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Low percolation threshold for conducting polymers

Epoxy composites based on aligned CVD-grown multi-wall carbon nanotubes with weight fractions ranging from as low as 0.001 up to 1 wt% were produced. The resulting electrical properties were analysed by AC impedance spectroscopy. The composite conductivity follows a percolation scaling law of the form sigma = constant x (p-pc)^t with the critical mean concentration pc to form a conductive network of approximately 0.0025 wt% and an exponent, t, of 1.2.

The experimental percolation threshold for the aligned nanotubes used in this study represents the lowest threshold observed for carbon-nanotube-based polymer composites yet reported.

Aligned multi-wall carbon nanotubes from an injection CVD process were dispersed as conductive fillers in an epoxy matrix. The resulting electrical properties were investigated by AC impedance spectroscopy and it is shown that sufficient conductivity for anti-static applications can be achieved at an average nanotube loading of approximately 0.005 wt%. The resulting bulk conductivity properties of the nanotube-epoxy composites arise from the formation of macroscopic nanotube aggregates. The experimental conductivity data follows a percolation scaling law. The use of the aligned multi-wall carbon nanotubes leads to a uniquely low percolation threshold, which is an order of magnitude smaller than best results previously achieved with entangled multi-wall nanotubes. Compared to carbon black particles, the use of nanotubes as filler represents a significant advance for epoxy systems, allowing anti-static

Fig. 6. Epoxy composite conductivity as a function of filler weight fraction for the aligned CVD-grown multi-wall carbon nanotubes compared to results previously achieved with entangled nanotubes and carbon black particles conductivities at loading fractions where the mechanical properties of the matrix are not degraded, as well as higher maximum conductivities. In addition, these ultra-low loading fraction anti-static materials may prove to have interesting, low EM-radiation profiles.

Based on the achievement of initially well-dispersed nanotube-resin mixtures, further work is aimed at establishing the percolation threshold as a function of nanotube aspect ratio as well as determining the nature of the interactions between individual carbon nanotubes as a function of processing conditions.

Reported by

Materials Science Dept
United Kingdom
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