Pyro-electricity

If the combination of X-ray and morphological evidence does not determine the space-group uniquely, additional information may be sought by tests for piezo-electric and pyro-electric properties, and by an optical examination for any evidence of rotation of the plane of polarization. (See Chapter VIII.) The results of such tests may settle the matter, since only certain crystal classes have these properties. Only positive results are decisive the apparent absence of piezo-electric or pyro-electric effects may be due to feeble phenomena. 

Pyro-electric and piezo-electric tests. When a crystal belonging to one of the non-centrosymmetric classes is heated or cooled, it develops electric charges and becomes positive at one end and negative at the other end of each polar axis. Therefore, if a crystal is found to be pyro electric, it must belong to one of the classes which lack a centre of symmetry. Various qualitative tests for pyro-electric character have been used. The three most suitable for small crystals are the following ... 

Only positive results are significant feeble pyro-electricity may escape detection by these tests. 

Beta The beta form develops under mechanical deformation of melt-processed materials, typically at temperatures close to its melting transition. This structure provides some unique properties for PVDF piezo- and pyro-electric activity. 

Poly(vinylidene fluoride) (PVDF) is a semicrystalline engineering polymer with very good resistance to chemicals, oxidation, and UV radiation (J. H. Yen, 2006). PVDF is known for its polymorphism crystalline structure and complicated microstructure. It is one of the most widely studied polymers due to its non-linearity, piezo- and pyro-electricity (L. T. Vo, 2007 K. Pramoda, 2005). PVDF can crystallize in at least five well-known crystalline phases (E. Giannetti, 2001, A. Lovinger, 1982 N. S. Nalwa, 1995) ... 

The thermoelectric voltage effect is found in thermo couples and thermopiles. A temperature dependence of capacitance in certain materials gives rise to the pyro-electric effect. 

Anyway, if a charging effect at the temperature variation (represented by scalar T) of the physical system appears at that moment, the cause (T) - effect (D) correlation will be made throughout a tensor, but of a first degree (marked with p), the described effect being called pyro-electricity ... 

Moreover, for the piezo- and pyro-electric effect, the physical nature (i. e., the appearance at the spatial ends of the system in focus, and so also for any crystal, of the opposite electric charges) is cancelled by any structure containing the inversion center, because its presence by the inversion of any pair of ends of the structure re-neutralizes it. Therefore, the appearance of such kinds of effects do not clearly indicate the crystallographic class (group), while existing 21 crystallographic groups without an inversion center, as shown in Table 2.11 (see also Table 2.8 for the presence of the symmetry elements and operations). 

Studies of the unique piezo- and pyro-electric properties of poled poly(vinylidene fluoride) have made a fundamental understanding of origin of the properties the subject of intense activity.Central to this study is the structure determination of the four phases, I, II, III, and IV, and their interconversion to test the correlation between orientation, crystal structure, and activity. The greatest piezoelectric activity is found with phase I, produced by subjecting phase II, the normal form produced on melt crystallizing, to intense electric fields up to 5 x 10 V cm, i.e. poling. The activity has been attributed to bulk polarization of the poly(vinylidene... 

The newest market for KYNAR resins is perhaps the most exciting and most challenging. This is the KYNAR Piezo Film. By definition, a piezo electric material is one that can change polarization in response to mechanical stress. The CH - CF repeat unit was found to exhibit the strongest piezo electric and pyro electric activity of all known polymers. This property was first reported in 1969 based on experiments using KYNAR PVDF. Since that time the piezo and pyro electric properties of PVDF have been the subject of many publications. 

One of the most extensively studied pdymers of medium crystallinity is poly-(vinylidene fluoride). The interest in this polymer results from its use as a piezoelectrical and pyro-electrical material (Nakamura and Wada, 1971 Hiqrakawa and Wada, 1973 Murayama, 1975 Murayama and co-workers, 1975 Muiayama and Hashizume, 1976 Mopsik and Broadhurst, 1975). Many dielectric studies have been made and recent work includes that of Koizumi and co-workers (1969), Sasabe and co-workers (1969), Yano (1970), Kakutani (1970), Osaki and cowoikers(1971), Nakagawa and Ishida (1973), Uemura (1974), Osaki and Iriiida (1974), Yano and co-workers (1974) and Brereton and co-workers (1977). It is evid it from these and eadier studies that the dielectric behaviour of this polymer is one of the most complex of the linear polymer systems. Studies have been made of samples of different (i) ciystal forms, (ii) orientations, (iii) crystallinity, (iv) thermal and electrical histories. At least four relaxation regions are observed, being labelled here as a, Oc,... 

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