Piezoelectric polymers
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Piezoelectric polymers Siegfried Bauer1, Simona Bauer-Gogonea1, Mario Dansachmüller1, Gilles Dennler2, Ingrid Graz1, Martin Kaltenbrunner1, Christoph Keplinger 1, Howard Reiss3, Niyazi Serdar Sariciftci2, Thokchom Birendra Singh2 and Reinhard Schwödiauer 1, 1 Soft Matter Physics, Johannes Kepler University Linz, Austria. 2 Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University Linz, Austria. 3 University of California, Department of Chemistry and Biochemistry, Los Angeles, USA. ABSTRACT The science and technology of piezoelectric polymers has long been dominated by ferroelectric polymers from the polyvinylidene fluoride (PVDF) family. The piezoelectricity in this polymer class arises from the strong molecular dipoles within the polymer chain and from the resulting change of the dipole density upon application of a mechanical stimulus. Ferroelectric polymers show moderate piezoelectric coefficients (d33 and d31,32 around 20-30 pC/N) in comparison to ceramic piezoelectrics, with an acoustic impedance comparable to that of water. The thermal stability of the piezoelectric effect is limited to below 100°C, though stability up to 125°C has recently been announced. Applications of ferroelectric polymers emerged in many niches. A good example of a success story for PVDF applications are clamp-on transducers used as pressure sensor for Diesel injection lines, with selling numbers over 50 million pieces per year. A relatively new development are relaxor ferroelectric polymers, based on electron-irradiated poly(vinylidene fluoride) trifluoroethylene copolymers or on terpolymers of vinylidene fluoride, trifluoroethylene and chlorofluoroethylene. Relaxor ferroelectric copolymers exhibit strong electrostriction and thus large piezoelectric coefficients, when used under electric dc-bias fields. Internally charged cellular polymer foam electrets (ferroelectrets) resemble close similarities to ferroelectrics. They display large intrinsic piezoelectric d33-coefficients well above 100 pC/N and very small d31 and d32 coefficients, coupled with a limited thermal stability up to 50°C in the polypropylene workhorse material. The materials are pioneered in Finland and already entered the market in niches, for example in musical pick-ups. They promise large area applications, for example in surveillance and intruder systems. Finally, organic semiconductors have shown a rather unusual electromechanical response, governed by a power law S=V3/2 of strain S versus voltage V, located in between traditional piezoelectricity and electrostriction. The field of piezoelectric polymers therefore received new stimulus, and the material class of piezoelectric polymers has been significantly broadened recently. INTRODUCTION Direct piezoelectricity was first described by Jaques and Pierre Curie in 1880, at the time of their discovery being only 25 and 21 years old at the beginning of their scientific careers. It is most interesting to note that the discovery was not made by chance, the Curie brothers have looked for the
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