T. Takada

Space charge trapping in electrical potential well caused by permanent and induced dipoles for LDPE/MgO nanocomposite

Space charge accumulation in low-density polyethylene film containing a small amount of MgO nanoparticles (LDPE/MgO nanocomposite film) subjected to an electric field greater than 100 kV/mm has been studied using an improved pulsed electroacoustic (PEA) system. No marked space charge accumulation was observed in LDPE/MgO nanocomposite films. To determine the mechanism of no space charge accumulation in the LDPE/MgO nanocomposite film, we compared electric potential wells produced by a permanent dipole moment such as that of carbonyl groups (C=0) and an induced dipole consisting of MgO nanoparticles (spherical dielectrics) under a high electric field to create a trapping site for electric charge carriers. The trapping depth created by the permanent dipole moment such as that of the carbonyl groups (C=0) of chemical defects is approximately 0.45 eV. However, the potential well induced by high-permittivity dielectric nanoparticles (MgO) is about 1.5 to 5.0 eV, which is much deeper than that induced by chemical defects. The suppression of space charge formation is explained using the potential well model consisting of a dipole induced by a high-permittivity dielectric nanoparticle. We explained the suppression mechanism of charge accumulation in the LDPE/MgO film that contains deep traps.

Effect of additives on morphology and space charge accumulation in low density polyethylene

The effects of an antioxidant additive on polymer morphology and space charge formation have been investigated, as a function of temperature, in melt-quenched films of low-density polyethylene. On quenching, the additive-free polymer crystallizes to give extensive spherulitic structures; similar objects are not seen in the system containing the antioxidant. The pulsed electro-acoustic method was then used dynamically to follow both the formation of space charge at high voltage and its subsequent decay under short circuit conditions in both material systems. Data were obtained at room temperature, 50 and 70/spl deg/C. Comparing the additive free material with identical polymer containing a standard anti-oxidant package indicates that the addition of the antioxidant markedly changes both space charge formation and decay processes. At room temperature, both materials were found to behave similarly, suggesting that the changes in gross morphology described above are of secondary importance. At higher temperatures, the addition of the antioxidant enhances negative charge accumulation in the material, implying its association with relatively deep traps for the negative charges within the bulk.

Pulse Acoustic Technology for Measurement of Charge Distribution in Dielectric Materials for Spacecraft

Recently, some accidents in spacecraft due to charging up by irradiation of charged particles have been reported. Some of them are caused by a surface discharge, which normally takes place in a plasma environment in space. Some others seem to be caused by a discharge due to an accumulation of charge in the bulk of materials in a relatively high-altitude environment. To investigate the charge accumulation in the bulk of materials, the authors have been developing an internal charge measurement system. The internal charge accumulation in dielectric materials under irradiation of artificial radioactive rays is observed using the developed system, which employs the so-called pulsed electroacoustic method and pressure wave propagation method based on pulse acoustic measurement techniques. The developed system is applicable to the measurement of the charge distribution in dielectrics in air atmosphere or vacuum environments under electron beam, proton beam, and gamma ray irradiations. Using the system, we measured bulk charge distributions in polymeric films and glass materials. In this paper, summaries of some typical measurement results are presented following a brief explanation of measurement principles

Space charge formation in LDPE/MgO Nano-composite under high electric field at high temperature

Time dependence of space charge formation under high dc electric field at various temperatures between 30 and 60degC in nanocomposite of low density polyethylene (LDPE) mixed with nano-size filler of magnesium oxide (MgO) were studied using pulsed electro-acoustic (PEA) method. In LDPE without MgO, it is found that huge amounts of both positive and negative packet-like charges injection at all measured temperatures, and they makes a local electric field in bulk higher. On the other hand, there is almost no such huge packet-like charge was observed in LDPE/MgO with MgO content of more than 1 phr even under high electric field at high temperature, while it was observed in that with MgO content of 0.2 phr. To study the reason of such space charge suppression effect by nano-filler, we tried to calculate the potential distribution which is induced by the difference of relative permittivities between LDPE and MgO under such high electric field. By a numerical analysis, it is found that the induced potential distribution induced around the nano-filler under high electric field works as a deep trap to capture and to prevent the movement of injected carriers from electrode.

Space charge and charge trapping characteristics of cross-linked polyethylene subjected to ac electric stresses

This paper reports on the result of space charge evolution in cross-linked polyethylene (XLPE) planar samples approximately 220 µm thick. The space charge measurement technique used in this study is the pulsed electroacoustic method. There are two phases to this experiment. In the first phase, the samples were subjected to dc 30 kVdc mm −1 and ac (sinusoidal) electric stress levels of 30 kVpk mm −1 at frequencies of 1, 10 and 50 Hz ac. In addition, ac space charge under 30 kVrms mm −1 and 60 kVpk mm −1 electric stress at 50 Hz was also investigated. The volts-off results showed that the amount of charge trapped in XLPE sample under dc electric stress is significantly bigger than samples under ac stress even when the applied ac stresses are substantially higher. The second phase of the experiment involves studying the dc space charge evolution in samples that were tested under ac stress during the first phase of the experiment. Ac ageing causes positive charge to become more dominant over negative charge. It was also discovered that ac ageing creates deeper traps, particularly for negative charge. This paper also gives a brief overview of the data processing methods used to analyse space charge under ac electric stress. (Some figures in this article are in colour only in the electronic version)

