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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 5 • Date Oct. 2006

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  • Table of contents - Oct. 2006 Vol 13 No 5

    Page(s): 1
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  • IEEE Dielectrics and Electrical Insulation Society

    Page(s): 0_2
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  • Editorial: Useful effects of space charge and dipole polarization: recent developments in polymer electrets and organic semiconductors

    Page(s): 939 - 941
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  • Bernhard Gross and electret research: his contributions, our collaboration, and what followed

    Page(s): 942 - 952
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    The 100th anniversary of Bernhard Gross was celebrated at the 12 th International Symposium on Electrets in 2005. Gross has inspired and stimulated electret research since the 1930's when he entered this field as a young scientist. The following article is, to a large part, a personal tribute to his contributions. In the first part, some of the seminal early work of Gross will be discussed. This is followed by reminiscences of the author to joint studies of radiation charging, carrier transport, and other aspects of electrets at Bell Laboratories and at the Darmstadt University of Technology. In the last part, recent work in Darmstadt on charge storage, charge transport, and piezoelectricity of cellular electrets, still in part stimulated and accompanied by Bernhard Gross, will be discussed View full abstract»

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  • Piezo-, pyro- and ferroelectrets: soft transducer materials for electromechanical energy conversion

    Page(s): 953 - 962
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    Cellular space charge polymer electrets significantly enlarge the basis of pyro- and piezoelectric as well as ferroic materials. Since the first review of the field has been published in this journal (R. Gerhard-Multhaupt, Less can be more: Holes in polymers lead to a new paradigm of piezoelectric materials for electret transducers, IEEE Trans. Dielectr. Electr. Insul. Vol.9, pp.850-859, 2002), progress has been achieved in the understanding, preparation and characterization of cellular space charge electrets. Advanced applications arise in stacked microphones, artificial sonar systems, and in the combination of piezo- and ferroelectrets with flexible electronics. In the present survey an update of the developments since 2002 is provided View full abstract»

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  • Novel heat durable electromechanical film: processing for electromechanical and electret applications

    Page(s): 963 - 972
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    New ferroelectrets were developed on the basis of foams from cyclo-olefin polymers and copolymers. The results obtained on the cyclo-olefin polymer foam demonstrate a significant improvement of the service temperature for ferroelectret transducer materials. Suitable compounding and preparation led to cyclo-olefin ferroelectrets with an electromechanical activity of around 15 pC/N, which is thermally stable at least up to 110degC. The properties in sensor and actuator applications are strongly dependent on the processing parameters related to film-making, sensor and actuator preparation, gas content and electric charging. The processing window for the film stretching was very narrow compared to the earlier developed polypropylene ferroelectrets. The film porosity, softness and thus the electromechanical activity are adjusted by gas-diffusion expansion. The activity of the electromechanically operating sensors and actuators was increased by stacking several layers of cellular cyclo-olefin film. For applications such as flat loudspeakers, the foamed films are tuned by tensioning them on a support frame. Correct tensioning was essential also for reducing the distortion levels View full abstract»

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  • Stacked piezoelectret microphones of simple design and high sensitivity

    Page(s): 973 - 978
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    Pressure-chamber and free-field measurements of high sensitivity piezoelectret microphones of two different designs are reported. High sensitivity is achieved by (1) an increase of the piezoelectric d33 -coefficient of the cellular polypropylene films by a pressure expansion procedure and (2) stacking of the films. The pressure expansion results in large d33-coefficients of up to 600 pC/N in the audio frequency range. Microphones consisting either of cemented stacks or edge-clamped but otherwise loose and non-cemented stacks of up to six expanded films were built. For all investigated microphones, sensitivities of about 2.3 mV/Pa per film were found, resulting in an open circuit sensitivity of 15 mV/Pa at 1 kHz for a six-film-microphone. The equivalent noise level is about 37 dB(A) for a single-film transducer and 26 dB(A) for a five-film microphone. Distortion is less than 1 % at a sound pressure level (SPL) of 164 dB. Advantages of the new piezoelectric transducers include their simple design, consisting essentially only of the cellular film or films and electrical shielding. Other advantages are low cost, small weight, as well as a large range of shapes and sizes possible with such microphones View full abstract»

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  • Piezoelectrets from thermo-formed bubble structures of fluoropolymer-electret films

