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A.    Formation of Methane Hydrates from Super-compressed Water and Methane Mixtures

      The goal of this study is to understand the compression rate dependence on the formation of methane hydrates under pressures, using d-DAC. The time-resolved optical images and Raman data indicate the pressure-induced formation of methane hydrate depends on the compression rate and peak pressure.

      In the high-pressure experiment (2.2 GPa in the stability field of ice-VII and MH-III), water and methane rapidly solidify into ice-VII and CH₄-I upon compression, instead of forming methane hydrates. On the other hand, MH-I forms upon the pressure release below 1.0 GPa – the melting point of ice-VI at ambient temperature. This result indicates that methane hydrate forms only from water – not from ice, probably due to slow diffusion rate of methane gas through ice. 

      In the intermediate-pressure experiments (1.5 and 1.6 GPa in the stability field of ice-VII and MH-III), we observed the formation of MH-II during compression to 1.5 and 1.6 GPa.  Note that in this pressure range, the rapidly compressed water at the rate of 0.8-1.6 GPa/s is super-compressed, without solidification, to well within the stability field of ice-VI. Therefore, it is likely that MH-II is formed from super-compressed water (not from ice-VI).

      The fact that MH-II phase forms from super-compressed water underscores the diffusion controlled process.  In this regard, it is interesting to note methane hydrate forms at intermediate pressures - not at low pressure (0.6 GPa in the stability field of MH-I). This is likely due to the fact that methane becomes more miscible with water as pressure increases, resulting in a faster dynamic of the formation of MH-II than MH-I. Furthermore, within the time scale we studied (10 GPa/s) we have not observed the formation of MH-III in its stability field. This is due to the fact that solidification of super-compressed water occurs much faster than formation of hydrate above 2 GPa. These experimental observations are consistent with a diffusion controlled growth mechanism for formation of methane hydrate under pressure, as well as our results showing fast dynamics of water solidification as described below in section C.

B.     Solid-Solid Phase Transitions of Methane Hydrates

      The goal of this study is to determine transition dynamics of solid-solid phase transitions in methane hydrates. In this study, we have focused on the transitions among three different methane hydrate phases (MH-I, II and III) using d-DAC. The subtle spectral and pressure changes across the MH-I-to-II transition indicate that transition is associated with how methane diffuses into water cages and alter the population of two different water cages. The large pressure drop associated with MH-II-to-III transition indicates that it is a reconstructive transition involving a fundamental structural change of water cages. These results are in excellent agreement with the previous x-ray data, showing a large volume collapse across the MH-II-to-III transition, but nearly no volume change across the MH-I-to-II transition.

      In this study, we have also determined kinetics of MH-I to MH-II transition at five different compression rates. Results show that, at low compression rates, transition pressure agrees with static transition pressure, as expected. At relative higher compression rates, kinetic phase boundary moves remarkably to higher pressure. Furthermore, conversion rates diminish dramatically with increasing compression rates. 

      The large pressure drop observed across the MH-II-to-III transition indicates a big structure change. The transition pressure is defined at peak pressure, and conversion rate is assumed to correlate with the rate of pressure drop. Results show that both transition pressure and conversion rate remarkably increase with increasing compression rates. The observation of a large pressure drop is confirmed with a large volume collapse in the x-ray study, indicating that MH-II-to-III transition is related to reconstruction of water cages.

C.    High Density Amorphous Discovered at Stability Field of Ice VI at Ambient Temperature

      The goal of this study is to understand stability and metastability of ice phases relevant to formation of methane hydrates between 0.1 and 3.0 GPa, at ambient temperature.  While relevant stable phases in this pressure range are water, ice VI, and ice VII, our results signify the presence of metastable ice-VII, which appear in the stability domain of ice VI, and of high density water (HDA), observed for the first time at ambient temperature in this study.  The study illustrated transition of water to “needle-like” ice VII and then to an amorphous ice whose Raman spectrum is the same with that of HDA previously observed at low temperatures. Furthermore, time-resolved pressure data indicate that water is super-compressed well into the stability field of ice VI (observed nearly up to 2.0 GPa) and then transforms into metastable ice-VII (not ice VI), as observed previously. The presence of ice-VII is confirmed by its needle-like morphology (contrast to a granular shape of ice VI) and characteristic Raman spectrum. The transition of super-compressed water to ice VII is due to smaller interfacial energy between ice VII and super-compressed water.

      The amorphorization process of ice (i.e., the ice VII to HDA transition in A) is analogous to melting/relaxation process (i.e., the ice VI to water transition in B). This finding is remarkably similar to the fact that previously observed HDA at low temperatures are at the vicinity of the extrapolated melting line of ice Ih to 77 K and 1 GPa. In this regard, the present study offers the insight into how the supercompressed water (or high density water, HDW) transforms into HDA, via metastable ice VII whose structure is rather similar to that of HDW.  The significance of the present study is the first observed HDA at ambient temperature.