ON THE REACTIVITY, STRUCTURE, AND POROSITY OF CARBON-FIBERS AND FABRICS

Carbon fibers and fabrics have been used extensively in fabricating carbon composites. From the perspective of the fiber, the adhesion between a fiber and matrix is controlled by the type of fiber, its heat-treatment temperature, and its surface properties. This article summarizes these properties on selected carbon fibers and fabrics; including the total surface area as determined by gas adsorption at 77 K, the active surface area as determined by oxygen chemisorption at 573 K, and microporosity as measured by CO2 adsorption at 298 K. The carbonized polyacrylonitrile (PAN) fiber and the graphitized rayon fabric as well as pitch fiber have small surface area (< 1.0 m2/g) and insignificant open porosity. By contrast, most of the carbonized rayon fabrics, at least the samples examined, have considerable micropore volumes. Oxidation rates of the fibers and fabrics in air have been measured at isothermal temperatures in the range 773-1223 K. With the PAN fiber, the oxidation process instantaneously opens the pores and the reaction is controlled by diffusion of O2 into the pores and of gasification products in the opposite direction. With the graphitized pitch fiber and rayon fabric, the reaction is controlled by the chemical reactivity of the surface below 1123 K, and by the diffusion limitation at higher temperatures. At 773 K, the catalytic effect of sodium on oxidation rates of carbonized rayon fabrics has been established, and a maximum concentration of the alkalis (1200 ppm) in the fabrics has been proposed.