Structure characteristics of electrical treeing in XLPE insulation under high frequencies

Electrical tree structure is one of the most important influencing factors for electrical treeing characteristics in polymers. In this paper, we focused on the structure characteristics of electrical treeing in cross-linked polyethylene (XLPE) insulation under high-frequency voltages. The tree structure characteristics include structure distribution characteristics and structure conversion characteristics. The influences of voltage, frequency, and pin-plane spacing on tree structure characteristics were analyzed based on the experimental results. It can be concluded that tree structures regularly change with the local electric field and frequency. The electric field in a very small zone near the needle tip is an important influencing factor for the formation of bush-like trees, and the lowest frequencies for the observed pure-vine-like trees increased with voltage. For double-structure trees, the local electric field at the transition location of the two structures remained almost unchanged with voltage and pin-plane spacing, but obviously increased with frequency. In order to investigate the relations of the growth rate and fractal dimension with tree structure characteristics, a new parameter, the energy threshold W_t, has been introduced and calculated for different tree structures.     

高频电压下交联聚乙烯电缆绝缘中电树枝生长的动力学模型

针对高压交联聚乙烯电缆绝缘试样,在1000—2000Hz 10kV峰值正弦电压下,采用计算机实时显微数字摄像技术进行了电树枝培养实验.基于半结晶绝缘材料中电树枝生长机理和电树枝结构的分形特征,提出了一个在高频范围定量预测电应力驱动下交联聚乙烯电缆绝缘中电树枝生长特性的动力学模型,获得了电树枝生长率方程和从电树枝生长到击穿过程的寿命公式.将该模型预测值与实验中获得的电树枝生长规律实验数据进行比较,其结果有较好的一致性,表明提出的模型化方法可以应用到交联聚乙烯电缆绝缘中电树枝老化规律的定量分析研究中. 关键词： 交联聚乙烯电缆绝缘 电树枝 施压频率 动力学模型     

交联聚乙烯电缆绝缘材料中电树枝的导电特性研究

利用光学显微观察、局部放电测量和共聚焦Raman光谱分析相结合的方法, 研究了交联聚乙烯(XLPE)电缆绝缘材料中两种典型电树枝的导电特性.尽管具有相似的培养条件, 两种电树枝却呈现出完全不同的形态,其中9 kV下典型电树枝为枝-松枝状, 11 kV下为枝状, 而且电树枝生长及局部放电规律呈现出明显的差异.枝-松枝状电树枝主干通道内存在无序石墨碳的沉积, 根据石墨碳G带与D带的相对强度,估算碳层厚度约为8 nm,树枝通道单位长度电阻小于 10 Ω· μm^-1,足以抑制电树枝内局部放电的发展,电树枝呈现出导电型电树枝特征. 枝状电树枝通道内观察到荧光背景,存在材料劣化的产物,但不存在无序石墨碳的聚集, 通道具有明显的非导电特性而不足以抑制电树枝内局部放电的连续作用. 最后提出了XLPE电缆绝缘材料中导电型和非导电型电树枝的单通道生长模型, 利用等效电路理论对XLPE电缆绝缘材料中两种不同导电特性电树枝的生长机理进行了探讨.     

Continuous spectrum generated by the excited dental X-ray tube

The continuous X-ray spectral profiles have been successfully produced using the excited dental X-ray tube, where used dental X-ray tube was employed in the experimental setup. The effect of parameters on the continuous spectrum is investigated, whereas the major parameters are the anode current and adjustable voltage. It was found that the optimum conditions for the threshold voltage were three times higher than the highest energy absorption edge with tungsten anode target on the spectral distribution of radiation energy, which was increased, and the short wavelengths limitation obtained are 1.03, 0.73, 0.59, 0.48 and 0.46 Å.. The relative sensitivity is increased from 12 to 25 kV, and it is decreased at 26 and 27 kV. The intensity near the short wavelength is increased by the double voltage value.     

