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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 4 • Date Aug 2000

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Displaying Results 1 - 19 of 19
  • TSDC in epoxy networks of well-controlled architecture: Experimental evidence for local and non-local relaxation process

    Page(s): 551 - 555
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    Local and non-local relaxation processes were distinguished by investigating the heating rate dependence of the thermally stimulated discharge current (TSDC) line-shape in the case of three epoxy networks of well-controlled architecture. These networks were chosen because their chain dynamics differ by the relative extent of their local and short-scale cooperative molecular motions, as a consequence of marked differences in crosslink density. As far as local motions are concerned, the TSDC evolution measured at low temperature can be accounted for by using a model based on a distribution of activation energies. On the other hand, the higher temperature part of the spectrum of the densely crosslinked network, which mainly results from cooperative motions, shows a much different behavior which cannot be fitted by such a distribution model View full abstract»

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  • A finite element method for the determination of space charge distributions in complex geometry

    Page(s): 556 - 560
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    Electrical breakdown in insulators very often initiate near high field regions of the structure, as found near small-radius impurities or at electrode defects. This is attributed to the development of localized space charges. For this reason many efforts have been made to determine such charge. Various techniques are now available, but they are not directly applicable to complex geometries where it is difficult to determine analytically the field configuration and thus the relation between the measured variables and the space charge distribution. To solve this problem, we propose to use a numerical simulation using a finite element method (FEM). In this paper we describe how it can be implemented in the case of the pressure wave propagation (PWP) method. It is shown that measured signals in insulating samples with divergent electric field regions are well fitted by simulations. We show that this allows for the determination of space charge distribution in such samples View full abstract»

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  • Space charge behavior in low-density polyethylene

    Page(s): 503 - 508
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    The space charge distribution in low-density polyethylene (LDPE) was measured with the pulsed electroacoustic (PEA) method. We used three types of LDPE: LDPE-L and LDPE-H were prepared by the high pressure process, and m-LDPE was polymerized with a metallocene catalyst. Space charge in LDPE strongly depended upon the electrode material. Semiconductive electrodes enhanced carrier injection into LDPE and, as a result, space charge. The density, polymerization process, applied field, temperature and so on also affected the space charge behavior. This space charge behavior was compared with the results of dc current measurements View full abstract»

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  • Molecular dynamics in homo, diblock and triblock polymers studied by dielectric relaxation spectroscopy

    Page(s): 509 - 516
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    The dynamics of poly(oxybutylene) chains was investigated in comparison to that of poly(oxybutylene) blocks in solid semicrystalline poly(oxyethylene)-poly(oxybutylene) (EB) diblock copolymers and poly(oxyethylene)-poly(oxybutylene)-poly(oxyethylene) (EBE) triblock copolymers, using broadband dielectric relaxation spectroscopy (10-2 to 109 Hz). The B homopolymers and the B blocks of the triblocks were dipole-inverted, whereas no dipole inversion occurred in the B blocks of the diblocks. For B blocks longer than a critical value Bcr, the main relaxation, which is related to the glass transition, exhibits similar characteristics in all three systems. The normal mode relaxation process due to chain motions of larger length scales than that of segmental motion, also was studied for the homopolymers and the copolymers. The results also imply that B blocks of triblock copolymers exhibit normal mode relaxation, where mainly normal eigenmodes of relatively high orders contribute. The mobility of the global chain motion is determined by the order of the normal eigenmodes with the highest intensities, whereas the whole chain conformations are mainly determined by the spatial confinement in the solid copolymers and the effects of fixed chain ends, irrespectively of the order of the most intensive eigenmodes. For very short B blocks, fixed chain end effects affect significantly the dynamics of both, main and normal mode relaxation View full abstract»

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  • Porous PTFE space-charge electrets for piezoelectric applications

