Superconductivity in deformed niobium alloys

A brief review of deformation structure in bcc metals is given and the main features of type II superconductivity are summarised. Microstructure and superconducting properties of deformed niobium-tantalum, niobium-titanium, and niobium-zirconium alloys were studied and compared with those of pure niobium. All these alloys are type II superconductors, which above a certain critical field, H _ic1, enter the mixed state, consisting of Abrikosov current vortices in a superconducting matrix. Dislocations introduced by deformation interact with current vortices to cause irreversible superconducting behaviour and contribute to a high current-carrying capacity. It is concluded from the results presented here that a strong vortex dislocation interaction is obtained only when a non-uniform dislocation distribution is present, and that a uniform distribution produces only weak pinning. Flux-pinning by dislocation tangles is treated as a special case of pinning by normal particles resulting from a variation in , the Ginzburg-Landau parameter.     

Superconductivity in new A-15 niobium alloys

A number of previously unreported A-15 Nb alloys have been studied, using samples prepared by dual target sputtering.T _c was measured resistively and structure determined by x-ray diffraction. Samples were “chemically” analyzed by anodization. New alloys include A-15 Nb₃Si and pseudobinary derivatives, as well as some derivatives of “Nb₃P.”T _c and the A-15 lattice parameter were surveyed for these, and for previously studied alloys related to Nb₃Al, Nb₃Ga, and Nb₃Ge. The “chemical” variation ofT _c was slight, except near pure Nb₃Si and for alloys based upon “Nb₃P,” where the samples show considerable disorder. Maxima inT _c occur for the alloys Nb₃Al_0.7Ge_0.3 and Nb₃Al_0.5Ga_0.5. The first is well known; trends measured here indicate that the second may be equally high. It is proposed that the dominant factor forT _c in this group of alloys is the state of order achieved in the samples. No evidence was seen to associate enhancedT _c with A-15 phase boundaries.     

Structural evaluation of phospholipidic nanovesicles containing small amounts of chitosan

In this study we present a full characterization of nanovesicles containing soybean phosphatidylcholine and polysaccharide chitosan. The nanovesicles were prepared by the reverse phase evaporation method, including the preparation of reverse micelles followed by the formation of an organogel, which is dispersed in water to yield the final liposomal particles. Structural changes as a function of the chitosan amount and the filter porosity used in the nanovesicles preparation were studied employing Static and Dynamic Light Scattering as well as Small Angle X-ray Scattering. The hydrodynamic radius of the nanovesicles ranged between 106 and 287 nm, depending on the chitosan contents and the filter porosity. A comparison with nanovesicles free of chitosan indicates the existence of higher contents of multilamellar structures that depends on the chitosan concentration in the vesicles containing chitosan. Typical spherical vesicles having nanometric diameters with polydispersity mostly desired in the biomedical area could only be achieved by filtration through a 0.45 microm porous filter.     

Internal oxidation in gold alloys containing small amounts of Fe and Sn

Internal oxidation was observed in gold-rich alloys as substrates for porcelain veneers in dental restorations, which contain small amounts of Fe and Sn. The internal oxidation proceeded with oxygen ions diffusing to the inner part of the alloy through Fe₂O₃ formed at the grain boundaries of the alloy matrix. SnO₂ was formed internally together with the Fe₂O₃. The external oxidation zone was composed of only Fe₂O₃ in a wide range of Fe and Sn concentrations. Fe₃O₄ was formed with Fe₂O₃ in the Sn-rich composition range by reduction of Fe₂O₃ in the presence of Sn. A band mainly composed of SnO₂ was formed at the inside of the internal oxidation zone in the composition range where Fe₃O₄ formed. In the Sn-rich alloys this internal oxidation band of SnO₂ moved to the external oxidation zone.     

