Dipole electrets

The short excursion into the history of electret research already shows that there are numerous different material types and materials with the ability to quasi-permanently store charges. Depending on the charge carrier type, electrets are divided into charge electrets and dipole electrets. The following section introduces both concepts and the most important polymers belonging to each category. 

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). 

Fig. 3 Schematic view of the classification of dipole electrets Amorphous dipole electrets (a) must contain preferentially oriented molecular dipoles that are either located as guest charges in the polymer (/), attached to the side (//), or located in the main chain (in). Semicrystalline dipole electrets (b) exhibit both oriented molecular dipoles and Maxwell-Wagner compensation charges at the interfaces to the crystalline regions...

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). 

Meunier M, Quirke N et al (2001) Molecular modeling of electron traps in polymer insulators chemical defects and impurities. J Chem Phys 115 276-288 Mopsik FI, Broadhurst MG (1975) Molecular dipole electrets. J Appl Phys 46(10) 4204 Neugschwandtner GS, Schwoediauer R et al (2001) Piezo-and pyroelectricity of a polymer-foam space-charge electret. J Appl Phys 89 4503-4511... 

An electret is a crystal which has dipoles oriented permanently in one direction. The crystal therefore is a macroscopic dipole. 

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. 

Solids can also be subdivided by their electrical polarization properties. The preponderant fraction of solids (crystalline or amorphous) are dielectric They have no net electrical polarization. If the individual components (molecules or clusters of ions) do have a net electric dipole moment, and these add nonlinearly, then one has electrets. There are also nanoferroelectrics. 

Despite this uncertainty about the existence of a ferroelectric transition, it is possible to produce an ice crystal with a permanent macroscopic dipole moment—an electret— by polarizing the crystal at about —15 °C and then cooling it to liquid-air temperature (Gelin Stubbs, 1965). These electrets are essentially stable at these low temperatures but discharge slowly at higher temperatures, the decay half-life being of the order of i min at — 60 °C, though the decay is not exponential. 

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. 

Figure 5.5. Diagram of a typical poling assembly in which permanent dipoles are aligned to form electrets.

Macroscopic organizations possessing a permanent electric dipole moment like electrets have a finite Maxwell field E which may be externally measured, for instance, as a plate capacitor field. 

In some polar media, the dipole moments are frozen in positions that statistically result in a net component in a given direction. They are called electrets, and they have a net internal P-field in the absence of an externally applied field. An electret is the electrical equivalent of a permanent magnet. In the form of a thin sheet, the resultant equivalent surface charge density may be of the order of 50 x 10 C/m. Slowly, the polarization will diminish, but the process may take many years. 

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