Molecular dynamics simulations with adaptive intermolecular reactive empirical bond order (AIREBO) potential are performed to investigate the effects of rectangular nanoholes with different areas, aspect ratios (length/width ratios) and orientations on the tensile strength of defective graphene. The simulations reveal that variation of area, aspect ratio and orientation of rectangular nanohole can significantly affect the tensile strength of defective graphene. For example, defective graphene with a larger area of rectangular nanohole shows a bigger drop in tensile strength. It was found that the tensile strength of both armchair and zigzag edged graphene monotonically decreases with area increases in rectangular nanohole. Changes in aspect ratio and orientation of rectangular nanohole, however, can either decrease or increase the tensile strength of defective graphene, dependent on the tensile direction. This study also presents information that the tensile strength of defective graphene with large area of nanohole is more sensitive to changes in aspect ratio and orientation than is defective graphene with small area of nanohole. Interestingly, variation of tensile strength of defective graphene from MD simulations is in good agreement with predictions from energy-based quantized fracture mechanics (QFM). The present results suggest that the effect of nanoholes on the tensile strength of graphene provides essential information for predictive optimization of mechanical properties and controllable structural modification of graphene through defect engineering.
Aqueous zinc‐ion batteries (AZIBs) have recently shown promise as prospective energy storage systems owing to their eco‐friendliness, low price, high security, and reversible storage. However, low specific capacity and the lack of cathode materials with a long life severely restrict the development of AZIBs. Herein, nanoflower‐like vanadium oxide nanoribbons are rapidly synthesized using a microwave‐assisted solvothermal method, and the layer spacing of synthesized nanoribbons is subsequently expanded with ammonium via high‐temperature calcination in an NH3 atmosphere. The increased layer spacing of vanadium oxide facilitates the intercalation/extraction of Zn2+ and accelerates the electrochemical kinetics, resulting in a highly reversible pseudocapacitive contribution (71% at a scan rate of 1 mV s−1). The ammonium‐embedded V2O5 with the carbon‐coated NH4V4O10/C (NHVO/C) cathode shows a high specific capacity (458.6 mAh g−1 at 0.1 A g−1) and excellent cycling stability (about 90% capacity retention after 2800 cycles at 10 A g−1), which is superior to most cathode materials for AZIBs. Furthermore, reversible Zn2+ intercalation/extraction in the NHVO/C cathode during electrochemical reactions is elucidated through in situ and ex situ material characterization. The findings are expected to inspire the development of advanced vanadium‐based cathode materials for green, safe, and dependable energy storage devices for commercial applications.
Understanding the structural evolution of covalent systems under rapid cooling is very important to establish a comprehensive solidification theory. Herein, we conducted molecular dynamics simulations to investigate the crystallization of silicon-germanium (SiGe) alloys. It was found that during crystallization, the saturation and orientation of covalent bonds are satisfied in order, resulting in three phase transitions. The saturation is satisfied during a continuous phase transition that occurs in the super-cooled liquid state. When the orientation was satisfied at the local scale, a novel state, the critical-nuclei crystalline (CNC) phase was obtained, where the local diamond structures increase in number with time and ultimately stabilize at an average size at the critical value. Finally with a coordinated rearrangement of atoms, the orientation is satisfied globally and a stable diamond crystal is produced. For SiGe alloys this CNC phase is universal and rather stable, and the stable temperature range has a certain relationship with the cooling rate and number fraction of atoms. This novel pathway is believed to be universal for such materials including carbon. The CNC state can explain the observation that diamond can be obtained without high pressure. These findings will significantly advance the understanding of the mechanism of phase transition, particularly for covalently bonded materials.
High-entropy alloys (HEAs) consist of five or more metallic elements in equal or near-atomic proportions to form multicomponent alloys with high configurational entropy. Recently, nanostructured HEAs have attracted considerable attention from both academia and industry for their extraordinary properties. Nucleation during solidification directly affects the properties of metals and alloys. Although experimental techniques to study the microstructure and nucleation growth during metal solidification continue to make remarkable developments, many unanswered questions remain in this field. Molecular dynamics (MD) simulation is an effective tool to describe the nucleation mechanism and microstructural evolution of HEAs during solidification processes. In this paper, we explore the atomic origins of the homogeneous and heterogeneous nucleation in the FeNiCrCoCu HEA using classical MD simulations. The results show an obvious difference between homogeneous and heterogeneous nucleation. A new growth pattern of crystals in HEA was discovered during the heterogeneous nucleation process. The mechanisms of heterogeneous and homogeneous nucleation and their control factors are revealed through the evolution of several crystalline structures and dislocation density.
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