Phase electret

The dependence of built-in field on film thickness is reported in Fig. 3.10. It is seen, that this field increases essentially with the film thinning as l/h. The estimations had shown that for the films of several tens nm thickness and reasonable material characteristics, can achieve several hundreds kV/cm as it is shown in the inset to Fig. 3.10. For the sake of comparison the thermodynamic coercive field is shown by dashed line in the inset. The emergence of electret phase due to built-in field will be discussed later. 

L and dependence on temperature and film thickness in Fig. 3.24. It follows from Fig. 3.24a, c, that at fixed temperature the average polarization L decreases for the film thinning (compare curves 1-5). Built-in field smears the temperature of phase transition and susceptibility maximum its influence increases with the films thinning (see Fig. 3.24b, d). Moreover, the order parameter behavior for the thinnest possible films resembles that for thin films of ordered ferroelectrics with the thickness less than critical one (compare the curves 5 in Fig. 3.24c, d with Fig. in the paper [54]). Built-in field induces order parameter in the film with maximal disorder, see dotted curves 5 in Fig. 3.24a, b. However hysteresis loops is absent on these curves so that the behavior resembles that of electret state. 

Glinchuk, M.D., Morozovska, A.N., Eliseev, E.A. Ferroelectric thin films phase diagrams with self-polttfized phase and electret state. J. Appl. Phys. 99,114102(12) (2006)... 

Ferro-, and/or pyro-, and/or piezoelectric polymer electret Charges in polar chemical bonds form molecular dipoles Crystalhtes with oriented dipoles move inside amorphons phase Interfacial charge and amorphons phase can modify the observed effects... 

In this context, the possibility to tune tire piezo- and pyroelectricity of specific composites (Floss et al. 2000) by means of separate poling of the inorganic particles and of the polymer crystallites should also be mentioned. In addition, piezo-, pyro-, and ferroelectric polymers such as PVDF and its relevant copolymers may be optimized by controlling fire poling of the amorphous and of the crystalline phase, as well as of the interface between fiiem (Maxwell-Wagner interface polarization) separately (Rollik et al. 1999). Furthermore, it is possible to follow the examples of the classical electret transducers (witti polymeric space-charge electrets) or of the dielectric-elastomer transducers (sometimes also called electro-electrets) and to... 

In contrast to charge electrets with their stored real charges, dipole electrets are characterized by a preferential orientation of their molecular dipoles. Here amorphous and semicrystalline dipole electrets are distinguished. Amorphous dipole electrets are characterized by a quasi-permanent dipole polarization in the amorphous phases (Fig. 3a), whereas semicrystalline ones show a permanent dipole orientation in the crystalline phases and additionally contain Maxwell-Wagner compensation charges at the interfaces between amorphous and crystalline regions for stabilizing the polarization (Fig. 3b) (Mopsik and Broadhurst 1975 Broadhurst et al. 1978 Sessler et al. 1992). 

In semicrystalline dipole electrets, polar crystallites are present in addition to the polar amorphous phase (Fig. 2b). In die technically most interesting semicrystalline dipole electrets such as polyvinyhdene fluoride (PVDF) and its copolymers with trifluoro ethylene (P(VDF-TrFE)) (Lovinger 1983) or hexafluoropropylene (P(VDF-HFP)), odd Nylons 7 and 11, polyureas, polyureflianes (PU), and some liquid crystalline polymers, the crystallites are ferroelectric (Vasudevan et al. 1979 Hattori et al. 1996). The terpolymer poly(vinyhdene-fluoride-trifluoroethylene— chlorotrifluoroethylene) (P(VDF-TrFE-CTFE)) has been shown to have relaxor ferroelectric properties as the CTFE group destabilizes die long-range order of the ferroelectric phase (Xu et al. 2001). 

Rychkov et al. studied the effect of modification of PE, PTFE, and FEP wifli various chemical solvents on their electret properties (Ryehkov et al. 2011a, b, 2012a, b, e, 2013). The chemicals for modification were used either in gas-phase state as vapors or in liquid state. 

Though most insulating polymers are not intrinsically ferroelectric, polar materials can be produced by charge injection into insulating polymers. Such electrets have been known, studied and utilized for many years. Notable exceptions are poly(vinylidene difluoride) (PVDF) and its copolymers which are ferroelectric. There have been numerous studies of orientation, poling, structure and phase transitions of these polymers. The large piezoelectric effect of PVDF when in the poled state has led to its application in acoustic and ultrasonic transducers. 

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