Nonstoichiometry and relative stabilities of Y2Ti2O7 polar surfaces: A density functional theory prediction

Density functional theory calculations were carried out to predict the surface structures and relative stabilities of (1 0 0) and (1 1 0) polar surfaces of a complex metal oxide, Y₂Ti₂O₇. Based on a thermodynamic defect model, surface stabilities were evaluated as a direct function of stoichiometry and environmental factors, i.e. oxygen partial pressure and temperature. Calculations show that, as the oxygen partial pressure increases, the most stable termination of the (1 1 0) changes from Y/Ti-rich to O-rich. For the (1 0 0) surfaces, the most stable termination changes from Y/Ti-rich to stoichiometric, and then to O-rich, with increasing oxygen partial pressure. All variants of the (1 1 0) surfaces were found to be more stable than (1 0 0) surfaces. In particular, non-stoichiometric (1 1 0) surfaces are always more stable than their stoichiometric counterparts.     

Zr-Ti合金相稳定性与弹性性质的第一性原理研究

利用特殊准随机结构(specialquasi-randomstructure,SQS)超晶胞模型及第一性原理计算研究了Zr_(1-x)Tix合金中Ti含量及相结构(hcp与bcc结构,α相与β相)对合金相稳定性及弹性性质的影响。研究表明:从弹性性质上看,Ti含量增加会使Zr-Ti合金的结构更加稳定,Zr-Ti合金α相的结构稳定性普遍高于β相;Zr-Ti合金普遍具有延展性,Zr-Ti合金的α相具有更高的硬度并且其硬度和韧性都与Ti元素浓度成正比,Zr-Ti合金的β相具有更高的韧性;从电子结构上看,Ti不会显著改变hcp结构和bcc结构Zr-Ti合金系统的态密度(DOS),Zr-Ti合金系统的DOS分布中的费米能级与纯Zr系统的相似,Ti含量对Zr-Ti合金的相稳定性影响不大。     

Equilibrium and stability of triple junctions in anisotropic systems

When a homophase or heterophase interface involves a crystalline solid, the interfacial energy is expected to depend upon the interface-plane orientation. In this paper, the equations governing equilibrium triple-junction configurations in anisotropic systems are reviewed. These equilibrium conditions were originally derived by considering the differential change in triple-junction interfacial energy associated with a differential change in triple-junction configuration and equating it to zero. However, the derived conditions do not distinguish between a triple-junction configuration that satisfies the equilibrium conditions by residing at a local energy minimum, a local energy maximum or at a saddle point. The present paper develops stability criteria for triple junctions with and without interfaces which have anisotropic energy, which can be used to determine whether triple-junction equilibrium conditions correspond to local energy minima (stable), maxima (unstable) or saddle points. For isotropic systems, there is a single solution to the triple-junction equilibrium conditions, and it is necessarily stable. For anisotropic systems, there are multiple solutions to the triple-junction equilibrium conditions, some of which may be stable, unstable or correspond to saddle points in triple-junction interfacial energy. Microstructure features and interface lengths/areas are expected to play a role in dictating the relative energies of stable configurations.     

Prediction on Phase Stabilities of the Zr–H System from the First-Principles

Basic fundamentals governing the hydrogenation of Zr and its alloys have both theoretical and practical importance.In this work,first-principles calculations have been performed to evaluate the relative stabilities of various possible phases in ZrH_x(x=1-2) under different temperatures and pressures.It was predicted that fct-γ and ε phases with various different H-atom configurations can be energetically favorable for ZrH_x(x=1,1.25 and 1.5),while ZrH_(1.75) and ZrH2 prefer fct-ε phase only.Fcc-δ phase is less favored in energy at any H concentrations,but can be mechanically stable in some cases.The thermodynamically stable and metastable phase stability diagrams were then constructed for a wide temperature and H concentration range,to predict the environment-dependent formation of ZrH_x during hydrogenation.     

LaS/CeS与γ-Fe异相界面性质的第一性原理计算

为了探讨LaS/CeS与γ?Fe两相之间的异相界面性质,本文采用边?边匹配(E2EM)模型计算了LaS/CeS与γ?Fe两相之间晶体学上的原子匹配情况,基于晶体学计算结果,采用基于密度泛函理论的第一性原理,从原子尺度计算了LaS/CeS与γ?Fe之间的界面结合性质与界面能。晶体学计算表明,LaS/CeS与γ?Fe之间沿匹配列的原子间距错配度最小值为10.63 %/10.52 %,密排面间距错配度最小值为2.04 %/3.32 %;LaS与γ?Fe之间粗略的位向关系为：LaS∥γ-Fe & LaS∥γ-Fe和LaS∥γ-Fe & LaS∥γ-Fe ;CeS与γ?Fe之间粗略的位向关系为：CeS∥γ-Fe & CeS∥γ-Fe和CeS∥γ-Fe & CeS∥γ-Fe。基于预测的晶体学位向关系,采用相干界面近似构建了6种不同终端界面模型,第一性原理计算表明,LaS/CeS与γ?Fe之间原子匹配错配度最低界面的粘附功为4.78J·m?2/3.65J·m?2,界面结合强度较高,界面键合以金属键为主。异相界面能计算表明,LaS/CeS与γ?Fe两相之间的原子匹配错配度越小,界面能越低,原子匹配错配度最小时,界面能分别为?0.58J·m?2/?3.43J·m?2,计算结果能够为LaS/CeS与γ?Fe之间的晶体学匹配提供能量学依据。     

