A theoretical study of surface lithium effects on the [111] SiC nanowires as anode materials.

CONTEXT: Silicon carbide nanowires (SiCNWs) are considered a promising alternative material for application in lithium-ion batteries, with researchers striving to develop new electrode materials that exhibit high capacity and high charge/discharge rate performance. To gain a deeper understanding of the application of SiCNWs in semiconductor material science and energy supply fields, we investigated the effects of nanoscale and surface lithiation on the electrical and mechanical properties of SiCNWs grown along the [111] direction. First-principles calculation was used to study their geometries, electronic structures, and associated electrochemical properties. Herein, we considered SiCNWs with full hydrogen passivation, full lithium passivation, and mixed passivation at different sizes. The formation energy indicates that the stability of SiCNWs increases with the increasing diameter, and the surface-lithiated SiC nanowires (Li-SiCNWs) are found to be energetically stable. The mixed passivated SiCNWs exhibit the properties of indirect band gap with the increase of lithium atoms on the surface, while the fully lithium passivated nanowires exhibit metallic behavior. Charge analysis shows that a portion of the electrons on the lithium atoms are transferred to the surface atoms of the nanowires and electrons prefer to cluster more near the C atoms. Additionally, Li-SiCNWs still have good mechanical resistance during the lithiation process. The stable open-circuit voltage range and theoretical capacity of these SiCNWs indicate their suitability as anode materials. METHOD: In this study, Materials Studio 8.0 was used to construct the models of the SiCNWs. And all the density functional theory (DFT) calculations were performed by the Vienna ab initio Simulation Package (VASP). The self-consistent field calculations are performed over a Monkhorst-Pack net of 1 × 1 × 6 k-points. The energy convergence criteria for the self-consistent field calculation were set to 10-5 eV/atom with a cutoff energy of 400 eV.

Effects of the temperature, strain rate, and loading conditions on the deformation behaviors and mechanical properties of the Ni/Ni3Al superalloy.

Using the molecular dynamics method, we comprehensively studied the effects of temperature, strain rate, and loading conditions on the deformation behaviors and the mechanical properties of the Ni/Ni3Al superalloy. Our investigation revealed that, an increase of the deformation temperature led to a significant improvement of plastic deformation capacity of the system, but the tensile strength and elastic modulus decreased. And the tensile strength and plastic deformation capacity of the system drastically increased with the strain rate. At high deformation temperature and strain rate, the loading conditions had a large effect on the deformation behaviors and the mechanical properties of the system. The difference of the mechanical properties of the system was mainly due to the different deformation mechanism of the system under different deformation temperature, strain rate and loading conditions. Our study offered a theoretical framework for explaining the difference of the mechanical properties for the Ni/Ni3Al superalloy at different service conditions.

Effects of crack-γ/γ' interface relative distributions on the deformation and crack growth behaviors of a nickel-based superalloy.

Using the molecular dynamics (MD) method, we investigated the effects of crack distributions on the deformation and crack growth of a nickel (Ni)-based superalloy. The results indicated that as the distance between two cracks increased, both tensile strength and plasticity decreased, while the crack growth rate significantly increased. In systems with short crack distances, strong interactions occurred between the dislocations that emitted from two cracks and the γ/γ' interface mismatched dislocation network. These interactions led to an overlap in the plastic zones ahead of the crack tips at the γ/γ' interface, which resulted in significant passivation at the front and middle regions of the cracks. Consequently, the two cracks merged in the X-direction to form a wide crack. The cracks coalescence consumed a lot of external deformation work, resulting in the highest tensile strength and plasticity. In this study, we proposed a potential approach to simultaneously enhance the strength and plasticity of multidefect systems, providing a theoretical basis for explaining deformation mechanisms and crack growth in these systems.

Theoretical studies on the electronic structures and optical properties of (Cu, C)-codoped rutile TiO2 from GGA+U calculations.

The electronic structures, formation energy and optical properties of Cu-doped, C-doped, and (Cu, C)-codoped TiO2 were investigated by the projector augmented wave (PAW) method within GGA + U approximation. The results show that the lattice distortion of the Cu@i1&C@i2 system is the largest in all doping systems. The optical absorption edges of the C@i system and the Cu@i1&C@i2 system appear a blue-shift, which is attributed to the band gap expansion and some deep states generation. The Cu@i system exhibits a reduction in band gap and a generation of the hole state, such as it emerges the highest optical absorption in all doping systems.

Effects of area, aspect ratio and orientation of rectangular nanohole on the tensile strength of defective graphene - a molecular dynamics study.

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.