Effect of residual stress on the surface flashover of aluminaceramics

The influence of surface residual stress on the surface flashover behavior of polycrystalline alumina in vacuum is investigated. The samples for the present investigation were prepared by using SiC and diamond abrasives of particle size ranging from 268 to 0.25 μm. Two types of polishing procedures, namely forward and reverse sequence polishing, are adopted in the present work. The surface residual stress of the specimens is determined by the two-exposure X-ray diffraction method. The flashover strength and the surface stress of the samples, subjected to forward sequence polishing (i.e., coarse to fine finish), are found to exhibit a random behavior with respect to the surface finish. Interestingly, a linear dependence between flashover strength and surface stress with respect to the surface finish is observed for the samples subjected to reverse sequence polishing (i.e., fine to coarse finish). The observed dependence between the residual stress and the flashover strength is discussed in terms of the high energy defect centers (e.g., deep traps) formed within the forbidden gap of the alumina insulator     

Structural,optical, and electrical characteristics of 70 Mev Si<Superscript>5+</Superscript> ion irradiation-induced nanoclusters of gallium nitride

We report the formation of nanoclusters on the surface of gallium nitride (GaN) epilayers due to irradiation with 70 MeV Si ions with the fluences of 1 × 10¹² ions/cm² at the liquid nitrogen temperature (77 K). GaN epilayers were grown using a metal organic chemical vapor deposition system. Omega scan rocking curves of (002) and (101) plane reflection shows irradiation-induced broadening. Atomic force microscopy imagery revealed the formation of nanoclusters on the surface of the irradiated samples. X-ray photoelectron spectroscopy confirms that the surface features are composed of GaN. The effects of ion-beam-produced lattice defects on the surface, electrical, and optical properties of GaN were studied and possible mechanisms responsible for the formation of nanoclusters during irradiation have been discussed.     

Effect of 80 MeV oxygen ion beam irradiation on the properties of CdTe thin films

Polycrystalline CdTe thin films were irradiated with 80 MeV oxygen (O⁶⁺) ions for various fluences and its effect on the composition, structure, surface topography and optical properties have been investigated. The as-grown films are found to be slightly Te-rich in composition and there is no significant change in the composition after irradiation. X-ray diffraction analysis shows a high degree of crystallite orientation along the (111) plane of cubic phase CdTe. Upon irradiation a large decrease in intensity of the (111) plane and a small shift in the peak position has been resulted. The shift in the peak position is correlated with the change in the residual stress. The surface roughness of the films get increased after irradiation. A decrease in the grain size was observed after irradiation due to ion-induced recrystallization. The optical band gap energy decreased from 1.53 eV for as-grown film to 1.46 eV upon irradiation. The photoluminescence (PL) spectrum is dominated by the defect band and the effect of irradiation has been discussed and correlated with the observed change in the XRD peak position and optical band gap.     

Near ideal electrical switching in fast ion conducting glasses: Evidence for an electronic process with chemical origin

Bulk AgI based fast ion conducting (FIC) glasses have been prepared by a novel microwave technique. Electrical switching characteristics of these glasses have been investigated for the first time. It has been found that AgI based FIC glasses exhibit a current-controlled high speed memory electrical switching behaviour. SEM, EDAX and ESR investigations have been performed on the virgin and switched sampies to understand the nature of the conducting state. A chemical model is proposed to explain the switching behaviour of these glasses, which is consistent with the observed results.