Search

Heat treatments were performed on an ITER type internal Sn Nb3Sn superconductor and stages occurring after such treatment were examined.

Electron beam deposition using Y metal was conducted to grow Y2O3 buffer layers and yttrium barium copper oxide (YBCO) films on cube-textured nickel tape. It was observed that the resulting Y2O3 buffer layers have excellent out-of-plane alignment of full width at half maximum (FWHM) of 2.3-4 degrees and good in-plane alignment with -11 degrees (FWHM). On the other hand, YBCO films grown on textured Ni tape demonstrate good in-plane alignment but have poor superconducting characteristics.

Predictions of superconductor applications made right after the discovery of high temperature superconductivity (HTS) mostly involved conductors for the development of motors, power cables and magnets. Breakthroughs were made in 1989, when supporters of the technology demonstrated HTS-based devices. These developments led to commercially-available kilometer-length BSCCO conductors as well as HTS-based fault current limiters, transformers, components and other devices.

A study was conducted on the influence of cold rolling and wire drawing on the texture development of (Bi,Pb)2Sr2CaCu2Ox (2212) phase and its effects on the subsequent (Bi,Pb)2Sr2Ca2Cu3Oy (2223) phase texture in Ag-clad tapes. The 2212 phase was developed at different stages of deformation through reactions in a mixture of (Bi,Pb)2CuO4 and SrCaCuO3 powders with overall 2223 composition. Results reveal a significant improvement in texture.

A study was conducted on artificial pinning center Nb-Ti superconductors with alloyed Nb pins. A magnetic interaction functions as the dominant fluxon-defect interaction between artificial and conventional superconductors. Magnetic field and pin size dependency was observed in the bulk flux pinning force of an Nb10W-pin composite. Results were in agreement with Nb47Ti superconductors that were conventionally processed.

Conventional and artificial pinning center Nb-Ti round wires were found to exhibit critical current density anisotropy. This was discovered when magnetization and wire aspect ratio were increased. The results, however, were different from the transport critical current density calculated from conventional wires' saturated-magnetization. The discrepancy is attributed to the low electric fields emitted by superconducting quantum interference device measurements and ribbon discontinuity along the wire length in conventional Nb-Ti wires.

A study was conducted on the metallurgical state and composition of Nb-Ti superconducting wires near the upper critical field to separate the irreversibility field, resistively measured transition field and the upper critical field. Measurements were conducted on small current transport, magnetization and extended electric field vs current density curves for transition analysis. Results show a strong correlation between bulk flux pinning, magnetization, extrapolation and extended electric field vs current density curves.

The superconducting characteristics of composites rods containing niobium-titanium (NbTi) matrix centers and niobium pins were investigated. Specifically, transport measurements and microscopic observations were carried out on NbTi rods fabricated via the artificial pinning center process. The measurements demonstrated the attainment of critical current densities of 19,000 A/mm2 at one tesla.

Several methods for performing direct measurements of artificial pinning center thickness of niobium-titanium superconductors were evaluated. The methods include direct techniques such as random linear intercept distance and minor axis as well as indirect techniques such as Feret diameter, skeletal thickness and shape models. They have been applied to examine the distribution of pin sizes in niobium-titanium monofilamentary and artificial pinning center microstructures.

Magneto-optic and current transport methods were employed to investigate the electromagnetic coupling inhomogeneity across the high angle grain boundaries of YBa2Cu3O(6+x) bicrystals fabricated via melt texturing and liquid phase removal. Specifically, it was proven that the grain boundary plane orientation has no effect on the coupling behavior across such boundaries. Moreover, it was shown that inter- and intra-grain zones contain 'a strong coupled component to the boundary.'

The impact of the cooling rate on the microstructural and superconducting properties of silver-sheathed BSCCO-2223 superconducting tapes was investigated. Specifically, several one- and two-powder 2223 tapes were reacted and slowly cooled in 7.5% oxygen. Consequently, it was found that this raised the critical current densities up to 50% despite the degradation of the 2223-phase purity due to the presence of 2212 grains in the microstructure.

