Electrets, polymer electricity

Electrical polarization is a fundamental property of polymer materials transformed into electrets in electrical fields [86-88]. Electrical polarization is used to promote adhesion between layers of antirust film materials, limit permeability of the layers, impede wetting, reduce waterproof properties, raise the quality of printing and intensify Cl saturation [68,70,88]. 

The described advantages of electret polymer coatings have not been called for until recently chiefly due to the lack of information and inaccessibility of electret techniques. It is taken for granted today that the electrically polarized state is a natural physical state of polymer dielectrics. The realization of merits that are intrinsic to polymer coatings may open new... 

The presence of dispersed fillers in the polymer material in low amounts may intensify electrization, increase the residual charge and change the friction coefficient. Introduction of the filler in the electret state exerts a still stronger effect on polymer electrization on frictional interaction with metals. Depending on the direction of the field intensity vector formed by the filler particles, the field generated by triboelectrization can be attenuated or intensified. This means that the principle of the electret-triboelectrization superposition is realized [49], which can be used to regulate the parameters of frictional interactions. Thus, by the introduction of the electret filler, e.g. mechanically activated F-3 powder, it is possible to decrease the friction force (Fig. 4.9). 

The polarization induced in polymer dielectrics by the application of an electric field or by other charging treatments can, in a few cases, persist for long (ca. months) periods at room temperature. The polymers thus treated are termed electrets. Polymer films that have been subjected to this polarization treatment or poled manifest piezoelectric and... 

Mellinger A, Camacho Gonzalez F et al (2004) Photostimulated discharge in electret polymers an alternative approach for investigating deep traps. IEEE Trans Dielectr Electr Insul 11 218-226... 

Piezoelectric Effects and ElectretS. When certain polymers, eg, PTFE and poly(vinylidene fluoride), are placed while hot in a d-c electric field (polarized), and cooled while the d-c field is still applied, stable trapped charges remain (71). Such permanently polarized materials, called electrets, may possess piezoelectric properties, which are useful in microphones and small loudspeakers. Heating an electret produces electrical discharges between opposed and shorted electrodes... 

They produced high performance electrets from thin polymer films metallized so as to yield high capacitance. Both electrical and mechanical properties of these transducers have been remarkable examples of how applications of science of solids, including knowledge of electron traps, conduction processes in insulators and the viscoelastic phenomena of semicrystalline polymers, can be combined.(6) Incidentally, similar ideas have been applied to optimization of the properties of particle microphones, through assemblies of perfectly microspherical polymer carbon systems. These have shown what limits of performance... 

Consideration of the structure of polyvinylidene fluoride (65) assuming a barrier of 3 kilo cal per mole for rotational minima of conformation of the chain by A. E. Tonelli (66) led to detailed conformation and its implications for dipole structure (Fig. 22). Indeed, the material can approximate a ferro electric. It is thus of interest in our expectations of the environments that polymers can provide for the creation of new phenomena. The total array of dipoles in polyvinylidene fluoride will switch in about 3 microseconds at 20°C with 200 megavolts per meter field. The system becomes much slower at lower temperatures and fields. But we do have a case of macroscopic polarization intrinsic to the polymer molecules, which thus supplements the extensive trapping and other charge of distribution phenomena that we have discussed in connection with electrets. 

Some of the insulating polymers such as poly (1, 1-difluoroethylene) meet the requirements of electret materials (the electric analogue of a permanent magnet) they respond to applied electric fields by limited movement of charge, and the charge does not leak back when the field is removed. 

Murayama.N. Piezoelectric and pyroelectric effects of polymer electrets. Microsymposium on Electrical Properties of Polymers, Tokyo (Jan. 1972). 

A spectacular relaxation phenomenon of polymeric materials is exhibited in a field-temperature treatment phenomenon. As was shown above, the polarisation of a polymeric material after application of an electric field increases after an instantaneous polarisation to the relaxed polarisation. If the electric field is subsequently released the polarisation decreases again with time. However, if the polarisation took place at a temperature above the glass transition temperature after which the temperature was decreased to a temperature well below Tg, then depolarisation is not possible, due to the immobilisation of the polymer molecules. In this way a so-called electret maybe formed, the electric counterpart of a magnet. 

I.M. Vertyachikh, V.A. Goldade, A.S. Neverov and L.S. Pinchuk. Effect of electrical field of a polymer electret on vapor sorption of organic solvent, Vysoko-molec. Soedinenia, 1982, Vol. B24, No. 9, pp. 683-687. 

Electrical charges generated by static electrization scatter not only over the surface but across the material bulk as well. The direct consequence of friction-induced electrization process in polymers is the emergence of the electret state. 

V.A. Goldade, L.S. Pinchnk and V.V. Snezhkov. Strnctnral orientation and electric polarization of polymers in stationary magnetic fields. Proc. 7th Intern. Symp. on Electrets, 1991, pp. 316-321. 

DRS is also valuable for studying the translational motion of chaige carriers. These effects are important in inhomogeneous materials such as biological systems, emulsions and colloids, porous media, composite polymers, blends, crystalline and liquid crystalline polymers and electrets. The results of DRS may be complemented by TSDC studies, which provide a way of probing the mobility of dipoles and electric charges over a wide temperature range. 

Dias, C. J., and Das Gupta, D. K., Inorganic ceramic/polymer ferroelectric composite electrets, IEEE Trans. Dielectr. Electr. Insul., 3, 706-734 (1996). 

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