Chen Zhiqian

Intermetallic phase formation and evolution during homogenization and solution in Al-Zn-Mg-Cu alloys

The effects of major alloy element contents of Zn, Mg, Cu in Al-Zn-Mg-Cu alloys on the formation and evolution of intermetallic phases during casting, homogenization and solution treatment have been investigated through using X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. Experimental results showed that a relatively higher Zn content with lower Mg and Cu contents was beneficial to the formation of MgZn2 phase instead of the Al2CuMg phase, which resulted in the unicity of the intermetallics in the Al matrix, and that the MgZn2 phase was easier for diffusion and dissolution during homogenization and solution than the Al2CuMg phase. Additionally, the results of the first-principles calculations gave support for explaining the experimental phenomena. A larger absolute value of formation enthalpy and a smaller value of binding energy of the MgZn2 phase, as compared with the Al2CuMg phase, give it priority to precipitate during casting and make it easier to re-dissolve during homogenization and solution treatment. What’s more, higher elastic constants with severe anisotropy of Young’s modulus make undissolved blocks of Al2CuMg phase act as crack initiation, which degrade the performance of the materials.

High-strength wrought magnesium alloy with dense nano-scale spherical precipitate

This paper reported the influences of Yb addition on the precipitate and mechanical properties of wrought magnesium alloy ZK60. The ingots of ZK60-1.78Yb (wt%, 0.26 at%) alloys were cast using permanent mould and extruded at 370°C. By means of TEM and HRTEM, it was observed that Yb affected the precipitate and precipitation of ZK60-1.78Yb alloys significantly. Dynamic precipitation occurred in the as-extruded alloy and spherical nano-scale precipitate with high density and homogeneity exhibited in the aged alloys. The precipitate particles were about 5–20 nm in diameter, 10–30 nm in average space length. The tensile test results showed that the ZK60-1.78Yb alloy had excellent precipitation strengthening response with the maximum tensile strength 417.5 MPa at ambient temperature.

Investigation of Elastic Properties, Hardness and Thermal Conductivity of New Superhard Material z-BC

采用第一性原理计算研究了超硬材料z-BC 2 N的弹性各向异性性质、应力-应变关系、硬度及最小热导率性质。计算得到的晶体力学行为判据B/G为0.87, 泊松比为0.084, 普适弹性各向异性指数为0.09992。[100]晶向上最大拉伸强度达到180 GPa, 应变方向上最大剪切强度达到160 GPa, 维氏硬度值为77.07 GPa。基于Cahill模型得到的最小热导率为6.811 W/(m·K)。结果表明: z-BC 2 N是脆性材料且力学稳定性良好, 有非常高的拉伸强度、剪切强度, 体弹模量为各向同性, 杨氏模量各向异性程度不大。z-BC 2 N的最小热导率低于金刚石的最小热导率。

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Black phosphorus (α-P), a new member of two-dimensional (2D) materials, which has the common advantages of graphene and transition metal dichalcogenides (TMDs), is of great concern. Based on the studies of black phosphorus, three new monolayer phosphorene polymorphs (e-P, ζ-P, and θ-P) have been predicted recently, which greatly enrich the phosphorene structures. Three new monolayer phosphorene polymorphs, composed of P4 square units (e-P, ζ-P) or P5 pentagon units (θ-P), all exhibit buckled honeycomb lattices akin to black phosphorus. Remarkably, the θ-P is equally stable as the black phosphorus, and more stable than the other two new phosphorene polymorphs. In this paper, electronic structures and optical properties of monolayer black phosphorus (α-P) and three new monolayer phosphorene polymorphs (e-P, ζ-P and θ-P) have been investigated based on the first-principles density functional theory (DFT). In comparison with Perdew-Burke-Ernzerh of generalized gradient approximation (GGA-PBE) with ultra-soft pseudopotential, hybrid density functional (HSE06) with norm-conserving pseudopotential is selected for subsequent calculation, because it is closer to experimental data. The calculation results show that, α-P and e-P are direct semiconductors with band gaps of 1.542 and 0.973 eV, respectively. ζ-P and θ-P are indirect semiconductors with band gaps of 1.874 and 1.772 eV, respectively. Based on the electronic properties, the optical properties are discussed in detail. It is found that α-P and three new monolayer phosphorene polymorphs have good permeability in the whole infrared and visible region, which means that they can be used as optical transparent materials. Moreover, the peaks of reflectance and absorption spectra are in the ultraviolet region, indicating that they can be used as ultraviolet protection materials or ultraviolet detection equipment. This research is aimed at enhancing the understanding of the photoelectric properties of α-P and three new monolayer phosphorene polymorphs, and providing an useful theoretical basis for the practical application as photoelectric materials.