Wetting characteristics of copper on niobium

Sessile drop experiments have been performed in a temperature range between 1090 and 1300° C, aiming to study the wetting of niobium by liquid copper and the influence of different atmospheres (argon, hydrogen, vacuum), crystallographic orientation, roughness, and oxygen doping of niobium on the wetting angle. At the peritectic temperature in the Cu-Nb system (1090° C) the contact angle is high,=67°, denoting poor wetting. With increasing temperature, decreases steadily for all samples. The wetting is at its lowest for the oxygen-doped samples and at its best for samples annealed under a hydrogen atmosphere. A new mechanism is suggested and discussed for oxygen degassing of niobium through liquid copper.     

Superconductivity of small metallic particles. I

The ground state energy and orbital magnetic susceptibility of a small-particle superconductor are calculated with the use of perturbation theory, which is expected to give correct results in the limit of small particle size, i.e., when the average one-electron energy level spacing is much larger than the energy gap of the bulk metal. The method is applicable up to particles of intermediate size for which / = 1/4–1/2 (e.g., aluminum particles of radius 25–30 Å). The magnetic susceptibility does not undergo any drastic change in the region ~ when the particle size is varied, and quantum size effects do not show up in the susceptibility.Supported by a Grant-in-aid from the Japanese Ministry of Education.     

Formation of ultrafine scale structures in aluminium containing small amounts of particles by conventional rolling deformation

The development of deformation microstructures during rolling has been studied in polycrystalline aluminium containing 0.8 vol.% of small Al₂O₃ particles and 2 vol.% of SiC whiskers. The morphology, size and misorientation of the ultrafine scale structures as a function of the imposed strain were characterised using transmission electron microscopy (TEM) technique. The observations were compared to those of cold rolled particle-free pure aluminium reported in the literature. The presence of particles enhances the rate and changes the nature of the development of the overall microstructure in comparison with pure aluminium. With the addition of second phase to pure aluminium, the ultrafine scale microstructure with its size less than 0.5 m and a fraction of high angle boundaries greater than 30% can be obtained at a comparatively low strain of 2.7. In contrast, complicated processes or severe deformation with a strain of more than 5 are required for obtaining such structures in pure aluminium. The results indicate that there is a considerable potential for extending the present investigation to other particle-containing materials. By selecting second phases with suitable particle size and volume fraction and deformation processes one can expect to develop composites with ultrafine scale microstructure at a comparatively low strain.     

Mechanical alloying of copper with niobium and molybdenum

Copper has been mechanically alloyed with the bcc structure type elements Nb and Mo in order to obtain a material with high strength, good ductility and good electrical conductivity. In the case of alloying Cu with Nb a solid solution is formed during milling, whereas in the case of alloying Cu and Mo a fine distribution of Mo particles within the copper matrix is observed. The different behaviour of these two alloys is related to the large difference of the elastic constants of the bcc elements. Niobium precipitates from the solid solution and molybdenum particles coarsen during a subsequent heat treatment.     

Hot-pressed β-Si3N4 containing small amounts of Be and O in solid solution

Single phase, hot-pressed Si₃N₄ ceramics with relative densities >95% and equiaxed grain structures have been prepared from high purity Si₃N₄ powders having specific surface areas of 8 to 20 m² g⁻¹ and oxygen contents ⩾2 wt % using a small amount of Be₃N₂ or BeSiN₂ as a densification aid. Densification depended sensitively on the concentration of Be and O in a given Si₃N₄ powder and on the usual hot-pressing parameters of pressure, temperature and time. A close association was found between densification and the conversion ofα- toβ-Si₃N₄ during hot-pressing. Based on the data presented, chemical reactions that occur during hot-pressing involve: (1) reaction of the densification aid with SiO₂ on the Si₃N₄ particle surfaces to form BeO and Si₂N₂O; (2) the further reaction of these two reaction products to give probable formation of a transient liquid phase (TLP); and (3) the reaction between TLP andα-Si₃N₄ particles to cause densification, probably by a solution-reprecipitation process, and conversion ofα-Si₃N₄ into aβ-Si₃N₄ solid solution. The chemical composition of a single phaseβ-Si₃N₄ solid solution prepared in this study by hot-pressing was approximately Si_2.9Be_0.1N_3.8O_0.2.     

