Effect of reflection from electrodes and of anisotropy of angular scattering on slow-electron diffusion in a gas. I

The effect of electron reflection from electrodes and of anisotropy of angular scattering on slow-electron diffusion in a gas is investigated theoretically and experimentally. An analytic expression is obtained for the diffusion currents by means of an approximate solution of the kinetic equation. Two electrode configurations are considered, infinite parallel planes and infinite coaxial cylinders. Measurements performed on an instrument with cylindrical electrodes are compared with results of theoretical calculations.     

Effect of electrode microstructure on gas-phase diffusion in solid oxide fuel cells

The relation between electrode microstructure and gas diffusion has been investigated with different morphologies of Pt electrodes by using AC impedance techniques. The measurements were carried out at temperatures of 873–1273 K and oxygen partial pressure (P_O2) of 0.01–1 atm.Gas-phase diffusion was observed only for high-performance electrodes at the high-temperature (1073–1273 K) and low-oxygen-partial-pressure regions (<0.1 atm P_O2). Considering the physical and electrochemical characteristics of impedance arcs, it was found that the arc at the frequency of below 1 Hz was related to gas conversion resistance, while the arc at the frequency of around 10 Hz represented pore diffusion resistance through the current-collecting part. For a thick electrode with a low porosity, however, gas diffusion resistance through pores of an electrode was observed at a frequency of around 100 Hz.From the results of a comparison of electrode performances with different electrode microstructures, electrochemical reaction sites (ERS) are supposed to be located at the peripheral line of Pt and YSZ as well as the Pt/YSZ interfaces where reaction gas can easily diffuse.     

LTD模块气体开关电极材料烧蚀性能实验

针对直线变压器驱动源(LTD)对开关的长寿命需求,基于前期研制的200kV低抖动多间隙气体开关电极几何参数以及开关通流水平,详细开展了不同电极材料(钼、黄铜、铜钨合金、高密度石墨、304和321不锈钢)的烧蚀性能实验。实验结果表明,在单次电荷转移量15.4mC,脉冲电流20kA条件下,体积烧蚀速率从大到小依次为:石墨、黄铜、铜钨合金、钼、不锈钢。电极烧蚀微观形貌分析表明,不锈钢是用于LTD开关相对较好的电极材料。根据不锈钢电极的体积烧蚀速率,可知理论上LTD开关的运行寿命可以超过1×106次,但前提是开关外壳需保持足够的绝缘强度。     

LTD模块气体开关电极材料烧蚀性能实验

针对直线变压器驱动源（LTD）对开关的长寿命需求,基于前期研制的200 kV低抖动多间隙气体开关电极几何参数以及开关通流水平,详细开展了不同电极材料（钼、黄铜、铜钨合金、高密度石墨、304和321不锈钢 ）的烧蚀性能实验。实验结果表明,在单次电荷转移量15.4 mC,脉冲电流20 kA条件下,体积烧蚀速率从大到小依次为：石墨、黄铜、铜钨合金、钼、不锈钢。电极烧蚀微观形貌分析表明,不锈钢是用于LTD开关相对较好的电极材料。根据不锈钢电极的体积烧蚀速率,可知理论上LTD开关的运行寿命可以超过1106次,但前提是开关外壳需保持足够的绝缘强度。     

FLTD三电极场畸变气体开关寿命特性

针对设计的一种场畸变气体开关,研究中间电极材料分别为不锈钢和黄铜条件下的烧蚀特性,结合开关寿命期间静态与触发特性的变化规律,获得决定开关寿命的关键因素,为三电极场畸变气体开关的性能优化提供理论支撑。研究结果表明,采用不锈钢和黄铜作为中间电极的烧蚀区域以及表面粗糙度均随着放电次数增加而增大,黄铜电极烧蚀较为严重且表面有明显的烧蚀圆斑,不锈钢电极则具有更高的表面粗糙度,阴阳极表面烧蚀存在明显差异,随着放电次数的增加,击穿点向电极边缘区域集中,影响开关的沿面绝缘特性,是导致开关寿命终结的主要原因。     

