B_2-AgMg金属间化合物点缺陷行为的理论研究

运用密度泛函理论研究了B_2-AgMg金属间化合物中Mg原子空位,反位和Al原子空位,反位缺陷对AgMg热力学性能的影响.结果表明:Mg和Ag反位缺陷由于与周围原子形成典型的共价键而更易于形成.这四种缺陷的出现,破坏了晶体的有序结构,导致了B_2-AgMg金属间化合物的硬度,脆性以及德拜温度出现了不同程度的降低.     

MgO高温高压特性及相变的第一性原理研究

应用第一性原理密度泛函理论计算了MgO在零温(0K)下和0~200GPa静水压范围内的晶体结构和弹性模量,以及B1、B4和B8相结构的MgO的声速随压力的变化。利用准简谐近似下的Debye模型,通过拟合三阶Birch-Murnaghan物态方程模拟了高温效应并对三个相在高温高压下的相稳定性做了研究。本工作的计算结果与前人的理论和实验结果符合较好,说明第一性原理结合准简谐Debye模型能够比较准确的模拟矿物如MgO在高温高压下的热力学性质。     

MgO高温高压特性及相变的第一性原理研究

应用第一性原理密度泛函理论计算了MgO在零温(0K)下和0~200GPa静水压范围内的晶体结构和弹性模量,以及B1、B4和B8相结构的MgO的声速随压力的变化。利用准简谐近似下的Debye模型,通过拟合三阶Birch-Murnaghan物态方程模拟了高温效应并对三个相在高温高压下的相稳定性做了研究。本工作的计算结果与前人的理论和实验结果符合较好,说明第一性原理结合准简谐Debye模型能够比较准确的模拟矿物如MgO在高温高压下的热力学性质。     

第一性原理研究Al-Cu-Li合金中T1相的腐蚀机理

利用密度泛函理论的第一性原理,讨论Al-Cu-Li合金中主要析出相T1相(Al6Cu4Li3)的表面性质,计算不同终结面的表面能和表面电子功函数,并探讨应力作用和常见合金元素对Al/T1界面的影响.结果表明:T1相的表面能与表面的原子排列有关,不同的表面通过应变释放重构,进而获得不同的表面能.表面电子功函数则与表面原子种类有关,由于Li的电负性最小,含Li原子的表面通常有较低的电子功函数,进而降低材料的耐蚀性.此外,在应力作用下,T1相一些表面的电子功函数变化与纯金属是相反的.压应力下T1相电子功函数降低,材料更加容易被腐蚀;张应力下T1相功函数增加,材料更加耐腐蚀.同时,通过计算Al/T1界面中Ag,Zn和Mg 3种合金元素的替位能,可以发现,这3种元素都有利于降低界面能,且Ag的作用最明显.     

复合纳米金属间化合物Ag2Al的制备及其结构表征

采用自悬浮定向流法制备出Ag2Al复合金属间化合物的纳米微粉,通过透射电子显微镜、X射线衍射仪和X射线能谱仪对纳米微粉的显微结构、粒度、相组成和成分构成进行研究。结果表明:所制备出的复合金属间化合物纳米颗粒呈规则球形,粒径分布在20～110 nm之间;纳米合金颗粒的主要组成相为Ag2Al,并伴有少量的Al;样品中的Ag,Al原子数比约为66.5∶33.5,金属间化合物纳米颗粒中Ag2Al晶粒尺寸约为33 nm,Al的约为21 nm。     

复合纳米金属间化合物Ag2Al的制备及其结构表征

采用自悬浮定向流法制备出Ag2Al复合金属间化合物的纳米微粉,通过透射电子显微镜、X射线衍射仪和X射线能谱仪对纳米微粉的显微结构、粒度、相组成和成分构成进行研究。结果表明：所制备出的复合金属间化合物纳米颗粒呈规则球形,粒径分布在20～110 nm之间;纳米合金颗粒的主要组成相为Ag2Al,并伴有少量的Al;样品中的Ag,Al原子数比约为66.5∶33.5,金属间化合物纳米颗粒中Ag2Al晶粒尺寸约为33 nm,Al的约为21 nm。     

MgSiO_3钙钛矿高温高压特性的第一性原理研究

本文采用第一性原理方法,计算了MgSiO_3钙钛矿在零温和0～150 GPa静水压范围内的晶体结构和弹性模量,并利用准简谐近似Debye模型,拟合三阶Birch-Murnaghan物态方程得到了其高温高压下的热力学性质.通过与现有的理论和实验的结果数据比较,确认在0～2000 K的温度区间内,第一性原理计算结合Debye模型能够较可靠地模拟在下地幔压力范围内MgSiO_3钙钛矿的热力学性质.     

