Nb/Al-AlOx/Nb约瑟夫森结薄膜沉积研究

Nb/Al-AlO_x/Nb约瑟夫森结构成的低温超导器件有着广泛应用，高质量的S-I-S约瑟夫森结制备的关键之一在于制备高质量的三层膜。三层膜的质量可由许多属性参数影响，比如剩余电阻比R.R.R、超导转变温度T_c、表面粗糙度、应力、铝膜厚度等等。通过对薄膜的溅射沉积条件与上述参数进行分析研究，确定最佳的生长条件。调节不同的氧化工艺条件，可以获得约瑟夫森结的不同临界电流密度。     

Nb/Nb5Si3复合材料的研究进展

Nb/Nb5Si3复合材料具有高熔点、低密度以及良好的室温韧性和高温强度,被认为是下一代航空发动机中极具竞争力的超高温结构材料.本文介绍了这种复合材料的主要制备方法、组织结构和力学性能,并展望了其发展前景.     

Scaling of Hc2⊥(T) in Nb/CuMn Multilayers

Measurements of the perpendicular upper critical magnetic field H _c2(T) are reported for several Nb/CuMn multilayers. It is found that, despite the magnetic nature of the samples, the data for samples with low Mn percentage in the CuMn layers are simply described by the Werthamer–Helfand–Honenberg theory for conventional type-II superconductors, neglecting both Pauli spin paramagnetism and spin orbit impurity scattering. For high Mn concentration a different theoretical approach is needed.     

Mechanism of the Interfacial Reaction of AI_2O_3/Medium Mn Steel Containing Nb

The mechanism of the interfacial reaction of Al_2O_3/medium Mn steel containing Nb was studied bymeans of the observation on the interfacial reaction phenomenon of Al_2O_3/medium Mn steel,andthe analyses on the interracial phases.The results show that when T≥1550℃,the interfacial reac-tion of Al_2O_3/medium Mn steel containing Nb happened.In the medium Mn steel matrix,Nb existsin the form of NbC.NbC are the nucleating base of CO gas bubbles.     

Properties of (Ti,Nb)Al–(Ti,Nb)5Si3 eutectic composite

Ti–40Al–5Si and Ti–39Al–5Si–2Nb (in at.%) alloys were studied as prospective high-temperature structural composites consisting of γ-(Ti,Nb)Al + α₂-(Ti,Nb)₃Al matrix and Ti₅Si₃ reinforcement. The alloys were prepared by arc melting under helium. Oxidation resistance was studied at 900 °C in air. Thermal stability of alloys was investigated by measuring room temperature hardness and compressive strength after long-term annealing at 900 °C. To prepare oriented composites, directional crystallization at rates of 5–115 mm/h was carried out by the floating zone technique. It was observed that the addition of 2% Nb to the Ti–40Al–5Si alloy does not modify eutectic structure. Niobium is almost uniformly distributed in all present phases. Both alloys show excellent oxidation resistance at 900 °C in air. The Nb-addition causes significant improvement of oxidation resistance due to the doping effect and increase of Al activity in the scales. Room temperature hardness and compressive strength of both as-cast alloys are similar – about 500 HV and 1600 MPa, respectively. Room temperature mechanical properties do not reduce significantly after 300 h annealing at 900 °C, due to a high morphological stability of eutectic silicides. Directionally solidified alloys consist of columnar Ti–Al grains elongated in crystallization direction and silicides. Niobium refines both Ti–Al grains and Ti₅Si₃ silicides. As a consequence, orientation and elongation of silicides in the Nb-containing alloy are reduced. In the Ti–Al–Si alloy directionally crystallized at 5–115 mm/h, the silicide interparticle spacing λ (in mm) is related to the crystallization rate R   (in mm/h) by a following expression: λ^1.33·R=0.32${\lambda }^{1.33}·R=0.32$. In the Nb-containing alloy, silicide interparticle spacing does not depend on the crystallization rate.     