Long-term deconditioning of gas-filled surge arresters

The aim of this paper is to identify parameters that influence the long-term deconditioning effect of gas-filled surge arrester (GFSA) and to provide practical recommendations for mitigating this effect. Namely, after some period of time, on order of hours or days, during which there is no activation due to overvoltage, the deconditioning of GFSA occurs. This effect was observed experimentally within the paper. The observed parameters that could influence the long-term deconditioning effect were the following: shape of voltage load, gas type, gas pressure, interelectrode distance, electrode material, electrode surface topography as well as GFSA design such as two- or three-electrode configuration. According to the results obtained, it has been shown that the occurrence of long-term deconditioning in an insulating system, insulated by a noble gas at a subpressure and with small interelectrode distances, is a phenomenon that always occurs when the insulating system is at rest for about an hour. It has been found that the type of noble gas does not influence the long-term deconditioning. Analysis of such insulating systems' parameters, with a prospect of being used as GFSAs, has demonstrated that this phenomenon is less pronounced at higher pressures (for the same value of the pressure (p) and interelectrode distance (d) product) and for electrodes with microscopically embossed surfaces. According to the results that were obtained by noble gases and their mixtures, as well as the results that were obtained by mixtures of SF6 gas with noble gasses, it can be claimed with confidence that the effect of the long-term deconditioning is an electrode effect. It has also been established that the deconditioning effect does not depend on the electrode material except in the case of electrodes made out of noble metals, which reduce the effect. Based on these results, it can be recommended that the working point of GFSAs be set (according to the DC breakdown voltage value) at a pressure that is as high as possible (with pd?=?const), and that the electrode active surface should have a marked microscopic topography. In addition to this, an essential conclusion for GFSA manufacturers is that long-term system deconditioning is caused by impurities and adsorbed gases that appear at electrode during the state of rest. Out of these two causes, the influence of impurities is probably the dominant one, which is proved by considerably reduced long-term deconditioning in the case of noble metal electrodes, not susceptible to corrosion. This has also been confirmed by a less distinct effect of long-term deconditioning in the case of sandblasted electrodes that have a stronger tendency towards gas adsorption and a weaker tendency towards corrosion. However, it has been shown that adding of the third electrode (that is concentric to the main electrode system) on a free floating potential along with usage of sandblasted electrodes and with smaller interelectrode distance significantly reduces the effects of the long-term deconditioning.     

Impedance studies on Pt|SrCe0.95Yb0.05O3|Pt under dried and humidified air,argon and hydrogen

Arrhenius plots of conductivities and capacitances associated with the bulk, overall electrolyte, charge transfer and electrode processes taking place in a symmetrical cell with well-defined Pt electrodes and electrolyte based on the ceramic proton conductor, SrCe_0.95Yb_0.05O₃, were obtained under wet and dry air, wet and dry hydrogen and wet and dry argon over the temperature range, 200 °C–800 °C. Results for all six gas atmospheres are discussed in order to gain insight into the conduction mechanisms taking place within the electrolyte and, especially, the conduction, sorption and diffusion processes occurring at the electrodes, in this model system.     

Hydrophobic properties of carbon fabric with Teflon AF 2400 fluoropolymer coating deposited from solutions in supercritical carbon dioxide

A method for uniform deposition of a hydrophobizing polymer from a solution in supercritical carbon dioxide (SC-CO₂) onto the surface of carbon fabric used for manufacturing gas diffusion layers of fuel cells is developed. This approach, based on using Teflon AF 2400, a SC-CO₂-soluble copolymer, is compared to the traditional method for hydrophobization of the gas diffusion layer of a fuel cell, based on the use of an aqueous dispersion of Teflon 30N. Hydrophobizing polymers were deposited on the surface of a highly rough carbon fabric (Saati), an electrically conductive gas diffusion layer material with good mechanical and resource characteristics. In one of the versions of the method of deposition from SC-CO₂, the hydrophobic film was subjected to additional annealing at a temperature above the glass transition temperature of Teflon AF 2400 amorphous copolymer. It is shown that this approach makes it possible to form a uniform thin fluoropolymer film on carbon fibers, which imparts the most stable superhydrophobic properties to the surface of the gas diffusion layer at very low amounts of deposited polymer. In this case, the contact angle reaches a value much greater than that previously reported in the literature for similar methods. Prolonged immersion in water (for 1000 h) or wash in the presence of detergent does not impair the superhydrophobicity of the gas diffusion layer. The developed gas-diffusion layer was used to prepare an electrode for phosphoric fuel cell, the current-voltage characteristic of which indicates a satisfactory performance. The results obtained show that adopted approach is promising for developing gas diffusion layers for fuel cell electrodes.     

TEA CO2激光器几种放电电极的比较

 从均匀场电极理论出发，计算并讨论了近似Rogowski电极、Chang电极(含紧凑式Chang电极)和Ernst电极的形状、理论电场分布和实际电场分布。对实验室所用的几种电极进行了脉冲放电实验，从而获得了TEA CO₂激光器的放电参数与输出特性，并对这些结果进行了比较和分析。计算表明，在基本参数相同的情况下，Ernst电极具有最紧凑的外形和最佳的均匀电场分布。实验表明，采用近似Rogowski电极的TEA CO₂激光器具有最大放电辉光范围和44 kV的最小放电击穿电压；采用Ernst电极的TEA CO₂激光器具有25 J的最高单脉冲能量和最大17.2%的斜率效率。最后提出了TEA CO₂激光器主电极的选择建议。     

