Pulsed HV vacuum breakdown of polished, powder coated, and e-beam treated large area stainless steel electrodes with 0.5 to 7 mm gaps

An investigation of the HV vacuum breakdown between polished, powder coated, and e-beam treated 304L and 316L stainless steel electrodes is described. Tests were performed with 160 ns, 1-cos(/spl omega/t), and 260 ns flat-top voltage pulses of up to 500 kV. The high voltage hold-off for the 160 ns pulse was /spl sim/130 kV/mm for 2 mm gaps for 80-mm diameter polished stainless steel electrodes, and 15% lower for 120-mm polished and e-beam treated electrodes. The longer 260 ns pulse gave 15% lower hold-off for 80-mm electrodes. These electrodes showed voltage hold-off that scaled as the square root of the gap between 0.5 and 7 mm. This total voltage effect has been interpreted in the past as due to accelerated particles. We analyze our data in terms of this mechanism and show that only nanoparticles of molecular size could be responsible. We also discuss how ions or background gas could affect the breakdown thresholds but existing models do not predict square root dependence. We test how extremely fine powers affect hold-off and show that contaminated surfaces have relatively constant reduced breakdown E-fields that intersect the clean-electrode voltage-dependent breakdown at critical gaps defined by the type and quantity of contamination. The hold-off was /spl sim/55 and 65 kV/mm with copper powder on the cathode and anode for 2 to 6.5 mm gaps, respectively, and /spl sim/95 and 75 kV/mm for talc powder on the cathode and anode for gaps <3.5 and 6.5 mm. Optical diagnostics show no difference in the light emission from clean and contaminated electrode breakdown arcs.     

Vacuum electrical breakdown characteristics and surface condition of Ti electrodes with oxidation conditions

Outgassing from an electrode surface is regarded as a major factor leading to electrical breakdowns in vacuum. Recently oxidation treatment at 200/spl deg/C was reported as an effective means of reducing Ti outgassing. In this paper, we report our measurement and comparison of the electrical breakdown characteristics of Ti electrodes with different oxidation conditions (without oxidation, oxidation at 200/spl deg/C, oxidation at 450/spl deg/C). In addition, we analyzed electrode surfaces before and after breakdown experiments in situ with X-ray photoelectron spectroscopy (XPS). Before oxidation, we machined the electrode's surfaces to the roughness of 0.8 /spl mu/m Rmax with diamond turning. Breakdown experiments demonstrated that the breakdown field is highest at the first application of voltage to electrodes with oxidized at 200/spl deg/C. Before breakdown experiment, surface analysis revealed that all the sample electrodes had a large amount of carbon originating from the hydrocarbons of contaminants, and after the experiments, they revealed that the carbons had disappeared. To obtain breakdown characteristics of electrodes with smoother surfaces, we conducted experiments on electrodes with a surface roughness of 0.05 /spl mu/m Ra. For these electrodes, the breakdown field was higher at first breakdown; the repetitions required to achieve saturated breakdown fields were significantly fewer, and the amount of carbon on electrode surfaces before breakdown was less.     

Parameters influencing breakdown characteristics of vacuum gapsduring spark conditioning

Experiments have been made to identify the key parameters in the increase of the breakdown strength of a vacuum gap through repetitive breakdowns. The current-voltage waveforms were observed, the residual stress in electrodes was measured, and the chemical composition of the electrode surfaces and the energy of the valence-band electrons were analyzed. These experiments revealed that prebreakdown currents (field emission currents) were sometimes observed and sometimes not. The residual stress before and after 500 breakdowns changed significantly. X-ray photoelectron spectroscopy analysis before and after 500 breakdowns showed that the spectral lines of oxygen and carbon due to contaminants on the electrode surface were completely removed and the copper ones appeared clearly after 500 breakdowns. The valence-band electron energy spectrum of the anode after 500 breakdowns shifted towards the lower binding energy to almost coincide with the theoretical spectrum, while the energy spectrum for the cathode shifted towards sometimes higher and sometimes lower binding energy     

