Electric conductivity in silicone-carbon black nanocomposites: percolation and variable range hopping on a fractal

Abstract

Effects of various experimental parameters on dc conductivity of Silicone-Carbon Black (CB) nanocomposites are measured and correlated to the fractal morphology of CB network. The influence of filler volume fraction, temperature and electric field strength, are discussed using percolation and variable range hopping on a fractal as theoretical background. One also investigates effects of curing rate and CB content on both surface’s morphology and conductivity. The fractal aspect of CB network is studied by optical microscopy (OM). Electric conductivity is measured using four points setup. AFM and STM are used to study respectively surface morphology and conductivity. Low percolation threshold ϕc = 0,15% and scaling exponent μ ≅ 2,6 of conductivity with CB volume fraction is correlated to the measured fractal dimension df = 2,4. Conduction at low temperature proceeds by mean of variable range hopping on the CB fractal network with a Mott’s exponent m = 0,65 and temperature T0 = 64 K. Non-Ohmic conductivity curves at different temperatures could be superposed into a unique master curve and the onset field of non-linearity Scales with temperature with a typical exponent XT ≅ 1,2. At low curing rate, capillary forces have time to bring down all CB particles from the surface into the bulk whereas at fast curing rate the roughness and conductivity of the surface remain high.