PTC materials composed of particles of conducting graphite and insulating cristobalite-form SiO2 or AlPO4 were prepared by the use of silicone rubber as a binder. The PTC effect was observed exactly at the reversible phase transition point with a large thermal expansion of cristobalite (ca. 270°C in SiO2 and ca. 230°C in AlPO4), and was reproducible on repeated runs. The magnitude of the PTC effect was high in composites having a middle composition between insulator and conductor in the percolation curve.