Superconductivity above 100 K in compounds containing Hg

The results of studies of new Hg based cuprate superconductors are reported. Several members of a new family of (Hg,Tl)(Ba,Sr)Ca_n–2Cu_nO_x high temperature superconductors have been synthesized. These compounds, which are analogs to the Hg-Ba-Ca- and Tl-Ba-Ca-layered cuprates, are multi-phased and have superconducting transition temperatures above 100 K. Incorporation of Hg appears to stabilize several of the Tl-compounds, including a double layer. Tl/Sr system, in a manner similar to the role that Pb plays in the Tl/Sr- and Bi/Sr-systems. It has been suggested that recent reports of resistive T_c's above 200 K in Hg based samples are due to the presence of free Hg. Magnetization measurements of such a sample confirm this hypothesis.     

Anodic polarization and corrosion of WC-Co hardmetals containing small amounts of Cr3C2 and/or VC

The anodic polarization and corrosion properties of commercial WC-Co hardmetals with different grain sizes and containing small amounts of Cr₃C₂ and/or VC in acid (pH 2.8), acid/3% NaCl, and neutral 3% NaCl solutions, have been measured using potentiokinetic and linear polarization conductance techniques. An increase in grain size increased the passive current density. Small additions of Cr₃C₂ increased substantially the corrosion resistance. Small additions of VC had little effect on the corrosion resistance, but in the presence of small additions of Cr₃C₂ it reduced the beneficial effects of chromium.     

Properties and microstructure of stress-relieved submerged-arc weld metal containing niobium

A review of previous investigations on the mechanical properties and microstructure of niobium micro-alloyed steel weldments indicated that there is a loss of ductility after post-weld heat treatment (PWHT). This effect has been ascribed to the precipitation of finely dispersed niobium carbonitrides. However, it can be shown that such precipitation is only observable when the niobium content in the weld metal exceeds 0.025 wt%. This limit appears to be confirmed in the current work by TEM observations on samples containing 0.03 wt% niobium, annealed for various times at 625° C.     

Superconducting 500 MHz accelerating copper cavities sputter-coated with niobium films

Thermal breakdown induced either by electron loading or by local defects of enhanced RF losses limits the accelerating field of superconducting niobium cavities. Replacing niobium with a material of higher thermal conductivity would be highly desirable to increase the maximum field. Therefore, cavities made of OFHC copper were coated by D.C. bias sputtering with a thin niobium film (1.5 to 5 μm). Accelerating fields up to 8.6 MVm^-1were obtained without observing any field breakdown, the limitation being due to the available rf power. The Q values achieved at 4.2 K and low field were similar to those of niobium sheet cavities (i.e.sim 2 times 10^{9}), but a fast initial decrease of Q to about 10⁹was reproducibly experienced. Subsequent inspection of regions of enhanced rf losses revealed defects the origin of which is under study. The apparatus used for coating the cavities and the results obtained are presented and discussed.     

Superconductivity in dilute Cu-Nb-Sn alloys containing Nb3Sn precipitates

The dilute Cu-Nb-Sn alloys containing small amounts of Nb and Sn less than 1 at % exhibited superconductivity after quenching from the liquid state and ageing. The best superconducting properties ( andJ _c=130 A cm^–2) in a Cu-0.30 at % Nb 0.15 at % Sn alloy were obtained when the sample was aged at 550° C for 384 h. This sample exhibited a structure of fine Nb₃Sn precipitates of 200 to 500Å diameter distributed homogeneously in the Cu matrix, and therefore it was concluded that superconductivity in these alloys resulted from the proximity effect of Nb₃Sn particles. In spite of the similar structure obtained by ageing at 800° C, the Cu-Nb-Sn alloys showed inferior superconducting properties compared to the Cu-0.4 at % Nb alloy and this would be explained qualitatively by the difference in the mean free path in the two alloys.     

