Percolation in Multi-Wall Carbon Nanotube-Epoxy Composites Influence of processing parameters, nanotube aspect ratio and
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Percolation in Multi-Wall Carbon Nanotube-Epoxy Composites Influence of processing parameters, nanotube aspect ratio and electric fields on the bulk conductivity Jan K.W. Sandler1, Alan H. Windle1, Christian A. Martin1,2, Matthias-Klaus Schwarz2, Wolfgang Bauhofer2, Karl Schulte3, Milo S.P. Shaffer4 1
Department of Materials Science and Metallurgy, University of Cambridge, UK Materials in Electrical Engineering and Optics, TU Hamburg-Harburg, Germany 3 Polymer Composites, TU Hamburg-Harburg, Germany 4 Department of Chemistry, Imperial College London, UK 2
ABSTRACT A simple mechanical stirring process leads to charge-stabilised dispersions of aligned, substrate-grown, CVD-grown multi-wall carbon nanotubes in an epoxy resin. Subsequent sample processing, after the addition of the hardener, can be used to induce the nanotube agglomeration necessary to achieve electrically conductive bulk composites at low loading fractions. Both the nanotube percolation threshold and the resulting bulk conductivity can be adjusted by selection of suitable processing parameters and nanotube aspect ratio. This behaviour of aligned CVDgrown multi-wall carbon nanotubes allows lower electrical percolation thresholds than are possible with entangled multi-wall carbon nanotubes, single-wall carbon nanotube bundles, or carbon black in an epoxy matrix. Furthermore, the application of electric fields during composite processing induces the formation of aligned multi-wall carbon nanotube networks between electrodes dipped into the dispersion. Such composites show an electrical conductivity above the anti-static level and retain a degree of optical transmissivity. INTRODUCTION The concept of bulk polymer composites based on electrically conductive nano-sized filler particles offers mechanical property advantages over alternatives, such as intrinsically conductive polymers or the application of thin conductive surface layers of indium tin oxide (ITO). The electrical percolation threshold (critical concentration of filler particles to achieve a three-dimensional pathway of conductive particles) in polymer composites can be reduced by increasing the aspect (length-to-diameter) ratio of the filler particles. The use of entangled catalytic multi-wall carbon nanotubes (eCGNT) leads to an electrical percolation threshold about two orders of magnitude lower than spherical carbon black particles in the same epoxy matrix [1]. Intense mechanical stirring over prolonged periods of time proved effective at breaking up existing aggregates for this particular nanotube material, yielding a dramatic increase in apparent volume; however, some nanotube entanglement remained. Macroscopic aggregation of these nanotube clusters occurred at lower weight fractions and led to a higher maximum composite conductivity. In order to further optimise the percolation behaviour, a high degree of initial dispersion of the individual particles in a given matrix is desired. When dispersing conductive particles with diameters below 1 µm in a medium of low viscosity, diffus
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