Dianxi Zhang

The electronic structures and optical properties of Mn-doped, Co-doped, and (Mn,Co)-co-doped Cu₂ZnSnS₄ were calculated and analyzed using the first-principles pseudopotential plane-wave approach. The results indicate that doping with Mn or Co increases the bond population and decreases the bond length of the S-Mn and S-Co bonds, respectively, enhancing their covalent character. The undoped Cu₂ZnSnS₄ exhibits a bandgap of 0.16 eV, whereas doping with Mn or Co introduces impurity levels near the Fermi level, resulting in bandgap narrowing. Within the visible light spectrum, the static dielectric constant ε₁(0) reaches its maximum value of 67.7 under co-doping conditions, and the absorption coefficient also attains its maximum value of 6.7 × 10⁴ cm⁻¹ under co-doping. Doping with Mn and Co induces a redshift (shift towards lower energy) in both the absorption peaks and dielectric function peaks, concomitantly increasing the probability of photon-induced electronic transitions. Conversely, doping shifts the reflectivity peaks towards higher energies (blue-shift), with the most pronounced blue-shift occurring under co-doping; the strongest reflectivity peaks remain below 43%. A prominent conductivity peak is observed at 1.7 eV. Doping shifts this peak position towards lower energies, with the maximum peak intensity reaching 1.6. These findings collectively suggest that Mn and Co doping effectively modulate key optical properties of Cu₂ZnSnS₄, such as its band gap and absorption coefficient, constituting an effective strategy for enhancing its optoelectronic transport characteristics.

In this study, the phase field method was used to numerically simulate single crystal and polycrystalline silicon under the action of forced convection. Following the construction of the phase field model, the effects of parameters such as the degree of undercooling and convection velocity on the growth morphology of monocrystalline and polycrystalline silicon were simulated. The crystal nuclei of different sizes and multi-grain growth conditions of different nucleation times were obtained. The results showed that the growth rate of the upstream tip was the fastest, followed by that of the horizontal direction, and the growth rate of the lower end was the slowest under forced convection. When the undercooling degree was small, anisotropy was not obvious, and grain growth was slow. When the undercooling degree was greater than 0.65, the interface became unstable, and side branches appeared in the direction of the horizontal dendrite arm, while growth was transformed from the non-small crystal plane to the small crystal plane. Under the action of forced convection, the larger the flow velocity, the faster the crystal growth, and the faster the growth speed in the upstream direction of the grain. During the early growth stage, the overall growth rate of the larger initial nuclei was slower than that of the smaller initial nuclei, and subsequently, the difference was not significant. As the grains continued to grow, the thermal diffusion layers of the adjacent grains were in contact with each other, resulting in competitive growth.

To enhance the performance of titanium plated coating on the surface of magnesium alloy AZ31, this study investigates the influence of N ion implantation dose on the structure, mechanical properties, and friction corrosion behavior of Ti film. The results reveal that N ion implantation leads to the formation of a new physical phase TiN and induces surface softening. However, with an increase in N ion implantation dose, microhardness of the Ti film increases due to the formation of TiN which enhances its hardness. Friction and wear experiments demonstrate that at maximum implantation dose, the coating exhibits minimal friction coefficient; however, an implantation dose of 5 × 10¹⁵ ion cm⁻² offers superior wear resistance. The electrochemical test results indicate the corrosion current density and self-corrosion potential of Ti coating decrease with the increase of implantation dose due to the formation of nitride and the presence of N element, and corrosion resistance of the modified coating has been significantly enhanced. The research results provide reference for improving the protection performance of Ti coating on magnesium alloy surface.

