After certain cellular polymers are internally charged, they behave like soft and sensitive piezoelectrics that can be used to interconvert acoustical or mechanical signals and electrical signals.

Charged polypropylene films with a cellular structure show pseudopiezoelectric properties. Measurements of the direct and inverse electromechanical transducer constants of such films, relating to the operation as sensors and as actuators, respectively, yield values of ≈200 pC/N. These values can be explained with a theoretical model by assuming reasonable charge distributions and charge densities. The experimental and the theoretical results show the reciprocity of the transducer constants.

A review of the physical properties of polyvinylidenefluoride (PVDF) with particular emphasis on its piezoelectric activity is given and the applications of this material are discussed. PVDF is a semicrystalline polymer whose crystalline domains appear in four different forms. These forms may be interconverted by the application of heat, electrical fields, and pressure. Thermal poling or corona poling will orient the molecular dipoles in the crystalline parts and thus yield a permanent polarization. This polarization causes, by means of differences in the dielectric and elastic properties of the amorphous and crystalline parts, the piezoelectricity of PVDF. The piezoelectric constant d31 reaches values of about 35 pC/N at room temperature. At decreasing temperatures a drop of d31 and d33 is observed. Compared to other piezoelectric materials, PVDF has such unique properties as flexibility, ruggedness, availability as thin films, and low acoustic impedance, but a somewhat smaller electromechanical coupling factor. Applications of PVDF are in transducers for audiofrequency, ultrasonic, underwater, and electromechanical use and in pyroelectric and optical devices.

Permanently charged films with a cellular or porous structure represent a new family of polymer electrets. These materials show piezoelectric properties with high piezoelectric constants. The electromechanical response equations of such films are derived for their operation as sensors and as actuators. Experimental results are also presented for cellular polypropylene (PP). In particular, measurements of the direct and inverse piezoelectric constants in the frequency range 0 to 10 kHz and of the variation of these constants across the surface of the films are discussed. These measurements, performed by direct application of stress or by the use of a profilometer, an accelerometer and an interferometer yield a frequency-independent piezoelectric $33 constant of 5 220 pC1N. Assuming reasonable charge distributions and charge densities, the calculated piezoelectric constants are in good agreement with the measured values. The theoretical model shows the reciprocity of the piezoelectric constants.

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111 den Gasen Argon, Luft und Wasserstoff wird bei hohen Frequenz/Druek (//p)-Werten (10 ~ bis 1011 1-Iz/Atm) die Schallabsorption und-dispersion berechnet und gemessen. Bei den niedrigsten f/p-Werten (10 7 bis 108 Hz/Atm) ergeben die Messungen eine Bestiitigung der klassischen Theorie. Wird die mittlere freie Wegliinge der Gasmolekiile gr6Benordnungsm~il3ig gleich der Wellenl~nge der Schallwelle (f/p ~ 10 s bis 2 -t 09 Hz/Atm), so mul? die klassische Theorie durch eine verbesserte Theorie ersetzt werden, deren erste und zweite N~herung (Bnrnettsche und ,,super"-Burnettsche Theorie) bekannt sind. Die Mel3ergebnisse stimmen bei Argon und Luft besser nit der Burnettschen, bei Wasserstoff teils (Dispersionsmessungen) besser nit der 13urnettsehen, teils (Absorptionsmessungen) besser nit der ,,super"-Burnettschen Theorie iiberein. Bei den h6chsten//p-Werten (101~ bis I0 n Hz/Atm), bei denen aus experimentellen Grtinden der Schallweg kleiner als die mittlere freie Wegliinge gew/ihlt werden ran13, gelten die Voraussetzungen der obengenannten Theorien nicht mehr. Die Absorp--tion und Dispersion wird auf molekularkinetischer Grundlage berechnet und stimmt nit den MeBwerten gut iiberein.

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Several layers of polytetrafluoroethylene and fluoroethylenepropylene films are fused such that small interfacial gas voids are formed between the layers. After proper charging and annealing, the fused multilayer films show large and thermally stable quasistatic piezoelectric d33 coefficients of about 1000pC∕N which are subject to a minor reduction of 3% per day if exposed to 90°C. Depending on sample processing, the piezoelectric coefficient is relatively independent of applied pressure in the range up to 20kPa and of applied frequency up to about 100kHz. Thermally-stimulated discharge measurements indicate that the decay of the piezoelectric activity at higher temperatures is at least partially due to charge drift along the surfaces of the voids. A microphone built with the cellular ferroelectrets has a sensitivity of 3mV∕Pa at 1kHz.