非晶态Mg_(70)Zn_(30)合金结构因子的预峰

利用X射线衍射仪对非晶态Mg70 Zn30 合金的结构进行了研究 ,获得了强度曲线、结构因子、双体分布函数和原子间最近邻距离 .结果表明 ,Mg70 Zn30 合金在小Q区间存在强烈的预峰现象 .根据预峰的特性 ,提出了Mg70 Zn30 熔体的结构模型 ,即Mg原子位于中心 ,8个Zn原子位于顶角所形成的简单立方结构模型 .该模型以共享顶点的方式相连接 ,能够满足预峰对Mg—Mg原子间距离的要求 .Mg70 Zn30 非晶合金中预峰的产生是相邻原子团中心Mg原子之间相互关联的表现     

Mg-Y-Zn合金三元金属间化合物的电子结构及其相稳定性的第一性原理研究

采用基于密度泛函的第一性原理平面波赝势方法计算Mg-Y-Zn合金三元金属间化合物X-Mg₁₂YZn 相和W-Mg₃Y₂Zn₃相的晶格常数、形成焓和电子结构. 形成焓的计算结果表明, X-Mg₁₂YZn相和W-Mg₃Y₂Zn₃相都具有负的形成焓, 并且W-Mg₃Y₂Zn₃相的形成焓更低; 电子结构的计算分析表明, W-Mg₃Y₂Zn₃相成键峰主要来自Mg的2p轨道、Zn的3p轨道和Y的4d轨道的贡献. 而X-Mg₁₂YZn相成键峰主要来自Mg的3s和2p轨道、Zn的3p轨道和Y的4d轨道的贡献. 对W-Mg₃Y₂Zn₃相(011)面和X-Mg₁₂YZn相(0001)面的电荷密度分析表明, 两相中Zn-Y原子间都形成了共价键, 且W-Mg₃Y₂Zn₃相的共价性比X-Mg₁₂YZn相的共价性更强. 在费米能级低能级处, W-Mg₃Y₂Zn₃相具有更多的成键电子数, 决定了W-Mg₃Y₂Zn₃相比X-Mg₁₂YZn相有更好的相稳定性.     

Mg掺杂ZnO所致的禁带宽度增大现象研究

采用第一性原理的超软赝势方法，研究了纤锌矿ZnO及不同量Mg掺杂ZnO合金的电子结构.理论计算表明，Mg的掺杂导致ZnO晶体的禁带宽度增大.研究发现，Zn 4s态决定导带底的位置，Mg的掺入导致Zn 4s态向高能端的偏移是导致禁带宽度增大的根本原因. 关键词： 密度泛函理论 赝势 Mg掺杂ZnO     

Structural, electronic, elastic and thermal properties of Mg2Si

First-principles calculations of the lattice parameter, electron density maps, density of states and elastic constants of Mg₂Si are reported. The lattice parameter is found to differ by less than 0.8% from the experimental data. Calculations of density of states and electron density maps are also performed to describe the orbital mixing and the nature of chemical bonding. Our results indicate that the bonding interactions in the Mg₂Si crystal are more covalent than ionic. The quasi-harmonic Debye model, by means of total energy versus volume calculations obtained with the plane-wave pseudopotential method, is applied to study the elastic, thermal and vibrational effects. The variations of bulk modulus, Grüneisen parameter, Debye temperature, heat capacity C_v, C_p and entropy with pressure P up to 7 GPa in the temperature interval 0-1300 K have been systemically investigated. Significant differences in properties are observed at high pressure and high temperature. When T<1300 K, the calculated entropy and heat capacity agree reasonably with available experimental data. Therefore, the present results indicate that the combination of first-principles and quasi-harmonic Debye model is an efficient approach to simulate the behavior of Mg₂Si.     

高温高压下Mg_xZn_(1-x)O固溶体的制备

研究了高温高压下制备MgxZn1-xO(0.30x0.60)固溶体的过程.在1000—2000℃和4—5.6GPa的条件下,制备出稳定的单一立方相MgxZn1-xO(x=0.4,0.5,0.6)固溶体,解决了常压下MgxZn1-xO的分相问题.通过X射线衍射仪和扫描电子显微镜等测试手段,对MgxZn1-xO样品进行了表征,阐明了立方相MgxZn1-xO的形成机制,给出了高压下MgxZn1-xO固溶体的温度与组分相图.     

The specific heat of iron titanium hydride at room temperature

Specific heat measurements were used to determine vibrational properties of hydrogen interstitials in the intermetallic alloy Fe_{0, 49}Mn_{0, 01}Ti_{0, 5}H_x. The specific heats are 0, 465 J/gK for x = 0, 0, 486 for x = 0, 31, 0, 515 for x = 0, 47, 0, 557 for x = 0, 66 at 280 K. The localized vibrational excitations of hydrogen have an energy of 100 meV for x = 0, 5 and 77 meV for x = 1. The Debye temperature at x = 0 is 300 K at a temperature of 280 K.     

Electronic structural, elastic properties and thermodynamics of Mg17Al12, Mg2Si and Al2Y phases from first-principles calculations

Electronic structures, elastic properties and thermal stabilities of Mg₁₇Al₁₂, Mg₂Si and Al₂Y have been determined from first-principle calculations. The calculated heats of formation and cohesive energies show that Al₂Y has the strongest alloying ability and structural stability. The brittle behavior and structural stability mechanism is also explained through the electronic structures of these intermetallic compounds. The elastic constants are calculated, the bulk moduli, shear moduli, Young's moduli and Poisson ratio value are derived, the brittleness and plasticity of these phases are discussed. Gibbs free energy, Debye temperature and heat capacity are calculated and discussed.     

