Ferroelectrets ferroelectret

Electret materials are meanwhile used in a large number of modern high-tech applications including microphones, acoustic sensors, transducers, radiation and pollution dosimeters, power generators, filters, and many more. Additionally, electret technology is of great interest in the field of biomaterials, for instance in callus formation and wound healing [10, 11], When used in cellular or in multilayer sandwich structures, polymer electrets can exhibit piezoelectricity. Such materials are ferroelectrets, as they show typical features of ferroelectric materials such as piezo-and pyroelectricity [12-17],... 

Bauer S (2006) Piezeo-, pyro- and ferroelectrets soft transducer materials for electromechanical energy conversion. IEEE Trans Dielectr Electr Insul 13 953... 

Bauer S, Gerhard-Multhaupt R, Sessler G (2004) Ferroelectrets soft electroactive foams for transducers. Phys Today 57 37... 

Bauer, S., Gerhard-Multhaupt, R. and Sessler, G. (2004) Ferroelectrets Soft electroactive foams for transducers. Physics Today, 57, 37-44. 

Wirges, W., Wagener, M., Varonina, O., et al. (2007) Optimized preparation of elastically soft, highly piezoelectric, cellular ferroelectrets from nonvoided poly(ethylene terephthalate) films, Adv. Funct. Mat., 17, 324—9. 

In real piezoeleetric materials, one phase will be stiffer and the other softer so that a nonzero response is observed. Equation 6 tells us how to design piezoelectric materials with two or more phases such as ferroelectrets from cellular foams (the dipole phase typically consists of air-filled foam cells or cavities) or Maxwell-Wagner piezoelectrets from particle-polymer composites (in which the dipole phase may consist, e.g., of hard particles that cannot be compressed very much). 

Equation 7 includes the prediction that the ratio of the piezoelectric coefficient and the remanent polarization P3 = Pr should be approximately equal to the elastic compliance /Ym of the matrix phase for the dipole-density effect or to the elastic compliance HYd of the dipole phase for the dipole-moment effect, respectively (or inversely proportional to the respective elastic modulus). First results assembled from the literature and from our own experimental data on PVDF and on cellular-foam PP and tubular-channel FEP ferroelectrets (Altafim et al. 2009) are shown in Fig. 5 (Qiu et al. 2013, 2014). They provide experimental evidence that the direct piezoelectric thickness coefficient of polymer materials can indeed be roughly approximated by the product of the remanent polarization in the poled material and of its overall elastic compliance. Additional data from the literature on other... 

Fig. 5 The ratio of the relevant component of the piezoelectric coefficient d and the respective polarization is plotted vs. the reciprocal of the respective elastic modulus Y (i.e., the relevant component of the elastic compliance). From left to right Literature data for inorganic ferroelectrics squares) barium titanate (BaTiOs), lead zirconate titanate (PZT), and lead zirconate niobate (PZN). Ferroelectric polymers triangles) polyamide-11 (PA-11), poly(vinylidene cyanide-vinyl acetate) (P(VDCN-VAc)), polyurea-5, poly(vinylidene fiuoride (PVDF)), poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), and poly(vinylidene-hexafiuoropropylene) (P(VDF-HFP)). Polymer ferroelectrets circles) cellular polypropylene (cellular-PP) and tubular-channel poly (fluoro-ethylene-propylene) (FEP) (Qiu et al. 2014)...

It is, however, not surprising that the prediction is in general only approximately Mfilled for the polymeric piezoelectrics and seems to break down completely for the inorganic piezoelectrics. The model apparently works best for polymers in which the elastic compliance of the phase that is essential for piezoelectricity clearly dominates the overall compliance (i.e., the soft amorphous matrix in semicrystalline polymers above their respective glass-transition temperatures or the gas-filled cavities in polymer ferroelectrets, respectively). When looking at the spring model and its comparison with selected experimental data from the literature (cf. Fig. 5), the following points should be taken into accormt ... 

A fundamental difference between ferroeleclret polymers and ferroelectric polymers is the large elastic anisotropy of the ferroelectrets which can usually be easily deformed (compressed or expanded) in tire fliickness direction, but not laterally Ferroelectric polymers are, however, essentially incompressible, which leads to a strong coupling between the longitudinal piezoeleclricity in tire fliickness direction and the transverse piezoelectricity that couples lateral elastic deformations with changes of the vertical electric polarization. 

Qiu X, Hollander L, Wirges W, Gerhard R, Basso HC (2013) Direct hysteresis measurements on ferroelectret films by means of a modified Sawyer-Tower circuit. J Appl Phys 113 224106 Qiu X, Wirges W, Gerhard R (2014) Polarisation and hysteresis in tubular-channel fluoro-ethylenepropylene-copolymer ferroelectrets. Ferroelectrics 472 100-109... 

Rychkov D, Altaflm RAP, Gerhard R (2014) Unipolar ferroelectrets - following the example of the electret microphone more closely. In Annual Report, IEEE Conference on Electrical Insulation and Dielectric Phenomena. IEEE, New York, pp 860-862. doi 10.1109/CEIDP.2014.6995907... 

Electrets are dielectric materials capable of quasi-permanently storing electric charges at their surface or in their bulk. This chapter presents a brief history of electret research, followed by a classification and introduction of the most important electret materials. The chapter also discusses ferroelectrets and recent developments in charge stability. 

Electret Polymer Ferroelectret Dipole Charge Stability... 

Fig. 2 Scanning electron microscope image showing a PP foam ferroeleciret cross-section (a) (courtesy of Mika Pajannen, VTT, Finland) and schematic depiction of engineered macrodipoles within the voids of the ferroelectret (b) and hysteresis loop exhibited when switching of the macrodipoles orientation (c) (After Bauer et al. 2004)...

Harris S, Mellinger A (2014) Towards a better understanding of dielectric barrier discharges in ferroelectrets Paschen breakdown fields in micrometer sized voids. J Appl Phys 115 163302... 

Qiu X, Mellinger A et al (2007) Barrier discharges in cellular polypropylene ferroelectrets how do they influence the electromechanical properties J Appl Phys 101 104112 Qiu X, Hollander L et al (2013) Patterned piezo-, pyro-, and ferroelectricity of poled polymer electrets. J Appl Phys 89 4503... 

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