60th Annual Report on Research 2015

This project seeks to provide a thorough characterization of the mechanical properties of gas hydrates through an atomistic-to-continuum approach. Gas hydrates are crystalline compounds in which hydrogen-bonded water molecule cages entrap gas molecules. They are important mainly because they store significant amounts of methane in the permafrost region and deep ocean sites; however, they also constitute a marine geohazard that can lead to landslides. They are also considered potential media for gas transportation due to their high energy density and relatively mild formation conditions. For these applications, the mechanical properties of gas hydrates are of paramount importance.

Significant progress has been made at the atomistic scale for the evaluation of the mechanical and associated thermophysical properties of hydrates, and work is in progress at the micro-scale.

At the atomistic scale, the ideal strength of methane hydrate has been determined, and ice Ih was considered for comparison. The ideal strength is the stress at the elastic instability of a perfect crystal which sets an upper limit for a crystals strength. The ideal strength can be approached near crack tips and is important in plastic deformation. Ab initio Density Functional Theory as implemented in SIESTA was used in which stresses were applied incrementally with a conjugate gradient relaxation algorithm.

Hydrostatic, shear, and uniaxial deformations were applied for methane hydrate, and tensile deformation was applied for ice Ih. The resulting strain vs. stress curves were generated, and structural analysis was done throughout the uniaxial deformation. It is found that the hydrate has a much higher compressive (negative) strength (where the figure only shows the lower limit for compressive strength) compared to tensile (positive). This reflects brittleness and agrees with experimental studiesIt is found that different slip systems were found equivalent with almost identical shear strengths due to the radial bond arrangement in hydrates which agrees with work on carbon clathrates . As for uniaxial deformation, It is found that that ice has higher strength compared to the hydrate due its less rigid structure. The O-O-O angle was calculated. For ice, there is a significant deviation from the ideal tetrahedral value unlike the hydrate which fails once it can no longer maintain its perfect water tetrahedral arrangement. Despite their strength difference, the hydrate and ice were found to approach almost the same hydrogen bond length near their tensile strength. This reflects the almost negligible effect of the gas molecule in the hydrate under high tensile stresses. Finally, the Poisson ratio was calculated using two different transverse directions. It is found that ice maintains its transverse isotropy under both tension and compression, but the hydrate loses it under high compressive stresses.

Furthermore, key thermos-physical properties of methane hydrate have been calculated and are in progress using ab initio molecular statics and dynamics as implemented in SIESTA. The pressure dependence of the second-order elastic constants has been determined using energy-strain analyses. The bulk modulus was found to vary linearly with pressure up to 1 GPa; this simplifies the development of equations of state for hydrates. Ab initio molecular dynamics (AIMD) simulations are in progress for the calculation of the heat capacity, compressibility, and thermal expansion coefficient as a function of temperature and pressure.

Finally, micro-scale classical molecular statics simulations have been started for the calculation of the pressure dependence of the Peierls stress in methane hydrate using LAMMPS. Edge and screw dislocations were simulated. Edge dislocations were found to dissociate under high pressures.

Gas hydrates have been heavily researched in the last few decades, and they have recently received commercial interest with Japans  plan for mass production before 2018. Thus, work on hydrates helps any researcher contribute to the booming industry of hydrates and play a role in their realization as potential energy resources and gas transportation media, among other applications. Previous work on the strength of hydrates has been experimental where the effects of different defects have been lumped together. Also, previous work on thermos-physical properties has been experimental or based on classical molecular dynamics. This work uses ab initio simulations and studies individual defects.

For the upcoming year, simulations from the past year will be finalized and new ones will be started. AIMD simulations for thermophysical properties and molecular statics simulations of screw dislocations will be finished. Work will start for the composition dependence of Peierls stress, and a dynamic model for dislocation flow will be developed using classical molecular dynamics. Also, different gas hydrate structures will be considered for comparison.