Thermodynamics of helium and hydrogen films adsorbed on single-walled carbon nanotube bundles

Abstract

Adsorption of Helium and Hydrogen on bundles of single walled carbon nanotubes (SWCNs) has been studied via heat capacity and volumetric isotherm measurements. These measurements allow conclusions regarding the growth of the films, the dimensionality as a function of density, and the structure of the films. Comparisons can then be made, both quantitative and qualitative, to theories regarding ideal and real one-dimensional systems, as well as the theory of adsorption specifically on the SWCNs.In this thesis I describe the experiments, focusing mostly on the heat capacity measurements. These are the first systematic measurements of the heat capacity of this system as a function of both coverage and temperature. The data support some of the earlier conclusions about growth of the films, in particular the 1-line → 3-line → monolayer progression with little or no adsorption in interstitial sites. There are also new mysteries presented. Most importantly we see no evidence of a gas phase at all, and instead have small and increasing heat capacity, indicative of a solid, or at least immobile system. This observation is inconsistent with all of the predictions so far, as well as with a previous experiment (Lasjaunias et al., 2003). Finally, the helium film is one dimensional at very low density, as expected, but also at roughly 3/4 of a monolayer, whereas hydrogen appears to be two dimensional at all coverages. These surprising facts need further theoretical development.