CHEMICAL VAPOR-DEPOSITION OF PYROLYTIC CARBON ON CARBON SUBSTRATES .1. EFFECT OF SUBSTRATE SURFACE CHARACTERISTICS ON THE KINETICS OF DEPOSITION

Chemical vapor deposition (CVD) of pyrolytic carbon on 25 different carbon substrates has been investigated at 1273-1338 K, using a mixture of 10% methane in argon flowing at 150 cc/min (STP). The substrates covered a wide spectrum of carbons (porous/nonporous, graphitic/nongraphitic, and small/large surface areas). The results indicate that there are at least four different categories of carbon substrates. The first category includes nonporous, graphitized, or carbonized substrates having a small surface area. The deposition rate increases with surface area. The second category includes microporous carbons with a small external surface area. The early deposition of pyrolytic carbon blocks the micropores, and the reaction continues on the external surface in a manner similar to the first category. The third category includes nonporous, graphitized, and carbonized substrates having a large surface area. The deposition rate on the graphitized substrates is smaller than the carbonized substrates provided that their initial surface area is the same. With carbonized substrates, the total and active surface areas are contributing differently to the CVD rate. However, the contribution of active surface area (ASA) is less pronounced than that of the total surface area (TSA). The fourth category includes activated graphitized carbons. The activation process enhances the ASA and develops microporosity. The newly developed ASA is apparently located inside the micropores since the deposition rate drops substantially after the commencement of reaction. When the micropores are blocked, the entrances to internal ASA are blocked and the reaction continues on the external surface in a manner similar to that of the third category.