Space charge trapping in electrical potential well caused by permanent and induced dipoles

Space charge formation in low-density polyethylene film containing a small amount of MgO nanoparticles (LDPE/MgO nanocomposite film) subjected to a high electric field greater than 100 kV/mm has been studied using an improved pulsed electroacoustic (PEA) system. No marked space charge formation was observed in LDPE/MgO nanocomposite films. To discuss the mechanism of no space charge accumulation in LDPE/MgO nonocomposite film, we assumed that both electric potential wells produced by a permanent dipole moment such as that of a carbonyl radical (C=0) and an induced dipole consisting of MgO nanoparticles (spherical dielectrics) under a high electric field create a trapping site for electric charge carriers. The trapping depth created by the permanent dipole moment such as that of the carbonyl radical (C=0) of chemical impurity defects is about 0.45 eV. However, that induced by high-permittivity dielectric nanoparticles is about 1.5 to 5.0 eV, which is much deeper than that of chemical impurity defects. The suppression of space charge formation is explained using the potential well model consisting of an induced dipole by a high-permittivity dielectric nanoparticle.

Space Charge Formation in LDPE/MgO Nano-composite Thin Film under Ultra-high DC Electric Stress

Nano-composite technology makes it possible to improve dielectric properties, such as resistivity, breakdown strength and so on, under high DC stress. However, the mechanism of the improvement in MgO nano-composite LDPE (LDPE/MgO) has not been clear, yet. We attempted to clarify the mechanism of the improvement on the electrical properties focused on the space charge formation. To investigate the influence of MgO nano-filler on space charge profiles, we measured the space charge distributions using PEA system under DC electric field of 50 to 250 kV/mm. In the case of LDPE without nano-filler, a positive packet-like charge injection with peak charge density of more than several hundred C/m3 were measured under more than 150 kV/mm. In the case of LDPE/MgO with MgO content of 0.2 phr, an oscillating behavior of packet-like charge was observed when the DC electric field of more than 200 kV/mm was applied to it. On the other hand, in the case of LDPE/MgO with MgO content of more than 0.5 phr, there is no remarkable charge injection. It is thought that space charge behavior is drastically changed by adding only more than 0.5 phr of MgO to LDPE. The nano-composite including more than 0.5 phr shows a superior property to prevent the injection of space charge into bulk.

Characteristic of charge accumulation in glass materials under electron beam irradiation

Space charge formation in various glass materials under electron beam irradiation was investigated. The charging of spacecrafts occurs in plasma and radiation environments. In particular, we focused on an accident caused by internal charging in a glass material that was used as the cover plate of a solar panel array, and tried to measure the charge distribution in glass materials under electron beam irradiation using a pulsed electroacoustic (PEA) method. In the case of quartz glass (pure SiO2), no charge accumulation was observed either during or after the electron beam irradiation. On the contrary, charge accumulation was observed in glass samples containing metal oxide impurities. It is found that the polarity of the observed charges depends on the contents of the impurities. To identify which impurity dominates the polarity of the accumulated charge, we measured charge distributions in several glass materials containing various metal oxide impurities. Furthermore, the dependence of the polarity of accumulated charges on the composition of glass materials is discussed using energy band models

Space charge in polyethylene under AC electric stress using the pulsed electroacoustic method

This paper reports on space charge in XLPE planar samples of approximately 220 /spl mu/m thick, subjected to dc 30 kV/mm and 1 Hz, 10 Hz and 50 Hz ac 30 kV/sub pk//mm conditions for 24 hours. Measurements of space charge were carried out using the pulsed electroacoustic (PEA) technique. In addition, experiments with higher applied electric stress of 60 kV/sub pk//mm at 50 Hz ac condition were also conducted. In order to investigate the effect of degassing on space charge distributions in samples under ac stress, both as-received and degassed, were used in this study.

Charge injection and transport analysis and measurements in highly purified water

Because of its high dielectric constant, low cost, and ease of handling and disposal, water and water mixtures are being actively investigated for use as the dielectric in pulse power capacitors, pulse forming and transmission lines, and switches [1]. Recent charge injection analysis, terminal voltage-current measurements and Kerr electro-optic field mapping measurements have shown significant space charge effects on the electric field distribution [2,3]. Recent measurements for a water/ethylene glycol capacitor with stainless steel electrodes found that open circuit high voltage decay curves had a dielectric relaxation time that decreased with increasing initial voltage at room temperature, while at −10° C, the open circuit voltage decay had a negative second derivative with time in contrast to the expected exponential decay which always has a positive second derivative [2,3]. The decay rate also depended on the electrode spacing unlike a pure ohmic dielectric with conductivity σ and permittivity ε which has a dielectric relaxation time τ =ε/σ independent of geometry and voltage. These results indicated that the charge transport time of order ℓ2/(μV0) was also an important parameter. It was hypothesized that this anomalous behavior was due to positive space charge injection with density q that increases the ohmic conductivity σ to σ + qμ.