Non-ohmic I-V behaviour of random metal-insulator composites near their percolation threshold

A large increase in electrical conductivity has been observed when a direct current voltage is applied to random metal-insulator composites near their percolation threshold. This reversible non-ohmic I-V behaviour, which is similar to that observed in zinc oxide varistors, has been studied in three metal/insulator systems including silver particles in a matrix of potassium chloride, and two different systems of nickel particles in a matrix of polypropylene. These composites have all been prepared by mechanically mixing metal particles with an insulator host in predetermined volume fractions. A physical model with a semi-phenomenological equation has been proposed to describe this non-ohmic I-V behaviour. The non-ohmic effect is postulated to arise from a localized reversible dielectric breakdown between narrowly separated metal clusters in the metal/insulator composite.