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Conductive silicon carbide particles were incorporated into an insulating cristobalite ceramic matrix to produce composite materials with a sizeable positive temperature coefficient (PTC) effect. A large drop in resistivity with silicon carbide content at room temperature, or percolation behaviour, was observed. The PTC effect of the composites, which resulted from the thermal expansion of the cristobalite ceramic matrix, was a maximum with five orders of magnitude for the specimen with 25 vol% silicon carbide. The PTC transition temperature of the composites was at 260 °C, which coincided exactly the reversible high-low phase inversion temperature of the cristobalite ceramic matrix.