Nb_2Al(100)表面的电子结构和相对稳定性（英文）

采用基于密度泛函理论的第一性原理计算,研宄了Nb_2Al(100)表面不同终止端的电子结构、表面能和热力学性质。计算结果表明,由于表面弛豫和表面态的形成,所有终止端表面的电子结构均表现出增强的金属性质和减弱的共价性质。根据不同终止端表面的表面能计算,分析了非化学计量比表面的稳定性。在富含Nb和Al的条件下,C终止端表面(Nb_(22)Al_(12))是热力学最稳定的表面。此外,计算了Nb_2Al(100)表面的功函数,表明该表面获得和失去电子的能力与形成前的纯元素表面相似。     

Si合金化对C15 NbCr2 Laves相稳定性和断裂韧性影响的第一性原理研究

基于第一性原理计算方法,通过对形成焓、结合能、原子自由体积和电子结构的计算,研究了Si合金化对C15 NbCr₂Laves相稳定性和断裂韧性的影响。位点占据能表示Si原子倾向于占据Cr位点。形成焓和结合能计算表明,随着Si含量的增加,Nb₈Cr_16-xSi_x(X= 0~ 5)相的形成能力和稳定性均得到提升且与Si含量保持线性相关性。原子自由体积计算表明,Nb₈Cr_16-xSi_x相的原子自由体积较NbCr₂基体相均得到增加,其中在Si含量为8.33 at%(Nb₈Cr₁₄Si₂)时,原子自由体积取得最大值,断裂韧性达到最优。电子结构计算表明,Si合金化使得DOS曲线右移,费米能级向赝能隙峰谷靠近,稳定了NbCr₂基体相,同时所有的成键峰变得下降和展宽,削弱了Nb-Cr原子的键合强度,使得剪切变形易于进行,从而提高韧性。     

钛种植体表面CuO和Nb2O5共掺杂TiO2涂层的制备及抗菌性能和生物相容性

利用等离子体喷涂技术在医用钛合金表面成功制备TiO₂、50% Nb₂O₅-TiO₂和1% CuO-49% Nb₂O₅-TiO₂涂层，并对涂层的微观结构、表面粗糙度、表面亲疏水性、化学稳定性进行表征。利用金黄色葡萄球菌和大肠杆菌来评价涂层的抗菌性能，利用小鼠颅顶前骨细胞亚克隆14细胞来评价涂层的细胞相容性。结果表明：等离子体喷涂TiO₂涂层主要由金红石和少量锐钛矿相组成，而在50% Nb₂O₅-TiO₂和1% CuO-49% Nb₂O₅-TiO₂涂层中均出现了Ti_0.95Nb_0.95O₄固溶体和特殊的棒状交织结构；Nb₂O₅和CuO的掺杂增加了TiO₂涂层的表面粗糙度，改善其亲水性。Nb₂O₅掺杂提高了细胞的增殖能力和成骨分化能力，CuO和Nb₂O₅共掺杂显著提高了TiO₂涂层的抗菌性能，但Cu离子的释放降低了50% Nb₂O₅-TiO₂涂层的细胞相容性。和TiO₂涂层相比，1% CuO-49% Nb₂O₅-TiO₂涂层的细胞相容性并未明显降低，说明Nb₂O₅掺入可一定程度上抑制了Cu离子对细胞的副作用。     

CO2 在 δ-Pu(100)表面吸附行为的第一性原理研究

结合密度泛函理论框架内的周期性平板模型,运用第一性原理计算方法研究了CO₂在δ-Pu(100)表面的吸附行为。结果表明,CO₂分子以C端向下和C-Pu、O-Pu多键结合的方式吸附在δ-Pu(100)表面。吸附类型属于强化学吸附,最稳定的吸附构型是H₁-C₄O₄,此时吸附能为-6.430 eV,吸附稳定性顺序为穴位>桥位>顶位。CO₂分子主要和表面Pu原子反应,而与其它3层Pu原子的反应较弱。更多的电子向CO₂ 2π_u轨道转移有利于C-O键的弯曲和活化。此外,CO₂分子和Pu原子之间的化学键主要是离子态,反应机理是CO₂的C 2s、C 2p、O 2s 和O 2p轨道与Pu 6p、Pu 6d、Pu 5f轨道发生了重叠杂化作用,产生了新的键结构。H₁-C₄O₄构型的功函数变化最小,表明其它电子容易从该构型表面逃逸,且需要的能量最小。     