Microstructure and texture of copper/niobium composites processed by ECAE

The structure refinement is a challenge for conductors used for the winding of resistive coils producing non-destructive pulsed magnetic fields over 80T. These nanocomposite conductors composed of a conducting multiscale Cu matrix embedding Nb reinforcing nanofilaments are usually manufactured by using a Severe Plastic Deformation (SPD) process based on hot extrusion, accumulative cold drawing and bundling (ADB) steps [Thilly et al. Philos Mag A 82:925, 2002]. Equal-channel angular extrusion (ECAE) is investigated here as an alternative route since it may provide faster refinement to obtain the ultrafine structure needed for optimized electrical and mechanical properties of the conductors. Therefore, copper-niobium specimens obtained by hot extrusion were processed by ECAE at room temperature. The specific die of the ECAE tool used here is constituted by a round channel with three angles corresponding to a total equivalent strain of about 2.5. Deformed samples were examined by optical microscopy and characterized by hardness profiles and x-ray diffraction (texture pole figures). After one ECAE-pass, the shape of the samples is modified but no trace of damage appeared at the Cu-Nb interfaces. An increase of the hardness values localized in the copper matrix is revealed whereas the hardness of the niobium remains unchanged. Prior to ECAE, the hot extrusion process induced a fibre texture for both copper and niobium. Two fibre texture components were observed for copper: <111> and <200>. A single <110> fibre texture component is evidenced for the niobium. After ECAE a significant variation of the texture is observed in relation with the strong shear induced by this process.     

低浓度Cu-0.5%Nb纳米弥散强化铜合金退火行为的研究

为了获得具有高强、高导及良好抗高温软化性能的铜合金,用机械合金化法制备了Cu-0.5%Nb纳米弥散强化铜合金.采用力学、电学性能测量、金相、透射电镜观察对该合金退火行为进行了研究.研究表明:随着退火温度的升高,合金硬度呈下降趋势,在900 ℃以下退火以回复过程为主.该合金相对电导率随退火温度升高而升高,400 ℃时达到峰值89%IACS;随后不断降低,到800℃时由于Nb的回溶量增加,其降低速率开始加快.TEM观察表明,该合金冷轧态位错密度很高,随退火温度升高不断降低.纳米Nb粒子可强烈阻碍晶界和位错运动,900 ℃退火后基体仍以亚晶组织为主,合金抗高温软化性能较好.     

Tantalum and niobium as a diffusion barrier between copper and silicon

The efficiency of Ta and Nb films as diffusion barriers between thin Cu film and Si substrate has been studied using Auger electron spectroscopy, X-ray diffraction, optical microscopy, scanning electron microscopy and sheet resistance measurements. Two kinds of system were prepared by electron-beam evaporation: Cu/Ta (or Nb)/Si and Cu/Ta (or Mb) SiO₂/Si. The samples were annealed at temperatures from 400 to 800C in a vacuum of 1 × 10^–7 torr (13 Pa) for 30 min. In the Cu/Ta (or Nb)/Si system, the thermal stability was determined by interdiffusion at local sites, forming suicides, whereas the Cu/Ta (or Nb)/SiO₂/Si system degraded by interdiffusion at the interface between Ta (or Nb) and Cu. It appears that Ta is a more effective diffusion barrier than Nb for both kinds of system. This difference in the barrier effect of the transition metals is attributed to differences between oxygen segregation at grain boundaries of barrier layers and differences between diffusion coefficients through barrier layers. It is suggested that the driving force for interdiffusion may play a major role in the reaction that determines the thermal stability of a given contact system; this suggestion is based on the fact that the interdiffusion in Cu/barrier/Si systems is suppressed by interposing an SiO₂ layer in the Si substrate.