The experimental determination of the onset of electrical and thermal conductivity percolation thresholds in carbon nano
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The experimental determination of the onset of electrical and thermal conductivity percolation thresholds in carbon nanotube-polymer composites Byung-wook Kim1, Steven Pfeifer2, Sung-Hoon Park2, and Prabhakar R. Bandaru2 1
Department of Electrical and Computer Engineering, University of California, San Diego, USA
2
Materials Science Program, Department of Mechanical Engineering, University of California, San Diego, USA
ABSTRACT We show evidence of electrical and thermal conductivity percolation in polymer based carbon nanotube (CNT) composites, which follow power law variations with respect to the CNT concentrations in the matrix. The experimentally obtained percolation thresholds, i.e., ~ 0.074 vol % for single walled CNTs and ~ 2.0 vol % for multi-walled CNTs, were found to be aspect ratio dependent and in accordance with those determined theoretically from excluded volume percolation theory. A much greater enhancement, over 10 orders of magnitude, was obtained in the electrical conductivity at the percolation threshold, while a smaller increase of ~ 100 % was obtained in the thermal conductivity values. Such a difference is qualitatively explained on the basis of the respective conductivity contrast between the CNT filler and the polymer matrix.
INTRODUCTION Polymer composites containing conducting fillers [1] such as carbon black [2], carbon fiber, and metal fibers have been extensively investigated for various applications such as electromagnetic interference (EMI) shielding [3], electronic packaging [4], radar absorption [5], etc., However, presently used composites require high filler to polymer loading ratios which deteriorates the overall mechanical properties, through a compromise of intrinsic matrix morphology [6]. A possible way to ameliorate the above problems, through using low filler volume fractions, incorporates carbon nanotubes (CNTs) in composites [7-10]. A concomitant large aspect ratio and tunable electrical conductivity would enable electrical percolation to be achieved at very small CNT volume fractions [9]. The use of small CNT volume fractions would also reduce the economic costs and clustering of the CNTs within the polymer matrix, important for large scale application. In this paper, we discuss the issue of the minimum volume fraction necessary, i.e., the percolation threshold (Ic) for the CNT-polymer composites to serve as electrically or thermally conductive materials through percolation theory [11]. Such a study could then lead to an understanding of the viability of such composites for a large number of applications.
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EXPERIMENTAL DETAILS We have sought to understand issues associated with the uniform dispersion of CNTs into polymer matrices through optimized synthesis/processing, involving enhanced nanotube-polymer interactions through chemical functionalization schemes. We chose for our initial experiments, a composite of CNTs and a Reactive Ethylene Terpolymer (RET: Elvaloy 4170) – Figure 1. Both pristine and carboxyl functionalized SWNTs (average diameter 1-2 n
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