Electrically Conductive CNT Composites at Loadings below Theoretical Percolation Values

Abstract

It is well established that dramatic increases in conductivity occur upon the addition of conductive filler materials to highly resistive polymeric matrices in experimental settings. However, the mechanisms responsible for the observed behavior at low filler loadings, below theoretical percolation limits, of even high aspect ratio fillers such as carbon nanotubes (CNT) are not completely understood. In this study, conductive composites were fabricated using CNT bundles dispersed in epoxy resins at diverse loadings, using different dispersion and curing protocols. Based on electron microscopy observation of the CNTs strands distribution in the polymeric matrices and the corresponding electrical conductivities of those specimens, we concluded that no single electron transfer model can accurately explain the conductive behavior for all the loading values. We propose the existence of two different conductive mechanisms; one that exists close to the percolation limit, from ‘low loadings’ to higher CNT contents (CNT % wt > 0.1) and a second for ‘extremely low loadings’, near the percolation threshold (CNT % wt < 0.1). The high conductivity observed for composites at low CNT loading values can be explained by the existence of a percolative CNT network that coexists with micron size regions of non-conductive material. In contrast, samples with extremely low CNT loading values, which present no connectivity or close proximity between CNT bundles, show an electrical conductivity characterized by a current/voltage dependence. Data suggests that at these loadings, conduction may occur via a material breakdown mechanism, similar to dielectric breakdown in a capacitor. The lessons learned from the data gathered in here could guide future experimental research aimed to control the conductivity of CNT composites.

Impact of Current and Temperature on Extremely Low Loading Epoxy-CNT Conductive Composites

“…Direct current (DC) and alternating current (AC) electrical testing of epoxy-CNT composites shows that electrically conductive levels, necessary for use in ESD and EMI applications, can be easily achieved. Remarkably, even at extremely low CNT loadings (<0.1 wt%), epoxy-CNT composites meet these conductivity values at ambient conditions [15,16,[19][20][21]. Prior observations indicate that the order of magnitude of the electrically conductive behavior, and the microstructure associated with such, are a function of CNT content; high to low loadings (>0.1 wt%) present connected CNT networks and resistivities in the order of~1 to 10 Ohm•cm, while extremely low CNT loading (<0.1 wt%) exhibit unconnected CNT strands and resistivities in the order of 10 2 to 10 4 Ohm•cm [15].…”

“…In previous work on DC measurements of epoxy-CNT composites, specifically EA9396 epoxy with Nancomp CNTs, it was clearly shown that resistivity values decreased as the concentration/loading of CNTs increased [15]. It was also reported that DC resistivity, as a function of current, is sensitive to CNT loading.…”

“…CNTs and EA9396 were combined to form epoxy-CNT composites based on methodology from previous studies [15]. Requisite amounts of Part A-EA9396 epoxy resin and CNTs were measured and added to either a Max 10 or PP50 Flaktek mixing cup (depending on sample size) [24].…”

“…All DC measurements supported the determination of a resistance value, that along with sample dimensions, yielded electrical resistivity values. The resistivity measurement and calculation process was the same as the method used in prior work [15].…”

“…Additionally, the ability to tailor or modify CNT concentrations and/or composite synthesis and fabrication processes can support the rapid change of electrical properties. In the case of CNT composites, many parameters, such as CNT concentration, dispersion methods, curing time/temperature, type of CNTs, etc., can significantly impact the conductive properties of epoxy-CNT composites [15][16][17][18][19]. Direct current (DC) and alternating current (AC) electrical testing of epoxy-CNT composites shows that electrically conductive levels, necessary for use in ESD and EMI applications, can be easily achieved.…”