    Page(s): 480 - 488
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    Porous polytetrafluoroethylene (PTFE) films were positively or negatively corona charged at room or elevated temperatures. Their charge storage behavior was investigated by means of isothermal surface potential measurements in direct comparison to nominally nonporous samples of the same polymer. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities. Direct piezoelectricity was studied on quadruple, double, and single layer samples by means of quasi-static measurements. For the determination of indirect piezoelectricity, frequency-dependent acoustical-transducer experiments were carried out. Both applications-relevant measurements yielded piezoelectric d33 coefficients of up to approximately 600 pC/N or 600 pm/V. These values are more than one order of magnitude higher than in conventional piezoelectric polymers such as polyvinylidenefluoride (PVDF) and almost comparable to the highest known values of inorganic piezoelectrics. Consequently, the novel piezoelectric porous-fluoropolymer spacecharge electrets exhibit an outstanding potential for various device applications that are very briefly discussed View full abstract»

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  • Effect of dipolar arrays on the localization of charge carriers in molecular materials

    Page(s): 531 - 536
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    The paper presents results of calculations of the depth and distribution of local states for charge carriers created by polar impurities in a model molecular material. Equations resulting from the electrostatic model due to Lyons have been employed in the calculations. The presence of polar species in a non-polar molecular material locally modifies the polarization energy, thus creating local states (traps) on neighboring molecules. Calculations performed for an isolated dipolar defect show that traps as deep as ~0.4 eV to ~0.5 eV can result in such a way, their depths and cross-sections depending on the dipole moment of the guest molecule. Results of similar calculations carried out for arrays of spatially connected dipoles indicate that local states of a considerable density may be created, modifying the density-of-states function, and hence influencing the effective mobility of charge carriers View full abstract»

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  • The influence of electrodes and conditioning on space charge accumulation in XLPE

    Page(s): 561 - 571
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    The accumulation of space charge in planar crosslinked polyethylene (XLPE) samples under dc electric fields at room temperature was investigated using the pulsed electroacoustic (PEA) method. Three different organic semiconductor (semicon) materials containing carbon black at concentrations ~30 %wt, and vacuum-evaporated gold, were used as electrodes. Three different conditioning procedures were investigated. Unconditioned samples developed heterocharge with density increasing from zero at the electrodes, to a maximum at roughly one-third of the thickness, and then falling to zero around the center. They also developed homocharge close to the electrodes. The choice of electrode material had little effect on the heterocharge profile in unconditioned samples. Conditioning by holding at 80°C for four days, at rotary pump pressure or at atmospheric pressure, suppressed the accumulation of heterocharge. Homocharge accumulation close to the electrodes in samples with semicon electrodes was affected little by this conditioning, but was reduced considerably in samples with gold electrodes. Conditioning by holding at room temperature for seven days at rotary pump pressure had little effect. The heterocharge originates in inhomogeneous polarization due to a spatially inhomogeneous distribution of permanent dipole molecules, probably acetophenone. The dipole concentration decreases in going from the electrodes towards the center of the samples, as a result of diffusion of the molecules in the opposite direction. The homocharge originates in electronic charge injection/extraction across the electrode/XLPE interfaces View full abstract»

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  • Piezoelectricity, pyroelectricity and ferroelectricity in biomaterials: Speculation on their biological significance

    Page(s): 466 - 473
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    Many thousands of papers have been published on the piezoelectric, pyroelectric and ferroelectric properties of single crystals, ceramics, polymers and liquid crystals. Yet fewer than 100 papers have appeared on these properties in biological materials. Piezoelectric and pyroelectric effects have been measured on a number of plant and animal tissues and the possible existence of ferroelectricity has been suggested also. This paper reviews some measurements and speculates on their significance. Specific examples include piezoelectricity in a crystalline material in the pineal gland of the brain, pyroelectricity in the heat sensing organs of certain varieties of snakes and beetles, and ferroelectricity in the conduction of action potentials in nerve and muscle cells View full abstract»

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  • The method of matching resonance frequencies in coupled transmitter PVDF/TRFE diaphragms

    Page(s): 498 - 502
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    The influence of thermal aging on the resonance frequency and electric impedance of polyvinylidenefluoride (PVDF) type polymer films was studied with the aim to apply thermal treatment to modify and tune the parameters of polymer electromechanical transducers. Results of our study show that thermal treatment of PVDF/TRFE 75/25 copolymer in the temperature range 50 to 125°C (10 min) can be applied for matching the resonance frequencies of coupled transducers. The resonance frequency of the copolymer was found to increase linearly with the rise of the annealing temperature by ~12%, whereas the related decrease in the electromechanical coupling coefficient does not exceed ~11% View full abstract»