On the variety of triggering mechanisms of high-voltage breakdown in vacuum

The paper is devoted to study of the mechanisms responsible for vacuum breakdown as a whole and the total voltage effect in particular. Experiments at DC and pulsed voltages were carried out. It has been shown that there is no manifestation of the total voltage effect at DC voltages up to 20 kV. The strong dependence of hold-off on anode temperature was recognized at DC voltages while pulsed hold-off turned out to be almost the same with heating the electrodes. This gives a basis to consider gas desorption as an insufficient factor in the initiation of pulsed breakdown. An attempt to enhance hold-off with electrostatic removing of loosely bound particles with assistance of electron flow from a thermionic cathode was undertaken in the work. The approach turned out to be ineffective.     

Relationship between electrode surface roughness and impulse breakdown voltage in vacuum gap of Cu and Cu-Cr electrodes

The relationship between the impulse breakdown voltage of vacuum gaps and electrode surface roughness was investigated for the purpose of controlling the surface roughness on HV conductors. The roughness of mechanically polished Cu and Cu-Cr electrodes was measured with a roughness meter, and the relationship between the breakdown voltage and surface roughness was obtained for plate-to-plate gaps. The discharge-conditioning effect increased with reduction in the surface roughness. The breakdown voltage depended not only on the roughness of the cathode but also on the anode surfaces. Reducing the surface roughness was found not to be an effective way to increase the breakdown voltage for non-uniform field gaps after discharge conditioning.     

Flashover voltage of alumina ceramic contaminated by metal vapor in vacuum interrupters

We investigated the influence of metal vapor contamination of ceramic surfaces on flashover voltage (FOV) in vacuum. First, disk shape alumina (Al/sub 2/O/sub 3/) ceramics with surface resistivity (/spl rho/) of 10/sup 2/-10/sup 15/ /spl Omega/ were produced using deposition phenomena of metal vapor emitted from CuCr contacts. The impulse FOV for the ceramics decreased, as /spl rho/ reduced; FOV, the conditioning effect on FOV, and the scattering of FOV decreased when /spl rho/ was below 10/sup 12/ /spl Omega/. Therefore, the criterion value /spl rho/, which maintains excellent flashover performances of ceramic surface, is 10/sup 12/ /spl Omega/. Second, experimental vacuum interrupters (VIs) were produced to measure breakdown voltage before and after forty short-circuit current switchings with 20-40 kA/sub rms/ and were disassembled to measure the /spl rho/ of their inner ceramic surface. In a VI, which has inside diameters at both ends of the main shield much larger than the contact diameter, /spl rho/ was reduced to 10/sup 4/ /spl Omega/, further decreasing breakdown voltage between terminals.     

Size effect and statistical characteristics of dc and pulsedbreakdown of liquid helium

Breakdown time lag in liquid helium is measured over a wide range of electrode sizes and pulsed electric field strengths. The breakdown time lag and dc breakdown strength are statistically analyzed by using the Weibull distribution function and weak link theory. It is found that the time lag depends on both electrical stress and the electrode surface area stressed above a critical level. It is supposed that breakdown triggering electrons are generated by field emission phenomena at small protrusion tips on the cathode surface. In higher external electric fields, a less sharper protrusion emits initial electrons with a shorter time lag and may become responsible for liquid breakdown. A theoretical equation is proposed to predict the electrode size and electrical stress dependency of the breakdown time lag, based on Fowler and Nordheim theory. It is shown that the equation is consistent with the Weibull distribution function under multiple stress of electric field and stressing time     