    Page(s): 979 - 985
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    Cellular and porous polymers with internal bipolar space charge can exhibit large piezoelectric thickness coefficients and have therefore led to significant advances in the understanding and the application of piezoelectricity in polymer electrets. As possible alternatives to these cellular ferroelectrets, other potentially useful configurations of space-charge electrets, such as bi-layer or multi-layer stacks of at least one "soft" porous and one "hard" non-porous electret film have been suggested and investigated. Extending the concept of cellular or porous polymer electrets with microscopic voids, we propose and describe novel piezoelectret structures with regular arrays of millimeter-sized bubbles that are formed between fluoro-ethylene-propylene (Teflonreg-FEP) films via a vacuum-assisted thermal process. After internal charging by means of high impulse voltages, the bubble structures exhibit rather large piezoelectric activities in their thickness direction, with phenomenological quasi-static d33 coefficients of up to 500 pC/N (or pm/V) which quite strongly decrease with the amplitude of the applied force. The electromechanical behavior of the new piezoelectrets has been modeled in analogy to the operation of electret microphones View full abstract»

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  • The surface potential of perforated dielectric layers

    Page(s): 986 - 991
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    The maximum value of the surface potential or of the related equivalent voltage of perforated dielectric sheets may be of special interest in application of materials such as electrostatic-polymeric electret filters, fabrics, non-wovens and others. Experimental investigations carried out on polymeric fabrics with application of the corona triode have shown that the surface potential is limited mainly by the back ionization (back corona) phenomenon. It was also shown that the corona triode may be a very useful tool for characterisation processes which occur in such materials. Particularly a simple correlation was found between the maximum value of the surface potential and the threshold voltage (i.e. the voltage of the discharge current increase of the sample polarised in the corona triode system) View full abstract»

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  • Development of porous polypropylene blends with NA11 particles and glass hollow spheres by biaxial stretching for electret applications

    Page(s): 992 - 1000
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    In the present work, the development of cellular films by stretching isotactic polypropylene (i-PP) filled with NA11 (2,2'-methylene-bis-(4,6-di-tert-butylphenyl)-phosphate) particles and hollow glass spheres is reported. Morphology, electret properties and piezoelectric activity are investigated. An enhancement of the charge storage stability for the cellular films is due to the formation of cavities, which retard the drift of charges through the volume of the film. Furthermore, the cellular stretched i-PP films with hollow glass spheres were made piezoelectric by suitable corona poling method. Films were stretched to draw ratios of 3.5:3.5 (medium stretched) and selected samples were drawn to ratios of 5:5 (highly stretched). The applied draw ratio was found to directly influence the cavity formations and by this the electret and piezoelectric properties. Excellent electret properties were obtained in the case of highly stretched i-PP films containing 10 wt% of NA11, known as a nucleating agent additive for i-PP. Additionally, piezoelectric activity (d33=179 pC/N) was observed in these films. i-PP films with glass spheres showed piezoelectric coefficients between 17 and 170 pC/N, depending on glass sphere size and concentration as well as on draw ratio View full abstract»

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  • Local states in organic materials: charge transport and localization

    Page(s): 1001 - 1015
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    The origin of local states capable of localizing charge carriers in molecular materials is discussed. The classical electrostatic model is employed to estimate the energies of chemical and structural traps in molecular materials, with particular attention being paid to polycrystalline and amorphous low-molecular weight and polymeric materials View full abstract»

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  • Simultaneous space charge and TSDC measurements on polarized polymer

    Page(s): 1016 - 1022
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    The present work is concerned with the development of an experimental device for simultaneous measurements of space charge profiles and thermally stimulated depolarization currents (TSDC) on polymer sheets. Space charge measurements are nowadays realized by nondestructive methods but TSDC are still a destructive one. However coupling the results from the two techniques allows to watch the evolution of the charges and finally their disappearance. The space charge measurement is based on the current response to a low frequency thermal wave applied on the sample by using the alternating thermal wave method (ATWM). Basically the TSDC require to heat the sample slowly and linearly. So, a controlled thermo-electrical module is used to generate the appropriate thermal profile and a temperature cycle including both the linear temperature rises and low frequency thermal waves around a chosen temperature level is generated. Both measurements are made on one and the same sample without handling it during the procedure. The space charge measurement accuracy is improved by varying the thermal wave frequencies from large to short wavelengths. The purpose is to scan successively the different layers in the sample. The use of an adapted mathematical model for the electric field representation and space charge distribution is helpful for the signal processing. Experimental results obtained on polymer materials are given and discussed to illustrate the change in density and position of the remaining space charge due to the TSDC effect View full abstract»

