Molecular Origins of Deformation in Amorphous Methane Hydrates

Abstract

Water and methane can stay together under low temperature and high pressure in the forms of liquid solutions and crystalline solids. From liquid and gaseous states to crystalline solids or the contrary processes, amorphous methane hydrates can occur in these evolution scenarios. Herein, mechanical properties of amorphous methane hydrates are explored for the first time to bridge the gap between mechanical responses of monocrystalline and polycrystalline methane hydrates. Our results demonstrate that mechanical properties of amorphous methane hydrates are strongly governed by our original proposed order parameter, namely, normalized hydrogen-bond directional order parameter. Followed by this important achievement, a multistep deformation mechanism core is proposed to explain mechanical properties of amorphous methane hydrates. Through an extensive detailed analysis of amorphous methane hydrates, our simulation results not only greatly enlarge our fundamental understanding for mechanical responses of amorphous methane hydrates in geological systems but also offer a fresh perspective in structure–property topics of solid materials in future science and technology.

Mechanical properties of amorphous CO2 hydrates: insights from molecular simulations

“…24,43 To understand structures of amorphous and crystalline hydrates, as well as aqueous solutions, the radial distribution function as a useful tool plays an important role. 19,47,48 By comparison, it is found that the first peak of g O–O ( r ) with a value of about 2.80 Å in amorphous CO 2 hydrates is larger than that in monocrystalline CO 2 hydrates. 48 However, the second peaks of g O–O ( r ) in both amorphous and monocrystalline CO 2 hydrates are around 4.5 Å.…”

“…In this work, the normalized hydrogen-bond directional order parameter (N-Hbond-DOP) 19 was used to understand structural alterations. It has been successfully applied to characterize structural alterations of amorphous methane hydrates under mechanical loads.…”

“…It has been successfully applied to characterize structural alterations of amorphous methane hydrates under mechanical loads. 19 Moreover, a global structural order parameter for CO 2 molecules in amorphous CO 2 hydrates was devolved in this work. Here, the global structural order parameter was called the normalized CO 2 directional order parameter (N-CO 2 -DOP).…”

“…Notedly, a multistep deformation mechanism core for amorphous methane hydrate systems has been developed, and it can well rationalize their mechanical properties. 19 Moreover, thermal and dielectric measurements revealed that the stability of amorphous hydrates at 1 GPa and 160 K is stronger than that on pressurization to 1.3 GPa below 135 K. 20 Interestingly, X-ray and Raman measurements have experimentally shown that tetrahydrofuran hydrates can transform into an amorphous state on pressurization of 1.3 GPa at 77 K, and their stability can be enhanced by heating to 150 K at 1.5 GPa. 21 The thermal stability of amorphous hydrates can be increased by removing crystalline remnants.…”

Electric Field-Controlled Structural Instability and Mechanical Properties of Methane Hydrates

“…Clearly, the first and second compressive deformation stages in Figure correspond to distortions of polyhedral water cages in methane hydrates (Figure a–c,e,f,i,j) under weak electric fields. This compression-induced distortion of polyhedral water cages in molecular structures differs from the tension-induced ones, resulting from the difference in the distinct relative angles of tetrahedral hydrogen bonds making to the loading direction. , Such molecular structural changes can well rationalize the asymmetry of tension–compression in mechanical properties of methane hydrates. At high compressive strains as shown in Figure c,d,g,h,k,l, once localized polyhedral water cages collapse, strain-induced collapses of polyhedral water cages propagate into other areas of methane hydrates, thereby resulting in the dissociation of methane hydrates.…”