Characterization of carbon aerogels by transport measurements

Carbon aerogels are a special class of low-density microcellular foams. These materials are composed of interconnected carbon particles with diameters of approximately 10 nm. The temperature dependence of the dc electrical resistivity and magnetic susceptibility (χ) from 4 K to room temperature, magnetoresistance (MR) in a magnetic field up to 15 T, and Raman scattering were measured as a function of aerogel density. While Raman scattering measurements are not sensitive to variations in density, the χ data show that there are more free carriers in samples of higher density. Aerogel samples with different densities all show a negative temperature coefficient of resistivity and a positive MR. The less dense samples exhibit a stronger temperature dependence of resistivity and a stronger field dependence of the MR, indicating that with decreasing density and increasing porosity, charge carriers are more localized. Data analysis precludes variable-range hopping in favor of nearest-neighbor hopping and fluctuation-induced tunneling as the most likely conduction mechanisms for carbon aerogels.