18 research outputs found
Lattice-gas Monte Carlo study of adsorption in pores
A lattice gas model of adsorption inside cylindrical pores is evaluated with
Monte Carlo simulations. The model incorporates two kinds of site: (a line of)
``axial'' sites and surrounding ``cylindrical shell'' sites, in ratio 1:7. The
adsorption isotherms are calculated in either the grand canonical or canonical
ensembles. At low temperature, there occur quasi-transitions that would be
genuine thermodynamic transitions in mean-field theory. Comparison between the
exact and mean-field theory results for the heat capacity and adsorption
isotherms are provided
Phonons and specific heat of linear dense phases of atoms physisorbed in the grooves of carbon nanotube bundles
The vibrational properties (phonons) of a one-dimensional periodic phase of
atoms physisorbed in the external groove of the carbon nanotube bundle are
studied. Analytical expressions for the phonon dispersion relations are
derived. The derived expressions are applied to Xe, Kr and Ar adsorbates. The
specific heat pertaining to dense phases of these adsorbates is calculated.Comment: 4 PS figure
Vibrations of a chain of Xe atoms in a groove of carbon nanotube bundle
We present a lattice dynamics study of the vibrations of a linear chain of Xe
adsorbates in groove positions of a bundle of carbon nanotubes. The
characteristic phonon frequencies are calculated and the adsorbate polarization
vectors discussed. Comparison of the present results with the ones previously
published shows that the adsorbate vibrations cannot be treated as completely
decoupled from the vibrations of carbon nanotubes and that a significant
hybridization between the adsorbate and the tube modes occurs for phonons of
large wavelengths.Comment: 3 PS figure
Rotational and Vibrational Dynamics of Interstitial Molecular Hydrogen
The calculation of the hindered roton-phonon energy levels of a hydrogen
molecule in a confining potential with different symmetries is systematized for
the case when the rotational angular momentum is a good quantum number. One
goal of this program is to interpret the energy-resolved neutron time of flight
spectrum previously obtained for HC. This spectrum gives direct
information on the energy level spectrum of H molecules confined to the
octahedral interstitial sites of solid C. We treat this problem of
coupled translational and orientational degrees of freedom a) by construction
of an effective Hamiltonian to describe the splitting of the manifold of states
characterized by a given value of and having a fixed total number of phonon
excitations, b) by numerical solutions of the coupled translation-rotation
problem on a discrete mesh of points in position space, and c) by a group
theoretical symmetry analysis. Results obtained from these three different
approaches are mutually consistent. The results of our calculations explain
several hitherto uninterpreted aspects of the experimental observations, but
show that a truly satisfactory orientational potential for the interaction of
an H molecule with a surrounding array of C atoms has not yet been
developed.Comment: 53 pages, 9 figures, to appear in Phys. Rev B (in press). Phys. Rev.
B (in press
Quantum states and specific heat of low-density He gas adsorbed within the carbon nanotube interstitial channels: Band structure effects and potential dependence
We calculate the energy-band structure of a He atom trapped within the
interstitial channel between close-packed nanotubes within a bundle and its
influence on the specific heat of the adsorbed gas. A robust prediction of our
calculations is that the contribution of the low-density adsorbed gas to the
specific heat of the nanotube material shows pronounced nonmonotonic variations
with temperature. These variations are shown to be closely related to the band
gaps in the adsorbate density of states
H2 in the interstitial channels of nanotube bundles
The equation of state of H2 adsorbed in the interstitial channels of a carbon
nanotube bundle has been calculated using the diffusion Monte Carlo method. The
possibility of a lattice dilation, induced by H2 adsorption, has been analyzed
by modeling the cohesion energy of the bundle. The influence of factors like
the interatomic potentials, the nanotube radius and the geometry of the channel
on the bundle swelling is systematically analyzed. The most critical input is
proved to be the C-H2 potential. Using the same model than in planar graphite,
which is expected to be also accurate in nanotubes, the dilation is observed to
be smaller than in previous estimations or even inexistent. H2 is highly
unidimensional near the equilibrium density, the radial degree of freedom
appearing progressively at higher densities.Comment: Accepted for publication in PR
Isotopic and spin selectivity of H_2 adsorbed in bundles of carbon nanotubes
Due to its large surface area and strongly attractive potential, a bundle of
carbon nanotubes is an ideal substrate material for gas storage. In addition,
adsorption in nanotubes can be exploited in order to separate the components of
a mixture. In this paper, we investigate the preferential adsorption of D_2
versus H_2(isotope selectivity) and of ortho versus para(spin selectivity)
molecules confined in the one-dimensional grooves and interstitial channels of
carbon nanotube bundles. We perform selectivity calculations in the low
coverage regime, neglecting interactions between adsorbate molecules. We find
substantial spin selectivity for a range of temperatures up to 100 K, and even
greater isotope selectivity for an extended range of temperatures,up to 300 K.
This isotope selectivity is consistent with recent experimental data, which
exhibit a large difference between the isosteric heats of D_2 and H_2 adsorbed
in these bundles.Comment: Paper submitted to Phys.Rev. B; 17 pages, 2 tables, 6 figure
Chirality of internal metallic and semiconducting carbon nanotubes
We have assigned the chirality of the internal tubes of double walled carbon nanotubes grown by catalytic chemical vapor deposition using the high sensitivity of the radial breathing ~RB! mode in inelastic lightscattering experiments. The deduced chirality corresponds to several semiconducting and only two metallic internal tubes. The RB modes are systematically shifted to higher energies when compared to theoretical values. The difference between experimental and theoretical energies of the RB modes of metallic tubes and semiconducting tubes are discussed in terms of the reduced interlayer distance between the internal and the external tube and electronic resonance effects. We find several pairs of RB modes corresponding to different diameters of internal and external tubes
Capillary condensation transitions in a slab geometry
A simple model, previously used to explore wetting transitions, is evaluated for the case of a slab geometry in which adsorption occurs between two semi-infinite solids, with parallel faces separated by a distance L. The model yields a universal description of possible wetting and capillary condensation (CC) transitions. The system’s thermodynamic behavior is predicted from the values of two dimensionless parameters: [Formula Presented] (the reduced gas-surface interaction strength, a function of temperature) and [Formula Presented] (the reduced separation). If [Formula Presented] negligible adsorption occurs at all pressures below saturated vapor pressure (SVP). For somewhat larger values of [Formula Presented] CC occurs for sufficiently small [Formula Presented] close to SVP. For very large values of [Formula Presented] an additional prewetting transition (formation of a film) is predicted for large [Formula Presented] this is accompanied by a CC transition close to SVP. The model is generally consistent with limited results of density-functional calculations for the He liquids at zero temperature. © 1999 The American Physical Society.Fil:Gatica, S.M. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Calbi, M.M. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina