Pre-flashover luminosity along an alumina/vacuum interface

Two preflashover luminosity phases, one near the voltage peak (phase I) and the other just prior to the onset of voltage collapse (phase II), are found to be associated with the surface flashover of alumina in vacuum. The light emission is also observed under the no-breakdown condition (phase III). A new ancillary experiment, performed using three high-sensitivity photomultiplier tubes (two viewing the dielectric surface head-on and the other viewing edge-on) indicates that the plasma corresponding to phases I and III does not expand away from the insulator surface; whereas the plasma of phase II expands away from that surface. The observed results suggest qualitatively that the phases I and III may be attributed to a solid state process whilst the phase II may be a mixed response of both solid state and gaseous processes     

Characteristics of preflashover light emission from dielectricsurfaces in vacuum

Luminescence from dielectric surfaces during preflashover in vacuum was observed under pulse excitation. By preflashover luminescence, we mean light emission under no flashover conditions. Study of light emission with respect to electrode configurations and spacing between the two electrodes shows that preflashover light emission is related to electron emission from the cathode and trapping/detrapping processes on the dielectric surface layer. Spectroscopic observations of the light emission indicate that the preflashover spectrum changes from one voltage shot to another voltage shot of the same amplitude and that the spectral distribution of the light emission depends on the dielectric material used. We believe that the light emission from preflashover is electroluminescence which may be attributed to de-excitation of excited species in the surface layers of the dielectric material     

真空中绝缘子沿面预闪络现象的研究

真空中绝缘子沿面闪络现象是制约真空系统电绝缘性能的一个重要因素。通过实时观察冲击电压作用下真空中绝缘子上的冲击电压波形,以及相应的冲击电流和绝缘子表面的发光信号,并在线测量真空中绝缘子的表面电荷分布,研究了高纯度Al2O3陶瓷绝缘子在真空中的沿面预闪络现象。研究结果表明,绝缘子预闪络现象与施加冲击电压的大小、施加次数等有关,同时,沿面预闪络现象也与表面电荷分布有关,根据试验结果提出了一种关于绝缘子预闪络现象的模型,该模型能够较好的用来解释真空中绝缘子沿面预闪络现象以及沿面闪络发展过程。     

Optical investigation of AC preflashover of alumina ceramic in vacuum

Optical phenomena accompanying the preflashover along a planar metal-alumina-metal structure were investigated under stepped AC voltage in vacuum. Two kinds of electrode contacts with and without sputtered gold films were employed. For sputtered alumina, the luminescence became observable at /spl sim/1 kV/sub peak/, and revealed two stages depending on the amplitude of applied voltage. For non-sputtered alumina the optical emission appeared at a much higher voltage and presented irregular and discrete light pulses. The energy band at the metal-alumina interface for the two kinds of electrode contacts is responsible for the relevant optical mechanisms. For non-sputtered contact, the light emission was initiated by field electron emission from the triple junction. While for the sputtered contact, prior to the electron emission, electrons/holes could be injected from electrodes into the surface layer of alumina and electroluminescence phenomena occur due to the radiative electron-hole recombination. Injected electrons form a long-term negative space charge region away from each electrode and hence at a critical applied voltage, the trapped electrons are detrapped resulting in intense light emission. These processes play a significant role in the development of flashover.     

Optical emission characteristics of polycrystalline diamond duringsurface flashover in vacuum

This work presents experimental results of optical emission from polycrystalline, as well as single crystal diamonds, during the surface flashover events in a vacuum environment. Optical emission spectra were recorded and the source of this luminosity was investigated. Analysis showed that the surface flashover events were associated with relatively high intensity optical emissions. The light emission started much earlier than the voltage collapse, increased very rapidly with the breakdown, and eventually ended after few seconds; much later than the voltage across the surface was recovered. The polycrystalline diamond sample used in the experiments was produced by a microwave plasma CVD (chemical vapor deposition), and the sample surface was polished to optical quality before the experiments. The electrode material was copper, and a dc voltage was applied between the electrodes. Surface flashover voltage characteristics and breakdown voltage waveforms of both samples were determined     

