Resistivity of alumina-graphite composite ceramics

Alumina-graphite composite ceramics were fabricated by normal sintering in a reducing atmosphere, and their bulk resistivity measured. Because the composites consisted of connected clusters of conducting graphite particles within an insulating alumina matrix, the experimental results were discussed in terms of percolation theory. The electrical resistivity increases as the volume fraction of graphite (x) decreases, following a well-accepted power law model. The observed critical exponent was larger than the expected universal value based on a three-dimensional structure, due to a weighted distribution of conductivity throughout the sample. The renormalized pressure coefficient of resistance, d(ln R)/dP, decreased as x decreased and was explained by a strong enhancement of d(ln ρ)/dx as the percolation threshold was approached.