Gerhard M. Sessler

Ferroelectrets : soft electroactive foams for transducers

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.

Piezoelectricity in polyvinylidenefluoride

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...

Electromechanical response of cellular electret films

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.

Piezoelectricity in cellular electret films

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 d/sub 33/ constant of /spl lsim/ 220 pC/N. 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.

Charge dynamics for electron‐irradiated polymer‐foil electrets

Charge buildup, storage, and decay in 25‐μm polyfluoroethylenepropylene (Teflon type A) foils due to irradiation with partially penetrating monoenergetic electron beams in the 10–50‐keV range are investigated. Currents from the irradiated front surface and the nonirradiated rear surface are measured separately by means of 1000‐A evaporated aluminum electrodes. Charge storage due to irradiation and a charge recall effect caused by irradiation of previously charged samples are observed. Discharge currents measured after termination of the irradiation indicate the importance of the delayed component of radiation‐induced conductivity. Measurements of the spatial depth of the charge are also reported. A mathematical model is developed which takes into account the radiation‐induced conductivity and the electrical and geometrical parameters of the sample. It allows for the calculation of current transients during and after irradiation.

Large and broadband piezoelectricity in smart polymer-foam space-charge electrets

Charged closed-cell microporous polypropylene foams are shown to exhibit piezoelectric resonance modes in the dielectric function, coupled with a large anisotropy in the electromechanical and elastic material properties. Strong direct and converse dynamic piezoelectricity with a piezoelectric d33 coefficient of 140 pC/N at 600 kHz is identified. The piezoelectric d33 coefficient exceeds that of the ferroelectric polymer polyvinylidene fluoride by a factor of 5 and compares favorably with ferroelectric ceramics. Applications of similar concepts should provide a broad class of easily fabricated “soft” piezoelectric materials.

Electrical conduction in polyimide films

Electrical conduction phenomena in polyimide (Kapton) films were studied with particular attention devoted to the separation of interface and bulk phenomena. The measurements were carried out with a variety of methods in the temperature range of 50 to 270 °C at electrical fields of 104 to 6×105 V/cm and at time intervals of up to 2×104 s after voltage application. Biased, two‐side metallized samples yield, after sufficiently long voltage application, interface‐controlled steady‐state currents described by Schottky injection, modified by space‐charge layers in the vicinity of the electrodes. The effective work function for aluminum‐polyimide is estimated to be 1.7 eV in the temperature range between 100 and 270 °C. A distinct dependence of these currents on electrode material is observed. Bulk phenomena were studied on one‐side metallized samples subject to positive‐corona charge injection. At temperatures below 200 °C, significantly larger currents than those for biased, two‐side metallized samples were o...

Charge distribution and transport in polymers

Experimental work on charge distributions and charge transport in polymer films is reviewed. For studying the charge distributions in the thickness dimension, thermal or acoustic techniques are used with resolutions down to /spl sim/1 /spl mu/m. The polymer samples usually are charged with corona, electron beam or thermal methods. Observation of the change of charge and polarization distributions with time allows one to draw conclusions about charge transport and charge injection phenomena. In some cases theoretical models have been established, based on excess-charge drift due to a finite carrier mobility and on charge compensation due to intrinsic or radiation-induced conductivity. Other models are concerned with the interaction of injected or intrinsic space charge with a dipole polarization. Comparisons of measured and calculated charge distributions and their change with time show the applicability of these models and shed new light on charge trapping and charge transport phenomena.

Self‐Biased Condenser Microphone with High Capacitance

A new kind of condenser microphone with solid dielectric between the electrodes is described. The system employs metallized dielectric foil electret and requires no external dc bias. The lower impedance eliminates the need of a cathode follower and allows the use of a long connecting cable. The frequency response of this system is flat within ±3 dB between 50 and 15 000 cps. The sensitivity of the system is −59 dBV for a sound pressure of 1 dyn/cm2 (74 db SPL).

Improvement of piezoelectric activity of cellular polymers using a double-expansion process

Expansion of cellular polypropylene films through an increase in gas pressure and subsequent pressure release at elevated temperatures prior to charging is known to enhance the piezoelectric d33-coefficient of the material. By means of a second pressure expansion the piezoelectric activity can be further increased by more than 40% in comparison with samples subjected to only a single expansion. The effectiveness of the double-expansion process must be attributed to the gain in thickness through the second expansion, following the charging and metallization processes. This thickness change causes a decrease in Youngs modulus and thus an increase in d33. Typical d33-coefficients of 1400 pC N−1 at 0.01 Hz and about 500 pC N−1 at 25 kHz have been achieved.