Pyroelectric behaviour

Athenstaedt, H. Permanent longitudinal electric polarisation and pyroelectric behaviour of collagenenous structures and nervous tissue in man and other vertebrates. Nature 1970. 228, 830-834. 

This highly stereoregular polymer displayed a very high relaxed dipole moment and poled films displayed pyroelectric behaviour with a pyroelectric response figure of merit superior to that of PVF2. By contrast, the Schrock initiator... 

Carbon nanotubes (CNT) are nanostructured materials with superior mechanical, thermal and electrical properties. They can be classed as single-walled nanotubes (SWNT) or multi-walled nanotubes (MWNT). The SWNT can be thought of as a one-atom-thick layer of graphite, whereas MWNT as multiple concentric rolled layers of carbon atoms. They are highly useful in contributing to the properties of nanoscale structures. Levi et al. (2004) have investigated the properties of aPVDF-CNT matrix. They used both SWNT and MWNT and claimed that the piezoelectric and pyroelectric behaviours were enhanced remarkably over their parent polymer. 

The pyroelectric effect may be defined as the change in spontaneous polarisation, s, as a function of temperature. The symmetry requirements for pyroelectricity are far more restrictive compared with SHG and piezoelectricity. To exhibit a spontaneous polarisation, the material in question must crystallise in one of ten polar crystal classes (1, 2, 3, 4, 6, m, mm2, 3m, 4mm, or 6mm). Thus, polarity is required for pyroelectric behaviour. Determining the pyroelectric coefficient may be done two ways - either measuring the pyroelectric current or the pyroelectric charge. Both techniques will be described. 

C Muralidhar and P. K. C Pillai, Pyroelectric behaviour in barium tilanalclpolyvinylidene flnonde (PVDF) compoaite, Proc. Sih Ira. Syrup. Electrets. Heidelbetg. 1963. pp. 863-870. R. E. Newnhan, Compoaite Electfaceramics. Ferroelectrtcs 68 1 (1966X ). Wotak. Dielectric behaviour of 03-lype piezoelectric oompositca. IEEE Trims. Elect InsuL 28 116 (1993). 

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 ... 

Because a ceramic is composed of a large number of randomly oriented crystallites it would normally be expected to be isotropic in its properties. The possibility of altering the direction of the polarization in the crystallites of a ferroelectric ceramic (a process called poling ) makes it capable of piezoelectric, pyroelectric and electro-optic behaviour. The poling process - the application of a static electric field under appropriate conditions of temperature and time -aligns the polar axis as near to the field direction as the local environment and the crystal structure allow. 

Asymmetric two-site model. If the two sites are not equivalent they may be unequally populated in equilibrium under no field. This will in general give rise to a temperature-dependent polarization in zero field, that is, to a variety of pyroelectricity. A set of pyroelectric elements can be arrayed in a material so that the behaviour cancels, whereas they all contribute to the polarization response to an electric field. 

G. Meslin said that potassium dichromate is paramagnetic. He found that the magnetic susceptibility of the powder is 0-13 x 10 mass unit and G. Quincke gave for a soln. of the dichromate between 18° and 20°, 0-76 Xl0 mass unit. J. Forrest measured the variations in the parallel and transverse components of the magnetization of the crystals. P. Weiss and P. Collet found the paramagnetism of a sohi. of the dichromate is constant between 14° and 50° and the subject was discussed by P. Weiss, L. A. Welo and A. Baudisch, and S. Berkman and H. Zocher. W. G. Hankel and H. Lindenberg found that the crystals exhibit pyroelectricity in that when warmed the (OOl)-face is usually positively electrified, and the (OOf)-face negatively electrified—the reverse behaviour is rare. 

Some crystals, however, exhibit relative permittivity values many orders of magnitude higher than found in normal dielectrics. By analogy with magnetic behaviour, this behaviour is called ferroelec-tricity, and the materials are called ferroelectrics. Ferroelectrics also possess a spontaneous polarisation, Ps, in the absence of an electric field and a mechanical distortion. They are, therefore, a subset of pyroelectrics and, as such, all ferroelectrics are also pyroelectrics and piezoelectrics. The feature that distinguishes ferroelectrics from pyroelectrics is that the direction of the spontaneous polarisation, Ps, can be switched (changed) in an applied electric field. 

The relaxational behaviour of polyvinylidene fluoride (PVDF) is of interest. Its p phase exhibits piezo- and pyroelectric properties and so it is used in various devices, such as sensors, transducers or actuators for different applications. Typically such materials are prepared through a technological process involving stretching and... 

Indeed, the Curie-type behaviour is in agreement with the experimental data [12] obtained by the pyroelectric technique on a SmC mixture with 600 statV/cm or 2 mC/m, see Fig. 13.7. However, the maximum time at Tch is Umited by the value of Ism 13ps. To account for this saturation the fourth order term B9" in the free energy has to be taken into account. 

Achiral smectic materials with anticUnic molecular packing are very rare [40] and their antiferroelectric properties have unequivocally been demonstrated only in 1996 [41]. The antiferroelectilc properties have been observed in mixtures of two achiral components, although no one of the two manifested this behaviour. In different mixtures of a rod like mesogenic compound (monomer) with the polymer comprised by chemically same rod-like mesogenic molecules a characteristic antiferroelectric hysteresis of the pyroelectric coefficient proportional to the spontaneous polarization value has been observed for an example see Fig. 13.27a. Upon application of a low voltage the response is linear, at a higher field a field-induced AF-F transition occurs. 

Although piezoelectric properties such as relative permittivity and piezoelectric coefficients change with the temperature, the total effect for the calibration of PZT sensors far below the Curie temperature is very small. However, PVDF foils show a significant temperature dependence of the pyroelectric properties in addition to their temperature depending piezoelectric features. Thus PVDF sensors are quite temperature sensitive, and in general, an appropriate temperature compensation is necessary. This behaviour, however, can be improved by special forming and spatial distribution of the PVDF sensors [100]. 

The occurrence of piezoelectric behaviour in LB films has been known for some time [57,58], and a 30 X-type layer LB film of (37) was found to give opposite signs of the piezoelectric strain coefficients d i and d [59], the latter having a value of 1.5 pC which is approximately an order of magnitude lower than that of the well-documented polymer poly(vinylidene fluoride) (PVDF). Values for 31 of 0.023 and 0.170 pC N have also been obtained for alternate-layer structures of 22-tricosenoic acid with docosylamine, and a ruthenium complex with docosanoic acid respectively [60]. As the use of pyroelectric materials in detector applications requires that the materials possess only low levels of piezoelectricity (high levels introduce problems of microphony), this suggests that the former materials would be better suited for pyroelectric detector applications, while the latter system would be more appropriate for piezoelectric-based applications. 

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