THE ELECTRIC-FIELD IN POROUS MATERIALS CALCULATED WITH THE GRAIN CONSOLIDATION MODEL

We have used the grain consolidation model (GCM) to study the spatial variation of the electric field inside a porous medium such as oil imp regnated cellulose. The calculations were primarily performed in order to determine the maximum electric field within the composite, which i s important for the overall dielectric strength of the material. In th e composite as a whole, the maximum field may be considerably higher t han in a homogeneous medium under the same applied external field. The contacts between the solid particles in the composite material are mo re realistically described in the GCM than in effective-medium models, and a comparison shows that the GCM yields higher maximum fields than effective-medium theory Two different S-dimensional geometries have b een used in the GCM calculations and the results indicate that the exa ct geometry is of minor importance. These models are isotropic, while many important insulation materials are highly anisotropic. In order t o illustrate the effects of different geometries, we have also perform ed calculations on a 2-dimensional geometry. In this case, it appears as if the lack of contacts between the solid particles mainly influenc es the maximum field in the phase which has the highest complex conduc tivity. If the conductivities of both phases are almost equal, the max imum field is close to the applied field. Increasing the difference in conductivity between the oil and the solid increases the maximum elec tric field. We have also shown that the GCM can be used to estimate th e electric field in the presence of interface effects.