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Abstract
Evaluating gas hydrates properties contributes valuably to their large-scale management and utilization in fundamental science and applications. Noteworthy, structure-H (sH) gas hydrate lacks a comprehensive characterization of its structural, mechanical, and anisotropic properties. Anisotropic and pressure dependent properties are crucial for gas hydrates’ detection and recovery studies. The objective of this work is the determination of pressure-dependent elastic constants and mechanical properties and the direction-dependent moduli of sH gas hydrates as a function of guest composition. First-principles DFT computations are used to evaluate the mechanical properties, anisotropy, and angular moduli of different sH gas hydrates under pressure. Some elastic constants and moduli increase more significantly with pressure than others. This introduces variations in sH gas hydrate’s incompressibility, elastic and shear resistance, and moduli anisotropy. Young’s modulus of sH gas hydrate is more anisotropic than its shear modulus. The anisotropy of sH gas hydrates is characterized using the unit cell elastic constants, anisotropy factors, and the angular dependent moduli. Structure-properties composition correlations are established as a function of pressure. It is found that compressing filled sH gas hydrates increases their moduli anisotropy. Differences in atomic bonding across a crystal’s planes can be expected in anisotropic structures. Taken together the DFT-based structure–properties–composition relations for sH gas hydrates provide novel and significant material physics results for technological applications.
Highlights
Structure-H gas hydrate is one of the main types of gas hydrates known by its hexagonal crystal symmetry that distinguishes it from the cubic sI and sII gas hydrates [1]
The lower hexagonal crystal symmetry of sH as compared with sI and sII gas hydrates is associated with significant anisotropies [3]
Characterizing the physical properties of sH gas hydrate adds to the fundamental understanding of the material physics of these guest-host compounds
Summary
Structure-H (sH) gas hydrate is one of the main types of gas hydrates known by its hexagonal crystal symmetry that distinguishes it from the cubic sI and sII gas hydrates [1]. The lower hexagonal crystal symmetry of sH as compared with sI and sII gas hydrates is associated with significant anisotropies [3] This guesthost hydrate encapsulates large molecules; it requires small guest (help gas) to stabilize its medium and small cages [4]. Determining the physical properties of gas hydrates contributes to the overall astrochemistry sciences and adds to the gas hydrates’ knowledge database with useful information that can serve different engineering and environmental applications. It improves the potential technological use of gas hydrate as a good source of energy, and as a potential geological and environmental challenge [5]. Density functional theory (DFT) computations provide access to a variety of a material’s elastic characteristics which compensates for the limitations of experimental procedures
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