Real-time and high-speed measurements of charging processes on dielectric surface in vacuum

In order to investigate the charging mechanisms of solid dielectrics in vacuum, we constructed real-time and high-speed measurement systems for charge density, current pulse, light emission and applied voltage. By applying a negative DC ramped voltage to an electrode with a cathode triple-junction (TJ), we measured the temporal variation of surface charge accumulation and current pulse waveforms expressing the electron emission from TJ, by electrostatic probes. From the measurement results, we explain the step like charging process and propose an electron emission model by taking the electric field and its relaxation by charging into account. In addition, we elucidate the relationship between the charging process and the location of electron emission.     

慢前沿冲击电流与放电通道光强度关系研究

采用冲击电流发生器对15 mm石墨棒状放电间隙放电,产生具有慢上升前沿的衰减振荡冲击电流波形,涉及4组电流共67个脉冲,含17个首次脉冲及50个后续脉冲,其峰值1.9～27.0 kA,10％ ～90％上升时间3.3～18.8μs,半峰宽度7.8～47.8μs.同步测量了石墨棒状放电间隙通道电流与光强度波形,开展了冲击电...     

Identification of the components of the electroluminescencespectrum of PE excited in uniform fields

The electroluminescence (EL) of insulating polymers is a subject of great interest because it gives insight into aging and breakdown processes. While most of the studies performed on polyolefins are under divergent field configurations, a uniform field is used more and more. However, a convincing interpretation of the emission spectrum has not been given up to now, and some discrepancies have appeared between the spectra recorded under divergent and uniform fields. Wavelength-resolved EL spectra are carried out on metalized polymeric films biased under different ac and dc voltage levels. We show that the light emission consists of two different components with typical field dependence and spectral distribution. One is associated with the luminescence of the polymer, the other is due to photon emission from the metallic layers through the radiative decay of surface plasmons excited by the injection current. The proposed interpretation reconciles the observations made under divergent and uniform fields and allows us to discuss the nature of the chromophores involved in EL     

Nano/Micro Dielectric Surface Flashover in Partial Vacuum

In this paper, we present experimental results of surface flashover characteristics of epoxy resin and epoxy resin cast with powdered Al₂O₃ (nano/micro-composite dielectric) samples in partial pressure of nitrogen. The flashover studies are conducted separately using DC and 20 kHz AC signals. The voltage, current, and light emission waveforms are obtained. The optical data is collected by a video camera and a photomultiplier tube (PMT). The nano/micro composite dielectric samples used in the experiments are produced in-house by mixing 1:10 weight ratio of AI₂O₃ powder with epoxy resin of known properties. Throughout this study, copper electrodes are used and are placed over the dielectric samples to initiate surface flashover events. The results of surface flashover voltages as a function of pressure are presented. Additionally, optical emission characteristics along with the voltage and current waveform data are also presented.     

High field emission efficiency surface conduction electron emitters

Two different surface conduction electron-emitter (SCE) structures with the nanogap of 90 nm wide fabricated by hydrogen embrittlement (HE) and focused ion beam techniques are simulated for the first time. We employ a three-dimensional particle-in-cell method coupling with finite-difference time-domain scheme to simulate the property of electron emission in these SCEs. Our calibrated simulation predicts high emission efficiency of the SCE structure which is fabricated by HE. Compared with the other SCE structure, it is observed that the proposed structure possesses low power consumption at the fixed emission current when the width of nanogap becomes narrower. The current-voltage characteristics including conducting mechanisms are investigated and explained.     

