Lesser-known piezoelectric and pyroelectric applications of electroactive polymers
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Lesser-known piezoelectric and pyroelectric applications of electroactive polymers Sidney B. Lang1 and Supasarote Muensit2 1 Dept. of Chemical Engineering, Ben-Gurion University of the Negev, Beer Sheva, Israel 2 Dept. of Physics, Prince of Songkla University, Hatyai, Thailand ABSTRACT
The piezoelectric effect was first observed in polyvinylidene fluoride polymer (PVDF) in 1969 and the pyroelectric effect was found several years later. A number of additional ferroelectric polymers have been discovered since that time including the copolymer PVDF with trifluoroethylene (P(VDF-TrFE)), and the odd-numbered nylons. A large number of applications of piezoelectricity and pyroelectricity have been developed. The magnitudes of the effects in polymers are much lower than those of ferroelectric ceramics (an exception is the piezoelectric effect in porous polymers). However, other factors make these very desirable materials for applications. The polymers have low permittivities, low acoustic impedances and low thermal conductivities. They are available in large area sheets and they are flexible and relatively low in cost. Major applications include microphones and loudspeakers, ultrasonic devices, SAW transducers, actuators, infrared detectors and many others. This review will describe some of the lesser-known applications of these materials in the fields of tactile devices, energy conversion, porous polymers, property measurement, pyroelectric infrared sensors, shock sensors and space science.
INTRODUCTION Pyroelectricity was first observed more than 2400 years ago by the Greek philosopher Theophrastus [1, 2] and piezoelectricity was discovered by Jacques and Pierre Curie in 1980 [3]. Piezoelectricity was first found in a polymer, polyvinylide fluoride (PVDF), by Kawai in 1969 [4]. The pyroelectric effect in PVDF was found two years later by Bergman et al. [5] and by Nakamura and Wada [6]. These effects have been observed in a number of additional polymers including polyvinylidene fluoride-trifluoroethylene copolymer (P(VDF-TrFE)), vinylidene cyanide, odd-numbered nylons and polyurea. However, only PVDF and P(VDF-TrFE) have been used significantly in applications. The physical properties of these two polymers are compared with that of the ferroelectric ceramic PZT-4 (lead-zirconate-titanate) in table I. Both the piezoelectric strain constants (d coefficients) and the pyroelectric coefficients of the polymers are very low in comparison to those of PZT-4. However, the low permittivities of the polymers lead to high values of the piezoelectric voltage constants (d/εε0) and pyroelectric voltage figureof-merit (proportional to p/ε). The acoustic impedances of the polymers are much lower than those of ceramic materials and much closer to those of water and air. The polymers can be prepared in large area, very thin sheets and they are relatively inexpensive. Consequently, they are used in a large number of applications. The pie chart of figure 1 shows the approximate distribution of papers published during the five-
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