利用第一性原理方法研究CuXSe_2(X=B,Al,Ga,In,Tl)力学性质

在广义梯度近似(GGA)和局域密度近似(LDA)下,采用第一性原理方法研究CuXSe2(X=B,Al,Ga,In,Tl)晶体结构的稳定性和力学性质.分析CuXSe2(X=B,Al,Ga,In,Tl)结构的晶格常数,弹性常数,体积模量,剪切模量,杨氏模量,泊松比,对比各材料的力学性质变换规律.计算结果表明,根据力学稳定判据,在零温零压下,CuXSe2(X=B,Al,Ga,In,Tl)的晶体结构是力学稳定的.经过同主族替换,发现晶格常数越大,体弹性模量就越小.这可以解释为X离子半径逐渐增大,晶格常数逐渐增加,晶体的可压缩性也增加.另外,CuTlSe2的剪切模量最小,不容易发生剪切形变.CuBSe2的杨氏模量最大,其刚度最高.由Pugh经验关系可知CuXSe2(X=B,Al,Ga,In,Tl)均属于韧性材料.     

利用第一性原理方法研究CuXSe (X=B，Al，Ga，In，Tl)力学性质

在广义梯度近似（GGA）和局域密度近似（LDA）下,采用第一性原理方法研究CuXSe2 (X=B,Al,Ga,In,Tl)晶体结构的稳定性和力学性质。分析CuXSe2 (X=B,Al,Ga,In,Tl)结构的晶格常数,弹性常数,体积模量,剪切模量,杨氏模量,泊松比,对比各材料的力学性质变换规律。计算结果表明,根据力学稳定判据,在零温零压下,CuXSe2 (X=B,Al,Ga,In,Tl)的晶体结构是力学稳定的。经过同主族替换,发现晶格常数越大, 体弹性模量就越小。 这可以解释为X 离子半径逐渐增大, 晶格常数逐渐增加, 晶体的可压缩性也增加。另外,CuTlSe2的剪切模量最小,不容易发生剪切形变。CuBSe2的杨氏模量最大,其刚度最高。由Pugh经验关系可知CuXSe2 ( X=B,Al,Ga,In,Tl)均属于韧性材料。     

第一性原理研究hcp-C3碳体环材料的力学性质

为探索新型高强度材料, 使用第一性原理方法研究了hcp-C3碳体环材料的晶体结构、电子性质与力学性质. 结构计算与电子性质分析表明, 基于特殊的分子结构, 碳体环结构中出现了变形的sp³杂化形式. 这使得hcp-C3碳体环结构中力学特性具有明显的方向依赖性. 力学性质计算表明, 沿着[0001]晶向, 碳体环结构的弹性模量达到1033 GPa, 抗拉强度达到124.17 GPa, 抗压强度达到381.83 GPa, 沿[2110]晶向的抗压强度达到了458.34 GPa, 从而显示了hcp-C3碳体环材料优秀的力学性质. hcp-C3碳体环材料可作为新型的高强度材料而使用.     

第一性原理研究bct-C4碳材料的强度性质

本文采用第一性原理方法,分析了bct-C4碳材料的力学性质,尤其是抗压强度性质,并在研究中分析了在压缩过程中bct-C4碳的 键长随压缩应变的变化规律.计算结果表明, bct-C4碳材料不仅具有优秀的弹性性质和硬度性质,也具有优秀的强度特性, 其沿[100]晶向的抗压强度高于金刚石6.9%.本文的工作表明, bct-C4碳是一种高强度的碳同素异形体, 可以用于压缩包括金刚石在内的各种物质,并可用于研究物质在高压下的结构变化以及力、电学等性质. 同时,作为一范例,本文研究同样可以为寻找力学性质超越金刚石的物质提供参照.     

Sr偏析Al晶界结构的第一性原理计算

采用基于密度泛函理论和局域密度近似的第一性原理赝势方法,计算了纯Al晶界和杂质Sr偏析Al晶界的原子结构和电子结构.结果表明Sr偏析引起了晶界膨胀和晶界处电子密度的大幅度降低,从而导致晶界结合力的减弱.这应为Sr杂质偏析引起的Al晶界脆化的主要根源所在.关键词：Al晶界Sr杂质偏析第一性原理计算     

含扭转晶界位错Al金属拉伸强度第一性原理预测

本文基于密度泛函理论第一原理方法,从影响力学性能本质的电子结构计算上,对含Σ 5{001}扭转晶界位错Al金属拉伸强度进行了预测,发现其理论拉伸强度达到8.73 GPa,临界应变为 24%.拉伸强度低于文献报道(Phys. Rev. B 75, 174101 (2007))的倾斜晶界位错Al金属的理论拉伸强度9.5 GPa,但其临界应变却远大于倾斜晶界的16%.本研究结果表明,通过工艺参数控制,改变缺陷形态,可极大地改变其力学性能.进一步地,从电子结构层次上, 分析了含晶界位错Al金属拉伸断裂行为的实质,通过分析电荷密度分布、键长变化等,发现其断裂处发生在晶界处;理论计算结果将对Al金属结构设计及力学性能改善具有重要的指导作用.     