Effect of surface current on photoelectron emission from mica

The relationship between photoelectron emission and the conduction mechanism on the surface of cleaved muscovite mica film was investigated; that is, under atmospheric conditions and subjected to UV irradiation, a low‐energy photoelectron counter measured the photothreshold energy of photoelectron emission from a surface treated by fast atom bombardment (FAB). Such emission was found to have a close relationship to the surface structure and surface conduction characteristics in that the photothreshold of the FAB‐treated mica surface varied from 5.0 to 5.8 eV, whereas no variation occurred in the untreated surface; the difference being attributed to different carriers of surface current. Results indicate that the surface states of mica play an important role in the electronic conduction of the surface, as well as the photoelectron emission from it. © 2000 Scripta Technica, Electr Eng Jpn, 133(1): 1–10, 2000     

无电解电容LED驱动电源纹波补偿控制策略综述

随着LED照明领域的不断拓展和人们对健康光源的迫切需要,LED光源的频闪问题备受关注。流经LED光源的纹波电流不但会引起频闪,还会对光度、比色性能和光效等造成不良影响。无电解电容LED驱动电源纹波补偿控制能有效兼顾长寿命和低纹波等性能指标的优化设计,从而为人们提供高效节能且更加健康的LED光源。通过归纳分析无电解电容AC-DC LED驱动电源的关键技术和纹波补偿控制策略,对大功率无电解化LED驱动电源合理化指标要求、拓扑结构、小信号模型、自适应数字电流预测控制、自适应纹波补偿控制和高频能量同步传输控制等进行了展望,并针对工程应用提出了相应的实现思路,以期助推绿色健康LED驱动电源的研究。     

Electroluminescence from polyolefins subjected to a homogeneous acfield

Electroluminescence associated with ac voltage aging of polyolefins has been reported under divergent field configuration. Neither the mechanism of excitation nor the nature of luminescent centers are well understood in these polymers. Homegeneous field experiments are desirable since they are easier to run and more light can be collected, thereby allowing spectral analysis to be made. We describe a suitable experimental configuration for electroluminescence detection from polyethylene and polypropylene films submitted to a homogeneous field and report on surface roughness, field, and time dependence of the light emission. Field and time dependence of the light spectra are also investigated. It is shown that the light originates from the polymer bulk and is controlled by the surface roughness of the films. The light spectrum appears to consist of two components which exhibit different behavior as a function of field and stressing time. A tentative explanation is given in terms of competition between two different excitation processes     

采用光学方法研究陶瓷绝缘材料的沿面闪络特性

对比研究了真空中交流电压下采用和未采用真空溅射金属电极的氧化铝陶瓷材料,在沿面闪络发生前不同的表面发光特性。基于固体的能带理论,提出了２类试品在金属电极－介质的界面处不同的能量状态分布模型;并在此基础上同时考虑电荷注入复合和场致电子发射对发光的影响,解释了２类试品发光的差异性,指出了采用和未采用真空溅射金属电极的氧化铝陶瓷存在不同的沿面闪络起始机理及发展过程。     

Spectral analysis of optical emission due to isothermal chargerecombination in polyolefins

Light emission from electrically stressed polymers, so called electroluminescence, is a subject of great interest because it is associated with electrical aging and dielectric breakdown of highly insulating materials. Radiative recombination of charge carriers on luminescent centers generally is evoked as the main contribution to the emission, but this is difficult to assess in polyolefins since the spectral features associated with this mechanism are not known. The reason is twofold. First, there is a lack of knowledge on the nature of the luminescent centers in these materials, and second, it is difficult to get wavelength-resolved electroluminescence spectra due to the low light level. By using an alternative activation method, we have isolated the emission spectrum associated with isothermal charge recombination in polyethylene and polypropylene. Electrical charges of both polarities are brought to the surface of the samples by contact with a cold plasma powered at a frequency of ≈5 kHz. The kinetic and spectral features of the recombination-induced emission are recorded after discharge switch off. The recombination mechanism is considered on the basis of the light decay kinetics whereas the chemical nature of the luminescent centers is discussed by a comparison with the photoluminescence spectrum of the polymer. It is shown that charges recombine by tunneling from traps to the luminescent centers which are unsaturated species. The chromophores involved in photo- and recombination-induced luminescence appear to be the same, but not the electronic transitions. Light emitted upon charge recombination has been assigned to transitions from the lowest lying triplet states of poly-enone sequences     

