Min-Rong An

As a new type of energy storage device, supercapacitors have the advantages of high-power densities, high safety factors, and low maintenance costs, so they have attracted widespread attention among researchers. However, a major problem with supercapacitors is that their energy densities are not high enough, which limits their application. Therefore, it is crucial to expand the application scenarios of supercapacitors to increase their energy density as much as possible without diminishing their advantages. The classification and working principles of supercapacitors are introduced in this paper. The electrochemical properties of pure NiS materials, NiS composites with carbon materials, NiS composites with sulfide materials, and NiS composites with transition metal oxides for supercapacitors are summarized. This paper may assist in the design of new electrode materials for NiS-based supercapacitors.

High-entropy alloys (HEAs) break the design concept of traditional alloys and exhibit excellent mechanical properties. However, as a new member of the alloy family in recent years, the dependence of the deformation behavior of the HEAs on alloy composition and twin boundary (TB) is still unclear, and many phenomena urgently need to be revealed. Here, the effects of TB spacing and Ni concentration on the mechanical properties and deformation behavior of the nano-twinned (CoCrFeCu)_1−XNi_X HEA (nt-HEA) under tensile loading are investigated by molecular dynamics simulation. The results show that with the decrease in TB spacing, the average flow stress of the nt-HEA changes from Hall–Petch strengthening to inverse Hall–Petch softening. When the TB spacing is greater than a critical value, the plastic deformation mechanism is dominated by the slip of partial dislocations. However, when the TB spacing is less than the critical value, the plastic deformation mechanism is transformed into the formation of voids induced by the amorphous phase, which becomes the key factor for the softening of the nt-HEA. It is also found that the mechanical properties of the nt-HEA can also change from strengthening to softening by adjusting Ni concentration, which is closely related to the change of stacking fault energy of the nt-HEA. In addition, the plastic deformation mechanism and voids formation mechanism of the nt-HEA are also discussed in detail.

Magnesium alloy is regarded as a lightest engineering structural metal material due to its low density, but its wide application is limited due to poor plastic deformation behavior. Therefore, the comprehensive mechanical properties of enhanced magnesium alloy have become a research focus in the material science. Here, the effect of graphene on the deformation behavior and that on the mechanical properties of magnesium under tensile loading are studied by molecular dynamics simulation. The results show that the introduction of graphene can significantly improve the mechanical properties of pure magnesium. Comparing with pure magnesium, the Young's modulus and the first peak stress of the graphene magnesium matrix (GR/Mg) composites are increased by about 27.5% and 36.5% respectively, which is mainly due to the excellent mechanical properties of graphene. The results also indicate that the embedded position of graphene has little effect on the Young's modulus or peak stress of the GR/Mg composites, but it will significantly affect the plastic deformation behavior of the GR/Mg composites after the second peak stress. With the increase of the embedded height of graphene, the average flow stress of the GR/Mg composites first increases in the later stage of plastic deformation. When the embedded height reaches 0.4L, the average flow stress of the GR/Mg composites reaches a maximum value, and then decreases. This phenomenon of the Gr/Mg composites can be explained by the plastic deformation behavior of the magnesium matrix above and below graphene. The embedded position of graphene has a great influence on the plastic deformation behavior of the upper and lower magnesium matrix of the GR/Mg composites. When the embedded height of graphene is small, the plastic deformation capability of magnesium matrix under graphene is strong and dislocation slip is easy to occur. And when the embedded height of graphene is large, the plastic deformation capabilities of the two parts of magnesium matrix above and below graphene are equal, and their plastic deformation behavior tends to be synchronous. The results show that the plastic deformation behavior of the GR/Mg composite is the same as that of pure magnesium, and the phase transition from HCP to BCC and then to HCP occurs in the process of the plastic deformation. The phase transition mechanism of magnesium matrix is also analyzed in detail. The results of this study have certain theoretical guiding significance in designing the high performance graphene metal matrix composites.

As the lightest metal structural material, magnesium alloy is known as the “green engineering material” of the 21st century. Especially, crystalline-amorphous dual-phase nanostructure magnesium materials exhibit excellent mechanical properties, though the mechanism of interaction between the dislocation in crystal and amorphous phase is still under the investigation. In the present work, the interaction between the edge dislocation and amorphous phase in nanocrystalline magnesium under shear load is studied by using molecular dynamics simulation. The result indicates that the interaction mechanism between amorphous phase and dislocation shows the size dependence. Compared with the sample with smaller amorphous size, larger amorphous size will lead to a large second strengthening effect. And the mechanism of the interaction between amorphous phase and dislocation is mainly attributed to the pinning effect of amorphous on the dislocation. For the samples with small amorphous size, the pinning effect of amorphous on the dislocation is limited and the pinning time is shorter. The interaction mechanism is contributed mainly by the dislocation bypassing amorphous phase. While for the samples with larger amorphous size, the pinning effect of amorphous on the dislocation is larger and the pinning time is longer. The interaction is due mainly to the cross slip mechanism of dislocation caused by amorphous phase. The results from this work have a certain reference value and guiding significance for designing and preparing the high-performance magnesium and its alloys.

Dual-phase nanoscale lamellar microstructures containing alternating regions of soft phase and hard phase are known to produce alloys with an exceptional combination of strength and plasticity. Here, the effect of layer thickness on the mechanical properties and deformation mechanism of the fcc/bcc dual-phase CoNiFeAl_xCu_1−x high-entropy alloys multilayers are investigated by the molecular dynamics simulation method. The results show that the deformation behavior of the multilayers is strongly related to the layer thickness. At the yield point, the deformation behavior of the small thickness multilayer is caused by the dislocation slip in the fcc phase, while the plastic deformation of the large thickness multilayer is initiated by the bcc → hcp phase transformation in the bcc phase. During the subsequent plastic deformation, the phase transformation of bcc → fcc also occurred in the bcc phase of the multilayer, which depended on the relative size of the bcc phase in the multilayer. Especially for the multilayer with the layer thickness of 2 nm, the bcc → fcc phase transformation promotes the formation of twins, and the fcc/bcc phase interface transforms into a perfect twin boundary. The twin formation mechanism and phase transformation mechanism are also discussed in detail.