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  • Spontaneous polarization and orientational dynamics of polar rod-like molecules in host/guest materials

    Page(s): 523 - 530
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    The polar domain structure and dynamics of host/guest materials which include polar rodlike molecules were investigated by pyroelectric and dielectric techniques. Using scanning pyroelectric microscopy (SPEM), spatially resolved information on a μm scale was obtained of the 3-D domain structure of the polarization caused by an acentric host lattice and/or the polar arrangement of dipolar guest molecules. Two classes of pyroelectric host/guest materials were examined with SPEM: (1) an inorganic host (zeolite AlPO4-5) which was loaded with p-nitroaniline (PNA) guest molecules by an adsorption process, and (2) an organic channel-type inclusion material, which establishes its macroscopic polarity by cocrystallization of perhydrotriphenylene (PHTP) with the polar guest 1-(4-nitrophenyl)piperazine) (NPP). Despite great differences in the chemical composition and morphology, both types of host/guest systems revealed 180° macro-domains, of which the particular shape and sign of the polarization were in accordance with models about crystal growth and/or directed adsorption of polar molecules. In order to assess the molecular dynamics of the included guest molecules, frequency dependent local pyroelectric measurement, based on SPEM, and broad-band dielectric relaxation spectroscopy were performed. Whereas guest molecules included in PHTP channels show no rotational dynamics, we found several relaxation processes in PNA-loaded AlPO4-5 crystals, which were assigned to local and cooperative relaxation modes of molecular chains of the hydrogen-bonded PNA guest molecules View full abstract»

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  • Polarization fatigue in ferroelectric polymers

    Page(s): 489 - 492
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    Ferroelectric materials are characterized by a spontaneous polarization which can be switched by an applied electric field. In many ferroelectrics the magnitude of the reversed polarization is observed to decrease when the polarization is switched many times. In ferroelectric polymers a significant decrease in reversed polarization may be observed after ~3000 cycles, and the reduction can be ~50% of the initial value. Infrared and X-ray observations show a conversion of the non-polar α crystal form to the ferroelectric β form in samples containing a mixture of the crystal phases. However there appear to be no crystalline changes of such a nature as would lead to fatigue. A decrease in dielectric loss at low frequencies is observed in all fatigued samples, suggesting that space charge trapping may be involved in the fatigue process View full abstract»

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  • Pyroelectric activity of ferroelectric PT/PVDF-TRFE

    Page(s): 517 - 522
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    This paper studies the pyroelectric coefficient of 0-3 composites consisting of 27%vol lead titanate (PT) powder embedded in a vinylidene fluoride-trifluoroethylene copolymer (PVDF-TRFE) matrix. The constituent phases of the composites have been polarized in four possible ways: only the copolymer polarized; only the ceramic polarized; the copolymer and ceramic phases polarized in the same direction; the two phases polarized in opposite directions. The pyroelectric coefficient was measured by a dynamic method at 5 mHz within the temperature range 20 to 90°C (which covers the ferroelectric to paraelectric phase transition temperature of the copolymer matrix). The composite with the copolymer and ceramic phases polarized in the same direction exhibits strong pyroelectric but relatively weak piezoelectric activity, and vice versa when the constituent phases are oppositely polarized. A theoretical model is used to analyze the pyroelectric coefficient of the composites in terms of the pyroelectric and dielectric properties of the copolymer matrix as determined from experiment, and those of the ceramic particles which are assumed to be temperature independent. The pyroelectric coefficient and dielectric permittivity of the ceramic particles are obtained as fitting parameters. The theoretical prediction is found to agree well with the experimental data View full abstract»

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  • Piezoelectricity in cellular electret films

    Page(s): 537 - 542
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    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 d33 constant of ≲ 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 View full abstract»

