A. W. P. Fung

Structural characterization of heat-treated activated carbon fibers

Raman scattering, x-ray diffraction, and BET measurements are used to study the effect of heat treatment on the microstructure of activated carbon fibers (ACFs) and to correlate the structural changes with the metal-insulator transition observed in the electronic transport properties of heat-treated ACFs. A sequence of events is identified, starting with desorption, followed by micropore collapse plus the stacking of basic structural units in the {ital c}-direction, and ending up with in-plane crystallization. The graphitization process closely resembles that depicted by Oberlins model, except that the final material at high-temperature heat treatment remains turbostratic. Because the metal-insulator transition was observed to occur at heat-treatment temperature {ital T}{sub HT}{congruent}1200 {degree}C, which is well below the {ital T}{sub HT} value (2000 {degree}C) for in-plane crystallization, we conclude that this electronic transition is not due to in-plane ordering but rather to the collapse of the micropore structure in the ACFs. Raman scattering also provides strong evidence for the presence of local two-dimensional graphene structures, which is the basis for the transport phenomena observed in heat-treated ACFs.

New characterization techniques for activated carbon fibers

Abstract Recent work on activated carbon fibers with specific surface area SSA ≥ 1000 m2/g is reviewed. Because of their heterogeneity, it is necessary to characterize activated carbon fibers by multiple characterization techniques, such as x-ray diffraction, Raman scattering, transport properties, magneto-resistance, photoconductivity, and electron spin resonance. Further insight into the structure-property relationships in activated carbon fibers is achieved through their study as a function of heat treatment temperature, using these same characterization techniques. Whereas transport-related properties are most sensitive to the specific surface area, the structure and Raman spectra are most sensitive to the state of graphitization.

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 ([chi]) 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 [chi] 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.

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.