M.J.R. Sandim

Annealing effects on the magnetic properties of a multifilamentary Cu–Nb composite

In this paper, the annealing effects on the magnetic properties of a multifilamentary Cu–15vol%Nb composite were investigated. During vacuum annealing, noticeable changes take place in the microstructure, mostly the partial spheroidization and further coarsening of the niobium filaments. Results show that spheroidization becomes noticeable at about 700 °C and, even after annealing at severe conditions, e.g. 1050 °C for 32 h, the continuity of the niobium-conducting path is partially preserved. The influence of these microstructural changes on the magnetic properties of the multifilamentary Cu–Nb composite conductor is discussed.

Composition gradient as a source of pinning in Nb-Ti and NbTa-Ti superconductors

Superconducting Nb–Ti and NbTa–Ti wires produced by solid-state diffusion were studied, correlating microstructural and transport properties. The most interesting result is that the diffusion layer sharp composition variation is an important source of pinning in these materials. Within this scenario, we suggest that the source of pinning centers in conventional NbTi wires should be re-examined, since the α-Ti precipitation creates a Nb gradient composition around it. We propose that in conventional superconducting wires there are actually two kinds of strong pinning centers. One is the well-known α-Ti normal interface and the new one now identified is the composition gradient around α-Ti precipitates.

Annealing effects on the electrical and superconducting properties of a Cu-15vol%Nb composite conductor

In this paper, annealing effects on the electrical and superconducting properties of a multifilamentary Cu-Nb composite were investigated. During annealing, noticeable changes in the microstructure take place, in special those related to recovery and recrystallization of copper and niobium followed by spheroidization and further coarsening of the Nb-filaments. Results show that spheroidization becomes significant only above 800/spl deg/C and, even after annealing at severe conditions, e.g., 1050/spl deg/C for 32 h, the continuity of the niobium-conducting path is partially preserved. The influence of these microstructural changes on the superconducting and normal properties of the multifilamentary Cu-15vol%Nb composite conductor is discussed.

Performance of as-reacted and multiple bent ('pre-bent') practical Nb3Sn bronze route wires with different architectures

Six practical composite wires of Nb3Sn (Furukawa Electric Co, Ltd) with different architectures (design and reinforcement) were compared from the normal state resistance R, critical temperatures Tconset, Tcoffset and ?Tc, upper critical field Bc2 (at 4.2?K) and critical current density Jc points of view. Wires were as follows: three of near-the-edge reinforcement design with Nb reinforcement of 0, 21, 50?vol% in the CuNb region, and two of central reinforcement design with Nb of 21 and 50?vol%. One wire with near the edge 50% vol Nb reinforcement had a different reinforcement/superconductor ratio. As-reacted wires show very different patterns of R, Tconset, Tcoffset, ?Tc, Bc2 and Jc. For the superconducting parameters this is probably due to different 3D thermal residual strains. Data suggest that the architecture of the as-reacted wire can control residual strain values and distribution. During multiple bending of the wires at room temperature (named pre-bending), introduction of the reinforcement improves relaxation of the 3D residual strain and especially of the lateral components. As a consequence, Bc2 and Jc versus pre-bending strain, ?pb, are enhanced to values closer to those of the Nb3Sn in the stress-free state. Relative enhancement of these critical parameters for the reinforced wires is higher than for the reinforcement-free wire. Evolution during pre-bending and maximum attained absolute values of the superconducting parameters can be grouped roughly as a function of near-the-edge or central reinforcement design. Variation of the superconducting parameters suggests that the pre-bent state may depend on the as-reacted one (which is a function of wire architecture and processing history); when Jc was low as in the case of as-reacted wires with central reinforcement, during pre-bending, despite a relatively high Jc?enhancement, this parameter attained lower absolute maximum values than for the other wires. Depending on the wire, when pre-bending strain ?pb = 0.8?1%, the enhancement of non-Cu Jc at 4.2?K in the pre-bent wire is by up to 43.5% at 15?T compared to the as-reacted case. The maximum absolute value of Jc is obtained for the near-the-edge reinforcement pre-bent wires with 21 and 50%?vol Nb. Bc2 of these wires was lower than for the other reinforced wires and higher than for the reinforcement-free wire. Pre-bent wires with near-the-edge reinforcement are identified as new and promising candidates for fabrication of react-and-wind coils with improved performance.