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  • LIMM study of space charge in crosslinked polyethylene

    Page(s): 1023 - 1029
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    Electric field/space charge profiles obtained using the laser-intensity-modulation-method (LIMM) are presented for XLPE samples machined from power cable insulation cross-linked using dicumyl peroxide. The samples, 110 mum thick with vacuum-evaporated aluminium electrodes, were subjected to DC fields of 48 kV/mm and both polarities at room temperature for at least 24 hr. The LIMM measurements were made at room temperature, and at various times after the end of poling. Electric field and space charge profiles obtained using the scale-transformation and polynomial regularization methods were in fair quantitative agreement over much of the sample thickness, except when the electric field showed a maximum or minimum. Simulation data for assumed electric field profiles strongly suggested that the polynomial regularization method is more accurate. Most of the space charge was concentrated near the electrodes, with densities of order 5-10 Cm-3 in those regions. Charge of the same sign tended to accumulate adjacent to a given physical electrode, for both poling polarities View full abstract»

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  • Thermal-pulse tomography of polarization distributions in a cylindrical geometry

    Page(s): 1030 - 1035
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    Fast, three-dimensional polarization mapping in piezoelectric sensor cables was performed by means of the novel thermal-pulse tomography (TPT) technique with a lateral resolution of 200 mum. The active piezoelectric cable material (a copolymer of polyvinylidene fluoride with trifluoroethylene) was electrically poled with a point-to-cable corona discharge. A focused laser was employed to heat the opaque outer electrode, and the short-circuit current generated by the thermal pulse was used to obtain 3D polarization maps via the scale transformation method. The article describes the TPT technique as a fast non-destructive option for studying cylindrical geometries View full abstract»

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  • A preliminary study of space charge distribution measurements at nanometer spatial resolution

    Page(s): 1036 - 1041
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    The continuous reduction in size of devices, such as integrated circuits or micro-electro-mechanical systems (MEMS), results in the need to control with better and better resolution the materials involved, and in particular the electrical properties of the insulating and semiconducting parts. In this paper we propose an approach applicable to space charge measurement methods for improving the resolution to the nanometer range by using femtosecond laser pulses. It is shown that a resolution of about 60 nm in SiO2 can be achieved with thermal and pressure wave propagation methods. Concerning the pulsed electro-acoustic method, the interfacial displacement as small as 100 fm can be measured at THz rate View full abstract»

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  • An explanation of the peculiar behavior of TSDC peaks at Tg: a simple model of entropy relaxation

    Page(s): 1042 - 1048
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    Thermally stimulated depolarization current (TSDC) signals measured in polymers at the glass transition Tg often exhibit intricate shapes that much depend on the thermal history and on the non-stationary condition used during experiments. This peculiar behavior is frequently explained in terms of a physical singularity of the molecular motions. We show that this singularity, i.e. unexpected values for activation energy and pre-exponential factor, obtained around Tg, result from the misuse of the Arrhenius law for treating the experimental data obtained in non-stationary experimental conditions. A simple model using time dependent configurational entropy for the material evolution and using a single Debye relaxation for the dielectric relaxation process is therefore proposed to explain the experimental behavior. The influence of the heating rate and of a non-Debye relaxation on the TSDC signal is also studied and clearly shows that the peculiar behavior of TSDC peaks can originate from the conjugated effects of entropy relaxation and non-Debye response View full abstract»

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  • Stabilization of positive charge in SiO/sub 2//Si/sub 3/N/sub 4/ electrets

    Page(s): 1049 - 1056
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    The stability of charge storage in electrets is a key parameter for their use in practical devices. For this purpose a new charging procedure for SiO2/Si3N4 thin film electret on a silicon carrier has been developed. It combines a heat-treatment at 450degC with a hexamethyldisilazane treatment before charging of the electret. The retention of a positive charge and the stability of its spatial trapping are investigated. The optimized technological parameters provide a lifetime of the charge on open air of about 200 years. No spatial charge retrapping has been observed after fabrication of the electret. A set of experiments has been carried out to characterize the properties of the charged electrets and to stabilize the charge retention using heat treatment at temperatures up to 450degC after charging, which is important for their packaging at elevated temperatures View full abstract»

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  • Dielectric properties during apoptosis in peripheral blood cells from chronic lymphocytic leukaemia patients