Breakdown conditioning characteristics of long gap electrodes in a vacuum

An improvement in dielectric strength is required in vacuum circuit breakers (VCBs) intended for use in higher voltage systems. In order to develop higher voltage VCBs, it is important to improve the dielectric strength in a vacuum based on consideration of the vacuum breakdown mechanism. Particularly for gaps longer than 10 mm, little is known about the breakdown mechanisms and their quantitative analyses in a vacuum. This paper discusses the breakdown conditioning characteristics of long gap electrodes, under a non-uniform electric field in a vacuum. We treat gap lengths of up to 50 mm in this paper. The conditioning characteristics are investigated under impulse voltage applications. A negative standard lightning impulse voltage was applied to rod-plane electrodes made of Cu-Cr and SUS304 for different tip radii and gap distances until the conditioning effect was completed. We observed illumination spots on electrodes at each breakdown during the conditioning process and calculated the corresponding breakdown field strengths. Experimental results revealed that the tendency of breakdowns associated with long gaps is different from that for the short gaps. As a result, we clarified that the breakdown field strengths are nearly constant at 110-120 kV/mm at the distances longer than 10 mm, and the breakdown field strength is at its maximum when the gap distance is about 5 mm.     

Surface switching characteristics of variable permittivity dielectrics

Flashover voltage, lifetimes, and switch performance of insulators utilizing square thin and thick film electrodes were examined to determine the viability of using thin electrodes for reliable surface discharge switching. Gold, silver, and platinum were sputtered (0.25 /spl mu/m) and screen printed (15 /spl mu/m) onto Al/sub 2/O/sub 3/, TiO/sub 2/, and modified BaTiO/sub 3/ (MBT), then tested in air at 10/sup 5/ Pa, under vacuum (10/sup -3/ torr), and while immersed in an insulating fluid, SF-2 (manufactured by 3M). For the measured range of 0.5 to 3 mm in air, the flashover voltage for all three insulators was found to have a linear dependence on the electrode separation distance with 15 /spl mu/m thick screen printed electrodes and a square root dependence with 0.25 /spl mu/m thick sputtered electrodes. Delay times of approximately 20 ns with a corresponding jitter of 6 ns were observed across all three insulators under triggered flashover. Insulators in air with sputtered electrodes had lifetimes of approximately 5 flashovers for dc flashover and 40 for triggered flashover. Screen printed TiO/sub 2/ and MBT had dc lifetimes of approximately 10 flashovers in air, and 3 flashovers in vacuum and SF-2. Screen printed TiO/sub 2/ and MBT had triggered lifetimes of greater than 200 flashovers in air, and <3 flashovers in vacuum and SF-2. Screen printed Al/sub 2/O/sub 3/ had dc and triggered lifetimes of greater than 200 flashovers in air, vacuum and SF-2. Insulator failure during dc flashover was determined to be due to the formation of a conductive channel between the anode and cathode. Formation of the channel was attributed to insulator thermal and dielectric properties and the presence of vaporized electrode species in the gap region during flashover.     

Changes of Cu electrode surface conditions and prebreakdown current characteristics caused by repetitive breakdowns in a vacuum

To clarify factors influencing the conditioning effect produced by repetitive breakdowns of a vacuum gap, changes of electrode surface conditions before and after 500 breakdowns caused by impulse voltage application were investigated. Changes of prebreakdown current characteristics were also investigated. The electrode material was copper. The electrode surfaces were analyzed by X‐ray photoelectron spectroscopy (XPS). XPS analysis revealed that the electrode surfaces were completely cleaned by repetitive breakdowns. Prebreakdown characteristics were classified into three patterns: (1) displacement current alone, (2) field emission current as well as displacement current, (3) explosive current increase. Patterns (1) and (4) were observed mainly in the early stage of repetitive breakdowns, while pattern (2) was found in the later stage. It has been concluded that the breakdown mechanism changes during conditioning by repetitive breakdowns. © 2000 Scripta Technica, Electr Eng Jpn, 133(1): 11‐18, 2000     