Lu2O3电子结构及物理性质的第一性原理研究

基于密度泛函第一性原理平面波赝势方法对Lu_2O_3六方、单斜和立方3种结构进行计算。结合能结果表明C型Ia3立方结构最稳定。立方Lu_2O_3的力学、热力学、电子结构以及光学性质计算揭示:Lu_2O_3有良好的韧性和弹性各向异性特征;热力学稳定性较好;Lu_2O_3为直接带隙,位于导带底的电子有效质量小,非局域程度高;价带顶到导带底跃迁主要源于Lu 4f和O 2p电子。Lu原子5d轨道和O原子2p轨道的强烈杂化形成Lu–O共价键;Lu_2O_3最大光反射率为0.36,在3~10 eV能量范围内其光吸收能力较强,近红外线和可见光范围内有优异透光性能,是良好光绝缘性材料。     

Stability of (Ti, M)C (M = Nb, V, Mo and W) carbide in steels using first-principles calculations

The lattice parameters, formation energies and bulk moduli of (Ti, M)C and M(C, Va) with the B1 crystal structure have been investigated using first-principles calculations, where M = Nb, V, Mo and W. The replacement at 0 K of Ti by Mo or W in the TiC lattice is found to be energetically unfavorable with respect to the formation energy. However, it decreases the misfit strain between the carbide and ferrite matrix, a factor which is of critical importance during the early stages of precipitation, thus favoring the substitution of Ti by Mo, as is observed in practice. The effect of Mo in enhancing the coarsening resistance of (Ti, Mo)C precipitates is discussed in terms of its role in the nucleation process, but followed by a more passive contribution during coarsening itself. The role of tungsten has been predicted to have a similar effect to molybdenum on the nucleation and coarsening process. Analysis of precipitates in Ti-, Ti-Mo- and Ti-W-bearing steels shows results consistent with the calculations.     

Cmcm空间群斜方SrHfO3的电子结构和光学性质理论计算

采用基于密度泛函理论(DFT)框架下广义梯度近似平面波超软赝势法,计算了Cmcm空间群斜方SrHfO3的电子结构和光学性质。计算得到的Cmcm斜方SrHfO3的平衡晶格常数均与实验值相近。同时,计算得到了Cmcm斜方SrHfO3的能带结构、态密度和电荷密度,其结果表明斜方SrHfO3属于直接带隙氧化物,Hf和O之间主要是共价键结合而Sr和O之间主要呈现离子键特性。计算还得到了斜方SrHfO3的复介电函数、折射率和消光系数等,且复介电函数的实部与虚部都与实验结果接近。     

Phase Stability,Magnetic Properties,and Martensitic Transformation of Ni2−xMn1+x+ySn1−y Heusler Alloy with Excess Mn by First-Principles Calculations

The phase stability, magnetic properties, martensitic transformation, and electronic properties of the Ni_2−xMn_1+x+ySn_1−y system with excess Mn have been systematically investigated by the first-principles calculations. Results indicate that the excess Mn atoms will directly occupy the sublattices of Ni (Mn_Ni) or Sn (Mn_Sn). The formation energy (E_f) of the austenite has a relationship with the Mn content: E_f = 135.27(1 + x + y) − 293.01, that is, the phase stability of the austenite decreases gradually with the increase in Mn content. According to the results of the formation energy of austenite, there is an antiparallel arrangement of the magnetic moment between the excess and normal Mn atoms in the Ni_2−xMn_1+x+ySn_1−y (x = 0 or y = 0) system, while the magnetic moment direction of the normal Mn atoms arranges antiparallel to that of Mn_Ni atoms and parallel to that of Mn_Sn atoms in the Ni_2−xMn_1+x+ySn_1−y (x, y ≠ 0) system. The martensitic transformation occurs in some Ni_2−xMn_1+x+ySn_1−y (x, y ≠ 0) alloys with large magnetic moments of ferrimagnetic austenite. Besides, the valence electrons tend to distribute around the Ni or Mn_Ni atoms and mainly bond with the normal Mn atoms. The results of this work can lay a theoretical foundation for further development of the Ni_2−xMn_1+x+ySn_1−y system as the potential ferromagnetic shape memory alloys.     

