Charge electrets

Keywords Charge storage properties Commodity polymers Corona charging Electret materials High performance polymers Physical aging Polymer films... 

Fig. 1 Schematic of a cross-section of a real-charge electret film with one electrode (top) and a dipolar electret film with two electrodes (bottom) [1]...

Temperature dependence of the surface potential of corona charged electrets... 

It is typical feature for the electret transducers that in most cases the electret is one-side metahzed and is placed between two plate electrodes short-circuited by a resistor. The charged electret surface gives rise to an electric field in the air gap which is formed between the non-metahzed electret surface and its adjacent electrode. This field can be found by an equation analogous to the one in (Palaia et al.) ... 

Space-charge electret (film, plate, fiber, etc.) a.k.a. electrophore Trapping on/in polymer film (image charges on electrodes) Polymer + backplate (harder part) plus air-fified cavities (much softer part) Mainly uniaxial pistonlike motion perpendicular to film plane... 

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... 

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. 

Fig. 1 Schematic view of classification of charge electrets Homogeneous polymers that carry excess charges at the surface (a) or within the bulk or volume (b) Heterogeneous charge electrets consist of a soft and a hard part. The noncentrosymmetry is either due to the sample geometry (c) or a result of bipolar charging (d)...

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). 

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... 

Nakayama M, Uenaka Y, Kataoka S, Oda Y, Yamamoto K, Tajitsu Y (2009) Piezoelectricity of ferroelectret porous polyethylene thin film. Jpn J Appl Phys 48 09KE05 Neugschwandtner GS, Schwddiauer R, Bauer-Gogonea S, Bauer S, Paajanen M, Lekkala J (2001) Piezo- and pyroelectricity of a polymer-foam space-charge electret J Appl Phys 89 4503... 

In this eontext, ferroelectrets actually represent a third elass of piezoelectric polymers in the sense that these cellular materials share features of charge electrets (real eharges) with those of ferroelectric polymers (hysteresis-type ED characteristics) (Bauer et al. 2004 Lekkala et al. 1996). Moreover, ferroelectrets are characterized by extremely high piezoelectric 33 coefficients as well as an extreme anisotropy in their mechanical and electromechanical properties, which partially require speeifie characterization techniques (Dansachmiiller et al. 2005). 

Naruse Y et al (2009) Electrostatic micro power generator fiom low frequency vibration such as human motion. J Micromech Microeng 19(9) 094002 Neugschwandtner GS et al (2000) Large and broadband piezoelectricity in smart polymer-foam space-charge electrets. Appl Phys Lett 77(23) 3827-3829 Paajanen M et al (1998) Electro-mechanical film EMFi-a new multipurpose electret material. Sensors Actuators A 84(l-2) 95-102... 

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