FRACTAL ANALYSIS AND SURFACE-ROUGHNESS OF NONPOROUS CARBON-FIBERS AND CARBON-BLACKS

Using the fractal analysis approach, the surface roughness and irregularities of nonporous carbon fibers (CF's) were estimated by analyzing the nitrogen adsorption isotherms at 77 K. These fibers were prepared from different precursors and subjected to different heat treatment temperatures. The fractal dimension, D, obtained by this method was correlated to the percent population, D', of (chemically reactive) carbon active sites available at the external surface of the fibers. The higher the value of D, the higher was D'. The fractal approach and the technique used to determine the population of carbon active sites are shown to be highly informative and consistent methods for quantifying the surface roughness of CF's. The extent of surface roughness as computed by D or D' is also in qualitative agreement with the images obtained on CF's using scanning tunneling microscopy. The nitrogen isotherms on the CF's and on various carbon blacks yield a set of adsorption isotherms ranging form pure van der Waals adsorption to pure capillary condensation (and intermediate cases) on fractal surfaces. In one extreme case (CF's), the surface exhibits fractal scaling over two decades of length and induces capillary condensation from a monolayer upward.