Localization Transport in Granular and Nanoporous Carbon Systems.

Porous carbon materials have long since been used in industry to make capacitors and adsorption agents because of their high specific surface area. Although their adsorption properties have been extensively studied, we have not seen the same vigor in the investigation of their physical properties, which are important not only for providing complementary characterization methods, but also for understanding the physics which underlies the manufacturing process and motivates intelligent design of these materials. The study of the new physics in these novel nanoporous materials also straddles the scientific forefronts of nanodimensional and disordered systems. In this thesis, we study the structural and electrical properties of two nanoporous carbons, namely activated carbon fibers and carbon aerogels. Specifically, we perform Raman scattering, x-ray diffraction, magnetic susceptibility, electrical transport and magnetotransport experiments. Results from other experiments reported in the literature or communicated to us by our collaborators, such as porosity and surface area measurements by adsorption methods, electron spin resonance, transmission electron microscopy, mechanical properties measurements and so on, are also frequently used in this thesis for additional characterization information. By correlating all the relevant results, we obtain the structure -property relationships in these nanoporous materials. This study shows that the transport properties of these porous materials can be used on one hand for sensitive characterization of complex materials, and on the other hand, for observing interesting and unusual physical phenomena. For example, as-prepared nanoporous carbon systems, exhibit in their low-temperature electrical conductivity a universal temperature dependence which is characteristic of a granular metallic system, despite their morphological differences. By studying further the magnetoresistance in these carbon materials, it is found that the variable-range hopping mechanism cannot be totally disregarded in the understanding of the low-temperature conduction process in some granular metals having a similar morphology. In the transport study of the heat-treated activated carbon fibers, the surprising observation of a negative magnetoresistance at room temperature has also provided some insight into the weak localization phenomenon in the percolation limit. In particular, the effects of anomalous diffusion in a percolating system is now included in the calculations of the weak-localization corrections to the conductivity and magnetoresistance, yielding a new temperature dependence of the dephasing distance. These localization phenomena in the nanoporous carbon structures studied here are mostly understandable in terms of the existing theories for disordered systems, but their detailed interpretations often indicate problems and shortcomings in some of these theories, at times because the physical properties of the nanoporous carbon materials studied here are unique among disordered materials. Hence, nanoporous carbons belong to a distinct class of disordered systems in their own rights. In the field of transport in disordered systems, porous media also seem to have been an oversight of the general research community, although theoretical percolation studies have often touched upon systems with similar morphologies. This thesis presents a study of the transport behavior in nanoporous carbons over the full spectrum of disorder, controlled by heat treatment, starting from the strong localization regime, then crossing the metal-insulator transition, and finally to the weak localization limit. In each regime of disorder, the existing theories are either adapted, and when necessary, extended to explain the observed transport behavior in these fascinating materials. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617 -253-1690.).