Piezoelectric Polymer Materials Research Articles

Piezo film: Form and function

Abstract Many excellent mathematical models exist for predicting the high-frequency response of piezoelectric transducers. Most of these, however, are designed to describe the behaviour of thickness-mode devices. With the advent of piezoelectric polymer material and, in particular, large-scale producion of the piezo form of polyvinylidene fluoride (PVDF), a new class of very useful transducers has arisen. These are more closely allied to strain gauges, but demonstrate very high voltage sensitivity to strain. In many circumstances low-frequency response is of more concern than the upper limits and a simple resistor/capacitor model suffices. But the unique properties of ‘piezo film’ allow operation into frequency ranges far beyond those possible using foil resistance gauges. The object of this paper is to demonstrate how the high-frequency response of planar piezoelectric strain sensors may be derived quite simply from inspection of the surface geometry of the elements themselves. The standard tools of signal theory are applied in an abstract manner, and the general form of the results allows prediction and construction of complex signal-processing elements using standard film patterning techniques. The author has worked with the applications engineering group of Pennwalt Corporation's Piezo Film Department since 1984, and has seen the development of PVDF from a laboratory curiosity into its current position of major importance as an electromechanical and pyroelectric transducer component.

A laboratory acoustic study of piezoelectric polymer hydrophones

This laboratory study was an evaluation of the acoustical and electrical properties of piezoelectric polymer (PVDF and PVF2/PVF3) materials. The task objective was to establish a technical framework for the future use of these materials in hydrophones. The technical approach was to investigate the polymer behavior in two configurations: (1) thick (500, 1000, and 1500 μm) copolymer (PFV2/PVF3) films bonded to various window and backing materials and (2) hollow cylinders with thin (28 μm) polymer (PVDF) film bonded to the inside wall of a selected window material. This study yielded copolymer hydrophones with a flat frequency response from less than 100 Hz to greater than 100 kHz and with receive sensitivities similar to those of standard ceramic (PZT) hydrophones (greater than − 200 dB re: 1 V/μPa). The conformable and lightweight nature of the polymer materials facilitated their use in the hydrophone fabrication. The broadband response and physical nature of the polymer films were in direct contrast with the highly resonant behavior and rigid nature of PZT ceramics.

Compensating sensor device for a charge amplifier circuit used in piezoelectric hydrophones

To compensate for the effect of external parameters on piezoelectric sensors connected to a charge amplifier, a small portion of this sensor is used to form the feedback capacitor of the amplifier. These parameters then have the same effect on the sensor and on the capacitor, thus making it possible to provide compensation to hydrophones made of piezoelectric polymer material.

Laminated piezopolymer plates for torsion and bending sensors and actuators

A set of piezopolymer devices has been developed based on a composite laminate theory for piezoelectric polymer materials. By using different combinations of ply angles and electrode patterns, a piezopolymer/metal shim plate structure was built that exhibited both bending and torsion deformation under an applied electric field. A set of torsion-beam sensor structures was also built that could distinguish between bending and torsion or between different vibration modes. These devices were based on a general theory of piezoelectric laminates. The experimental results agreed quite closely with the theoretical predictions. These integrated sensor–actuator devices may find application in the control of microactuators or may be used for modal control of larger continuous structures.

The dynamics of piezoelectric polymer materials at high frequencies measured with conversion electron Mössbauer spectroscopy (CEMS)

Conversion Electron Mossbauer Sideband Spectroscopy is presented as a new and the most accurate method to study the dynamics of piezoelectric polymer materials at high frequencies. With raising amplitude of driven vibrations, complete oscillating behaviour of the Mossbauer sideband-intensities is found. The amplitudes of vibrations can be measured with an accuracy of 0.005 A. Variations of the piezoelectric constant with temperature and frequency can be determined to better than 1%. Beside piezoelectricity the thermally induced dynamic behaviour of polymer surfaces can also be studied by CEMS.

Diaphragm comprising at least one foil of a piezoelectric polymer material

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高分子トランスデューサー 圧電・焦電変換高分子材料

高分子トランスデューサー 圧電・焦電変換高分子材料

Recent measurements on improved thick film piezoelectric pvdf polymer materials for hydrophone applications

Abstract A broad outline is given of THORN EMI Central Research Laboratories’work in converting PVdF materials into sensitive hydrostatic-mode hydrophone elements. Emphasis is placed on the piezoelectric coefficients and voltage sensitivities obtained. Some data is also presented on ageing characteristics at normal ambient and elevated temperatures and for repeated pressure cycling between ambient and high pressures (∼20 MPa). Hydrostatic-mode hydrophone sensitivities of -195 dB rel 1VμPa−1 are reported for 630μ thickness highly voided PVdF which greatly surpasses the typical -205 dB rel 1VμPa−1 for natural draw ratio material calculated at the same thickness.

Miniature piezoelectric polymer ultrasonic hydrophone probes

The advantages of using piezoelectric polymer (plastic) materials as the sensitive element in miniature ultrasonic hydrophone probes are described, and proposed designs are critically reviewed. A novel needle type of probe design is described in detail, together with calibration results of the sensitivity and directivity of 0.6 mm and 1 mm diameter probes. The former typically have an end of cable sensitivity of −271 dBV μPa −1 ± 2 dB between 1 and 10 MHz, the latter of −264 dBV μPa −1 ± 1.5 dB. The linearity, temporal stability and thermal stability of those probes are also discussed. They are shown to be small and reliable instruments for obtaining good spatial and temporal resolution in ultrasonic field measurements.

Integrating hydrophone sensing elements

An integrating hydrophone sensing element is provided having improved sensitivity and capability of withstanding high pressures. The sensing element consists of a rigid cylinder with semicylindrical sensing surfaces rigidly connected to the cylinder at diametrically-opposite points. A membrane of piezoelectric polymer material extends diametrically across the cylinder and is attached to the cylinder at points displaced 90° from the connections of the sensing surfaces. Vibration of the cylinder by acoustic signals transmitted from the sensing surfaces causes vibration of the membrane and produces an electrical output signal.

Application of piezoelectric polymers to audio transducers

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.

Applications of modern transducer principles

A discussion of piezoelectric polymer, ceramic, and electret materials and transducer principles will be presented. Applications of these transducers in a variety of fields, such as audio and ultrasonics (microphones, earphones, and phonograph cartridges), gas analysis, data transmission, and telephony will also be discussed.