Electrical Discharges under High Voltage on Surface of Silica‐Nickel Ceramic Composites

Abstract

Cermets have a variety of important technological applications. Two-phase cermets composed with two powders with different electrical conductivities can be used as resistors, sensors transducers or igniters. [1,2] Their global electrical conductivity depends on many parameters as the quantity of the most conductive phase and its statistical distribution into the low conductive material, to form a more or less ordered structure. Besides, particle shape, size and packing density also influence the global electrical properties. In general, the increase of the quantity of metallic particles into an insulating ceramic matrix phase results in the increase of the global electrical conductivity. For a narrow range of the metallic powder content, the electrical conductivity exhibits a percolative behaviour, which is characterized by an abrupt increase in conductivity ( Fig. 1). This percolation threshold is also characterized by the abrupt variation of many material properties as the thermal conductivity. Most of these properties are related to the material volume, but electrically conductive cermets also exhibit peculiar surface properties as an electronic field emission on surface from a large density of emission sites. A similar behaviour was observed with conductive film for electrode. [3] Basically, field emission sites are often micro-protrusions (Figs. 2a and 2b) where the electric field is enhanced by a factor b to a value (~10 9 ±10 10 Vm ±1 ), which is the threshold for this type of emission to occur. Then breakdown is generally attained under macroscopic electric field, which doesn't exceed 10 6 Vm ±1 that it is significantly lower than for a similar air gap.This electrical process last about 100 ls, with two main successive stages for firstly the surface ionisation and secondly the breakdown, when a plasma-like process is obtained. But surface events are related to volume characteristics since the global conductivities of cermet extends to about 10 order of magnitude, from 10 ±4 Scm ±1 to 10 4 Scm ±1 , as a function of the metallic phase proportion, type and connectivity.The purpose of this work is to prepare nickel-silica cermets using micro-sized Ni grains as conductive phase and silica glass as matrix phase. The Ni grain size was carefully selected in order to limit the matrix cracking near the surface due to the thermal expansion coefficients mismatches during the breakdown. For electric measurements, nickel-silica cermets were connected with metal electrodes as in Figure 3.Results: The variation of conductivity under a low electrical field (Fig. 1) below and above the threshold is similar to conductivity of various composites as nickel-zirconia cermet [4,5,6] or polymer matrix composites. [7] Fitting these data show the typical behaviour or our composite, in comparison to other composite types. A number of models [8] from the literature indicate that the current percolation in a two-phase composite depends on both the amount of conductive particles, their shape and their spatial distribution within the in...