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  • Polarization and space charge behavior of unaged and electrically aged crosslinked polyethylene

    Page(s): 474 - 479
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    The behavior of crosslinked polyethylene (XLPE) for HV cable insulation under electrical stress is discussed in the paper, referring to specimens unaged and electrically aged. Polarization and space charge features are considered. The behavior of the examined insulation is evaluated through the `universal' model for polarization and space-charge associated quantities, such as the dc threshold for space charge build up and the total charge accumulated into insulation. It is shown that aging affects the magnitude and the frequency of low and intermediate frequency polarization peaks, as well as the magnitude of accumulated space charge and threshold. Charge injected or formed in the bulk, as well as microstructural modifications, could explain changes of polarization and space charge magnitude with aging View full abstract»

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  • Preparation and characterization of novel piezoelectric and pyroelectric polymer electrets

    Page(s): 578 - 586
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    The preparation and characterization of novel piezoelectric and pyroelectric electrets is discussed with respect to their electret properties. Charged heterogeneous electrets, like closed-cell microporous polypropylene (pp) foams and `soft-hard' fluoropolymer hybrids are shown to yield large quasi-static and dynamic piezoelectric responses, indistinguishable from true piezoelectricity. Piezoelectric coefficients comparable to that of ceramic counterparts have been achieved. For the investigation of the piezoelectric and pyroelectric properties of the electret materials, the thermal-pulse technique has been extended towards temperature-dependent investigations and measurements on planar, cylindrical or spherical samples. The potential of the thermal-pulse technique not only for the measurement of electric-field and polarization distributions, but also for acoustical, electromechanical and pyroelectric properties is outlined View full abstract»

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  • Dielectric and electrical properties of polycrystalline rocks at various hydration levels

    Page(s): 493 - 497
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    The dielectric properties of low porosity natural polycrystalline rock material (marble) were investigated using broadband 10-2 to 106 Hz) dielectric relaxation spectroscopy at several hydration levels, from dry to fully hydrated samples, with the water content varying from 0 to 0.08% g water/g dry material. The measured dielectric response is characterized by large low frequency dispersions, which depend strongly on the water content. Upon varying the sample thickness, dielectric and water sorption measurements reveal that bulk rather than electrode/sample interface effects are responsible for the main part of the high values obtained for the complex permittivity ε*. Comparative analysis of the experimental data within the complex permittivity ε*, impedance Z* and modulus M*, allows a detailed investigation of the AC conductivity mechanisms existing in the rocks and of water content dependency. Concerning the rock-water interaction, a critical water content hc, (hc~0.04%, the water content for ambient humidity conditions) has been found View full abstract»

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  • A novel displacement component in PVDF and its role in ferroelectric switching

    Page(s): 543 - 550
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    In this paper we describe the detection of a novel frequency dependent displacement current in polyvinylidenefluoride (PVDF). This component was derived from electrical switching experiments performed by the application of voltage pulses of different duration, followed by defined short circuit times. By means of the results so obtained, a complete account of all relevant displacement contributions could be derived. It was possible to separate the remanent polarization, the ε-related displacements and the conduction currents. It was found that the ε-related displacement contains a frequency dependent component which to our knowledge has not been discussed in the literature. By means of this component it is possible to explain the previously observed temporal lag of the remanent polarization and the total displacement which led to the charge trapping model proposed by Eisenmenger and coworkers View full abstract»

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  • Corona poling and electroactivity in a side-chain methacrylate copolymer

    Page(s): 572 - 577
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    The corona poling process and the decay in electro-optic activity were investigated for side-chain methacrylate (MA) copolymers functionalized with different weight percentages of chromophore disperse red-13. In order to obtain the maximum electro-optic activity, the efficiency of the corona poling process was investigated. The poling time, temperature and grid voltage of the corona triode were varied. Also, it was found that the electro-optic decay depends on chromophore content, temperature and the cooling rate of samples at the end of the poling process. Experimental data were fitted using the Kohlrausch-Williams-Watts (KWW) stretched exponential function and parameters depending on temperature and thermal treatment, and their relationship with the poling process is briefly discussed View full abstract»

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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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