Modification of semiconductor materials using laser-produced ion streams additionally accelerated in the electric fields

The laser-produced ion stream may be attractive for direct ultra-low-energy ion implantation in thin layer of semiconductor for modification of electrical and optical properties of semiconductor devices. Application of electrostatic fields for acceleration and formation of laser-generated ion stream enables to control the ion stream parameters in broad energy and current density ranges. It also permits to remove the useless laser-produced ions from the ion stream designed for implantation.For acceleration of ions produced with the use of a low fluence repetitive laser system (Nd:glass: 2 Hz, pulse duration: 3.5 ns, pulse energy:∼0.5 J, power density: 10¹⁰ W/cm²) in IPPLM the special electrostatic system has been prepared. The laser-produced ions passing through the diaphragm (a ring-shaped slit in the HV box) have been accelerated in the system of electrodes. The accelerating voltage up to 40 kV, the distance of the diaphragm from the target, the diaphragm diameter and the gap width were changed for choosing the desired parameters (namely the energy band of the implanted ions) of the ion stream. The characteristics of laser-produced Ge ion streams were determined with the use of precise ion diagnostic methods, namely: electrostatic ion energy analyser and various ion collectors. The laser-produced and post-accelerated Ge ions have been used for implantation into semiconductor materials for nanocrystal fabrication. The characteristics of implanted samples were measured using AES.     

Polarization-dependent gain and saturation energy density in a TE N2-laser amplifier

The polarization-dependent gain, g₀, and saturation energy density, E_s, in a TE N₂-laser amplifier were measured, using an oscillator-amplifier laser system for different amplifier electrode gap separations, d_AMP, of 7, 9 and 4 mm and gas pressure of p = 77, 60, and 165 Torr, respectively. It was realized that for the amplifier with the gap separation of 7 and 9 mm, where the pd_AMP-value has its optimum-value of 54 Torr cm, the gain-coefficient for the input beam with the polarization parallel to the discharge electrodes (P-polarized beam) is slightly higher than the case when the beam polarization is perpendicular to the discharge electrodes (S-polarized beam). In this case, the depolarization ratio for d_AMP = 7 mm is the range of ∼0.998 to ∼0.962 as the input voltage increases from 12 to 15.5 kV, having a minimum of 0.937 around 14 kV. For the E_s-parameter, the reversed order is true. Also, it was found that the saturation energy densities for three states of polarization are linearly related to the output energy densities, having different slopes of 0.11, 0.14, and 0.17 for R (randomly), P- and S-polarization, respectively. The present measurement supports qualitatively the prediction of polarization-inhomogeneity model for the stimulated emission cross-section, showing that randomly oriented dipoles exhibit slightly larger gain on the direction of the electric field.     

Electron emission characteristics of Al-AlN granular films

An electron conduction emitter of Al-AlN granular films was proposed for surface conduction electron emission device in this paper. The Al-AlN granular films with thickness of 30 nm were prepared between two co-planar electrodes with gap of 10 μm by magnetron sputtering. After electroforming the Al-AlN granular films, the films’ structure could be recovered by applying the periodic device voltage (V_f). Stable and uniform electron emission was observed with turn-on voltage of 5.3 V and threshold voltage of 9 V. The emitter emission current (I_e) of 4.84 μA for 36 cells was obtained with the anode voltage of 2.5 kV and the device voltage of 12 V. In addition, Fowler-Nordheim plots for I_e-V_f properties showed that the electron emission mechanism should be field emission.     