Ce-La-Th合金高压相变的第一性原理计算

采用第一性原理计算对Ce_(0.8)La_(0.1)Th_(0.1)在高压下fcc-bct的结构相变、弹性性质及热力学性质进行了研究讨论.通过对计算结果的分析,发现了合金在压力下的相变规律,压强升高到31.6 GPa附近时fcc相开始向bct相转变,到34.9 GPa时bct相趋于稳定.对弹性模量的计算结果从另一角度反映了结构相变的信息.最后,利用准谐德拜模型对两种结构的高温高压热力学性质进行了理论预测.     

Elastic constants and thermodynamic properties of Mg2SixSn1-x from first-principles calculations

<正>This paper stuides the elastic constants and some thermodynamic properties of Mg₂Si_xSn_n-1（x=0,0.25,0.5, 0.75,1） compounds by first-principles total energy calculations using the pseudo-potential plane-waves approach based on density functional theory,within the generalized gradient approximation for the exchange and correlation potential. The elastic constants of Mg₂Si_xSn_n-1 were calculated.It shows that,at 273 K,the elastic constants of Mg₂Si and Mg₂Sn are well consistent with previous experimental data.The isotropy decreases with increasing Sn content.The dependences of the elastic constants,the bulk modulus,the shear modulus and the Debye temperatures of Mg₂Si and Mg₂Si_0.5Sn_0.5 on pressure were discussed.Through the quasi-harmonic Debye model,in which phononic effects were considered,the specific heat capacities of Mg₂Si_xSn_1-x at constant volume and constant pressure were calculated.The calculated specific heat capacities are well consistent with the previous experimental data.     

Electronic and thermal properties of B2-type AlRE intermetallic compounds: A first principles study

The ground state electronic structure and thermal properties of B₂-type intermetallic compounds AlRE (RE: Pm, Sm, Eu, Tb, Gd and Dy) have been studied using a self-consistent tight-binding linear muffin-tin orbital (TB-LMTO) method at ambient as well as at high pressure. These compounds show metallic behavior under ambient condition. The band structure, total energy, density of states and ground state properties like lattice parameter, bulk modulus are calculated in the present work. The Debye-Grüneisen model is used to calculate the Debye temperature and the Grüneisen constant. The calculated results are in good agreement with the reported experimental and other theoretical results. The variation in the Debye temperature with pressure has also been reported. We present a detailed analysis of the role of f electrons of RE in the AlRE system.     

Thermodynamic properties of OsB under high temperature and high pressure

The energy-volume curves of OsB have been obtained using the first-principles plane-wave ultrasoft-pseudopotential density functional theory (DFT) within the generalized gradient approximation (GGA) and local density approximation (LDA). Using the quasi-harmonic Debye model we first analyze the specific heat, the coefficients of thermal expansion as well as the thermodynamic Grüneisen parameter of OsB in a wide temperature range at high pressure. At temperature 300 K, the coefficients of thermal expansion α_V by LDA and GGA calculations are 1.67×10⁻⁵ 1/K and 2.01×10⁻⁵ 1/K, respectively. The specific heat of OsB at constant pressure (volume) is also calculated. Meanwhile, we find that the Debye temperature of OsB increases monotonically with increasing pressure. The present study leads to a better understanding of how the OsB materials respond to pressure and temperature.     

Fe2+取代对MgSiO3钙钛矿高温高压物性的影响

(Mg, Fe)SiO₃钙钛矿是下地幔中最主要的候选矿物成分之一, 关于其高温高压特性的研究对于深层地幔状态的理解和地震波变化规律的探索具有重要意义. 应用第一性原理计算了MgSiO₃和(Mg_0.75, Fe_0.25)SiO₃在0–140 GPa静水压范围内的晶体结构和弹性模量, 并由Voigt-Reuss-Hill方程计算了地震波速随压力的变化, 利用准简谐近似下的Debye模型模拟了高温效应, 分析了Fe²⁺取代Mg²⁺后镁铁钙钛矿弹性和热学性质的变化, 推断Fe²⁺取代行为软化了MgSiO₃等含镁的地球深部矿物的地震波速. 为解释地幔中某些区域的地震波速软化现象提供了一个有力的理论依据. 关键词： 钙钛矿 铁 高压 波速     

Electronic and thermodynamic properties of ReB2 under high pressure and temperature

First-principles calculations of the crystal structures of rhenium diboride (ReB₂) have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The quasi-harmonic Debye model, using a set of total energy versus molar volume obtained with the first-principles calculations, is applied to the study of the thermal and vibrational effects. The structural parameters, thermal expansions, heat capacities, Grüneisen parameters and Debye temperatures dependence on the temperature and pressure are obtained in the whole pressure range from 0 to 70 GPa and temperature range from 0 to 2000 K as well as compared with available data.