保温时间对瞬时液相扩散连接的Ti3Al /Ti2AlNb接头微观结构及力学性能的影响

采用Ni-Ti复合箔片作为中间层,在990 ℃、低连接压力（0.1 MPa）下,通过瞬时液相（TLP）扩散连接制备了Ti₃Al/Ti₂AlNb异种合金接头。分析了保温时间（10~90 min）对Ti₃Al/Ti₂AlNb接头微观结构及力学性能的影响,并研究了TLP扩散连接接头的界面演变和形成机制。结果表明,Ti₃Al/Ti₂AlNb接头具有典型的“Ti₃Al | Al_0.5Nb_0.5Ti₃ | 残余 Ni | NiTi | NiTi₂ | 残余 Ti | Al_0.5Nb_0.5Ti₃ | Ti₂AlNb”多层梯度结构。随着保温时间的延长,接头的抗剪切强度先增大后减小,当保温时间达到60 min时,Ti₃Al/Ti₂AlNb接头的抗剪切强度最大,达到167±12 MPa。另外,接头的断裂主要发生在Ti₂AlNb/Ti附近的NiTi₂层,并向Ti层延伸,呈现出脆性断裂的特征。     

Ti和Al的交互作用对Nb-Ti-Si-Al合金微观组织和力学性能的影响

Nb-Si基超高温合金由于具有低密度和高熔点,有望取代当前先进的Ni基高温合金,成为新型的高温结构材料。Ti和Al均为对Nb-Si基合金有益的合金化元素。在文献中,通常对低含量(≤ 3 at. % )Al进行合金化研究,在此情形下不会出现Nb-Al金属间化合物相。Nb-Al金属间化合物中Nb₃Al和Nb₂Al具有良好的强度和抗蠕变性能,NbAl₃则是重要的高温结构材料,而目前公开文献中关于Nb-Si基合金中含有Nb-Al金属间化合物的报道较少。因此,本文重点关注高Al含量对合金中Nb-Al金属间化合物形成的影响,研究两个高Al含量的Nb-Ti-Si-Al合金(A2: Nb-18Ti-14Si-9Al、A4: Nb-21Ti-14Si-9Al)的微观组织和力学性能,其中设计A4合金是为了分析Ti含量变化的影响。A2合金由(Nb)、Nb₅Si₃和Nb₃Al三相组成,而A4合金由(Nb)和Nb₅Si₃两相组成。A2和A4合金的室温断裂韧性分别为11.1 MPa.m_1/2和10.9 MPa.m_1/2,同时对两个合金进行微压痕测试,以表征合金在微观尺度的力学性能。     

电阻焊接技术制备Nb3Sn-NbTi 混合超导接头

在强磁场超导磁体应用中,特别是具有高稳定度高场超导磁体,具有极低电阻的超导接头发挥着重要作用。通过电阻焊接方法制备了Nb₃Sn 和NbTi的异种材料超导接头。首先通过烧结法制备了Nb₃Sn的超导接头,然后通过电阻焊接将Nb₃Sn超导接头与NbTi超导体焊接成型。分析了Nb₃Sn超导接头制备过程中Nb₃Sn的微观组织结构和相变。利用无损检测CT对接头结构进行三维重构,分析了Nb₃Sn 和NbTi超导接头内部的缺陷和2种不同材料间的微观连接状态。最后,在背景磁场下对超导接头的电阻性能进行了测试和研究。结果表明,与传统超导焊料制备的超导接头相比,电阻焊接制备的超导接头显示出良好的耐磁场性能,在1.5 T背景磁场下具有较低的电阻值。     