High field performance of thin-wall spacers in a vacuum gap

This paper reports the electrical conduction and breakdown characteristics of thin-wall ceramic spacers for a field emission display (FED). These spacers bridge two thin-film electrodes, which represent the FED cathode and the phosphor anode in a FED. Techniques to set up a high aspect-ratio thin-wall spacer without glue were developed. An extra-low light detection 3D-imaging system using an intensified CCD camera was developed which was able to identify the location of low-level light activity in the stressed vacuum-gap, indicative of imminent device failure. Thin wall spacers made of various ceramics were investigated extensively. The scanning electron microscopy (SEM) surface investigation showed that zirconia spacers exhibited a smoother surface morphology compared to all other materials studied; however, their breakdown voltages were rather low. The breakdown voltages of alumina spacers were severely limited by triple junction effects. At HV, breakdown at the edge of the thin-film electrode was observed. This edge breakdown can be used to explain the saturation of the breakdown voltage vs. vacuum gap spacing. The results of this work are highly encouraging in that an ~1000 μm tall spacer can support ~18 kV dc, at least 80% above the expected operational voltage of HV FED. The spacer breakdown voltage is expected to improve through surface treatment and elimination of the electrode edge-breakdown and triple junction effects     

Vacuum insulation and discharge phenomena in high-power deviceswith magnetic confinement of electron flow

Results of the experimental investigation of processes determining vacuum insulation maintenance and discharge phenomena in high-power crossed-field and electron-beam devices are presented. Data from crossed-field devices on accumulation and steady-state parameters of the space charge between cold cathode and anode, spatial, temporal and energy characteristics of electron flows onto surrounding electrodes are given. Techniques of gun breakdown prevention and formation of a dense nonrelativistic long-pulsed electron beams with energy density ⩽100 kJ/cm² are described, as well as the effect of RF discharge development on the beam transport in this system. Models of observed phenomena considering the effect of nonuniform electromagnetic fields, oscillatory processes and generation of secondary particles accompanying electron bombardment of electrodes are also proposed     

Estimation of cathode-initiated breakdown in vacuum

Cathode initiation of vacuum breakdown in DC and pulse modes is considered. New criteria and methods for evaluating the cathode initiation mechanism are proposed. They are based on consideration of relative changes in cathode surface state and electric strength. The elaborated criteria and methods have been verified with experiments results using DC and pulse modes of high-voltage treatment of electrodes. Accuracy of the proposed methods exceeds known methods precision. It is shown that optimum modes of pulsed conditioning can be used for definition of top boundary of cathode initiation mechanism applicability in DC mode.     

Charging characteristics and electric field distribution on alumina as affected by triple junctions in vacuum

This paper describes the dependence of the charging characteristics on the electric field distribution on the alumina (Al/sub 2/O/sub 3/) surface as affected by the triple junction in vacuum. For HV electrical insulation design of vacuum interrupter, surface flashover in vacuum is very important problem to be solved. Attention should be paid to the fact that the insulation characteristics on the dielectric surface are strongly influenced by field emission of electrons from the triple junction and the accumulated charges on the dielectric surface. In order to clarify the charging mechanism, we measured the charging characteristics for various types of triple junctions. In particular, we focused on the influence of the electric field distribution along the solid dielectrics and near the cathode triple junction (CTJ) on the charging characteristics. The results confirmed that the electric field distribution strongly affected the 2-dimensional (2D) distribution of the surface charge on the dielectric sample. Consequently, it was found that positive charging was generated on alumina, when the incident angle of the electric line of force on the alumina surface became >60/spl deg/.     

大电流真空电弧磁流体动力学模型与仿真

为了对大电流真空电弧进行深入研究,以真空电弧双温度磁流体动力学模型为基础,通过计算流体动力学软件FLUENT,采用控制容积法,对大电流真空电弧特性进行了仿真研究。对于大电流真空电弧而言,等离子体的流动处于亚音速状态,因此,在阴极和阳极边界条件的选择上将区别于超音速流动的真空电弧。同时对等离子体密度、轴向电流密度、等离子体速度、马赫数、离子温度、电子温度、离子压力、等离子体压力以及注入阳极的能流密度分布的形成机理进行了分析。从仿真结果可以发现,大电流真空电弧等离子体压力的最大值出现在阴极附近,等离子体将在压力梯度的作用下从阴极到阳极做加速运动,这一点明显区别于超音速流动的真空电弧。另外,仿真结果与高速CCD照片也是吻合的。     