Structure and stability of 2.4 nm period amorphous Ni–Nb/C multilayers

Amorphous Ni–Nb/C multilayers with a period of 2.4 nm were prepared by pulsed laser ablation deposition. The as-deposited multilayers were found to have an interdiffused Ni_1/3Nb_1/3C_1/3 layer present at the two interfaces; Ni_1/2Nb_1/2/C and C/N_1/2Nb_1/2. The specular reflectivity and diffuse scattering studies show that the interface roughness is chemical and not morphological in origin. The structural studies performed using X-ray scattering techniques after a period of 2.5 years from deposition show that the behaviour does not change with time. These results indicate that the multilayered structure is temporally stable in spite of the strong composition dependent driving force for chemical homogenization. The structural stability against homogenization is due to the presence of an amorphous Ni_1/3Nb_1/3C_1/3 layer present at the interfaces which acts as a diffusion barrier.     

Emergence of NbTi as supermagnet material

The discovery and emergence of NbTi as a high field superconductor is reviewed. The prehistory and setting for its discovery are described, and an anecdotal history follows its development up to the first successful large scale applications.     

Processing of Nb–Cu metal matrix composites

Abstract

During the development of new processing routes for Nb₃Sn superconductor, factors influencing the workability of two-phase metallic composites have been investigated. The ease with which such composites can be fabricated depends strongly on the relative hardnesses of the phases. Production of a regular, uniform filamentary structure is promoted by low hardness ratios in the initial composite.

MST/547     

Mo对于Nb/Nb5Si3原位复合材料室温韧性的影响

利用真空电弧熔炼制备了Nb/Nb5Si3复合材料,在1200℃×100h热处理后,利用单边切口悬臂梁法(SENB)测定了Nb-Si合金的室温断裂韧性.研究了合金化元素Mo对Nb/Nb5Si3复合材料微观结构和室温韧性的影响.结果发现适量Mo的加入明显地改善了Nb-18Si的室温韧性.     

Nb/Mg3SbBi界面层热稳定性研究

Zintl相Mg₃(Sb,Bi)₂基热电材料因在中低温区(27～500℃)表现出优异的热电性能而受到广泛关注。然而,由于Mg、Sb元素比较活泼,在长期高温服役下易与电极发生剧烈界面扩散反应,导致热电器件的性能和服役寿命衰减。因此,选择能有效阻挡元素剧烈互扩散并且具有低界面接触电阻率阻挡层材料至关重要。本研究首先利用热压工艺制备出300℃最高ZT～1.4的n型Mg₃SbBi(Mg_3.2SbBi_0.996Se_0.004)样品,然后采用Nb粉作为扩散阻挡层一步烧结制备Mg₃SbBi/Nb/Mg₃SbBi“三明治”结构样品,系统研究界面层的组成、微结构以及电阻随老化时间演变过程。加速老化实验(525℃/70 h; 525℃/170 h; 525℃/360 h)研究发现, Nb阻挡层中的Mg-Sb/Bi组分发生偏析,表面产生裂纹,抛光处理后界面连接完好,无裂纹和孔洞,界面扩散层厚度随老化时间延长缓慢增加至1.6μm。Nb/...     

Strain-induced precipitation kinetics of Nb(C,N) and precipitates evolution in austenite of Nb–Ti micro-alloyed steels

In this article, the precipitation–temperature–time (PTT) curves were obtained by plotting 1/A _r-time curves (A _r represents the area under the flow curves, and some details were clarified in the section “1/A _r method”) using two-stage interrupted compression testing. Based on the measured PTT curves, the L_J model has been applied to calculate the complete PTT curves by adjusting the modification factor, f, to be 0.022, from which it can be seen that they are both typical C-shaped with the nose temperature ranging from 900 to 925 °C. By using additivity rule, the continuous-cooling-precipitation (CCP) curves were established. And the critical cooling rates for precipitation start were also determined for different cooling start temperatures, and the effects of cooling rates and cooling start temperatures on CCP curves were systematically clarified. The evolution of precipitates was found to be in good agreement with PTT curves. Numerous fine and round precipitates were formed during holding after first compression deformation, which could have hindered static recrystallization kinetics and accounted for the plateaus on 1/A _r-time curves. In addition, The cubic precipitates are mainly undissoloved TiN-rich (Ti,Nb) (C,N) carbonitrides. And based on the CCP and continuous-cooling-transformation curves of tested steel, the effect of cooling paths on precipitation and transformation were also discussed in details.     