Two types of lightning discharges to a high stack on the coast ofthe sea of Japan in winter

To clarify the characteristics of lightning on the coast of the Sea of Japan in the winter season, the current waveform and the progression of lightning discharge to a high stack were observed. Observation results from 1989 to 1994 show that: (1) there are two types of lightning discharges-one with large currents and strong luminosity in the lightning path, and the other with small currents of long duration and weak luminosity of lightning path; (2) the parameters of lightning currents of the two types of discharges are quite different; and (3) there is a quantitative correlation between the change of luminosity in a lightning path and the lightning current waveform     

Spectroscopic study of red light emission in porous silicon

Since bulk silicon does not emit light in the visible part of the spectrum, the discovery of visible luminescence from porous silicon has been quite surprising and has generated significant interest. This material differs from bulk silicon in one important way, in that it consists of interconnected silicon nanostructures, having very large surface to volume ratios. The first emission mechanism proposed involved carrier recombination within quantum size silicon particles, but more recent work has shown that surface emission models may be more likely. The problems with the quantum confinement model will be discussed in view of current data, and an oxygen center luminescence model will be discussed, with supporting experimental data. A direct correlation between the presence of these centers and the red photoluminescence in both as-made and oxidized PSi will be presented     

GaN-based light-emitting diodes using tunnel junctions

We demonstrate GaN-based light-emitting diodes (LEDs) with tunnel junction (TJ) structure and surface-emitting light-emitting diodes with TJ current aperture for lateral current confinement. The p/sup +//n/sup +/ GaN TJs are located in the upper cladding layers of conventional devices, allowing n-type GaN instead of p-type GaN as a top contact layer. The reverse-biased tunnel contact junction provides lateral current spreading without a semitransparent electrode and spatially uniform luminescence exhibiting an improved radiative efficiency. Also, the current confinement aperture for the lateral injection current in the LEDs was defined by mesa etching of a TJ structure and regrowth of the current blocking layer surrounding the TJ mesa. The very uniform light emission just through a buried TJ aperture confirms that the buried TJ structure acts very effectively as a confinement aperture of lateral current injection, particularly in GaN-based vertical-cavity surface-emitting lasers.     

Resonant-cavity optoelectronic devices for fluorimetry

Vertical cavity, semiconductor optoelectronic devices have been designed specifically for applications in fluorescence spectroscopy. The devices emit and detect light in a direction normal to their surface and could be readily integrated into lab-on-a-chip formats with extremely close proximity coupling to the analyte. The emission is narrow band and centered at 645 nm, whereas the detection response is broadband extending from 645 to 870 nm. A resonant cavity structure has been used to independently control the emission and detection characteristics, and a comparison is given between structures with a cavity enhancement at either the emission or the detection wavelength. In both cases, an enhancement by a factor of greater than 15 is achieved due to the presence of the optical cavity. In emission, this provides micrometer-scale devices with power levels in the 50-/spl mu/W range. When the cavity is used to enhance detection, a minimum detection power level of 100 nW is achieved.     

Influence of sintering temperature on the flashover performance of alumina insulators in vacuum

This paper reports on the influence of sintering conditions on the flashover/pre-flashover characteristics of alumina insulators in vacuum under an impulse voltage. Four kinds of alumina insulator were studied, which were prepared using the same formula with different sintering conditions. The surface pre-flashover and flashover phenomena of different samples in vacuum have been observed under impulse voltage. Based on the experimental results, the characteristics of pre-flashover and flashover vary with different kinds of test samples. It was found that the insulators with a higher sintering temperature had a lower flashover voltage, and more active preflashover. From a scanning electron microscope (SEM) analysis of alumina insulators, we conclude that the methods of preparing alumina samples can affect their microstructure resulting in a change in the pre-flashover and flashover performance as alumina insulators.