Nb-Ti-Al合金高温氧化机理电子理论研究

采用递归法计算了Nb合金的电子态密度、原子镶嵌能、亲和能和团簇能等电子结构参数,研究Nb合金高温氧化机理.研究表明,氧在Nb合金表面的吸附能较低,易在合金表面吸附,并逐渐扩散到Nb合金的基体中.氧在合金基体中镶嵌能为负值,氧的态密度和Nb相似,在Nb中具有很高的溶解度.Ti,Al在合金晶内的镶嵌能均高于各自在合金表面的镶嵌能,Ti,Al从合金内部向合金表面扩散,最终在Nb合金表面偏聚,形成富Ti,Al的表层.团簇能计算结果表明Nb合金表面的Ti,Al原子各自均有聚集倾向,分别形成Ti和Al原子团.氧与合金     

Characterization of Fe–Nb sputtered thin films

Structure, composition and chemical behavior of co-sputtered Fe–Nb thin films are analyzed by different techniques, as conversion electron Mössbauer spectroscopy, x-ray photoelectron spectroscopy and x-ray diffraction. It is shown that oxygen is determinative in hindering the Fe–Nb alloy formation and, as a result, Fe_1−xO and Nb₂O₅ occur in significant amounts, even in vacuum. In spite of the oxygen role, a Fe–Nb alloy is formed in little amounts, which increase as the Nb content is increased. The increase of the Nb content is also related with the increase of Fe_1−xO and with a decreasing of the metallic Fe present. Mössbauer data indicate the Fe–Nb phase present is the Fe₂Nb Laves phase.     

Auger electron spectroscopic study of the surface oxidation of uranium-niobium alloy {U-6 wt.% Nb} in a UHV environment containing primarily H2, H2O and CO

Auger electron spectroscopy has been used to study the surface oxidation of a uranium-niobium alloy, U-6 wt.% Nb, in a UHV environment containing primarily H₂, H₂O and CO. For comparison, the reaction of the oxidative species with pure uranium and niobium metals has also been investigated. The observed changes of the shape and position of the Auger transition peaks for U, Nb and O following exposures this UHV environment up to 200 L, and under a continuous electron bombardment indicated that oxidation of the alloy leads, first, to the formation of a surface oxide consisting of UO_2.0. The formation of this oxide was followed by the development of Nb-oxides (NbO, NbO₂ and Nb₂O₅) at the metal-oxide interface. Pure uranium was found to exhibit faster oxidation kinetics than the alloy, while pure niobium oxidised at a much lower rate than the uranium based materials.     

First-principles study of electronic properties and stability of Nb5SiB2（001） surface

The density functional calculations are performed to study the electronic structure and stability of Nb 5 SiB 2(001) surface with different terminations.The calculated cleavage energies along the(001) planes in Nb 5 SiB 2 are 5.015 J · m 2 and 6.593 J · m 2 with the break of Nb-Si and Nb-NbB bonds,respectively.There exists a close correlation between the surface relaxation including surface ripple and the cleavage energy:the larger the cleavage energy,the larger the surface relaxation.Moreover,the surface stability of the Nb 5 SiB 2(001) with different terminations has been investigated by the chemical potential phase diagram.From a thermodynamics point of view,the four terminations can be stabilized under different conditions.In chemical potential space,NbB(Nb) and Nb(Si) terminations are just stable in a small area,whereas Si(Nb) and Nb(NbB) terminations are stable in a large area(the letters in brackets represent the subsurface atoms).     

Calculation of Chemical Shifts of X-Ray-Emission Spectra of Niobium in Niobium(V) Oxides Relative to Metal

Chemical shifts of the K_α1 and K_β1 lines of X-ray-emission spectra of niobium in oxides (Nb₂O₅)_n, n = 1–4, relative to metal Nb have been calculated. Stoichiometric clusters (Nb2O5)n the electronic structure of which was calculated using accurate relativistic pseudopotentials and two-component version of the density functional theory are considered as prototypes for modeling different crystal forms of niobium(V) oxide. The chemical shifts were calculated using the method based on using the property of approximate proportionality of valence spinors in the core region of the heavy atom [11]. Corrections to values of chemical shifts have been determined with allowance for deviations from the abovementioned proportionality. Rapid convergence of results with respect to the size of the niobium oxide cluster has been demonstrated.     

