Directional Breakdown in Alkali Halide Crystals

Abstract

The dc breakdown strength of sodium chloride, shown by von Hippel and Davisson to be independent of orientation at room temperature, also proves to be direction-independent at liquid nitrogen temperature. Experiments at room temperature reveal the existence of three types of breakdown paths: anodic, cathodic, and homogeneous field paths. Anodic paths are initiated by prebreakdown discharges at the anode and are precisely orientated with respect to the crystallographic axes. Cathodic breakdown paths are initiated at point cathodes; their direction dependence differs from that of anodic paths. Finally, in strictly homogeneous fields the paths follow the direction of the field.Measurements on various alkali halides between -160\ifmmode^\circ\else\textdegree\fi{} and 325\ifmmode^\circ\else\textdegree\fi{}C under strictly controlled field conditions confirm the findings of Davisson that the anodic path directions follow the sequence: random \ensuremath{\rightarrow} [100] \ensuremath{\rightarrow} [111] \ensuremath{\rightarrow} [110] with increasing temperature. [111] paths, observed only in sodium salts, are usually accompanied by [110] or [100] paths. While the random \ensuremath{\rightarrow} [100] transition takes place gradually over a temperature range of more than 100\ifmmode^\circ\else\textdegree\fi{}C, the [100] \ensuremath{\rightarrow} [110] transition in potassium and rubidium salts is sharp. With overvoltage this acuteness of the transition disappears, the path directions are shifted towards lower temperature forms and the temperature range of [111] paths in sodium salts is greatly extended. Mechanical deformation has little effect on the path directions. A phonon-Brillouin zone picture, as used by Offenbacher and Callen, but modified for inhomogeneous fields, is in good qualitative agreement with our observations. The electronic-Brillouin zone boundary and the crystalline potential may also influence the path directions.