γ-TiAl合金表面Al2O3/Al复合涂层的抗高温熔盐腐蚀性能研究

采用磁控溅射技术于γ-TiAl合金表面制备Al₂O₃/Al复合涂层。在850 °C下、 100 wt.% Na₂SO₄熔盐中观测Al₂O₃/Al复合涂层的高温腐蚀行为。结果表明,Al₂O₃/Al复合涂层具备由Al₂O₃表层、富Al中间层以及互扩散层组成的梯度结构,因而有效地提高了基体γ-TiAl合金的抗高温腐蚀性能。在腐蚀实验后,涂层试样表面相结构为Al₂O₃,TiO₂和TiAl₃。致密的Al₂O₃/Al复合涂层有效地抑制了O_2-,S_-和Na₊对基体γ-TiAl合金的侵蚀。并且,Al₂O₃/Al复合涂层的梯度结构亦使其表现出了优异的抗开裂和抗剥落性能。     

过渡元素改善B4C/Al材料界面润湿性的机理研究

B₄C/Al复合材料是目前最理想的中子吸收材料,但工业上常用的液态搅拌法制备过程中存在着界面润湿性差的问题。本文结合实验及第一性原理的方法,通过研究Al(111)/AlB₂(0001)和Al(111)/TiB₂(0001)界面的结构来分析工业上添加过渡元素Ti对B₄C/Al界面润湿性的改善机制。通过计算发现,Al(111)/TiB₂(0001)界面相对Al(111)/AlB₂(0001)界面具有更高的粘附功值,说明其界面结合更强。进一步对比Ti掺杂二硼化物和AlB₂的偏态密度结构,发现Ti掺杂体具有较低的反键态,表明Ti-3d和B-2p轨道电子杂化后,在B、Ti原子间形成了较强的化学键,从而促进了Al(111)/TiB₂(0001)界面处的强结合作用,提高了Al(111)/TiB₂(0001)界面粘附功,故而改善了B₄C/Al界面的润湿性。根据同样的理论依据,V掺杂体也具有较低的反键态,V和B之间的强结合效果或许能够改善B₄C/Al界面的润湿性,成为又一理想的溶体改性掺杂元素。     

凝固速度对Ni47Ti44Nb9铸态合金组织和织构的影响

对比研究了Ni₄₇Ti₄₄Nb₉合金快冷铸锭和慢冷铸锭的宏、微观组织和织构特点,分析了凝固速度对组织和织构的影响机理。采用光学显微镜(OM)和扫描电子显微镜(SEM)观察了微观组织,采用X射线衍射(XRD)分析了织构。结果表明,铸态Ni₄₇Ti₄₄Nb₉合金具有<113>方向的择优生长取向;凝固速度对Ni₄₇Ti₄₄Nb₉合金铸态组织和织构的形成具有重要影响,快速冷却条件下获得均匀的树枝晶组织,铸锭的横、纵截面均形成{113}织构,慢速冷却条件下形成具有特定方向的柱状树枝晶,纵截面织构以{113}<361>、{113}<332>和{113}<141>为主;凝固过程中热量方向性传导造成的树枝晶定向生长是影响合金铸态组织和织构形成的主要原因。     

Electronic structure and related properties of Pd/TiAl membranes

First principles calculations reveal that the L1₀ TiAl bulk is energetically favorable among several crystal structures and the Ti–Al interaction has different effects on the electronic structures of Ti and Al. Calculations also show that a number of four surface layers is sufficient to get a converged work function and surface energy for both fcc Pd and L1₀ TiAl, and the Al-terminated (100) and (110) TiAl surfaces possess lower surface energies than other surface configurations. In addition, it is revealed that interface orientation and atomic configuration have some effects on various properties of Pd/TiAl interfaces. Interestingly, the high bond strengths and negative interface energies suggest that the Pd/TiAl interfaces would be energetically favorable and thermally stable, which seem good for the lifetime of Pd/TiAl membranes.     

Development and oxidation resistance of air plasma sprayed Mo-Si-Al coating on an Nbss/Nb5Si3 in situ composite

A Mo-Si-Al coating, which is mainly composed of Mo(Si,Al)₂ and Mo₅(Si,Al)₃, was developed to protect a Nb_ss/Nb₅Si₃ in situ composite by air plasma spraying. After oxidation at 1250 °C, the oxidation curve followed parabolic law and even after oxidation for 100 h, the weight gain of Mo-Si-Al coating was 8.24 mg/cm². The surface of the oxidized samples became flatter and smoother as time increased due to the formation of SiO₂ glass. Moreover, the microstructure of Mo-Si-Al coating changed and a layer structured interdiffusion zone was formed at the substrate-coating interface after oxidation.     