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The electronic structures, elastic properties, minimum thermal conductivity at high temperature, and optical properties of three monoclinics, namely, δ-V2O5, ζ-Nb2O5, and B-Ta2O5, are examined using the density-functional theory plane-wave method. Studies have indicated that bonds between the atoms of M2O5 (M = V, Nb, Ta) include covalent and ionic bonds. Good mechanical properties, high hardness, and high ductility tend to increase in the sequence of ζ-Nb2O5→B-Ta2O5→δ-V2O5. The elastic anisotropy of these three materials is significant. The three monoclinics have low minimum thermal conductivity at high temperature. The three-dimensional thermal conductivity diagram of the Clark model based on Young’s modulus in each crystal orientation is anisotropic. In terms of optical properties, the static dielectric constants of δ-V2O5, ζ-Nb2O5, and B-Ta2O5 are 7.89, 4.78, and 4.01, respectively. δ-V2O5 exhibits better absorption of light with longer wavelengths; the absorption wavelength extends to 614 nm. In the infrared and visible spectral regions, the reflectivity and refractivity of δ-V2O5 are larger than those of ζ-Nb2O5 and B-Ta2O5. In the region with a wavelength of less than 394 nm, ζ-Nb2O5 and δ-V2O5 exhibit the maximum and minimum refractivity, respectively. Moreover, the loss function of M2O5 in the visible light region is extremely low; a maximum value is found between 6.00 and 17.00 eV.摘要本文采用基于密度泛函理论和平面波方法, 计算了δ-V2O5, ζ-Nb2O5和B-Ta2O5的电子结构、弹性性质、高温下的最小热导率以及光 学性质. 研究表明M2O5 (M = V, Nb, Ta)包含共价键和离子键. 材料力学性质的优良性、硬度以及韧性都按ζ-Nb2O5→B-Ta2O5→δ-V2O5的顺 序增加. M2O5 (M = V, Nb, Ta)都有非常显著的弹性各向异性. 而这三种材料在高温下都有较低的热导率, 并且基于杨氏模量的Clark模型计 算得到的三维热导率图形在各个晶向上都呈各向异性. δ-V2O5, ζ-Nb2O5和B-Ta2O5的静介电常数分别为7.89、4.78和4.01. δ-V2O5在长波长 范围内展现出较好的吸收性质, 而且波长范围扩展到了614 nm. 与ζ-Nb2O5和B-Ta2O5相比, 在红外和可见光区δ-V2O5的反射率和折射率较 大. 在小于394 nm的波长范围内, δ-V2O5和B-Ta2O5分别出现了折射率的最大值和最小值. 且M2O5 (M = V, Nb, Ta)在可见光区域的损失函数 都非常小, 而在6.00−17.00 eV范围内出现了最大值.

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采用第一性原理计算研究了超硬材料z-BC 2 N的弹性各向异性性质、应力-应变关系、硬度及最小热导率性质。计算得到的晶体力学行为判据B/G为0.87, 泊松比为0.084, 普适弹性各向异性指数为0.09992。[100]晶向上最大拉伸强度达到180 GPa, 应变方向上最大剪切强度达到160 GPa, 维氏硬度值为77.07 GPa。基于Cahill模型得到的最小热导率为6.811 W/(m·K)。结果表明: z-BC 2 N是脆性材料且力学稳定性良好, 有非常高的拉伸强度、剪切强度, 体弹模量为各向同性, 杨氏模量各向异性程度不大。z-BC 2 N的最小热导率低于金刚石的最小热导率。

gt;N

本文运用基于密度泛函理论框架下的第一性原理平面波超软赝势和模守赝势研究了四种晶相HfO 2 的弹性、慢度和热导率及其各向异性, 通过计算得到四种结构的弹性常数、体弹模量、剪切模量、杨氏模量和泊松比. 根据泊松比和Pugh模量比分析了四种材料的键合属性; 根据弹性各向异性分数比 A B , A G 和 A E 以及剪切各向异性因子 A 1 , A 2 和 A 3 的计算值, 说明四种晶相HfO 2 是弹性各向异性的; 又通过弹性模量的三维立体图形展示了四种晶相HfO 2 的弹性各向异性. 计算得到的声学波速以及不同晶向上的最小热导率, 显示了四种晶相HfO 2 声学波速和热导率的大小及其各向异性, 并绘出慢度在 XY , YZ 和 ZX 平面上的图形.The elasticity, slowness, thermal conductivity and theiranisotropiesoffourHfO2 polymorphs are investigated based on density functional theory plane wave method with ultrasoft and norm-conserving pseudo-potentials. The elastic constants, bulk modulus, shear modulus, Youngs modulus and Poissons ratio are obtained by first-principles calculations. The bonds properties of the four materials are analyzed through Poissons ratio and Pugh modulus ratio ( G / B ). Both calculated values of elastic anisotropic ratios ( A B , A G and A E ) and shear anisotropic factors ( A 1, A 2and A 3) indicate the four HfO2 being anisotropic. The details of anisotropies are demonstrated in 3D plots. Sound speed and minimum thermal conductivities along different directions are calculated. Slowness on XY , YZ and ZX faces are solved and plotted.本文运用基于密度泛函理论框架下的第一性原理平面波超软赝势和模守赝势研究了四种晶相HfO 2 的弹性、慢度和热导率及其各向异性, 通过计算得到四种结构的弹性常数、体弹模量、剪切模量、杨氏模量和泊松比. 根据泊松比和Pugh模量比分析了四种材料的键合属性; 根据弹性各向异性分数比 A B , A G 和 A E 以及剪切各向异性因子 A 1 , A 2 和 A 3 的计算值, 说明四种晶相HfO 2 是弹性各向异性的; 又通过弹性模量的三维立体图形展示了四种晶相HfO 2 的弹性各向异性. 计算得到的声学波速以及不同晶向上的最小热导率, 显示了四种晶相HfO 2 声学波速和热导率的大小及其各向异性, 并绘出慢度在 XY , YZ 和 ZX 平面上的图形.

First-principles studies electronic structures and opticalproperties of four new phosphorene polymorphs

We have observed the thermodynamic properties of metallic superconductive nano-particles in the grand canonical ensemble; and the level distribution and the level correlation between the discrete electronic energy levels are considered in the calculation of the electronic spin susceptibility of the ensemble numerically. The quantum effect, even–odd effect and other special effects existing in the metallic nano-particles are also studied in this article.