MgCu4Sn型稀土镁合金相力学和热力学性质的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法,系统研究了典型立方结构合金相LaMgX4(X=Co, Ni, Cu)的力学特性和热力学特性。根据广义胡可定律计算了合金相的单晶弹性常数;根据单晶弹性常数和Hershey’s averaging方法计算了合金相的多晶弹性模量、泊松比、Zener各向异性因子和德拜温度。采用基于准谐近似的Gibbs2代码计算了LaMgX4(X=Co, Ni, Cu)合金相的吉布斯自由能、熵和等体热容与温度的关系。LaMgNi4的计算结果与其他文献计算结果及实验结果符合的很好。结果表明：LaMgX4 (X= Co, Ni, Cu)合金均为延展性、塑性和弹性各向异性材料。德拜温度按以下顺序递减：LaMgNi4>LaMgCu4>LaMgCo4。     

第一性原理研究Zn偏析对CuΣ5晶界的影响

采用基于密度泛函理论的第一性原理方法研究了Zn偏析Cu晶界的原子构型和电子结构,分析了Zn偏析对Cu晶界力学性能的影响.结果表明,Zn以替换方式偏析到晶界处,Zn—Cu与Cu—Cu的成键方式类似,均为含有共价成分的金属键.Zn偏析导致少量电荷集聚于Zn与近邻Cu之间,有限地增强了晶界的结合.拉伸过程中Zn的d轨道定域性增强,Zn与近邻Cu间的电荷密度下降,削弱了Zn—Cu键,导致晶界断裂发生在Zn—Cu间.     

第一性原理研究Al掺杂Mn4Si7的电子结构及光学性质

基于密度泛函理论框架下,利用第一性原理计算方法,对本征Mn₄Si₇以及不同Al掺杂浓度下Mn₄Si₇的电子结构及光学性质进行系统计算分析. Mn₄Si₇晶胞中有16个Mn原子及28个Si原子,建立4种Mn_16-xAl_xSi₂₈(x=0,2,4,8)的掺杂模型,计算结果表明：本征Mn₄Si₇的禁带宽度E_g =0.775 eV,属于间接带隙半导体,Al的掺入导致了Mn₄Si₇费米能级附近的电子结构发生改变,导带向低能方向发生偏移,价带向高能方向发生偏移,禁带宽度由0.775 eV降至零,呈现出金属性.计算还表明,在光子能量低能区域,Al的掺入使Mn₄Si₇的介电函数、折射率、吸收及反射系数等光学性质有所提升,改善了Mn₄S...     

Al晶界的第一性原理拉伸试验

基于密度泛函理论和局域密度近似的第一性原理方法,进行了Al晶界的第一性原理拉伸试验.得到Al晶界的理论拉伸强度为9.5 GPa,对应的应变为16%.根据价电荷密度、键长和原子构型随应变的变化,我们证实断裂发生在晶界面,其特征是所有界面键的断裂.同时还发现在周围原子键的数目减少的情况下,界面重构的Al-Al原子键具有共价键的性质.因此Al晶界依然保持着较高的界面强度.关键词：Al晶界第一性原理拉伸试验理论拉伸强度     

Grain Boundary Motion during Ag and Cu Grain Boundary Diffusion in Cu Polycrystals

Grain boundary (GB) motion in high-purity Cu material (5N8 and 5N Cu) is investigated using the results of radiotracer GB diffusion measurements with tracers exhibiting fundamental differences in the solute-matrix atom interactions. The results on GB solute diffusion of Ag (revealing a miscibility gap in the Ag-Cu phase diagram) and Au (forming intermetallic compounds with Cu) in Cu and on Cu self-diffusion are analyzed.The initial parts of the Ag and Cu penetration profiles turned out to be substantially curved. The profile curvature is explained via the effect of GB motion during ^110m Ag and ⁶⁴Cu GB penetration. The activation enthalpies of GB motion in these two independent measurements occurred to be very close, 95 and 103 kJ/mol, respectively. Moreover, these values turn out to be close, but still somewhat larger than the activation enthalpy of Cu GB self-diffusion in Cu material of the same very high purity, Q ^Cu _gb = 72 kJ/mol. Although tracer diffusion measurements of Au GB diffusion in Cu yielded only limited information on GB motion, the absolute values of GB velocities are consistent with those calculated from the Ag and Cu GB diffusion data.     