Effect of Nb on the Interfacial Wettability in Al_2O_3/Medium Mn Steel

To develop medium Mn steel (MMS) matrix composites reinforced by AI2O3 particles, the effect of Nb on the interfacial wettability of AI2O3/MMS and its mechanism were investigated in this paper. The results show that the wetting angle of the specimens with different Nb contents are bigger than that of the specimen without Nb at the first stage, and then decreases with time at 1450℃. At certain time, the wetting angle is lower than that of the specimen without Nb. At 1550 and 1600℃, the wetting angle of the specimens containing Nb decreases quickly with time at first stage. After 10 min, the wetting angle reaches a steady state, and hardly changes with time. The mechanisms of Nb to improve the wettability can be attributed to the enrichment of Nb at the interface and Nb serves as surface active agent of MMS at T<1550℃, and as catalyzer for the interfacial reaction of AI2O3/MMS at T>1550℃, and reduces the interfacial energies.     

Aging response of the Ti–Nb system biomaterials with β-stabilizing elements

Effects of aging temperature and the contents of β-stabilizing elements on the composition of martensite α′′ in two Ti–Nb alloys and the resulting mechanical properties were investigated for biomedical applications. The microstructures were examined by means of optical microscopy (OM) and X-ray diffraction (XRD). Vickers hardness, compressive elastic modulus and the yield strength have been measured. The results show that the decomposition mode of the martensite α′′ in two studied alloys depends on aging treatment and the contents of β-stabilizing elements. Various microstructures such as α, (α + β) and (β + ω) phases were observed to precipitate in the studied alloys after the aging treatments performed at 523 K, 773 K, 883 K and 1023 K for 0.5 h, respectively. Afterwards, the Ti–24Nb–6Zr–7.5Sn–2Fe alloy was aged at 773 K for 1 h. The compressive elastic modulus and mechanical properties of the two alloys are found to be sensitive to the microstructural change caused by aging temperature. For the Ti–24Nb–6Zr–7.5Sn–2Fe alloy, after aging at 773 K for 1 h, its yield strength, compressive elastic modulus and Vickers hardness reach 846 MPa, 26 GPa and 398 HV, respectively. This aged alloy exhibits proper comprehensive mechanical property and strength-to-modulus ratio for biomedical implant applications.     

Effects of Nb content in Ti–Ni–Nb brazing alloys on the microstructure and mechanical properties of Ti–22Al–25Nb alloy brazed joints

Vacuum brazing was successfully used to join Ti–22Al–25Nb alloy using Ti–Ni–Nb brazing alloys prepared by arc-melting. The influence of Nb content in the Ti–Ni–Nb brazing alloys on the interfacial microstructure and mechanical properties of the brazed joints was investigated. The results showed that the interfacial microstructure of brazed joint consisted of B2, O, ?3, and Ti2 Ni phase, while the width of brazing seams varied at different Nb contents. The room temperature shear strength reached359 MPa when the joints were brazed with eutectic Ti40Ni40Nb20 alloy at 1180?C for 20 min, and it was321, 308 and 256 MPa at 500, 650 and 800?C, respectively. Cracks primarily initiated and propagated in ?3compounds, and partially traversed B2+O region. Moreover, the fracture surface displayed typical ductile dimples when cracks propagated through B2+O region, which was favorable for the mechanical properties of the brazed joint.     

Impacts of accelerated aging on the mechanical properties of Cu–Nb nanolaminates

Accelerated aging (30 min at 400 °C) has been shown to alter the mechanical properties of Cu–Nb nanolaminate systems. The Cu–Nb nanolaminates produced were 1,000-nm thick with alternating 20 or 100-nm-thick individual layers, which were fabricated by magnetron sputter deposition. Unaged Cu–Nb systems increased in hardness (from 4.3 to 5.5 GPa) with decreasing layer thickness. After aging, the nanolaminates with 20 nm layers softened greatly (5.5 GPa decreased to as little as 1.3 GPa), yet nanolaminates with 100 nm layers hardened slightly (4.3–4.8 GPa). Both nanolaminate structures exhibited significant residual tensile stress, which was further increased by up to 70 % (100 nm layers) and 120 % (20 nm layers) after accelerated aging. X-ray diffraction showed the presence of primary textures and high stress in niobium layers for unaged systems.     

