Dielectric quantification of conductivity limitations due to nanofiller size in conductive powders and nanocomposites

Conducting submicron particles are well suited as filler particles in nonconducting polymer matrices to obtain a conducting composite with a low percolation threshold. Going to nanometer-sized filler particles imposes a restriction to the conductivity of the composite, due to the reduction of the density of states involved in the hopping process between the particles, compared to its value within the crystallites. We show how those microscopic parameters that govern the charge-transport processes across many decades of length scales can accurately and consistently be determined by a range of dielectric-spectroscopy techniques from a few hertz to infrared frequencies. The method, which is suited for a variety of systems with restricted geometries, is applied to densely packed 7-nm-sized tin oxide crystalline particles with various degree of antimony doping and the quantitative results unambiguously show the role of the nanocrystal charging energy in limiting the hopping process.