主族元素掺杂对SmCo5合金结构和磁性能影响的第一性原理计算

建立了以Al、Ga、In、Sn等元素为例的主族元素掺杂SmCo₅合金的计算模型,基于第一性原理结合统计热力学方法研究了添加元素本征特性、掺杂浓度和温度对合金物相结构和磁学性能的影响。计算结果表明,主族元素的优先占位受元素理化性质和掺杂体系占位空间大小两方面的影响;Al和Ga的添加有利于SmCo₅体系保持结构稳定性,且Al的占位概率随温度变化不明显,适用于较宽的温度范围。几种主族元素添加均削弱SmCo₅体系的总磁矩,而In掺杂体系具有相对较大的总磁矩,主要原因是In原子半径较大,引起掺杂体系晶格畸变,使In周围次近邻的Co原子出现磁矩增大的现象,对体系的总磁矩下降具有弥补作用。基于计算结果分析优选出利于SmCo₅体系结构稳定性和磁性能的主族元素Al和In,且预测了Al和In的最佳掺杂浓度范围。     

Electron-induced surface modification of hydroxyapatite-coated implant

Here, the authors report on surface free energy modulation of a hydroxyapatite-coated titanium femoral implant that is performed by a newly-developed method using a low-energy electron irradiation. They observe pronounced increase of hydrophobicity of irradiated samples that occurs in several stages and is characterized by various mechanisms. Bacterial adhesion on electron modified hydroxyapatite samples is studied, by considering different approaches. The authors show that bacterial adherence is selective and depends on the surface free energy components, which were determined from detailed surface free energy analysis. The selective bacterial adhesion, together with the ability to define the surface energy properties, suggests that this newly-developed method opens an avenue for protection of implants from bacterial infections.     

化学氧化处理对不锈钢表面性质的影响

研究了化学氧化处理对不锈钢（1Cr18Ni9Ti）表面亲水性、表面自由能及表面化学结构的影响,并提出了此三者之间的关系。研究结果表明：化学氧化处理可以有效改善不锈钢的表面亲水性,且亲水性随处理时间的延长和处理温度的升高而升高,最佳工艺条件是处理温度为75℃,处理时间为8min;经化学氧化处理后,不锈钢的表面自由能γgs及其极性分量γgs^p显著增大,使不锈钢表面呈现出极强的亲水性;多功能电子能谱分析表明,表面自由能γgs及其极性分量γgs^p的增加与表面及一定厚度范围内含氧极性基团的增加有关。     

ZrCr2 Laves相弹性性质和堆垛层错能的第一性原理计算

采用缀加平面波加局域轨道方法和广义梯度近似对立方C15结构的ZrCr2Laves相金属间化合物的弹性性质，包括弹性常数和弹性模量，以及层错能进行理论计算。结果表明：计算得到的ZrCr2Laves相的弹性性质与实验结果相近，其泊松比和弹性各向异性系数大小说明ZrCr2中原子键合的方向性并不强烈；ZrCr2Laves相的内禀和外禀层错能分别为112mJ／m^2和98mJ／m^2。并计算了层错与位错的弹性交互作用。对ZrCr2Laves相的力学特性和变形机制进行了讨论。     

十八酸包覆鳞片状锌粉性能的研究

以制备超细鳞片状锌粉为目的，对羧酸烃类表面活性剂进行了研究。研究发现，十八酸能显著降低粉体材料的表面能，粉碎性能优良，固体分散性较好；但是十八酸的高温挥发性差、化学稳定性及相溶性皆不够，制出的粉体光亮度较差，所以在粉体材料的制备过程中，加入十八酸的量不能太多，应控制在0．5％～1．0％范围内，且应与其它表面活性剂配合使用。     

钛合金中原子迁移性质的第一原理研究

钛合金作为一种重要的航空材料,具有比强度高,耐蚀性好等优异性能,并具有一定的高温蠕变抗力。但是在更高温度下,有限的蠕变抗力限制了钛合金的进一步应用。实验研究表明,钛合金稳态蠕变过程蠕变激活能与合金的表观扩散激活能非常接近,说明原子的扩散与蠕变过程密切相关。本文采用第一性原理的方法,计算了钛合金中常见的杂质及合金原子的迁移能垒。结果表明,对以空位机制扩散的合金原子,其在基面内迁移的能垒从高到低为Al、V、Ti、Sn、Ta、Mo、Nb、Zr,面间原子迁移由难到易为Al、Sn、V、Ti、Ta、Mo、Nb、Zr。以间隙机制迁移的Co、Fe、Ni迁移能垒较低,与实验观测到的这些原子为快扩散原子相符。