Electronic structure and physical properties of hcp Ti3Al type alloys

According to the basic information of sequences of Ti and Al characteristic atoms in hcp Ti-Al system, the compositional variations of the electronic structure, atomic potential energies, atomic volumes, lattice constants and cohesive energies of the ordered hcp Ti3Al type alloys were calculated by the framework of systematic science of alloys（SSA）. The electronic structure of the hcp Ti3Al compound consisted of ψ4h^Ti and ψ0h^Al atoms is 0.75[Ar] （3dn）^0.573（3dc）^2.1685（4sc）^0.972（4sf）^0.3093＋0.25[Ne]（3sc）^1.32（3pc）^1.19（3sf）^0.49. The factors of controlling lattice stability are electronic structure, atomic energies and atomic concentration. The ψ4h^Ti atoms play a determinative role in forming D019 structure with a=0.287 2 nm, c=0.456 4 nm, atomic cohesive energy e=4.810 8 eV/atom and heat of formation △H=-0.332 8 eV/atom. These calculated values are in good agreement with experimental values （a=0.287 5 nm, c=0.46 0 nm, △H=-0.27, -0.29 eV/atom）. The calculated cohesive energy of the hcp Ti3Al compound is slightly bigger than that of the fcc Ti3Al.This is a good sign that makes it feasible to stabilized L 12 structure of the hcp Ti3Al compound by ternary element, The new element should have more dc-electrons than Ti-metal and occupy at the Ti-lattice points.     

Ag表面性质的第一原理计算

采用第一原理赝势平面波方法,计算了Fcc-Ag晶体及其Ag(111)、Ag(110)与Ag(100)自由表面的能量、几何与电子结构.根据表面能的计算,预测了Ag表面的结构稳定性,结果表明密排Ag(111)面结构稳定性最好,低指数奇异面Ag(100)面次之,Ag(110)面的结构稳定性最差.通过对不同表面几何与电子结构的比较,初步分析了其结构稳定性差异的产生原因.表面原子驰豫不仅引起表面几何结构的变化,而且使表面层的电子结构与键合特性发生改变.驰豫后表面层原子的部分价电子跑到了表面层以上的真空区,使表面层原子的电子态密度峰形发生变化,还新形成了表面态,这是表面能产生的主要原因,而Ag(110)表面相对于Ag(111)与Ag(110)表面具有高表面活性的主要原因则源于其表面层原子显著的结构驰豫.     

A study of the effects of Hf and Sn additions on the microstructure of Nbss/Nb5Si3 based in situ composites

The phase stability and microstructures of Nb_ss/Nb₅Si₃ based in situ composites alloyed with Hf and Sn have been investigated. The Nb₅Si₃ silicide in the β and γ forms was the primary phase in the Nb–24Ti–18Si–5Cr–5Al–5Hf–2Mo and Nb–24Ti–18Si–5Cr–5Al–5Hf–5Sn–2Mo alloys studied in this work. In the as cast alloys, the formation of the Nb₃Si phase was suppressed by the addition of Hf. The structure of the Nb₅Si₃ phase was mainly affected by the Hf addition. The Hf-rich regions in the 5–3 silicide probably corresponded to the γNb₅Si₃. This silicide was stable up to 1500 °C in the presence of Hf. The Ti solubility in the Hf-rich Nb₅Si₃ was higher than that in the Nb₅Si₃. The Si concentration in the Nb₅Si₃ phase increased slightly and shifted closer to its stoichiometric composition by the addition of Hf. The alloying elements Hf and Sn preferentially substituted for Nb in the Nb₅Si₃ and Nb_ss, respectively. The Sn addition had a significant effect on the niobium solid solution leading to the formation of Sn-rich and Sn-poor parts in the solid solution in the as cast microstructure. In the presence of Sn, the Si solubility in Nb_ss increased considerably whilst the Cr solubility decreased. As a result of the decrease of the Cr solubility in Nb_ss, the Sn addition promoted the formation of Si-rich C14 Cr₂Nb Laves phase and stabilised this phase at 1300 °C. TiN and HfO₂ were formed below the surfaces of the heat treated alloys.     