Study of oxidation process of Cr/Cu/Cr thin film electrodes

Understanding the oxidation process of Cr/Cu/Cr laminated thin film electrode is important for developing new oxidation-resistant electrodes. By studying the evolution of crystal structure, morphology and electric resistivity of Cr/Cu/Cr thin films and electrodes that were post-annealed at different temperatures, the oxidation mechanism and process of Cr/Cu/Cr electrode were proposed. Copper is first oxidized into Cu₂O at low temperatures (<310 °C), and converts to CuO phase at higher temperatures. Two pathways for oxygen diffusion were identified: diffusion from the protective Cr layer, and diffusion from the sidewall of electrode, of which the latter one leads to total oxidation of copper interlayer at high temperatures (>310 °C). As a result, the passivation of Cu metal at electrode sidewalls is crucial in reducing oxidation of Cr/Cu/Cr electrodes or designing new copper-containing oxidation-resistant electrodes.     

TEA CO2激光器几种放电电极的比较

从均匀场电极理论出发,计算并讨论了近似Rogowski电极、Chang电极(含紧凑式Chang电极)和Ernst电极的形状、理论电场分布和实际电场分布。对实验室所用的几种电极进行了脉冲放电实验,从而获得了TEA CO2激光器的放电参数与输出特性,并对这些结果进行了比较和分析。计算表明,在基本参数相同的情况下,Ernst电极具有最紧凑的外形和最佳的均匀电场分布。实验表明,采用近似Rogowski电极的TEA CO2激光器具有最大放电辉光范围和44 kV的最小放电击穿电压;采用Ernst电极的TEA CO2激光器具有25 J的最高单脉冲能量和最大17.2%的斜率效率。最后提出了TEA CO2激光器主电极的选择建议。     

基于可逆固体氧化物电池堆的电能高效存储

研究了固体氧化物电池堆中电能的可逆存储与产生,通过相变金属存储燃料电池模式下的热能并在电解池模式下加以利用. 系统的荷电状态(即氢燃料百分比)可显著增加开路电压,系统压力的增加有效提高了开路电压. 较高的系统压力可促进电极表面的物质扩散和输运,相应地改善电极的极化电阻. 通过有效的热能管理,系统的电能可逆存储的循环效率可高达92%.     

甚高频等离子体增强化学气相沉积大面积平行板电极间真空电势差分布研究

采用二维准平面电路模型研究了甚高频等离子体增强化学气相沉积(VHF-PECVD)大面积平行板电极间真空电势差分布.计算结果表明：随平行板电极尺寸增加和激发频率提高,电势驻波效应成为影响电极间电势差非均匀分布的重要因素.在尺寸为1.2m×0.8m的大面积平行板电极上应用40.68和60MHz两种激发频率,通过功率馈入点数量和位置优化,计算获得非均匀性分别为±2.5%和±5.5%的真空电势差分布.这些数值计算结果为大面积平行板电极在VHF-PECVD中应用提供了重要的理论指导. 关键词： 二维准平面电路模型 甚高频等离子体增强化学气相沉积 电势驻波效应     

Preparation and electrochemical characterisation of supporting SOFC–Ni–YZT anodes

Symmetrical cells consisting of Ni–Y_0.20Ti_0.18Zr_0.62O_1.90 (Ni–YZT) cermet electrodes on a Ni–YSZ support have been investigated with respect to the hydrogen/water partial pressures. Impedance spectra at open circuit potential were obtained as function of temperature and analysed in terms of a fractal finite length Gerischer Impedance. For fine and coarse microstructures of the Ni–YZT electrodes, a consistent set of model parameters could be obtained. The results indicate that surface diffusion rather than bulk diffusion plays a role in the hydrogen/water reaction but also that a fine-grained fraction in the mixed conducting YZT phase is advantageous for the overall electrode performance and the surface exchange reaction.     

Electrochemical characteristics of the dimercaptan-poly(ethylene oxide) grafted polyaniline electrodes

The electrochemical and transport properties of the 2,5-dimercapto-1,3,4-thiadiazole (DMcT)/poly(ethylene oxide) (PEO) grafted polyaniline electrodes and the DMcT/polyaniline electrode interfaced with the poly(acrylonitrile) (PAN) based solid-polymer-electrolyte (SPE) containing lithium perchlorate and ethylene carbonate were studied. Compared with the electrochemical and transport properties of the DMcT/polyaniline electrode, the capacitance and voltammetric current density, obtained by cyclic voltammetry, were increased for the electrode with low grafted polyaniline (less than 3 mol. %), while decreased when the applied copolymers were highly grafted ones. The charge transfer resistance obtained from impedance measurements was much smaller in the DMcT/PEO grafted polyaniline electrode than that in the DMcT/polyaniline electrode, and more pronounced reduction of charge transfer resistance was observed for the electrode with low grafted polyaniline. The diffusion coefficient of lithium cation in the electrode was increased when the PEO grafted polyaniline was used as an electrode material, however, the increase of the diffusion coefficient was less significant at higher graft degrees. All these changes in electrochemical and transport characteristics by the employment of PEO chains upon polyaniline backbones were attributed to the enhancement of lithium ion solvation and enlarged free volume in the electrode.     