Faceting of the Σ 3 coincidence tilt boundary in Nb

Due to their unusual properties, the grain boundaries (GBs) with the lowest possible inverse density of coincidence sites Σ = 3 play a special role in the GB engineering. The as-grown shape of the cylindric tilt grain boundary (GB) in Nb bicrystal grown by the floating zone method has been studied with the electron back-scattering diffraction method. Both grains form the superlattice called coincidence site lattice (CSL) with the lowest possible inverse density of coincidence sites Σ = 3. Four different CSL facets (100)_Σ3CSL, 110_{Σ 3CSL}, 120_{Σ 3CSL} and 210_Σ3CSL were observed simultaneously. Flat facets (100)_Σ3CSL, 110_Σ3CSL, 120_Σ3CSL and 210_Σ3CSL form smooth edges (no slope discontinuity) with rounded rough GB portions. Rough surface curves away from the plane of the (–1, 2, 0)_Σ3CSL facet at the edge with (–1, 1, 0)_Σ3CSL facet as x^β with β = 1.61 ± 0.09. At the edge between (210)_Σ3CSL and (–1, 2, 0)_Σ3CSL facets β = 1.46 ± 0.09. Both values reveal the GB roughening belonging to the Pokrovsky-Talapov universality class. It has been shown for Pb surfaces [K. Arenhold, S. Surnev, P. Coenen, H.P. Bonzel and P. Wynblatt, Surf. Sci. 417 (1998) L1160] that the β value depend on the details of the steps interaction at the vicinal surface. In our case the difference between measured β for two different facet edges can be due to the similar details of GB steps.     

Microwave dielectric properties and applications of Ba(Zn1/3Nb2/3)O3–Ca(Zn1/3Nb2/3)O3 composite ceramics by one-step synthesis method

(1 ? x) Ca(Zn_1/3Nb_2/3) O₃ ? xBa(Zn_1/3Nb_2/3)O₃ (short for CZN/BZN, x = 0–1.0) ceramics were prepared and investigated through the “one-step synthesis method” method. The structure of the system was analyzed using X-ray diffraction. The microstructure of the sintered pellet was analyzed using scanning electron microscopy. Dielectric constant (ε_r), temperature coefficient of resonant frequency (τ_f) and the unloaded quality factor (Q × f) were measured in the microwave frequency region. Two dielectric properties were firstly in the rising tendency and then decreasing with the increased x. On the other hand, a good combination of microwave dielectric properties (ε_r = 24, Q × f = 23,510 GHz τ_f = ?9 ppm/°C) were obtained at x = 0.1. The compositions have excellent microwave dielectric properties and hence are suitable for ceramic capacitors or dielectric resonators applications.     

Inter-dependence of grain growth,Nb segregation,and chemical ordering in FeSiNb nanocrystals

It is known that small Nb additions help stabilize Fe₃Si against grain growth. We studied the microstructural stability of (Fe₃Si)_.95Nb_0.5 during annealing by Mössbauer spectrometry, x-ray diffractometry, and transmission electron microscpy. We measured the development of grain growth, chemical ordering, and the segregation of Nb atoms. We identified inter-relationships between these processes. Chemical ordering was found to be linked to Nb segregation; after the segregation of Nb atoms, DO₃ chemical order developed in the Fe₃Si crystallites. Grain growth was found to be impeded when Nb segregation occurred, although the development of DO₃ order could be involved.     

Effect of Nb?+?Ti coating on the wetting behavior, interfacial microstructure, and mechanical properties of Al/Al2O3 joints

The subject of the work was to study the effect of Nb + Ti thin film deposited by PVD method on alumina substrates on the wetting behavior, bond strength properties, and structure of interface in the Al/Al₂O₃ joints. Applying the sessile drop method, the wetting behavior of molten Al (99,999%) on coated alumina substrates was studied in the temperature range between 953 and 1373 K under a vacuum of 0.2 mPa for 30 min of contact. The sessile drop samples were used to examine the interface structure, shear strength, and interfacial fracture toughness under the concentrated load. The introduction of the thin Nb + Ti film layer of 900 nm thickness: (1) greatly improves the wettability of alumina by molten Al at above 1223 K and the shear strength of Al/Al₂O₃ joints produced at 1223 K, (2) has positive effect on structure transformation in the interface and leads to fabrication of reliable metal–ceramic joints. Microstructural investigations of the interface indicated that the precipitates of Nb and Ti-rich intermetallic phases were formed at the Al/Al₂O₃ interface, which influenced strengthening of these joints. Hence a conclusion can be drawn that the interface structure influences the durability increase in Al/Al₂O₃ joints.