Fabrication,characterization and biocompatibility of TiO2 nanotubes via anodization of Ti6Al7Nb

Ti6Al7Nb has been used as an implant material because of its good corrosion resistance and high mechanical properties. However, the presence of aluminium (Al), which may lead to ostemalacia, anaemia and nervous system disorders, limited its wide clinical use. In this study, a titanium oxide (TiO₂) nanoporous layer was fabricated on a Ti6Al7Nb alloy using an electrochemical anodic oxidation method. The structure of the TiO₂ nanoporous layer was examined by scanning electron microscopy. The chemical compositions of the samples were analysed by X-ray photoelectron spectroscopy (XPS). Biocompatibility was evaluated by culturing rat osteoblast cells. The result showed that TiO₂ nanoporous layers comprise a mixed oxide containing TiO₂ and a small amount of nobium oxides (Nb₂O₅) and almost no elemental aluminium. The outer layer of the TiO₂ nanoporous layer comprises highly ordered nanotubes and the inner layer forms disordered nanopores. The TiO₂ nanoporous layer could support the adhesion, proliferation, differentiation and gene expression of osteoblast cells. Therefore, a TiO₂ nanoporous layer could enhance the biocompatibility of Ti6Al7Nb alloy and is as a promising candidate for Ti6Al7Nb alloy implants.     

On the oxidation and on the superconductivity of niobium

Most superconducting rf cavities consist on Nb and most Josephson tunnel junctions are Nb based. Due to the very small dissipation in these superconducting cavities and tunnel junctions quantitative and qualitative new surface effects emerge, which are related to the oxidation of Nb.The oxidation of Nb is dominated by the solution of oxygen in Nb and the growth of Nb₂O₅ microcrystallites. Both effect heavily strain the Nb-Nb₂O₅ interface creating O segregates and serrating the interface, growing with oxidation. These effects yield a serrated and eroded transition superconductor — insulator, which affects the properties of superconducting rf cavities and tunnel junctions significantly.Based upon a habilitation thesis, University of Karlsruhe (1985)     

Structure and apatite induction of a microarc-oxidized coating on a biomedical titanium alloy

An oxide coating with nanostructure was prepared by micro-arc oxidation (MAO) on a biomedical Ti-24Nb-4Zr-7.9Sn alloy. Chemical composition of the coating mainly includes O, Ti, Nb, Ca, P, Na, Zr and Sn, where the ratio of Ca/P is about 1.6. Ti, Nb, Zr and Sn participate in the oxidation to form TO₂, Nb₂O₅, ZrO₂ and SnO₂ nanocrystals, while Ca, P and Na are present in the form of amorphous phases. After alkali treatment, the surface of the MAO coating becomes rough, and Na concentration increases remarkably while P disappears basically. The alkali treated coating shows better apatite forming ability than the untreated one, as evidenced by apatite formation after SBF immersion for 7 days. The improvement of apatite forming ability of the modified coating is attributed to the formation of a sodium titanate layer and numbers of submicron-scale network flakes. The enhancement of the surface wettability of the alkali treated coating also plays an important role in promoting the apatite forming ability.     

超导Nb-Ge溅射膜的俄歇电子能谱及电子能谱化学分析

利用表面分析仪器俄歇电子能谱(AES)仪和电子能谱化学分析(ESCA)仪对超导Nb-Ge溅射膜进行表面和深度剖面分析,以探讨在溅射沉积成膜过程中A15结构Nb₃Ge能够稳定生长的机制,以及影响其转变温度T_c的因素。分析结果表明,超导Nb-Ge膜中含有O,C和Al等杂质,其中O的含量对T_c有重要影响。表面上未发现有聚集的Ge。Nb/Ge密度比由表面层的3.1向内部逐渐降至2.26。X射线光电子能谱(XPS)的分析表明:Nb,Ge,C和O的谱峰沿深度均有不同程度的化学位移和峰形变化,这明这些元素的化学态在膜内是复杂的。但其中Nb的变化最小,可能对高T_cA15Nb₃Ge相的生长起稳定作用。 关键词：     

Effect of Nb and Cr incorporation on the structural and magnetic properties of rapidly quenched FeCoSiB microwires

Rapidly quenched microwires with a nominal composition of Fe₃₉Co₃₉Si₈B₁₄ (#A_O), Fe₃₇Co₃₇Nb₄Si₈B₁₄ (#A_N) and Fe₃₆Co₃₆Nb₄Cr₂Si₈B₁₄ (#A_NC) have been investigated. Devitrification of as-quenched microwires showed that crystallization temperatures increased with simultaneous incorporation of Nb and Cr as in #A_NC alloy. Addition of these elements also contributed to an increase in activation energy in #A_N and #A_NC alloys. Nb addition reduced the particle size, which became much finer in the case of the Cr-containing alloy. Although Nb addition did not have much effect on lowering the Curie temperature T_C of the amorphous phase, Cr substitution lowered T_C to 698 K from high values of 785 K and 787 K observed in the #A_O and #A_NC alloys, respectively. However, the Cr addition revealed a better Giant magneto-impedance (GMI) response compared to the other alloys. Such improved GMI properties in the Cr-containing alloy are attributed to lower values of the coercivity and magnetostriction in the alloy containing both Nb and Cr.     

