POLYCRYSTALLINE METHANE HYDRATE - SYNTHESIS FROM SUPERHEATED ICE, ANDLOW-TEMPERATURE MECHANICAL-PROPERTIES

We describe a new and efficient technique to grow aggregates of pure m ethane hydrate in quantities suitable for physical and material proper ties testing. Test specimens were grown under static conditions by com bining cold, pressurized CH4 gas with granulated H2O ice, and then war ming the reactants to promote the reaction CH4(g) + 6H(2)O(s --> 1) -- > CH4 . 6H(2)O (methane hydrate). Hydrate formation evidently occurs a t the nascent ice/liquid water interface on ice grain surfaces, and co mplete reaction was achieved by warming the system above the ice melti ng point and up to 290 K, at 25-30 MPa, for approximately 8 h. The res ulting material is pure, cohesive, polycrystalline methane hydrate wit h controlled grain size and random orientation. Synthesis conditions p laced the H2O ice well above its melting temperature while reaction pr ogressed, yet samples and run records showed no evidence for bulk melt ing of the unreacted portions of ice grains. Control experiments using Ne, a non-hydrate-forming gas, showed that under otherwise identical conditions, the pressure reduction and latent heat associated with ice melting are easily detectable in our fabrication apparatus. These res ults suggest that under hydrate-forming conditions, H2O ice can persis t metastably to temperatures well above its ordinary melting point whi le reacting to form hydrate. Direct observations of the hydrate growth process in a small, high-pressure optical cell verified these conclus ions and revealed additional details of the hydrate growth process. Me thane hydrate samples were then tested in constant-strain-rate deforma tion experiments at T = 140-200 K, P-c = 50-100 MPa, and (epsilon) ove r dot 10(-4)-10(-6) s(-1). Measurements in both the brittle and ductil e fields showed that methane hydrate has measurably different strength than H2O ice, and work hardens to an unusually high degree compared t o other ices as well as to most metals and ceramics at high homologous temperatures. This work hardening may be related to a changing stoich iometry under pressure during plastic deformation; X-ray analyses show ed that methane hydrate undergoes a process of solid-state disproporti onation or exsolution during deformation at conditions well within its conventional stability field.