ac breakdown properties of alumina, aluminum nitride, and glass ceramic (Si02-Al,03-MgO-Ti02) materials have been investigated. The breakdown strength of glass ceramics ( -65 kV/mm) was found to be higher than that of alumina ( -31 kV/mm) and aluminum nitride ( -16 kV/mm). The specimens were characterized for their density, porosity, and microstructural parameters, such as grain size and distribution, as they are believed to influence the breakdown properties significantly. In all the cases, the prebreakdown and breakdown conduction were found to be associated with visible-light emission at the electrode-specimen interface. The samples that underwent breakdown were found to exhibit irregular puncture channel, terminated with craters at both ends. Several types of crystallization structures were observed at the rim of the craters and in some cases at the edges of the breakdown channels. It is suggested that the breakdown process of the ceramic materials is a combination of electronic, electromechanical, and thermal processes.

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The one-dimensional Klein-Gordon equation for equal vector and scalar -parameter hyperbolic Poschl-Teller potential is solved in terms of the hypergeometric functions. We calculate in detail the solutions of the scattering and bound states. By virtue of the conditions of equation of continuity of the wave functions, we obtained explicit expressions for the reflection and transmission coefficients and energy equation for the bound state solutions.

A thermochemical etching procedure, comprising a short chemical etch followed by a low-temperature thermal anneal (typically 10 min at 570% of the sintering temperature), has proved successful for the preparation of alumina, aluminum nitride, and nickel zinc ferrite ceramics for microstructural studies. Good-quality results have been achieved in times much shorter than those normally associated with chemical etching and at temperatures lower than those usually used for thermal etching. Under such conditions, grain growth and surface modification are believed to be minimal.[

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