Electrically Conductive Bulk Composites through a Contact-Connected Aggregate

Ahsan I Nawroj,Aaron M Dollar,John P Swensen,Aristides Docoslis

doi:10.1371/journal.pone.0082260

Abstract

This paper introduces a concept that allows the creation of low-resistance composites using a network of compliant conductive aggregate units, connected through contact, embedded within the composite. Due to the straight-forward fabrication method of the aggregate, conductive composites can be created in nearly arbitrary shapes and sizes, with a lower bound near the length scale of the conductive cell used in the aggregate. The described instantiation involves aggregate cells that are approximately spherical copper coils-of-coils within a polymeric matrix, but the concept can be implemented with a wide range of conductor elements, cell geometries, and matrix materials due to its lack of reliance on specific material chemistries. The aggregate cell network provides a conductive pathway that can have orders of magnitude lower resistance than that of the matrix material - from 1012 ohm-cm (approx.) for pure silicone rubber to as low as 1 ohm-cm for the silicone/copper composite at room temperature for the presented example. After describing the basic concept and key factors involved in its success, three methods of implementing the aggregate into a matrix are then addressed – unjammed packing, jammed packing, and pre-stressed jammed packing – with an analysis of the tradeoffs between increased stiffness and improved resistivity.

Highlights

There are a large number of reasons why it might be desirable to enable a component or structure to be electrically conductive without limiting the choices of materials to metals or the small number of conductive non-metals
While these cells might be made from any conductive material and in a wide range of geometries, we demonstrate the concept with copper cells that are fabricated into coils-of-coils, which have a relatively low stiffness and low volumetric density while allowing for a large number of contacts with surrounding cells (Figure 1)
Resistance changes with stress and sample size Increasing applied stress on the samples increases the intercellular contact forces, which drives down the contact resistance and the bulk resistance of the entire sample

Summary

Introduction

There are a large number of reasons why it might be desirable to enable a component or structure to be electrically conductive without limiting the choices of materials to metals or the small number of conductive non-metals. Concept Implementation The central design idea is to embed a large number of conductive cells made of a highly conductive metal in a base material matrix with very high intrinsic resistivity These conductive cells come in direct physical contact with each other during the fabrication of the composite, and through their number and adjacency, create a connected network of contact resistances. Composites of the third type, ‘‘Jammed with pre-stress’’, are cured while a known mass (calculated to apply 4 kPa of stress on the cell network) is placed on top of the packing The choice of this stress value was made after observing a plateauing effect in resistivity of jammed packings at applied stresses greater than 4 kPa (Figure 9). These values give a mean error of 3.48% for the resistance measurements across all samples, and a maximum error of 1 mV (on the smallest recorded resistance of 0.029V, measured on a jammed and prestressed packing)

Findings

Discussion

Conclusions and Future Work