R. Gerhard-Multhaupt

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.

Less can be more. Holes in polymers lead to a new paradigm of piezoelectric materials for electret transducers

The review begins with a very brief history of electrets and of piezo and pyroelectricity in nonuniform space charge electrets. The development of cellular propylene as an electret-transducer material since the mid-eighties is described, and the accompanying research into the origins of (quasi-)ferro-, pyro- and piezoelectric effects in voided polymer electrets is discussed in some detail. In this context, the most relevant charging and measuring techniques for piezo and pyroelectric polymer-foam electrets are introduced. Porous fluoropolymer electrets and their investigation as single or multiple-layer piezoelectrics are considered in view of their better thermal and temporal stabilities. Finally, some proposed or demonstrated applications of voided polymer electrets are briefly mentioned.

Understanding the role of the gas in the voids during corona charging of cellular electret films - a way to enhance their piezoelectricity

The influence of the corona-charging process on the piezoelectric transducer coefficient d33 of a cellular electret film has been investigated. An increased corona voltage can be considered as a way to enhance the charge density and thus also the resulting piezoelectric effect. Higher corona-charging voltages are possible with increased ambient pressure or in suitable dielectric gases. The effect of the gas inside the voids has also been studied. Enhanced transducer coefficients were obtained by corona charging in N2 or N2O gas atmospheres at 100-450 or 100-140 kPa pressures, respectively. The highest transducer coefficients of about 790 pCN-1 were obtained when N2 gas was filled into the voids of a cellular polymer film by means of consecutive vacuum and high-pressure treatments at 295 or 313 K.

Controlled inflation of voids in cellular polymer ferroelectrets : optimizing electromechanical transducer properties

When exposed to sufficiently high electric fields, polymer-foam electret materials with closed cells exhibit ferroelectric-like behavior and may therefore be called ferroelectrets. In cellular ferroelectrets, the influence of the cell size and shape distributions on the application-relevant properties is not yet understood. Therefore, controlled inflation experiments were carried out on cellular polypropylene films, and the resulting elastical and electromechanical parameters were determined. The elastic modulus in the thickness direction shows a minimum with a corresponding maximum in the electromechanical transducer coefficient. The resonance frequency shifts as a function of the elastic modulus and the relative density of the inflated cellular films. Therefore, the transducer properties of cellular ferroelectrets can be optimized by means of controlled inflation.

Nondestructive Laser Method for Measuring Charge Profiles in Irradiated Polymer Films

Relative charge-density distributions in the thickness direction of thin (>10¿m) polymer films are determined directly and accurately with a new laser-induced pressure-pulse (LIPP) method: A pressure pulse is generated in the sample by applying a short (70ps) and energetic (1-11mJ) light pulse from a Nd:YAG laser to a specially coated surface of the sample. Stress effects within this surface layer and possibly the recoil caused by ablation of some target material generate the desired pressure pulse of less than 1ns duration which propagates through the sample. From the electrode currents, the charge distribution can be evaluated. The new method is described in detail and some results for 20 to 50¿m thick films of Teflon FEP, Mylar PETP, and Kynar PVDF are given.

Investigation of piezoelectricity distributions in poly(vinylidene fluoride) by means of quartz‐ or laser‐generated pressure pulses

The direct determination of charge, polarization, or piezoelectricity profiles in thin dielectrics is now possible if piezoelectrically generated pressure steps or pulses and laser‐induced pressure pulses are employed. These recently developed high‐resolution methods were applied to the same piezoelectric poly(vinylidene fluoride) (PVDF) samples. Comparison of the respective results demonstrates the feasibility, the advantages, and the limitations of the new techniques for the study of piezoelectricity distributions. It is confirmed that, for relatively low poling fields, the piezoelectric activity of thermopoled PVDF foils is often confined to a layer near the positively biased surface. The same effect is found for poling with a positive corona discharge. For high‐field corona poling, the piezoelectric activity extends throughout the PVDF film.

