Yuan-shing Liu

Influence of electrode contact on luminescence from alumina ceramic surface under ac electric field in vacuum

The luminescence from a planar metal–alumina–metal structure was investigated under ac voltage application in vacuum. Two kinds of electrode contacts, i.e., with/without sputtered gold film, showed quite different optical phenomena. For the nonsputtered alumina samples, no luminescence was detected until extremely irregular light pulses resulted from partial discharges under higher voltage. While for the sputtered samples, from far lower applied voltage, there was faint but stable light emission due to electroluminescence (EL) observed. Based on the band theory of solids, we proposed a model to explain the interesting phenomena qualitatively. Taking account of the sputtering process, a marked reduction of the potential barrier between electrodes and alumina surface would result. Thus, charge carriers can be easily injected from the electrodes into the surface layer of alumina, and EL will emit due to the radiative recombination of electrons and holes. It is considered that EL has a significant influence o...

Investigation of Charge Injection in Gas-Impregnated Polyethylene by Measurement of Electroluminescence under AC Voltage

Electroluminescence (EL) in polyethylene (PE) impregnated with N2, O2 or SF6 gas has been measured under ac voltages to investigate the effect of impregnation gas on charge injection. EL is strongly dependent on the processes of electron injection and transport in the surface region of PE. The results of our measurements of EL indicate that the process of electron injection is greatly affected by the type of impregnation gas in PE. In particular, O2 gas which is electronegative and chemically reactive markedly affects the charge injection process, causing a reduction in EL inception voltage and barrier height at the electrode-polymer interface. The surface states induced by the presence of O2 gas or by oxidation of PE enhance the electron injection. However, electrons injected into such surface states likely contribute little to EL. N2 gas also causes a reduction in the EL intensity but does not induce a change in the EL inception voltage and the barrier height in comparison with those in the degassed case. N2 gas probably affects an electron transport process such as intermolecular conduction in the surface region of PE rather than the electron injection process.

LIGHT EMISSION BEFORE AND DURING PREBREAKDOWN ON POLYTETRAFLUOROETHYLENE SURFACE WITH METALLIZED ELECTRODES UNDER AC VOLTAGE APPLICATION IN VACUUM

Light emission before and during electrical breakdown from the surface of polytetrafluoroethylene (PTFE) with metallized electrodes has been investigated with ac electric field application along the polymer surface, using a photon counting method, in order to understand the initiation mechanism of the prebreakdown. Two distinct stages of light emission were observed depending on the applied voltage: a low-level stable light emission, electroluminescence (EL), before prebreakdown and an irregular intense light emission during prebreakdown. Before prebreakdown, charge injection from the electrode directly into the polymer surface layer results in EL emission and the formation of long-term electron space charges away from the electrode. The crucial factor of prebreakdown initiation is the strong modification of the local electric field near the electrode because of space charge formation in the surface layer before prebreakdown. The prebreakdown with intense light emission is initiated by detrapping the long-term trapped electrons toward the electrode via the vacuum and/or the surface layer in the positive half-cycle of ac voltage.

Preflashover and flashover phenomena of silicon–vacuum system under pulsed excitation

Rectangular n+nn+-type silicon blocks with different surface treatments, i.e., one surface is chemically etched and the other is unetched, were used to investigate the characteristics of preflashover and flashover of semiconductor under a high voltage pulse in a vacuum. The metallic contacts were employed to keep a fine contact between the sample and the electrode. For unetched samples, the preflashover current obeyed Ohm’s law, and for etched samples, it was dominated by the space-charge limited current. After flashover shots, two kinds of silicon samples showed quite distinct tracks. It is considered that all the phenomena are attributed to the different density and distribution of surface states for two kinds of samples. The developing process of surface flashover across a semiconductor could be explained as the current filamentation which is induced by thermal effects. It is suggested that the flashover occurs in the interface layer of silicon butted to electrodes, and in the lateral layer of silicon ...

Optical Investigation of Fast Pulsed Discharge on Surface of Ceramics for Environmental Applications

Pulsed surface discharge on the surface of ceramics was investigated experimentally. Photon counting method was applied to clarify preceding phase of the discharges. The results indicated the effect of charge injection from electrodes. Pulsed surface discharge reactor for decomposition of nitrogen oxides utilizing honeycomb ceramics was proposed. Framing and streak observation was carried out with image converter cameras. Emission spectra from the discharge were investigated with spectrometer. Uniform surface discharges on honeycomb ceramics were successfully obtained utilizing pulsed power generator with a SI-thyristor.

High-Energy-Density Plasma Produced by Pulsed High Current Discharge of Powder

A plasma created from copper powder was successfully obtained in pulsed high current discharges. Powder flow was injected transiently between the electrodes in vacuum. The powder particles are spherically shaped and are accelerated by pulsed electrostatic field. Diverging powder flow was controlled to form a directed flow with a constant diameter by applying an extra focusing electrode. The powder particle trajectory is numerically analyzed to design the focusing electrode. Conical shaped light emission from the particles was observed in framing photographs during the early stage of discharge.