Development of YBCO power devices in Japan

A new Japanese national project, called M-PACC, to develop high temperature superconducting electric power devices started in June last year (FY2008–FY2012). This project aims to develop three different types of electric power devices that are expected to provide stable power supplies with large capacity and small size by using YBCO coated conductors. The first program is the development of a 2 GJ class superconducting magnetic energy storage system to control stable electric power systems. It is planned to develop several sets of element coils for a 20 MJ class system as a technological feasibility study for a 2 GJ class coil. The second program is the development of two different types of power cables with higher performance than existing power cables; one is a three-core 66 kV–5 kA class large current cable and the other is a single-phase 275 kV–3 kA class high voltage cable. These cable were required several technological developments, namely, large current and low AC loss, high voltage insulation and low dielectric loss, and power and heat balance for both cables. The third program is the development of a 20 MVA class power transformer with 66 kV/6.9 kV as a distribution transformer. In this project, power transformer systematization technology including 2 kA class large current coil technology, anti short-circuit wire winding technology, AC loss reduction technology, and winding technology will be developed.     

Charge transport along luminescent oxide layers containing Si and SiC nanoparticles

The electrical conductivity of silicon oxides containing silicon and silicon-carbon nanoparticles has been investigated. By use of sequential Si⁺ and C⁺ ion implantations in silicon oxide followed by an annealing at 1100 °C, luminescent Si nanocrystals and SiC nanoparticles were precipitated. The characterization of the electrical transport has been carried out on two kinds of structures, allowing parallel or perpendicular transport, with respect to the substrate. The first type of samples were elaborated by means of a focus-ion-beam technique: electrical contacts to embedded nanoparticles were made by milling two nanotrenches on the sample surface until reaching the buried layer, then filling them with tungsten. The distance between the electrodes is about 100 nm. The second type of samples correspond to 40 nm thick typical MOS capacitors.The electron transport along the buried layer has shown a dramatic lowering of the electrical current, up to five orders of magnitude, when applying a sequence of voltages. It has been related to a progressive charge retention inside the nanoparticles, which, on its turn, suppresses the electrical conduction along the layer. On the other hand, the MOS capacitors show a reversible carrier charge and discharge effect that limits the current at low voltage, mostly due to the presence of C in the layers. A typical Fowler-Nordheim injection takes place at higher applied voltages, with a threshold voltage equal to 23 V.     

Synthesis of LECBD grown cluster assembled SeO2 thin films

Cluster assembled selenium oxide (SeO₂) thin films, as a function of oxygen flow pressure (OFP) have been synthesized by a low energy cluster beam deposition (LECBD) technique. The OFP dependent surface morphology leading to well separated nanoclusters (size ranging from 50 to 200 nm) and fractal features are confirmed from transmission electron microscopic (TEM) measurements. A diffusion limited aggregation (DLA) mediated fractal growth with dimension as 1.71 ± 0.01 has been observed for high OFP (60 mbar). Structural analysis by glancing angle X-ray diffraction (GXRD) and selected area diffraction (SAD) studies identify the presence of tetragonal phase SeO₂ in the deposit. Micro-Raman studies indicate the shifts in bending and stretching vibrational phonon modes in cluster assembled SeO₂ as compared to their bulk counter part due to the phonon confinement effect.     

Thermal shock induced nanocrack as high efficiency surface conduction electron emitter

Significant emission current enhancement has been achieved for surface conduction electron emitter, due to the special three-dimensional nanocrack structure fabricated by the thermal shock process. The three-dimensional configuration strongly changed the electric field distribution and controlled the emission electron trajectory. Thermal shock treatment was also used to increase the edge roughness of the nanocrack and thereby dramatically improved the field emission characteristics. Stable and uniform electron emission was observed with turn-on voltage of 150 V. The surface conduction current of 400 μA for 6 cells was obtained with the detector voltage of 1 kV and the gap voltage of 170 V.     

The effect of an axial external magnetic field on the output power of a small-bore CuBr laser

In this work, the effect of an axial external moveable magnetic field on the output power of a CuBr laser with small-bore tube has been investigated. In all experiments, by applying an EMF along the tube axis, the laser output power has been decreased and by moving the EMF toward the cathode region, more substantial decrease of output power has been observed. The effect is more significant at a magnetic field intensity of 1100 G, Ne gas pressure of 35 Torr, frequency of 19 kHz and voltage of 3.8 kV, such that there was no laser emission when the EMF was placed around the cathode.     