Physical properties of tetragonal transition-metal borides Nb2MB2 (M = Mo,W, Re or Os) with a new superstructure

The mechanical and thermodynamic properties, chemical bonding characteristics and electronic structure of Nb₂MB₂ (M = Mo, W, Re or Os) with a new tetragonal U₃Si₂-type superstructure (space group P4/mnc, no. 128) were studied by means of density functional theory calculations. All Nb₂MB₂ structures studied were demonstrated to be thermodynamically and mechanically stable. The bulk, shear and Young's moduli, Poisson's ratio, Debye temperature and anisotropy factors were derived for ideal polycrystalline Nb₂MB₂ aggregates. Among these compounds, Nb₂WB₂ was found to have the highest shear modulus and hardness. The electronic densities of state and electronic localization function analysis revealed the metallicity and strong covalent B–B, Nb–B and M−B bonding in Nb₂MB₂. Moreover, these results reveal that the covalence between Nb 4d, M nd (n = 4 for Mo and 5 for W, Re and Os) and B 2p states is the cause of the relatively higher elastic modulus and hardness of the Nb-based compounds. Finally, thermodynamic properties, including the bulk modulus, heat capacity and thermal expansion coefficient of Nb₂WB₂ were obtained systematically under high temperature and pressure.     

Elastic,mechanical, electronic properties and hardness of Nb2AsC from first principles

The study aims at the elastic, mechanical, electronic properties and hardness of Nb₂AsC using first principles based on the density functional theory method within the generalised gradient approximation. The calculated lattice parameters of Nb₂AsC are in good agreement with the experimental data. The five independent elastic constants are firstly calculated as a function of pressure, and our results indicate that it is mechanically stable in the applied pressure. The elastic anisotropy is examined through the computation of the direction dependence of Young's modulus. The pressure dependences of the bulk modulus, shear modulus, average velocity of acoustic waves and Debye temperature of Nb₂AsC are systematically investigated. The band structure and density of states are discussed, and the results show that the strong hybridisations C p–Nb d and As p–Nb d would be beneficial to the structure stability of Nb₂AsC. Based on the Mulliken population analysis, the hardness of Nb₂AsC is predicted.     

Electronic structure of AgNbO3 and NaNbO3 studied by X-ray photoelectron spectroscopy

The XPS examinations of the AgNbO₃ and NaNbO₃ single crystals and ceramics allowed estimate their average composition as Ag_1.1Nb_0.9O₃ and Na_1.2Nb_0.9O_2.9. The valence bands of the AgNbO₃ compound, formed mainly of the Nb 4d, Ag 4d and O 2p states, show an energy gap about 3 eV while for the NaNbO₃ compound consist of the O 2p states hybridized with the Nb 3d states and show an energy gap about 4 eV. The chemical shifts of these compounds suggest a mixed ionic and covalent character of the bonds. The broadening of the core level lines of AgNbO₃ suggests a stronger structural disorder in comparison with NaNbO₃ compound.     

External strain effect on the electronic and mechanical properties of the superconductor Nb2InC

We use first-principles method to investigate the effects of external strain ε on the structural, mechanical and electronic properties for the superconductor Nb₂InC. The results show that the tensile strain induces an isostructural phase transition in Nb₂InC. The elastic constants C_ij, bulk modulus B, shear modulus G, Young's moduli E, and Poisson ratio v_ij of Nb₂InC were also investigated in the range from ε=−10% to ε=10%. It indicates that Nb₂InC is mechanically stable under external strain, and its brittle–ductile transition occurs at ε=3.5%. Moreover, Nb₂InC gets a negative Poisson ratio at ε=4%. The calculated electronic structures indicate that the Nb–C bonding is stronger than Nb–In bonding in Nb₂InC. The energy band structures and densities of states of strained Nb₂InC were also calculated and discussed in detail. From these calculations, it is clear that the related properties of Nb₂InC can be easily tuned by strain. Therefore, our findings are very useful to tailor the physical properties of Nb₂InC by using strain engineering.     

