Piezoresistance behavior of silicone-graphite composites in the proximity of the electric percolation threshold

Composites of a silicone matrix charged with graphite powder of micrometric size in volume fractions around the electric percolation threshold (25-35%) have been investigated with regard to their piezoresistance properties. The elastic modulus changes with graphite content, reaching a maximum at 30 vol.%. From measurements of electric resistivity, the percolation threshold was determined as 31 vol.% at a compressive strain of 2%. The threshold value was found to be dependent on the applied compressive strain so that an insulator in the unstrained condition may become a conductor when subjected to a small pressure. The property can be exploited for contact sensors. Further, the electric resistance of a composite, charged a little beyond the percolation threshold, is also strain dependent, according to an equation of the type R = R0exp(βɛ), where β was found to be about 51.5. This value of β corresponds to a very high electric sensitivity of the material to an applied strain and makes it a candidate for application as a logarithmic strain transducer. Owing to the visco-elastic behavior of the elastomer matrix, there is a retardation of the electric response on unloading of about 2 s. The electric response to an applied stress follows an exponential law on loading and undergoes a corresponding retardation on unloading.