Application of piezoelectric polymers to audio transducers

Abstract

Recently, piezoelectricity and pyroelectricity of polymer materials have attracted attention to the possibility of their application as new transducer materials. The first study of the application of piezoelectric biological polymer materials, such as whale bones and tendons, was done by Fukada in phonograph cartridges. Later, synthetic polymer films, poly(γ-methyl L-gultamate), were used as transducing elements of experimental microphones and headphones: however, these transducers were not commercialized, because of their low sensitivity. In 1969, Kawai had found large piezoelectric effects in stretched and polarized poly(vinylidene fluoride) films. The stretched poly(vinylidene fluoride) films exhibit hysteresis characteristics similar to those of ferroelectric crystals under high alternating electric fields of 50 Hz. After being subjected to a polarizing static field of about 600 kV/cm at 100°C for 40–60 min, the film possesses large piezoelectric and pyroelectric constants; for example, in the case of the transverse effect, the d constant of the film is more than ten times larger than that of quartz, namely, 8×10−7 cgs esu. Reversible changes in infrared spectra before and after the poling process imply that the dipolse in the β crystals of poly(vinylidene fluoride) are aligned along the direction of the applied electric field. The alignment is strongly affected by the properties of the amorphous region in the film. The coersive field in the P-E hysteresis curve increases rapidly near the glass transition temperature. Not all of the dipolse in the β crystals, however, can orient themselves along the poling field. Furthermore, this orientation is influenced by the crystal size and the degree of the crystallinity. This soft, thin piezoelectric polymer film has been applied to the design of various transducers, such as stereophonic headphones, direct radiator high frequency loudspeakers, microphones, phonograph cartridges, and very small accelerometers. These transducers have very simple structures because the diaphragms are piezoelectric films and possess the transducing function. Particularly, in the case of the loudspeakers, a pulsating cylinder may be easily obtained which has omnidirectional patterns up to 20 kHz. Some of these transducers have been commercialized already.