Oxidation and nitridation of niobium in air above 1150°C

When niobium is heated in static air, layers of oxides and nitrides form on the surface, depending on the temperature. At 1150–1500°C, a layer of solid Nb₂O₅ forms with linear growth kinetics. At 1500–1900°C, a layer of Nb₂N forms under a two-phase layer of liquid oxide+solid NbO₂. The two-phase oxide layer has linear growth kinetics, while the Nb₂N layer has parabolic kinetics. Above 1900°C, successive layers of Nb₂N, NbN, and liquid oxide form, with both nitrides exhibiting parabolic growth kinetics. The activation energies of layer growth are as follows: Q=105 kJ/mol for Nb₂O₅, 120 kJ/mol for the two-phase oxide layer, and 220 kJ/mol for Nb₂N (Q for liquid oxide and NbN layer formation was not determined). Above 1500°C, the nitride layers are somewhat protective, in that they slow the oxide formation rate and prevent ignition.     

The origin of hole injection improvements with MoO3/Al bilayer electrodes in pentacene thin-film transistors

The electronic structures of pentacene/MoO₃/Al and pentacene/Al were studied using in situ photoemission spectroscopy. The secondary electron cutoffs, highest occupied molecular orbitals (HOMOs) and core level changes were measured upon deposition of the pentacene and MoO₃ to study the electronic structures at the interface. We obtained complete energy level diagrams for each interface, showing that the insertion of the MoO₃ layer between pentacene and Al induces the HOMO level shift of pentacene toward lower binding energies compared to that without MoO₃. This results in reduction of the hole injection barrier, which is responsible for the improvements in electronic device performance.     

Structural and mechanical properties of Fe–Al compounds: An atomistic study by EAM simulation

The structural properties, formation enthalpies, and mechanical properties of Fe–Al compounds (FeAl, Fe₂Al, Fe₃Al, FeAl₂, FeAl₃ and Fe₂Al₅) are studied by using embedded-atom method (EAM) which is acquired by Mobius lattice inversion. The potential is transferrable and therefore does well for studying different Fe–Al compounds. The calculated lattice parameters and cohesive energies of Fe–Al compounds agree with the experimental and some EAM results. According to elastic constants restrictions, all the six Fe–Al compounds are mechanically stable. The calculated bulk moduli of the compounds increase with the increasing Fe concentration. Furthermore, results showed that FeAl, Fe₃Al, FeAl₃, FeAl₂, Fe₂Al₅ have lower ratios of shear modulus to bulk modulus and Fe₂Al has higher ratio.     

Temperature Dependence of the Surface Energy of the Low Index Planes of UO<Subscript>2</Subscript> and ThO<Subscript>2</Subscript>

The temperature dependence of the surface energy γ of the low index (111), (110), and (100) planes of UO₂ and ThO₂ was calculated for the first time, using a simple method based on physical and thermodynamic quantities and considerations. The extrapolated to 0 K surface energy values agreed well with the available theoretical data reported in literature for the (111) and (110) surfaces, whereas they were significantly lower for the (100) surface. The γ ₁₀₀/γ ₁₁₁ ratio for UO₂ and ThO₂ revealed the formation of crystals with truncated octahedron structure in thermodynamic equilibrium. This structure shifts to octahedral with increasing temperature. Comparison with experimentally determined surface energies of the polycrystalline oxides showed that their values are in the range between γ ₁₁₁ and γ ₁₀₀.     

Thermodynamic properties of Al,Ni, NiAl,and Ni3Al from first-principles calculations

The thermodynamic properties of Al, Ni, NiAl, and Ni₃Al were studied using the first-principles approach. The 0-K total energies are calculated using the ab initio plane wave pseudopotential method within the generalized gradient approximation. The contribution to the free energy from the lattice vibration was calculated using the phonon densities of states derived by means of the ab initio linear-response theory. The thermal electronic contribution to the free energy was obtained from the one-dimensional numerical integration over the electronic density of states. With the deduced Helmholtz free-energy, the thermal expansion and enthalpy as a function of temperature were calculated and compared with the experimental data. Our calculations show that the enthalpies of formation are slightly temperature dependent with a slope of −1.6 J/mol/K for NiAl and −1.2 J/mol/K for Ni₃Al. For Ni, the inclusion of thermal electronic excitation results in a 10% increase in thermal expansion and 15% increase in enthalpy at 1600 K.