The evolution of Nb recrystallization in a Cu matrix: morphology of Nb and modulation of its critical fields in a wire configuration

We report on the microstructural evolution in Cu15%Nb multifilamentary wires upon annealing and the corresponding effects on their magnetic properties. During annealing at temperatures higher than 800 °C thermal instability mechanisms take place in the microstructure of the Cu15%Nb composite, leading to the spheroidization of niobium filaments. Another important consequence of annealing is the recrystallization of Nb, that has a straightforward influence on the magnetization data. When compared to the as-drawn composite, in the annealed Cu15%Nb samples the bulk upper critical field Hc 2(T) is reduced significantly, while the lower critical field Hc 1(T) is increased as a function of the annealing temperature and time. This indicates that we may employ this process as an efficient method to modify the basic superconducting parameters of Nb, namely the coherence length ξ(T) and the penetration depth λ(T).

The influence of annealing on the microstructure and electrical resistivity of jelly-rolled Cu-Nb composite wires

In this paper, the annealing effects on the microstructure and electrical resistivity of jelly-rolled Cu-x% vol. Nb (x=25, 33, 50, and 63) composite wires were investigated. During annealing, noticeable changes take place in the microstructure, including recovery and recrystallization of copper and niobium, followed by spheroidization and further coalescence of niobium filaments. With increasing annealing temperature, the diffusion-controlled coalescence of niobium filaments is enhanced due to their proximity. The behavior of the electrical resistivity in the normal state of the Cu-Nb composite shows a noticeable change at a specific value of temperature in the range between 50K and 70 K. This temperature varies with the niobium volume fraction. Such a behavior is discussed in terms of the decrease of the amount of Cu-Nb interfaces promoted by coarsening and by electron scattering effects at these interfaces.

EBSD Characterization of Pure Iron Deformed by ECAE

Polycrystalline iron was deformed by eight ECAE passes using the route Bc to a total strain of 9.2. After deformation the material was annealed at temperatures up to 800oC. Scanning electron microscopy (SEM) and high-resolution electron backscatter diffraction (EBSD) were used to characterize both deformed and annealed structures. In the as-deformed state, the mean grain size is 650 nm and the volume fraction of high angle boundaries (VHAB) is 56%. Upon annealing there is a pronounced softening above 300oC. At the beginning of recrystallization, at about 400oC, the VHAB increases to 71%. The results indicate that discontinuous recrystallization is the main softening mechanism in severely deformed iron.

Microstructural and Magnetic Characterization of

In this work we focus on the microstructural and magnetic characterization of wires with different architectures (design and reinforcement). The microstructural characterization was performed using scanning electron microscopy. AC magnetic susceptibility was measured with field applied both parallel and perpendicular to the wire axis. The heat treatment performed to form the A-15 superconducting phase leads to partial spheroidization followed by coarsening of the Nb filaments in the reinforcement material. The differences concerning the microstructure of the reinforcement material among the investigated wires were reflected in the broadening of the superconducting transition of Nb, more evident for a field applied parallel to the wire axis. From the magnetic data the wires were also compared in terms of the superconducting volume fraction.

{\hbox{CuNb}}/{\hbox{Nb}}_{3}{\hbox{Sn}}

The present work presents the steps for the manufacture of an Nb?Ti artificial pinning centre (APC) multifilament wire for AC applications. The wire production starts from a Ti bar inserted in a Nb tube. The superconductor Nb?Ti alloy was obtained using heat treatments during the wire production steps through the growth of the diffusion layer between the pure Nb and Ti regions. The final wire is composed of many filaments dispersed in a Cu?Ni matrix. The transport critical current density was measured in samples for different conditions of heat treatment, followed by wire diameter reduction. Magnetization hysteresis loops were measured to extract the hysteretic losses in wires with filament diameters of 76 and 92??m and compared to the results in commercial wires with comparable filament diameters. We obtained an Nb?Ti APC wire with hysteretic losses of the same order of magnitude as the commercial wires. However, critical current densities Jc are higher for fields lower than 2?T, where AC applications are most important. The Jc results confirmed a previous result that the decrease of normal region sizes is a less important pinning source in this material than the pinning from variations in the composition of the diffusion layer.

Wires With Different Architectures

We have measured transport properties at low temperatures for the granular Sm1.82Ce0.18CuO4-d samples belonging to the family of electron-doped superconductors. The effect of applied electrical current on the resistive behavior is investigated. The experimental data are analyzed using a modified form of the theory for a field-tuned superconductor-insulator transition in 2D superconductors. The results suggest a possible electrical-current driven superconductor-insulator transition.