Electric power generation using vibration of a polyurea piezoelectric thin film

An energy harvesting system is proposed, in which mechanical energy is converted to electrical energy through the piezoelectric effect of a polymer polyurea film on the device. Electrical energy harvesting methods that use piezoelectric elements have been reported by several groups, and lead zirconate titanate (PZT) is predominantly employed as the piezoelectric material. An energy harvesting device with a polyurea thin film formed through vapor deposition polymerization with 4,4′-diphenylmethane diisocyanete (MDI) and 4,4′-diamino diphenyl ether (ODA). The conversion efficiency from mechanical to electrical energy was calculated using finite elemental analysis (FEA) of the cantilever configuration. Higher conversion efficiency was obtained using a thinner and shorter cantilever configuration with increased resonance frequency of the device. Experiments were conducted using an electric power generation device with a 3 μm thick polyurea thin film attached to a 0.1-mm-thick, 18-mm-long beryllium copper cantilever. Vibration in the vertical direction, which induces the bending vibration on the cantilever, was applied to the device and the output voltage was measured by connecting load resistances. The output power was measured with a change in the load resistance from 10 kΩ to 10 MΩ, and an optimum output was obtained at 1 MΩ, which corresponds to the value calculated using FEA. The conversion efficiency was improved by changing the cantilever length and an efficiency of 0.233% was obtained with a 4-mm-long cantilever.     

外电场下交联聚乙烯电介质材料分子结构变化及其电老化微观机理研究

为了从微观角度分析交联聚乙烯(XLPE)材料的电树枝老化,本文采用分子模拟方法计算并优化得到了XLPE分子结构.沿着聚乙烯链施加不同大小电场强度,分析交联聚乙烯分子的几何结构、偶极矩、极化率、电荷分布、前线轨道能量和红外光谱变化规律.计算结果表明,随着外电场的增大,交联聚乙烯分子红外光谱发生较大变化;当外施电场达到0.026a.u.后,红外光谱图中出现虚频,表明分子空间结构不再稳定,易发生断键;另外从前线轨道图的变化可以看出断键现象最先发生在交联聚乙烯链端部;沿着电场方向,原子所带电荷量由交联处向端部转移,当外施电场达到0.029a.u.后,链端部的C-H和C-C键断裂产生H·和CH_3·自由基.游离的自由基会形成空间电荷并发生积聚,产生局部较大场强,从而进一步影响交联聚乙烯链的空间结构.而电介质内部微观特性的变化必定会导致交联聚乙烯材料绝缘性能的下降,这些变化对揭示交联聚乙烯电缆电树枝形成的微观规律具有重要研究意义.     

Technology aspects of GaN-based diodes for high-field operation

This work reports important aspects of technology development and characterization for GaN based diodes operating at high electric fields. The considered operation conditions result, in comparison to III–V semiconductor devices, from the higher values of threshold field for intervalley transfer of electrons. This lies above 150 kV/cm and requires correspondingly higher biasing voltages and currents through semiconducting layers of transferred electron devices, switches or NDR (negative differential resistance) diodes. Mesa-based vertical and lateral devices using GaN layers on sapphire substrate were considered for current–voltage characteristics under very high electric field conditions. A systematic investigation of MOCVD-grown diode structures with regular, tapered mesa designs and variable dimensions was carried out under pulsed-bias condition. The current–voltage characteristics showed threshold voltages for saturation corresponding to electric fields well above the critical value of 150 kV/cm in the active layer. Self-heating and electromigration effects have been addressed in relation with biasing and metallization conditions.     