    Page(s): 1057 - 1062
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    Previous work in our laboratories showed that dielectric measurements reveal the very early physical events on damaged DNA which lead to the disruption of the DNA weak stacking forces. We followed up the evolution of this early physical damage into chemical damage within human cells, and were able to study the early stages of membrane alteration, associated with apoptosis, by measuring modifications in the dielectric properties of the cell membrane. It was found that in cells from patients with the B-chronic lymphocytic leukaemia (CLL-cells), which exhibit a higher rate of apoptosis, the values of the real and the imaginary part of the dielectric permittivity, epsiv*, i.e. epsiv' and epsiv" respectively, and of the alternate field specific conductivity, sigmaac, as a function of frequency, were higher than those of the healthy donors, in which apoptosis occurs at lower rate. Finally, large differences were found between the respective epsiv', epsiv" and sigmaac, values observed in the cells isolated from the two leukaemic patients. These differences were matching the large differences observed between the corresponding apoptosis rates at the two leukaemic patients View full abstract»

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  • Space charges and traps in polymer electronics

    Page(s): 1063 - 1073
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    A review is undertaken of the role of space charges and traps in polymer electronic devices. The origin of the space charges considered include electrode injection leading to space-charge-limited currents in organic light emitting diodes (OLEDs), ionized impurities leading to rectification at Schottky junctions and mobile ion effects. Also considered are the effects of traps on the performance of organic metal-insulator-semiconductor field-effect transistors (MISFETs) View full abstract»

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  • Temperature dependent impedance spectroscopy on polyaniline based devices

    Page(s): 1074 - 1081
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    Polyaniline (PANI) sandwich structures with Al and/or Au electrodes have been investigated using temperature dependent I-V measurements and impedance spectroscopy in the 50 mHz -1 MHz frequency range. The I-V measurements on the Au/PANI/Au structure have shown a practically Ohmic behavior, while the Au/PANI/Al structure has presented a clear non-linear behavior in the voltage range studied. From the complex impedance data analysis we have observed that while in the Au/PANI/Au structure the conductive process is best described with a single distribution of relaxation times, associated only to the bulk region, in the Au/PANI/Al structure the active layer is divided in two regions with distinct resistances: one associated to the bulk region, and the other, much higher, associated to a thin region close to the PANI/Al interface. We have also analyzed the real capacitance data of the Au/PANI/Al structure using two models based on an abrupt cut-off frequency concept. The application of these models has enabled the determination of some important parameters, such as: Debye length (ap20 nm), energetic position (ap320 meV) and associated density of states (ap2times1018 eV-1.cm-3) of the bulk Fermi level, width of the space charge region (ap70 nm), built-in potential (ap780 meV), and the gap states distribution. Correlating the results obtained from the I-V measurements, impedance and capacitance spectra, we have concluded that a highly resistive layer is formed in the vicinity of the polymer/Al interface in the Au/PANI/Al structure, which is responsible for the presence of a high density of localized states. In the Au/PANI/Al structure, such localized density of gap states dominates the electronic transport, in contrast to the Au/PANI/Au structure, where this resistive layer can be neglected and the bulk effects dominate the electronic transport View full abstract»

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  • Electrical characteristics of metal-insulator-semiconductor diodes and transistors with space charge electret insulators: towards nonvolatile organic memories

    Page(s): 1082 - 1086
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    Organic field-effect transistors (OFETs) consist of a gate dielectric and an organic semiconductor film. The performance of organic electronic devices substantially depends on the dielectric properties of the insulating gate layer. Only a few key parameters, primarily the dielectric constant and the resulting device capacitance, have been regarded to be of central importance. Many insulating layers are however not simple dielectrics, an example are space charge gate electrets with internally trapped charges. Space charge gate electrets affect the electrical characteristics of diodes and transistors in a much more sophisticated manner, they are for example the key element in flash memories. We present impedance measurements of an organic metal-insulator-semiconductor (MIS) diode and corresponding measurements of a related organic field effect transistor. Both devices have a comparable design, with polyvinylalcohol as gate electret and the methanofullerene PCBM as organic semiconductor. Pronounced electret effects of charge injection and trapping are observed by impedance measurements of the MIS structure and these effects are found equally expressed in the electrical characteristics of the OFET configuration, reflecting first steps towards organic flash memories View full abstract»

