Now showing 1 - 2 of 2
  • Publication
    Dynamical cage behaviour and hydrogen migration in hydrogen and hydrogen-tetrahydrofuran clathrate hydrates
    (American Institute of Physics, 2012-01-24) ; ;
    Classical equilibrium molecular dynamics(MD) simulations have been performed to investigate dynamical properties of cage radial breathing modes and intra- and inter-cage hydrogen migration in both pure hydrogen and mixed hydrogen-tetrahydrofuran sII hydrates at 0.05 kbar and up to 250K. For the mixed H2-THF system in which there is single H2 occupation of the small cage (labelled ‘1SC 1LC’), we find that no H2 migration occurs, and this is also the case for pure H2 hydrate with single small-cavity occupation and quadruple occupancy for large cages (dubbed ‘1SC 4LC’). However, for the more densely-filled H2-THF and pure- H2 systems, in which there is double H2 occupation in the small cage (dubbed ‘2SC 1LC’ and ‘2SC 4LC’, respectively), there is an onset of inter-cage H2 migration events from the small cages to neighbouring cavities at around 200 K, with an approximate Arrhenius temperature-dependence for the migration rate from 200 to 250 K. It was found that these ‘cage hopping’ events are facilitated by temporary openings of pentagonal small-cage faces with the relaxation and reformation of key stabilising hydrogen bonds during and following passage. The cages remain essentially intact up to 250 K, save for transient hydrogen bond weakening and reformation during and after inter-cage hydrogen diffusion events in the 200 to 250 K range. The ‘breathing modes’, or underlying frequencies governing the variation in the cavities’ radii, exhibit a certain overlap with THF rattling motion in the case of large cavities, while a there is some overlap of small cages’ radial breathing modes with lattice acoustic modes.
      529Scopus© Citations 31
  • Publication
    Mechanisms for thermal conduction in hydrogen hydrate
    (American Institute of Physics, 2012-01-23) ; ;
    Extensive equilibrium molecular dynamics (MD) simulations have been performed to investigate thermal conduction mechanisms via the Green-Kubo approach for (type II) hydrogen hydrate, at 0.05 kbar and between 30 and 250 K, for both lightly-filled H2 hydrates (1s4l) and for more densely-filled H2 systems (2s4l), in which four H2 molecules are present in the large cavities, with respective single- and double-occupation of the small cages. The TIP4P water model was used in conjunction with a fully atomistic hydrogen potential along with long-range Ewald electrostatics. It was found that substantially less damping in guest-host energy transfer is present in hydrogen hydrate as is observed in common type I clathrates (e.g., methane hydrate), but more akin in to previous results for type II and H methane hydrate polymorphs. This gives rise to larger thermal conductivities relative to common type I hydrates, and also larger than type II and H methane hydrate polymorphs, and a more crystal-like temperature dependence of the thermal conductivity.
      655Scopus© Citations 23