Colloidal processing, hot pressing and characterisation of electroconductive MWCNT-alumina composites with compositions near the percolation threshold

Multiwall Carbon Nanotubes (MWCNT)-alumina nanocomposites have been fabricated by colloidal processing and uniaxial hot pressing. In the nanotube sols, the electrical conductivity is particularly high, even for low concentrations (<= 0.7 vol.%). Classical conductivity models fail to explain this particular behaviour, which is likely to be related to the high aspect ratio of the nanotubes (> 70). Aqueous colloidal processing was performed optimising electrostatic repulsion and conserving the homogeneity by freeze-drying. Inhomogeneities of about 50 mu m appeared in the composites and a thermodynamic explanation is suggested based on the free volume of elongated and spherical particles, respectively and considering the persistence length of the nanotubes. The densification after hot pressing is incomplete (92-93%) even for the low nanotube concentrations considered (< 1.4 vol.%). The composites show electrical conductivity (2.5 S/m) and the percolation threshold is <= 0.6 vol.%. The conductivity is maintained up to 500 degrees C in air, degradation of the nanotubes due to oxidation at higher temperatures is likely to occur, decreasing the conductivity. (C) 2008 Elsevier Ltd. All rights reserved.