This study evaluated the electronic structure and optical properties of Fe-doped, Ni-doped, and (Fe,Ni)-co-doped Cu₂ZnSnS₄ through the first-principles pseudopotential plane-wave method based on density functional theory. The results indicated that Fe single-doping and Ni single-doping Cu₂ZnSnS₄ can reduce the charge transfer number of adjacent S atoms, enhancing covalent bonding in Fe–S and Ni–S bonds and reducing the bond length, lattice constants a and c, and unit cell volume v. The formation energies for Fe-doping, Ni-doping, and (Fe,Ni)-co-doping were 1.0 eV, 0.58 eV, and 0.78 eV, respectively. Both Fe and Ni-doping introduced 3d electrons near the Fermi level, resulting in new impurity levels and a gradual decrease in the band gap of Cu₂ZnSnS₄ from 0.16 eV. The conduction band density of Cu₂ZnSnS₄ was primarilycontributed by Sn 5s, Sn 5p, and a portion of S 3p orbital electrons, whereas the valence band density mainly stemmed from Cu 3d, Sn 5p, and S 3p orbital electrons. Fe and Ni-doping also partly contributed to the 3d layer electrons. In the case of (Fe,Ni)-co-doping, the maximum static dielectric constant was 100.49, and the dielectric peak shifted toward the low-energy direction in the presence of both Fe and Ni. Within the visible light range, Fe-doping, Ni-doping, and (Fe,Ni)-co-doping in Cu₂ZnSnS₄ exhibited absorption coefficients greater than 10⁴ cm⁻¹, with the maximum absorption coefficient being 1.6 × 10⁵ cm⁻¹ in the case of (Fe,Ni)-co-doping. In the energy range from 1.5 to 6.3 eV, the reflectivity of Cu₂ZnSnS₄ doped with Fe, Ni, or both was lower than 30%. Notably, a high conductivity peak at 1.9 eV indicated that Cu₂ZnSnS₄ possesses good photoconductivity in the visible range. Fe-doping and Ni-doping resulted in a slight shift of the conductance peak position towardthe low-energy direction, accompanied by an increase in the peak value.

By using the pseudopotential plane-wave method of first principles based on density functional theory, the band structure, density of states and optical properties of Cu₂ZnSnS₄ under isotropic stress are calculated and analyzed. The results show that Cu₂ZnSnS₄ is a direct band gap semiconductor under isotropic stress, the lattice is tetragonal, and the band gap of Cu₂ZnSnS₄ is 0.16 eV at 0 GPa. Stretching the lattice causes the bottom of the conduction band of Cu₂ZnSnS₄ to move toward lower energies, while the top of the valence band remains unchanged and the band gap gradually narrows. Squeezing the lattice causes the bottom of the conduction band to move toward the high-energy direction, while the top of the valence band moves downward toward the low-energy direction, and the Cu₂ZnSnS₄ band gap becomes larger. The static permittivity, absorption coefficient, reflectivity, refractive index, electrical conductivity, and energy loss function all decrease when the lattice is stretched, and the above optical parameters increase when the lattice is compressed. When the lattice is stretched, the optical characteristic peaks such as the dielectric function shift to the lower-energy direction, while the optical characteristic peak position shifts to the higher-energy direction when the lattice is compressed.

The use of the magnesium alloy AZ31 is common in aviation and biomedicine; however, this alloy has poor friction and corrosion resistance. Here, mechanical grinding, ultrasonic rolling, and ultrasonic rolling + ion implantation were performed on the magnesium alloy surface to study the effect of the treatment process on the friction and corrosion resistance of the magnesium alloy surface. The results show that the surface roughness of the magnesium alloy treated by ultrasonic rolling + ion injection is reduced more than mechanical grinding and ultrasonic rolling. The friction coefficient is the lowest, the wear resistance is the best, and new phase nitrogen compounds appear on the surface. The results of SBF (simulated body fluid) solution immersion showed that the sample treated via this composite process had the lowest corrosion rate, which was 62.45% and 58.47% lower than that of the mechanically ground samples. The surface was relatively intact after the corrosion test, and the corrosion resistance was the best. These results can provide a new strategy for magnesium alloy surface protection.

To further improve the performance of the coated tools, we investigated the effects of low-energy nitrogen ion implantation on surface structure and wear resistance for TiC coatings deposited by ion plating. In this experiment, an implantation energy of 40 keV and a dose of 2 × 10¹⁷ to 1 × 10¹⁸ (ions/cm²) were used to implant N ions into the TiC coatings. The results indicate that the surface roughness of the coating increases first and then decreases with the increase of ion implantation dose. After ion implantation, the surface of the coating will soften and reduce the hardness, and the production of TiN phase will gradually increase the hardness. Nitrogen ion implantation can reduce the friction coefficient of the TiC coating and improve the friction performance. In terms of wear resistance, the coating with an implant dose of 1×10¹⁸ ions/cm² has the greatest improvement in wear resistance. Tribological analysis shows that the improvement in the performance of TiC coatings implanted with N ions is mainly due to the effect of the lubricating implanted layer. The implanted layer mainly exists in the form of amorphous TiC, TiN phase, and sp²C–C phase.

This paper briefly reviews the history of low pressure casting and summarizes the major production processes of low pressure casting. It briefly introduces the quality management and control of low pressure cast aluminum alloy. The main processes include are: preparation of raw materials, Melting, refining, physical and chemical analysis, K-mode inspection, sand core, mold, heat treatment and so on.