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  • Transparent pyroelectric sensors and organic field-effect transistors with fluorinated polymers: steps towards organic infrared detectors

    Page(s): 1087 - 1092
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    Polymer electrets from the polyvinylidene family are highly suitable materials for plastic electronics. The high dielectric constant of such polymers in both the ferro- and non-ferroelectric phase is most suitably employed in gate dielectrics in organic field-effect transistors. The pyro- and piezoelectric response makes ferroelectric polymer electrets also an ideal choice for organic temperature, infrared radiation and pressure sensors. Here we report steps towards transparent organic pyroelectric infrared sensors: The infrared sensor element is based on poly (vinylidene fluoride), the top contact pentacene transistor on poly(vinylidene-fluoride tetrafluoroethylene hexafluoropropylene) and poly(vinyl cinnamate) as gate dielectric. Steps for combining the ferroelectric polymer sensors and transistors to a fully organic temperature and infrared sensing element are outlined. The sensor concept may be easily scaled up to the production of sensor arrays for thermal imaging View full abstract»

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  • Photogenerated minority carrier trapping and inversion layer formation in polymer field-effect structures

    Page(s): 1093 - 1100
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    We describe the results of a photocapacitance study of Metal-Insulator-Semiconductor (MIS) capacitors formed from semiconducting polymers. In this technique, capacitance-voltage (C-V) plots of the devices are obtained prior, during and after illumination with light of energy greater than the optical band gap of the semiconductor. When the capacitors are biased into depletion and simultaneously exposed to light, optically-generated minority electrons drift to and become trapped in states at the semiconductor-insulator interface, resulting in a significant shift of the C-V plot to more positive voltages. For devices employing a polysilsesquioxane insulator clear and unambiguous evidence is obtained for the formation of an inversion layer at the interface. Upon terminating the illumination, the devices relax back to the initially-obtained dark C-V plots as the trapped electrons are thermally excited from their trapping states. By tracking the voltage required to maintain a constant capacitance the dynamics of charge detrapping can be followed leading to an estimate for the energy of the interface state View full abstract»

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  • Layer-by-layer films of poly(o-ethoxyaniline), chitosan and chitosan-poly(methacrylic acid) nanoparticles and their application in an electronic tongue

    Page(s): 1101 - 1109
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    Layer-by-layer (LbL) films have been produced with poly(o-ethoxyaniline) (POEA), chitosan and chitosan-poly(methacrylic acid) (CS-PMAA) nanoparticles. Because the adsorption of LbL films depends on ionic interactions and H-bonding, optimized conditions had to be established for the growth of multilayer films. Unusually thick films were obtained for POEA and CS-PMAA, thus demonstrating the importance of using chitosan in the form of nanoparticles. These nanostructured films were deposited on chromium electrodes to form a sensor array (electronic tongue) based on impedance spectroscopy. This system was used to detect copper ions in aqueous solutions View full abstract»

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  • Recent developments of polar piezoelectric polymers

    Page(s): 1110 - 1119
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    Following the extensive studies on the piezoelectric, pyroelectric and ferroelectric properties of polyvinylidene fluoride and its copolymers, a vast number of polymers were investigated for these properties. Piezoelectric thin films of polyurea were prepared by vapor deposition polymerization in vacuum and stable up to 200 degC. Piezoelectric odd nylon was characterized by the field induced rotation of amide dipoles in the molecular sheet crystalline structure. Completely amorphous vinylidene cyanide copolymers produced a remanent polarization proportional to the large dielectric relaxation strength during poling. Drawn films of polylactic acid exhibited the large shear piezoelectricity due to the crystalline structure of helical molecules, however, a remnant polarization was also produced by poling at high temperatures. Various polymers containing dipolar groups of carbonyl, urethane, urea and thiourea bonds were poled and their ferroelectric hysteresis was observed. Application of magnetic field as high as 10 Tesla to the liquid crystalline state of helical molecules of poly-y-benzyl-L-glutamate during film formation resulted in a very high shear piezoelectric constant 26 pC/N. Unprecedented high piezoelectric constant such as 1000 pC/N was found in the highly hydrated cornea membrane consisting of collagen fibrils View full abstract»

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Aims & Scope

IEEE Transactions on Dielectrics and Electrical Insulation contains topics concerned with dielectric phenomena and measurements with development and characterization of gaseous, vacuum, liquid and solid electrical insulating materials and systems.

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Editor-in-Chief
Reuben Hackam