Correlation between morphology and electrical conductivity of dried and carbonized multi-walled carbon nanotube/resorcinol–formaldehyde xerogel composites

Monolithic multi-walled carbon nanotube (MWCNT)/resorcinol–formaldehyde (RF) composite xerogel was synthesized by sol–gel polymerization of RF monomers in a surfactant-stabilized MWCNT aqueous suspension. The resulting composite wet gel was dried under ambient conditions and pyrolyzed in a N₂ atmosphere to obtain MWCNT/carbon (C) nanoporous xerogel. TEM images showed that the nanotubes had been completely exfoliated in the starting solution before addition of the reagents. Optical microscopy and SEM micrographs revealed that there were aggregates of MWCNTs with diameters of up to 50 µm in the final composites. Nitrogen adsorption analysis and mercury porosimetry of the pristine MWCNT-free and composite gels showed that the density and pore texture of the xerogel materials were not modified by the nanotubes, but were strongly influenced by the addition of surfactant (sodium dodecyl benzene sulfonate, NaDBS). Electrical conductivity was measured for the organic (MWCNT/RF) and carbonized (MWCNT/C) composites and evaluations determined that this parameter increased in different ways as a function of MWCNT concentration. A percolating scaling law of the form σ ∝ (ϕ−ϕ _c)^t was obtained with a volume percolation threshold of ϕ _c = 2.5 × 10⁻⁴ and a critical exponent (t) of 1.0 for the organic nanocomposite. After pyrolysis, the carbonized xerogels exhibited an entirely different behavior with a continuous sharp increase in electrical conductivity with nanotube loading (4.5-fold increase at 1.64 v % MWCNT). Comparison of the data for experimental conductivity with the effective medium theory enabled estimation of an average length for the conductive strings formed by connected nanotube aggregations throughout the carbon matrix.