Equilibrium spin current induced by spin-orbital interaction in a quantum dot system

We report a theoretical study of the equilibrium spin current flowing in a quantum dot system. Two electrodes are the two-dimensional electron gas with Rashba or Dresselhaus spin-orbital interaction. By using the Keldysh Green's function technique, we demonstrated that a nonzero spin current can flow in the system without bias. At the weak coupling between electrodes and the quantum dot, the spin current is approximately proportional to the cross product of two average pseudo-magnetizations in two electrodes, which agrees with the result of the linear response theory; whereas at the opposite case, the strong coupling between the quantum dot and electrodes can lead to a non-sinusoidal behavior of the equilibrium spin current. These behaviors of the equilibrium spin current are similar to the Josephson current.     

Nasicon sensor for NO detection in the automotive exhaust

Development of a sensor where both electrodes are exposed to the same gas offers simple design with no need for a separate reference gas with a complicated sealing system. Such sensors are still in research stage. The NASICON electrolyte, which has high ionic conductivity at the temperature of the exhaust gas mixture, allows such sensors to work without a heater, making a simple and miniaturised construction technique. Using two asymmetric electrodes, where each electrode has a different catalytic activity at a given temperature, can result in anemf, which is a function of partial pressure of NO. A NASICON sensor with SmFeO₃ as a sensing electrode and Au as an oxygen electrode was calibrated as a NO sensor and tested in oxidising atmosphere in simulated exhaust gases. The sensor responded quickly with reproducibleemf values. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

TEA CO2激光器非对称电极系统的电场分布

 设计了一种非对称电极,这种非对称电极的阳极是Chang电极或Ernst电极,阴极是由直线和圆弧构成的简单电极。通过比较和分析非对称电极和对称电极的特点发现,非对称电极系统中简单电极的边缘电场起伏最大。采用有限元方法计算了非对称电极系统中的简单电极取不同的直线长度和弧线半径时,非对称电极系统的电场分布,给出了选择非对称电极系统中的简单电极参数的依据。结果表明：这种非对称电极系统结合了解析电极和简单电极的优点,通过选择非对称电极系统的简单电极参数,它可以在TEA CO₂激光器中产生边缘电场起伏小于0.06,且满足电极表面均匀场面积宽度要求的电场。     

TEA CO2激光器非对称电极系统的电场分布

设计了一种非对称电极,这种非对称电极的阳极是Chang电极或Ernst电极,阴极是由直线和圆弧构成的简单电极。通过比较和分析非对称电极和对称电极的特点发现,非对称电极系统中简单电极的边缘电场起伏最大。采用有限元方法计算了非对称电极系统中的简单电极取不同的直线长度和弧线半径时,非对称电极系统的电场分布,给出了选择非对称电极系统中的简单电极参数的依据。结果表明：这种非对称电极系统结合了解析电极和简单电极的优点,通过选择非对称电极系统的简单电极参数,它可以在TEA CO2激光器中产生边缘电场起伏小于0.06,且满足电极表面均匀场面积宽度要求的电场。     

Effects of gas pressure on plasma characteristics in dual frequency argon capacitive glow discharges at low pressure by a self-consistent fluid model

A self-consistent fluid model for dual radio frequency argon capacitive glow discharges at low pressure is established.Numerical results are obtained by using a finite difference method to solve the model numerically, and the results are analyzed to study the effect of gas pressure on the plasma characteristics. It shows that when the gas pressure increases from 0.3 Torr(1 Torr = 1.33322×10~2 Pa) to 1.5 Torr, the cycle-averaged plasma density and the ionization rate increase;the cycle-averaged ion current densities and ion energy densities on the electrodes electrode increase; the cycle-averaged electron temperature decreases. Also, the instantaneous electron density in the powered sheath region is presented and discussed. The cycle-averaged electric field has a complex behavior with the increasing of gas pressure, and its changes take place mainly in the two sheath regions. The cycle-averaged electron pressure heating, electron ohmic heating, electron heating, and electron energy loss are all influenced by the gas pressure. Two peaks of the electron heating appear in the sheath regions and the two peaks become larger and move to electrodes as the gas pressure increases.