基于X射线量变化评估真空灭弧室真空度的方法

为了找到一种操作简便且准确度高的方法来进行真空度评估,笔者提出了一种不需外施磁场、利用测量X射线量变化来判断真空灭弧室内部真空度的方法。对于真空灭弧室在指定开距下施加工频电压时,场致发射电子会以很高的速度从阴极表面逸出,在触头间电场的作用下加速轰击阳极,基于轫致辐射的原理会从阳极表面释放出X射线。这时,真空度的差异会使产生的X射线释放强度以及变化情况有所不同,因此,利用测量释放的X射线量的变化可判断真空灭弧室内部的真空度。笔者通过理论分析,得出了X射线量与真空度的关系,并进行了试验验证。     

Visualization of late breakdown in vacuum by ICCD camera

Experimental observations of late breakdowns in a vacuum circuit breaker were made using an intensified charge coupled device camera (ICCD). These late restrikes can cause arc reignition and interruption failure. This type of failure is rare and can be initiated by a number of mechanisms. This paper illustrates the method used to observe this phenomenon. Some key image examples of the phenomenon occurring during breakdown initiation are shown.     

Relation between breakdown voltage and prebreakdown current in vacuum gap

Two different surface treatments (mechanical polishing, thin film deposition) were performed on cathode surfaces, and the field emission currents from the cathodes were measured with a microchannel plate. In order to discuss the relationship between the breakdown voltage and prebreakdown current in the vacuum gap, the breakdown voltage was measured after field emission measurement. The V–I characteristics of the field emission and breakdown voltage were influenced by surface treatment, and the breakdown voltages of mechanical polished cathodes were lower than those of the thin film deposited cathodes. It was found that the probability of breakdown increased when the field emission current reached 10^–11 A. Atomic force microscope (AFM) measurements showed numerous protrusions on the cathode surface in the case of thin film deposition treatment, but we estimated by the finite element method that these protrusions make the field enhancement effect low. It was inferred that the breakdown voltage in vacuum gaps could be increased by the thin film deposition method. © 2000 Scripta Technica, Electr Eng Jpn, 131(4): 11–18, 2000     

LiCoO2-SiO/石墨锂离子电池循环衰减机理研究

研究了LiCoO₂正极和氧化亚硅/石墨复合负极（LiCoO₂-SiO/石墨）软包锂离子电池体系（LIBs）循环衰减机理,通过循环过程中电化学阻抗（EIS）、增量容量分析（ICA）、正负极形貌等分析了循环的影响因素。结果表明,硅基负极材料在完全嵌锂状态下的体积膨胀不仅会导致SiO负极的颗粒破碎,与电解液的副反应加剧,其膨胀应力还会造成电极的导电网络和粘结剂网络的破损,从而导致正负极活性物质利用率降低,降低SiO负极材料的循环性能。此外,SiO负极的充放电电压平台较高,与石墨材料复合使用时,容易造成电池正极的过充和放电容量损失,正极过充会加剧正极材料结构破裂。而随着循环的进行,过充程度和放电容量损失会愈发严重,加速电池循环性能衰减。     

The relation of trap distribution of alumina with surface flashover performance in vacuum

Alumina ceramic samples prepared under different sintering temperatures and varied additives were measured to indicate the trap density and trap energy located in alumina materials by using thermally stimulated current (TSC). The surface charges on alumina in vacuum after applying a negative pulse voltage (0.7/4 /spl mu/s), and flashover performances of the materials in vacuum also were measured. We found that the trap distribution in alumina has a correlation with surface charges and flashover performances in vacuum. It is shown that the higher is the trap density in the material, the higher is the surface charge density, and the lower is the flashover voltage on alumina surface. It is believed that the trapping and de-trapping mechanisms of carriers could play an important role during the development of the discharge processes, together with the secondary electron emission mechanism.