Frequency and spatial characteristics of the electrophysical parameters of a living tree trunk

The method of vertical electrical sounding is applied to derive the spatial characteristic of the dc resistivity ρ of a living pine trunk, ρ = L-0.85, and the frequency characteristic of the ac resistance R of the same pine trunk, R = f-0.053. The results are simulated by the methods of fractal geometry, according to which ρ = L ^{−h + 2} and $ R \sim f^{ - \tfrac{3} {h} + 1} $ R \sim f^{ - \tfrac{3} {h} + 1} , where h is the walk dimension of the electric current in the trunk. From comparison between the experimental and theoretical results, it follows that h = 2.85 and fractal dimension D = 1/h = 0.35. Since D = 1, conducting layers form a Cantor set.     

Fractal structure of equipotential curves on a continuum percolation model

We numerically investigate the electric potential distribution over a two-dimensional continuum percolation model between the electrodes. The model consists of overlapped conductive particles on the background with an infinitesimal conductivity. Using the finite difference method, we solve the generalized Laplace equation and show that in the potential distribution, there appear quasi-equipotential clusters   which approximately and locally have the same values as steps and stairs. Since the quasi-equipotential clusters have the fractal structure, we compute the fractal dimension of equipotential curves and its dependence on the volume fraction over [0,1]$[0,1]$. The fractal dimension in [1.00, 1.246] has a peak at the percolation threshold _pc$p_c$.     

Growth of TiN films at low temperature

Thermodynamic analysis on growth of TiN films was given. The driving force for deposition of TiN is dependent on original Ti(g)/N(g) ratio and original partial pressure of N(g). TiN films were deposited by ion beam assisted electron beam evaporation system under suitable nitrogen gas flow rate at 523 K while the density of plasma varied with diverse discharge pressure had been investigated by the Langmuir probe. TiN films were characterized by means of Fourier transform infrared absorption spectrum (FTIR), X-ray diffraction (XRD) and observed by means of atom force microscopy (AFM). The results of these measurements indicated preferential TiN(1 1 1) films were deposited on substrate of Si(1 0 0) and glass by ion beam assisted electron beam evaporation system at low temperature, and it was possible for the deposition of TiN films with a preferential orientation or more orientations if the nitrogen gas flow rate increased enough. Sand Box was used to characterize the fractal dimension of surface of TiN films. The results showed the fractal dimension was a little more than 1.7, which accorded with the model of diffusion limited aggregation (DLA), and the fractal dimension of TiN films increased with increase of the temperature of deposition.     

Enhanced electrical properties of pentacene-based organic thin-film transistors by modifying the gate insulator surface

A reliable surface treatment for the pentacene/gate dielectric interface was developed to enhance the electrical transport properties of organic thin-film transistors (OTFTs). Plasma-polymerized fluorocarbon (CFx) film was deposited onto the SiO₂ gate dielectric prior to pentacene deposition, resulting in a dramatic increase of the field-effect mobility from 0.015 cm²/(V s) to 0.22 cm²/(V s), and a threshold voltage reduction from −14.0 V to −9.9 V. The observed carrier mobility increase by a factor of 10 in the resulting OTFTs is associated with various growth behaviors of polycrystalline pentacene thin films on different substrates, where a pronounced morphological change occurs in the first few molecular layers but the similar morphologies in the upper layers. The accompanying threshold voltage variation suggests that hole accumulation in the conduction channel-induced weak charge transfer between pentacene and CFx.     

Second-order optical nonlinearity in thermally poled Pyrex borosilicate glass

The thermal poling method was utilized to create second-order optical nonlinearity in Pyrex borosilicate glass. The distribution and amplitude of the induced nonlinearity were characterized with second harmonic microscopy. The induced optical nonlinearity was found in a thin layer around 1.9 μm under the anode surface with a magnitude as high as 0.24 pm/V, comparable to that observed in fused silica samples. SEM observation of the cross-section of the poled glass region, after it had been etched in diluted hydrofluoric acid for several minutes, revealed an etched trench, ∼1.8 μm under the anode edge and ∼0.3 μm in width; while in post-annealed samples, no such etched trench could be observed. The effect of poling voltage on the magnitude of the induced nonlinearity was also studied, where the results showed that higher poling voltage resulted in higher nonlinearity with a threshold of ∼0.9 kV.