AN INVESTIGATION OF VAPOR-GROWN CARBON-FIBER BEHAVIOR TOWARDS AIR OXIDATION

Thermogravimetric analysis together with scanning electron microscopy observations have been used to study the behavior of vapor-grown carbo n fibers towards air oxidation. We show that different peaks are prese nt in the rate of gasification curves and we attribute this result, on the one hand to the presence of traces of iron in the fibers and on t he other hand to the tripler structure of these kind of fibers which a re composed of a central carbon filament, and of two regions of chemic al vapor deposited (CVD) carbon: an inner core, and an outer shell. Ox idation rates have been measured isothermally in the range 500-950 deg rees C. The gasification reaction is free of diffusion limitations bel ow 760 degrees C, and controlled by mass transfer (film diffusion regi me) at higher temperatures. Oxidation experiments performed in a dynam ic mode show differences in the gasification rate according to the fib er diameter. X-ray diffraction measurements and temperature programmed desorption experiments supported the fact that this difference in rea ctivity has not to be interpreted in terms of crystalline perfection b ut in terms of initial total or active surface area. (C) 1997 Elsevier Science Ltd.