Z. H. Wang

The effects of external conditions on the internal structure of carbon aerogels

Carbon aerogels with various particle (grain) sizes, mass densities and heat-treatment temperature were studied. Effects of these externally controllable parameters on the internal structure and properties of the carbon aerogel material were examined using techniques such as room-temperature Raman spectroscopy, temperature-dependent magnetic susceptibility and temperature-dependent dark- and photo-conductivities. The results show that the in-plane microcrystallite size increases with heat-treatment temperature, the electrical conductivity increases with mass density, and the particle size affects the unpaired spin disorder present. The transport mechanism common to all the samples is attributed to a variable-range hopping about a Coulomb gap.

Relationship between particle size and magnetoresistance in carbon aerogels prepared under different catalyst conditions

Abstract Previous studies of the temperature dependence of the high-field positive magnetoresistance were carried out for carbon aerogels with a fixed [resorcinol]/[catalyst] ( R C ) molar ratio of 200 but different mass densities. The present work focuses on the effect of the particle size on the transport properties of carbon aerogels of both high and low mass densities by varying the R C ratio within the range of 50–300, the smaller R C ratio corresponding to smaller particle size. Correlations of new results with SQUID and Raman findings identify the nanosize particles to be the localization sites for transport processes. The particle size can be estimated from the Coulomb-gap variable-range hopping model, which has been successful in explaining the low-temperature magnetoresistance data obtained for all carbon aerogels samples. The particle size thus extracted is consistent with transmission electron microscopy measurements on the carbon aerogels and is directly correlated with the R C ratio. In polymeric carbon aerogels ( R C = 50 ), the average particle size becomes comparable with the width of the glassy-carbon-like ribbons within the particles, indeed blurring the distinction between particles and inter-particle links.