Solution of the tunneling-percolation problem in the nanocomposite regime

TL;DR

This paper develops a comprehensive model for nanocomposite conductivity that captures the intermediate tunneling-percolation regime, incorporating particle shape effects and providing analytical formulas validated by experimental data.

Contribution

It introduces a generalized critical path method including particle shapes, enabling accurate modeling of intermediate tunneling-percolation regimes without arbitrary conductance cut-offs.

Findings

The model reproduces key features of nanocomposite conductivity.

Analytical formulas are derived for various regimes.

Experimental data on different nanomaterials validate the approach.

Abstract

We noted that the tunneling-percolation framework is quite well understood at the extreme cases of percolation-like and hopping-like behaviors but that the intermediate regime has not been previously discussed, in spite of its relevance to the intensively studied electrical properties of nanocomposites. Following that we study here the conductivity of dispersions of particle fillers inside an insulating matrix by taking into account explicitly the filler particle shapes and the inter-particle electron tunneling process. We show that the main features of the filler dependencies of the nanocomposite conductivity can be reproduced without introducing any \textit{a priori} imposed cut-off in the inter-particle conductances, as usually done in the percolation-like interpretation of these systems. Furthermore, we demonstrate that our numerical results are fully reproduced by the critical path method, which is generalized here in order to include the particle filler shapes. By exploiting this method, we provide simple analytical formulas for the composite conductivity valid for many regimes of interest. The validity of our formulation is assessed by reinterpreting existing experimental results on nanotube, nanofiber, nanosheet and nanosphere composites and by extracting the characteristic tunneling decay length, which is found to be within the expected range of its values. These results are concluded then to be not only useful for the understanding of the intermediate regime but also for tailoring the electrical properties of nanocomposites.