George Stan

Uptake of gases in bundles of carbon nanotubes

Model calculations are presented that predict whether or not an arbitrary gas experiences significant absorption within carbon nanotubes and/or bundles of nanotubes. The potentials used in these calculations assume a conventional form, based on a sum of two-body interactions with individual carbon atoms; the latter employ energy and distance parameters that are derived from empirical combining rules. The results confirm intuitive expectation that small atoms and molecules are absorbed within both the interstitial channels and the tubes, while large atoms and molecules are absorbed almost exclusively within the tubes.

Low coverage adsorption in cylindrical pores

Abstract We present a theoretical exploration of the adsorption of rare gases in carbon nanotubes. In both the classical and the quantum cases, nanotube adsorption provides a nearly ideal realization of quasi-one-dimensional (1D) matter. We have studied the adsorption potentials, the gas-surface virial coefficient and the isosteric heat of adsorption. Comparison shows a much stronger binding of the adsorbate in the tubes than at the planar surface of graphite. As a consequence, one can easily adsorb sufficiently many atoms to be measurable in a thermodynamic or scattering experiment. In studying the low coverage adsorption we find great sensitivity to the species, the assumed potential model, and the radius of the tubes. The effect of interactions between the adsorbed particles is evaluated in the 1D classical case.

Hydrogen Adsorption in Nanotubes

Model calculations are presented of the adsorption of hydrogen in carbon nanotubes. Using a phenomenological interaction potential, we compute the low coverage thermodynamic properties, showing explicitly the quantum effects in the Feynman (semiclassical) effective potential approximation. The effects of interactions are evaluated with a quasi-one dimensional classical Lennard-Jones approximation.

Condensation of Helium in Nanotube Bundles

Helium atoms are strongly attracted to the interstitial channels within a bundle of carbon nanotubes. The strong corrugation of the axial potential within a channel can produce a lattice gas system wherein the weak mutual attraction between atoms in neighboring channels induces a transition to an anisotropic condensed phase. At low temperatures, the specific heat of the adsorbate phase (with fewer than 2% of the atoms) greatly exceeds that of the host.

WETTING TRANSITIONS OF NE

We report studies of the wetting behavior of Ne on very weakly attractive surfaces, carried out with the grand canonical Monte Carlo method. The Ne-Ne interaction was taken to be of Lennard-Jones form, while the Ne-surface interaction was derived from an ab initio calculation of Chizmeshya et al. [J. Low Temp. Phys. 110, 677 (1998)]. Nonwetting behavior was found for Li, Rb, and Cs in the temperature regime explored (i.e.,

Interstitial He and Ne in Nanotube Bundles

Tl42 \mathrm{K}).

Axial Phase of Quantum Fluids in Nanotubes

Drying behavior was manifested in a depleted fluid density near the Cs surface. In contrast, for the case of Mg (a more attractive potential) a prewetting transition was found near

Threshold criterion for wetting at the triple point

T=28 \mathrm{K}.

COMPUTER SIMULATIONS OF THE WETTING PROPERTIES OF NEON ON HETEROGENEOUS SURFACES

This temperature was found to shift slightly when a corrugated potential was used instead of a uniform potential. The isotherm shape and the density profiles did not differ qualitatively between these cases.

Helium mixtures in nanotube bundles

We explore the properties of atoms confined to the interstitial regions within a carbon nanotube bundle. We find that He and Ne atoms are of ideal size for physisorption interactions, so that their binding energies are much greater there than on planar surfaces of any known material. Hence high density phases exist at even small vapor pressure. There can result extraordinary anisotropic liquids or crystalline phases, depending on the magnitude of the corrugation within the interstitial channels.