Light-valve technologies for high-definition television projection displays

Abstract For proper viewing, high-definition television (HDTV) images should be bright as well as large. Light-valve projection, whose feasibility is proven by several existing commercial devices (e.g. the Gretag Eidophor, the GE Talaria, the Sodern Titus, the Hughes LCLV and several Japanese liquid-crystal light-valve (LCLV) projectors), offers high resolution with large image areas and high brightness, and it thus appears to be most suitable for HDTV displays. The increasing availability of active-matrix-addressed liquid-crystal light-valve projectors for standard television, and the continuing dominance of ‘classical’ electron-beam-addressed light-valve projectors for the most demanding professional applications, are the starting points of a survey on the state of the art. After the two groups of light-valve projectors have been briefly reviewed, the major light-valve technologies are explained in some detail. Transmissive and reflective liquid-crystal light valves with various types of addressing are described first; the trend toward active-matrix addressing and high resolution is demonstrated. Commercial light-valve projectors with electrooptic and oil-film control layers are briefly introduced, and more recently developed technologies, such as metallized viscoelastic layers, deformable micromechanical mirrors, and polymer-encapsulated liquid crystals, are discussed. The authors research on deformable viscoelastic light-valve control layers for use in schlieren optical systems and on Si-Mosfet active matrices for addressing these is reported in some detail. The deformation behaviour of thin metallized elastomer layers has been analysed by the combination of the theories of electrostatics, linear viscoelasticity and thin plates; the calculated time behaviour and spatial-frequency response are found to be, at least in principle, sufficient for HDTV projection displays. Viscoelastic and metal layers have been manufactured by spin coating and vacuum evaporation, respectively; the deformation has been determined experimentally, mainly by phase-shift interferometry, and it is found to be in reasonable agreement with theoretical predictions. Sample Mosfet active matrices with picture-element densities suitable for HDTV have been designed and fabricated. In addition, the concept of a full-colour schlieren optical projection system for the above reflective VSLMs is briefly described. Finally, in view of its importance for the field, some general aspects of active-matrix addressing are briefly examined.

High surface-charge stability of porous polytetrafluoroethylene electret films at room and elevated temperatures

Porous polytetrafluoroethylene films were positively or negatively corona-charged at room or elevated temperatures and their charge-storage behaviour was investigated by means of isothermal surface-potential and thermally stimulated discharge-current measurements. In addition, electron micrographs of the sample morphology were taken and the influence of high humidities on the surface-charge decay was investigated. For comparison, nominally non-porous polytetrafluoroethylene films were studied in the same manner. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities if the relative humidity is not too high. Further investigations are under way in order to better understand this behaviour and to employ it for electret applications.

Pulsed laser deposition of stoichiometric potassium‐tantalate‐niobate films from segmented evaporation targets

The preparation of epitaxial potassium‐tantalate‐niobate (KTa0.7Nb0.3O3, KTN) films on strontium‐titanate substrates by means of pulsed excimer laser evaporation in vacuum is reported. In the most successful deposition experiments, a segmented evaporation target consisting of a semicircular KTN single crystal and a semicircular potassium nitrate pellet was utilized−to our knowledge for the first time; it was thus possible to overcome the otherwise observed partial loss of the volatile potassium and to avoid potassium deficiency in the KTN films. Rutherford backscattering spectrometry and x ray diffraction results indicate that the samples have the desired stoichiometric composition as well as the orientation prescribed by the substrate crystal.

Two-step inflation of cellular polypropylene films: void-thickness increase and enhanced electromechanical properties

In cellular, electromechanically active polymer films, the so-called ferroelectrets, the cell size and shape distributions can be varied through a controlled inflation process. Up to now, high-pressure treatments were usually performed at elevated temperatures. There are, however, significant experimental limitations and complications if the pressure and temperature treatments are performed at the same time. Here, we demonstrate the controlled inflation of cellular polypropylene films by means of separate pressure and temperature treatments. Separate procedures are much easier to implement. Excellent electromechanical properties were achieved with such a two-step inflation process. The technique has significant potential for inflating large-area transducer films for electromechanical and electroacoustical applications.