A first principles study of constitutional and thermal defects in D8m-Si3Nb5

First principles calculations of point-defect enthalpies of formation are performed in the D8_m-Si₃Nb₅ compound. Four sublattices are introduced to account for the D8_m structure. Based on a statistical–thermodynamic model, the defect concentrations are calculated as function of temperature and deviation from stoichiometry. For the stoichiometric composition, all the point-defect atomic concentrations remain very low even at the compound melting temperature. In the Si-rich D8_m-Si₃Nb₅, the constitutional defects are Si antisites on the Nb 4b sites of the D8_m structure. The thermal defects are Si antisites on the 16k sites of the D8_m structure, but their concentration remains very small even at high temperature. In the Nb-rich D8_m-Si₃Nb₅, the constitutional defects are Nb antisites on the 8h Si positions. The formation Gibbs energy of the D8_m-Si₃Nb₅ phase as well as the Si and Nb chemical potentials are obtained as function of the deviation from stoichiometry for various temperatures. The extension of the one-phase domain of D8_m-Si₃Nb₅ in the Si–Nb phase diagram is discussed.     

氮化铌表面氧化物的XPS研究

本文用X射线光电子能谱(XPS)对室温下自然氧化和射频氧化的氮化铌薄膜表面进行了成分分析。指出:与在类似条件下纯铌膜的氧化不同,NbN膜的表面氧化物中不存在NbO和NbO₂,而是以Nb₂O₃作为从NbN到Nb₂O_5-y的过渡相。氮化铌在氧化过程中表现出一种抑制Nb₂O,NbO等低价传导性氧化物生成的能力,这对于用氧化物作势垒的超导隧道结的研制具有实际参考价值。 关键词：     

XPS valence band spectra of Nb3Sn,Nb and Sn

XPS valence band spectra of Nb₃Sn, Nb and Sn have been obtained under high resolution. The 4d band structure in the Nb₃Sn spectrum is similar to that seen in Nb metal.     

The effect of Nb addition on the thermal stability and magnetic properties of Fe-based amorphous alloys with a wide supercooled liquid region

We have examined the effect of Nb addition on the thermal stability of the supercooled liquid before crystallization, the glass-forming ability and magnetic properties for the amorphous alloy series Fe₇₃Nb_xAl_5-xGe₂P₁₀C₆B₄ and Fe₆₃Co₇Nb_xZr_10-xB₂₀. The supercooled liquid regions (ΔT_x=T_x-T_g) have maximum values of 66 and 79K, respectively, for Fe₇₃Nb₁Al₄Ge₂P₁₀C₆B₄ and Fe₆₃Co₇Nb₄Zr₆B₂₀ alloys. The saturation magnetization σ_s of the Fe₇₃Nb_xAl_5-xGe₂P₁₀C₆B₄ alloy system is much higher than that of the Fe₆₃Co₇Nb_xZr_10-xB₂₀ alloy system, and is almost unchanged with an increasing Nb content from 0 to 3 at%. But the σ_s of the Fe₆₃Co₇Nb_xZr_10-xB₂₀ alloys decreases linearly with increasing Nb content. The coercive force H_c of the two alloy series can have a minimum value of 31A/m for Fe₇₃Nb₁Al₄Ge₂P₁₀C₆B₄ and 37A/m for Fe₆₃Co₇Nb₄Zr₆B₂₀. All these values are for the ribbons annealed at 773 and 873K for 15min in vacuum.     

Nb 3d and O 1s core levels and chemical bonding in niobates

A set of available experimental data on binding energies of Nb 3d_5/2 and O 1s core levels in niobates has been observed with using energy difference (O 1sNb 3d_5/2) as a robust parameter for compound characterization. An empirical relationship between (O 1sNb 3d_5/2) values measured with XPS for Nb⁵⁺-niobates and mean chemical bond length L(NbO) has been discussed. A range of (O 1sNb 3d_5/2) values possible in Nb⁵⁺-niobates has been defined. An energy gap ∼1.4–1.8 eV is found between (O 1sNb 3d_5/2) values reasonable for Nb⁵⁺ and Nb⁴⁺ states in niobates.     

Chemically shifted surface core-levels and surface segregation in EuAu and YbAu alloys

Chemically shifted surface core-level binding energies are observed for the rare-earth alloy component in Eu_1?xAu_x and Yb_1?xAu_x. Furthermore, these shifts are different from the chemical shifts of the bulk 4f levels. The surface core-levels are used to identify surface segregation of the lanthanide metal in the present alloys.