04_Zhang_optical_ac_flash - Zhang et al Optical...

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Zhang et al.: Optical In © estigation of ac Preflasho © er of Alumina Ceramic in Vacuum 498 Optical Investigation of ac Preflashover of Alumina Ceramic in Vacuum Guan-Jun Zhang, Wen-Bin Zhao, Zhang Yan School of Electrical Engineering Xi’an Jiaotong University, Xi’an 710049, P. R. China and Yuan-Shing Liu Department of Electrical Engineering Cheng-Shiu Institute of Technology, Neau-Song 833, Taiwan ABSTRACT 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 ; 1 kV , and re - peak vealed two stages depending on the amplitude of applied voltage. For non-sputtered alumina the optical emission appeared at a much higher voltage and presented ir - regular 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 mech - anisms. For non-sputtered contact, the light emission was initiated by field elec - tron emission from the triple junction. While for the sputtered contact, prior to the electron emission, electrons r holes could be injected from electrodes into the sur - face layer of alumina and electroluminescence phenomena occur due to the radia - tive 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 pro - cesses play a significant role in the development of flashover. Index Terms — Alumina ceramic, preflashover, flashover, vacuum. 1 INTRODUCTION N a solid insulating material between energized elec- I trodes in vacuum, surface flashover often occurs across the insulator, and its onset voltage is much lower than the breakdown voltage of the vacuum gap with the same length. Since the flashover limits the overall performance of a vacuum-dielectric system, it has received much atten- wx tion for more than half a century 1 . A flashover event begins with electron emission from the cathode triple Ž junction CTJ, the proximity of cathode, insulator and . vacuum , giving rise to secondary electron emission SEE due to the impact of emitted electrons on insulator sur- face and ends with a breakdown occurring in a desorbed surface gas layer 2 . Recently, a mathematical model was developed for calculating the time-dependent current of dc flashover on a nanosecond scale, which clearly showed that the above surface effects played a dominant role in flashover development 3 . Another accepted model was Manuscript recei ® ed on 6 March 2003, in final form 28 August 2003.
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04_Zhang_optical_ac_flash - Zhang et al Optical...

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