The densification of the powder cores in two samples of silver-sheathed Bi2Sr2CaCu2Ox (BSCCO-2212) wires and tapes were characterized as a function of drawing strain and packing density. Similar measurements were also carried out for the Vickers hardness and silver:superconductor ratio. The results, which contained no evidence of a linear correlation beween microhardness and density, showed that a maximum density of approximately 80% was attained at intermediate strain values of about four.

The use of low-temperature hot isostatic pressing (HIP) to raise the critical current density and minimize barrier reaction effects (Pb,Sn)-based bulk and wire samples of the Chevrel phase is investigated. The experimental results reveal that increasing the temperature and time under pressure of the HIP process improves the density, hardness, critical temperature and magnetization critical current density of the superconductor.

The transverse cross-section of the filament-matrix region in niobium titanium wires were investigated using the Scanning Electron Microscope-Backscatter Electron Imaging and Electron Micro Probe Analysis techniques. The observations included the average effective alpha-titanium (alpha-Ti) precipitate diameter and volume fraction of alpha-Ti to evaluate the impact of heat treatment on the microstructure of the flux pinning sites of niobium titanium composites.

In recent years there has been an increasing effort in improving the performance of Nb3Sn for high-field applications, in particular for the fabrication of conductors suitable for the realization of the Future Circular Collider (FCC) at CERN. This challenging task has led to the investigation of new routes to advance the high-field pinning properties, the irreversibility and the upper critical fields (HIrr and Hc2, respectively). The effect of hafnium addition to the standard Nb-4Ta alloy has been recently demonstrated to be particularly promising and, in this paper, we investigate the origins of the observed improvements of the superconducting properties. Electron microscopy, Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) and Atom Probe Tomography (APT) characterization clearly show that, in presence of oxygen, both fine Nb3Sn grains and HfO2 nanoparticles form. Although EXAFS is unable to detect significant amounts of Hf in the A15 structure, APT does indeed reveal some residual intragrain metallic Hf. To investigate the layer properties in more detail, we created a microbridge from a thin lamella extracted by Focused Ion Beam (FIB) and measured the transport properties of Ta-Hf-doped Nb3Sn. Hc2(0) is enhanced to 30.8 T by the introduction of Hf, ~ 1 T higher than those of only Ta-doped Nb3Sn, and, even more importantly the position of the pinning force maximum exceeds 6 T, against the typical ~ 4.5–4.7 T of the only Ta-doped material. These results show that the improvements generated by Hf addition can significantly enhance the high-field performance, bringing Nb3Sn closer to the requirements necessary for FCC realization. [ABSTRACT FROM AUTHOR]

We investigated the melt-growth behavior of YbBa2Cu3O7−δ (Yb123) powder under different atmospheres for potential application in Ag-sheathed Yb123 wires. We chose Yb123 because it has the lowest melting temperature of the RE123 (RE = rare earth) phases. Yb123 needs to be melted under low oxygen partial pressure (pO2) to depress its melt temperature below that of Ag. Here we report that adding Ag powder to Yb123 powder or encasing Yb123 powder in a Ag-sheath further decreases the melting temperature of Yb123. We designed a heat treatment process in which we varied the pO2 during the heat treatment to melt the Yb123 in a Ag-sheathed powder-in-tube (PIT) wire and then regrow Yb123 on cooling. Based on these studies, we have developed a model for Yb123 melting and growth. [ABSTRACT FROM AUTHOR]

A novel melt-growth heat treatment of YbBa2Cu3 O7-δ (Yb123) powder in Ag-sheathed tape has been established as a necessary first step to form aligned Yb123 grains in a powderin-tube (PIT) conductor. The heat treatment parameters including the rate at which the atmosphere was changed from Ar to pure O2, the heat treatment temperature, and the time during which the melt transformed back to Yb123 were investigated to find the optimum parameters for reforming Yb123 in Ag-sheathed tape. The Yb123 purity after being reformed is insensitive to the rate at which the atmosphere was changed. The optimized parameters are 910 °C for 48 h to reform Yb123. [ABSTRACT FROM AUTHOR]