Electrical conductivity of an insulator matrix (alumina) and conductor particle (molybdenum) composites

Different concentrations of molybdenum (from 5 to 35 vol.%) were added to alumina matrices and the resulting mixture hot-pressed to produce dense metal toughened-ceramic composites. Their resulting electrical properties were measured using two-probe impedance spectroscopy, in the range 25–1000 °C. Experimental and theoretical studies were done to examine how the volume fractions and the morphology of the embedded metal particles affect the electrical behaviour of these composite materials. Due to the percolation effect, a sharp increase in the electrical conductivity of these composites was observed for compositions with molybdenum contents higher than 20 vol.%. The experimental data were fitted into the GEM equation. It was shown that the conductivity of these metal-toughened-ceramic composites is strongly affected by the amount, and less by the size of the metal particles. For low concentrations of molybdenum only one semicircle was readily identified with impedance spectroscopy analysis. This indicates that the matrix is still the main conductor phase. For higher concentrations of molybdenum, two readily resolvable arcs were present. For concentrations of metal higher than 30 vol.% the second semicircle might be an overlapping of two semicircles and therefore the impedance spectra for these composites can be resolved with three semicircles. The presence of this third semicircle can be explained in terms of molybdenum clusters.