ADSORPTION STUDIES OF METHANE FILMS ON CATALYTIC CARBON NANOTUBES ANDON CARBON FILAMENTS

Adsorption measurements of CH4 on catalytically produced carbon nanotu bes were used to determine the wetting behavior of the films, the pres ence of capillary condensation, and the specific surface area of the t ubes. Two groups of carbon tubes were produced. The set of larger tube s had inner diameters on the order of 1 mu m, while the set of narrowe r tubes had inner diameters on the order of 10-100 nm. The narrower ca rbon tubes were either oxidized in nitric acid, or subjected to high-t emperature treatment at 2400 degrees C in vacuum, or left in as-produc ed condition. The activation processes were used to open the ends of t he tubes. Surface area determinations for untreated and treated tubes established that the specific surface area increased as a result of ac tivation. Isothermal adsorption-desorption cycles were measured on tre ated and untreated tubes. Hysteresis loops, indicative of the formatio n of a capillary condensate on the substrate, were present only for th e open-ended treated tubes; no hysteresis loops were present on the un treated tubes. The wetting behavior of methane films above and below t he bulk triple point was determined. Solid films incompletely wet the tubes. This behavior contrasts with the complete wetting exhibited by CH4 on exfoliated graphite, in spite of the fact that both the tubes a nd the exfoliate have essentially the same composition. Liquid methane completely wets the nanotubes.