Grain boundary interdiffusion in the case of concentration-dependent grain boundary diffusion coefficient

The grain boundary diffusion in a binary system which exhibits a grain boundary phase transition is considered in the framework of Fisher's model. The kinetic law of the growth of the grain boundary phase and the distribution of the diffusant near the grain boundary are calculated. The method of determining of the concentration dependence of the grain boundary diffusion coefficient from the experimentally measured penetration profiles of the diffusant along the grain boundaries is suggested. The experimental results on Zn diffusion in Fe(Si) bicrystals, Ni diffusion in Cu bicrystals and grain boundary grooving in Al in the presence of liquid In are discussed in light of the suggested model.     

3He流体的低温输运性质综述

收集整理了400余篇文献中的3He流体低温输运性质(粘度和热导率)实验数据,总结并分析了3He流体低温输运性质的相关理论,为建立3He流体低温输运性质的方程及完善3He数据库(He3Pak)奠定基础,也可供3He低温研究人员参考。     

Effect of Grain Boundary Segregation and Migration on Diffusion Profiles: Analysis and Experiments

In many experimental studies, curved penetration profiles are observed for grain boundary diffusion performed in the B kinetics regime in contrast to the shape expected from the solutions of the second Fick's equation. To explain these curvatures the effects of grain boundary structure, grain boundary migration, and grain boundary segregation have been successively proposed in the literature. Using previous data for Cu–Ag and Cu–Ni and new ones on Cu–Fe and Cu–Zn systems we will show how it is possible to separate all these possible contributions and how, knowing the true origin of the curvature, one can deduce much quantitative information impossible (or very difficult) to obtain by other techniques.     

温度梯度对晶粒生长行为影响的相场模拟

利用相场法建立了一个可应用于研究温度梯度影响下的晶粒生长行为的二维模型,模拟了多晶材料退火过程中由温度梯度引起的非均匀晶粒生长和定向晶粒生长行为.结果表明:退火过程中,在静态温度梯度的影响下,体系的晶粒呈现不均匀生长,且从晶粒生长指数来看,不同程度地偏离了正常晶粒生长;在动态温度梯度的影响下,体系内部常出现柱状晶粒生长,柱状晶粒前端持续生长至温度最高位置;柱状晶粒生长与动态热源的移动速率密切相关,只有当动态热源的移动速率处于最小和最大晶粒生长速率之间时,柱状晶粒才会出现.关键词：晶粒生长相场法晶界迁移率局部退火     

The Effect of Pressure on Indium Diffusion along 〈001〉 Tilt Grain Boundaries in Copper Bicrystals

The effect of pressure on the diffusion of indium along two grain boundaries (GBs) in copper bicrystals has been investigated. The GBs studied were of symmetrical tilt type with a 001 misorientation axis. The tilt angle of one GB was 36.5°, which corresponds to the near 5 coincidence orientation, where is the inverse density of the coincidence sites in the two misoriented crystal lattices. The other GB was a general boundary with a tilt angle of 45°. The diffusion along the 001 tilt axis has taken place at the temperature of 923 K and under argon gas pressures up to 1 GPa. The following activation volumes have been found: 0.94±0.11 _Cu for bulk diffusion,–0.5 ±0.7 _Cu for the diffusivity of the near 5 GB and –0.6±0.3 _Cu for the diffusivity of the 45° 100 GB, where _Cu is the atomic volume of copper. Two interpretations of this result seem plausible. The first interpretation is that the In atoms diffuse along the 001 tilt axis by an instertitialcy mechanism. It is known that for such a diffusion mechanism the activation volume is close to zero. The alternative explanation bases on the observation that the parameter measured is the GB diffusivity, which depends on the product of the GB diffusion coefficient and the segregation factor. An enhancement of the In segregation under pressure would lead to an increase of the GB diffusivity. It seems plausible that both interpretations are true and the fact that the atomic volume of In is a factor of 2.2 larger than that of Cu plays an important role both for the increased segregation of In under pressure and the diffusion mechanism.