SCATTERING PROCESS OF GAS MOLECULES ON VERTICALLY ALIGNED SINGLE-WALLED CARBON NANOTUBES

Scattering process of helium molecules on vertically aligned single-walled carbon nanotubes (VA-SWNTs) was investigated using the molecular beam technique. The samples were grown by alcohol catalytic CVD method and their thicknesses were approximately 0.1 and 4 mu m. We found that the modification of quartz surface with VA-SWNT films significantly enhances the energy transfer between gas molecules and the surface at room temperature and makes the energy accommodation coefficient of helium, which tends to be small even for contaminated surfaces because of the large mass mismatch between helium and surface atom, close to unity. The high porosity of the film enables gas molecules to penetrate into the film and suffer multiple collisions with carbon nanotubes (CNTs), leading to the efficient energy transfer. Although the accommodation coefficient slightly decreases for the thinner film, it is still high enough for practical applications. Our results demonstrate a potential application of VA-SWNTs as nanoscale fin structures to enhance heat transfer between gas phase and solid surfaces in rarefied gas flows. As the surface temperature increases, however, the energy accommodation becomes less efficient. This would be attributed to the small adsorption energy, which could reduce the trapping probability of helium on CNT bundles.