Percolation in metal-insulator composites of disordered jammed spherocylindrical nanoparticles

While classical percolation is well understood, percolation effects in random jammed structures are much less explored. Here we investigate both experimentally and theoretically the electrical percolation in a binary system of disordered jammed spherocylinders. Experimentally we determine the percolation threshold and conductivity critical exponent for composites of conducting (CrO₂) and insulating (Cr₂O₃) nanoparticles that are geometrically identical. Simulations and modeling are implemented through a combination of the mechanical contraction method and a variant of random walk (de Gennes ant) approach, in which charge diffusion is correlated with the system conductivity via the Nernst-Einstein equation. The percolation threshold and critical exponents are identified through finite size scaling and are in good agreement with the experimental results. Interestingly, the calculated percolation threshold for spherocylinders with an aspect ratio of 6.5, p_c= 0.312 ± 0.002, is very close (within numerical errors) to the one found in two other distinct systems of disordered jammed spheres and simple cubic lattice, an intriguing and surprising result. We also explored tunneling percolation in a system of CrO₂ nanoparticles with variable thickness of insulating Cr₂O₃ shell barriers.