupled with very low dielectric permittivity. In addition, the materials can successfully be applied in optic and optoelectronic systems due to their wide transparency range. High transparency in the ultraviolet region enables use of the materials as multipliers of laser radiation frequencies up to the second, and even fourth optical harmonic generation. 

The FT-IR technique using reflection-absorption ( RA ) and transmission spectra to quantitatively evaluate the molecular orientation in LB films is outlined. Its application to some LB films are demonstrated. In particular, the temperature dependence of the structure and molecular orientation in alternate LB films consisting of a phenylpyrazine-containing long-chain fatty acid and deuterated stearic acid (and of their barium salts) are described in relation to its pyroelectricity. Pyroelectricity of noncentrosymmetric LB films of phenylpyrazine derivatives itself is represented, too. Raman techniques applicable to structure evaluation of pyroelectric LB films are also described. 

Sources and detectors Specific discussions of sources and detectors have been covered elsewhere in this article. The issues here are more service and performance related. Most sources have a finite lifetime, and are service replaceable items. They also generate heat, which must be successfully dissipated to prevent localized heating problems. Detectors are of similar concern. For most applications, where the interferometer is operated at low speeds, without any undesirable vibrational/mechanical problems, the traditional lithium tantalate or DTGS detectors are used. These pyroelectric devices operate nominally at room temperature and do not require supplemental cooling to function, and are linear over three or four decades. 

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. 

The pyroelectricity or depolarization current is closely related to the piezoelectricity of Groups (B) and (C), because the pyroelectric current is caused by the thermal activation of charges in the film. The application of polypeptide film to an acoustic transducer was proposed by Fukada, Tamura, and Yamamoto (1968). 

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. 

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