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Clathrate hydrates show wide applications in energy recovery and storage, CO2 capture and storage, and other sustainable technologies. Water vacancy in clathrate hydrates is a common defect; however, its effects on the mechanical properties of clathrate hydrates, especially CO2 hydrates, have not been well studied. Herein, the mechanical characteristics of CO2 hydrates with three different types of water vacancy defects are investigated for the first time through molecular dynamics simulations with several popular water force fields. It turns out that the mechanical properties of CO2 clathrate hydrate vary with the type of water vacancy and water force field. Upon critical strains, a variety of unconventional cages of 425862, 425863, 425864, 4151062, 4151063, and 4151064 form, of which 4151062 predominates and is identified to be transient and a clathrate intermediate in forming 425862, 425863, and 425864. Moreover, diverse cage transformations of 51262 ↔ 4151063, ↔ 4151062, ↔ 425863, ↔ 425864 and 512 ↔ 4151062, ↔ 425862 occur via two distinct transformation mechanisms including insertion/removal and rotation of a pair of water molecules. This study provides new perspectives on the mechanics and microstructural transformations of CO2 hydrate, which are crucial for evaluating the formation and mechanical stability of CO2 hydrate-bearing sediments as well as the CO2 geological storage by hydrate-based technologies.

Resourceful beyond-graphene two-dimensional (2D) carbon crystals have been proposed/synthesized; however, the fundamental knowledge of their melting thermodynamics remains lacking. Here, the structural and thermodynamic properties of nine contemporary 2D carbon crystals upon heating are investigated using first-principle-based ReaxFF molecular dynamics simulations. Those 2D carbon crystals show distinct evolution of energetic and Lindemann index that distinguish their thermal stabilities. There are two or three critical temperatures at which structural transformation occurs for non-hexagon-contained 2D carbon allotropes. Analysis of polygons reveals that non-hexagon-contained 2D carbon crystals show thermally induced hex-graphene transitions via mechanisms such as bond rotations, dissociation, and reformation of bonds. The study provides new insights into the thermodynamics and pyrolysis chemistry of 2D carbon materials, as well as structural transitions, which is of great importance in the synthesis and application of 2D materials in high-temperature processing and environment.

The melting thermodynamic characteristics of 2- to 20-layered onion-like fullerenes (OLFn) (C60@C240 to C60@···@C6000···@C24000) are comprehensively explored using first-principles-based ReaxFF atomistic simulations and random forest machine learning (RF ML). It is revealed that OLFn shows lower thermal stability than the counterparts of single-walled fullerenes (SWFn). The melting point of SWFn increases monotonically with increasing size, whereas for OLFn, an unusual size-dependent melting point is observed; OLFn with intermediate size shows the highest melting point. For small OLFn, the melting occurs from the inner to the outer, whereas for large OLFn, it nucleates from the inner to the outer and to intermediate fullerenes. The melting and erosion behaviors of both SWFn and OLFn are mainly characterized by the nucleation of non-hexagons, nanovoids, carbon chains and emission of C2. RF ML model is developed to predict the melting points of both SWFn and OLFn. Moreover, the analysis of the feature importance reveals that the Stone-Wales transformation is a critical pathway in the melting of SWFn and OLFn. This study provides new insights and perspectives into the thermodynamics and pyrolysis chemistry of fullerenic carbons, and also may shed some lights onto the understanding of thermally-induced erosion of carbon-based resources and spacecraft materials.