Piezo- and Pyroelectric Polymers

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],... 

If a polymer has a larger dipole moment in its molecule than can be aligned to form a polar crystal, there is every likelihood it will exhibit strong piezo- and pyroelectricity. However, though many polar... 

Pyro- and Piezoelectric Properties The electric field application on a ferroelectric nanoceramic/polymer composite creates a macroscopic polarization in the sample, responsible for the piezo- and pyroelectricity of the composite. It is possible to induce ferroelectric behavior in an inert matrix [Huang et al., 2004] or to improve the piezo-and pyroelectricity of polymers. Lam and Chan [2005] studied the influence of lead magnesium niobate-lead titanate (PMN-PT) particles on the ferroelectric properties of a PVDF-TrFE matrix. The piezoelectric and pyroelectric coefficients were measured in the electrical field direction. The Curie point of PVDF-TrFE and PMN-PT is around 105 and 120°C, respectively. Different polarization procedures are possible. As the signs of piezoelectric coefficients of ceramic and copolymer are opposite, the poling conditions modify the piezoelectric properties of the sample. In all cases, the increase in the longitudinal piezoelectric strain coefficient, 33, with ceramic phase poled) at < / = 0.4, the piezoelectric coefficient increases up to 15 pC/N. The decrease in da for parallel polarization is due primarily to the increase in piezoelectric activity of the ceramic phase with the volume fraction of PMN-PT. The maximum piezoelectric coefficient was obtained for antiparallel polarization, and at < / = 0.4 of PMN-PT, it reached 30pC/N. 

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... 

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... 

Piezo- and pyroelectric coostanls and dielectric relaxation strengths VDCN copolymers are listed in TU>ie 1 [1438]. Piezoelectric constant seems to be dneedy pn>-poitional to residual polarization (F,), and further related to dielectric lelaxathm strength. Generally, piezoelectric constant in polymer electreta is proportional to the remanent polarization as related by following equatioo ... 

Therefore a composite oon i of highly piezo- and pyroelectric ceramic material combined with a polymer would be the ideal rcpiacemem to obtain the properties of both... 

The ways of preparing and general properties of the composites of various types will first be described. Then their dielectric and electrect properties will be discussed, since they determine the piezo- and pyroelectricity of polymer-ceramic composites. The last-mentioned properties will be described in the last part of this chapter. 

Wb will try to suggest the answer to the question, What causes the piezo- and pyroelectricity in electrets—dipoles (or spontaneous polarization), charges, or both " The exact answer to this question was not found during the Special Discussion Sesskm at ISE 7 in Berlin, 1991. The investigation of relaxation processes by means of the TSD method seem to lead to the answer to the rtrave question conceming polymer-ceramic compodtes. it will be shown by the example of the electret and piezoelectric properties of some multilayer systems [17-19]. 

Polymers exhibit many unique properties which open up numerous technological opportunities in the fields of optics and electrooptics. They combine the possibility of easy processing with an infinite potential for functionalization. They can thus be ferroclectrics, conductors or semiconductors, mesomorphous and can exhibit photoconductivity, piezo or pyroelectricity or nonlinear optical properties. These polymers give rise to a tremendous number of potential applications such as optical data recording, optical communications, displays... 

PVDF copolymers have been investigated for their piezo properties and for uses in various applications such as sensors. An example of a PVDF copolymer is polyvinylidenefluoride-co-trifluoroethylene [P(VDF-TrFE)], which is a ferroelectric, crystalline polar polymer that exhibits inherent piezoelectric and pyroelectric responses with low acoustic impedance. Such properties provide an optimistic approach towards the use of these polymers for various applications in the near future. Higashihata et al. (1981) compared the piezoelectric craistants of PVDF and P(VDF-TrFE) and observed that much larger values were obtained for P(VDF-TrFE) under the same polarizing conditions. The special interest in this copolymer is also due to the evidence reported by Furukawa et al. (1981) that the PVDF-TrFe copolymer can be annealed to 100% crystallinity, as opposed to 50% in PVDF. Other copolymers have also been explored to determine an enhanced piezo effect (Poulsen and Ducharme, 2010). 

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