Conductive filler networks in flexible conductive composites

Abstract text (incl. figure legends and references)

Flexible, electronic hybrid, and composite materials combine mechanical elasticity and electrical conductivity by forming a conductive particle network. The performance of such "network materials", i.e., particle-based composites, battery materials, etc., is dominated by the morphology of the conductive filler particle network and the contact resistances between particles.

Here, we study the structure of the particle network of conductive silver spheres in a silicone elastomer matrix. A conductive network with macroscopic conductive paths is formed at the concentration required to meet the vol% of the percolation threshold in the electrically insulating. We investigate selected morphological parameters as a function of filler volume fraction and correlate the trends with macroscopic electrical conductivities.

Spherical silver particles (average diameter 2.5µm) were dispersed in a polydimethylsiloxane (PDMS) elastomer matrix with different silver volume fractions from 25 - 45 vol%. Macroscopic four-point measurements were performed at different volume fractions to analyze the electrical conductivity of the composites. We use Focused Ion Beam – nanotomography (FIB-nt) of volumes with a statistical number of particles to reveal the 3D particle network structure for each silver volume fraction (Figure 1). Post-processing of the 3D reconstruction provided information such as geometric tortuosity distributions, particle coordination number distributions, percolation fractions, particle-particle contact area distributions, and the number of shortest paths.

The mean tortuosity decreased with increased filler volume fraction, indicating shorter conductive paths with fewer interfaces. The mean coordination number increased with the volume fraction, increasing the number of potentially conductive contacts. We observe an increase of the average contact area with volume fraction which indicates an increasingly jammed packing of the particles above the percolation threshold. The overall increase in conductivity could be explained by a more interconnected network with reduced particle contact resistances.