Masayuki Nagata

Filamentary Nb3Sn Superconductor Manufactured by the Solid-Liquid Diffusion Method

There are a number of production processes for the fabrication of multifilamentary Nb3Sn superconductors. The most established process is the bronze process which is carried out by reaction of Nb cores and Sn in a bronze matrix.1 As is well known, to obtain a high critical current density throughout the conductor, it is desirable that the Sn content in the bronze matrix be higher than 13 wt%, which is the solubility limit of Sn in Cu. If the Sn concentration is increased over this limit, break down of the wire at drawing by work hardening of the bronze matrix is likely to occur.

The Fabrication and Properties of Multifilamentary Nb3Sn Superconductors by the Solid-Liquid Diffusion Method

There are a number of production processes for the fabrication of multifilamentary Nb3Sn superconductors. Most of them use bronze with an Sn concentration of near 13 wt.%. Many efforts have been made to increase the Sn concentration in order to increase the overall critical current density of superconductors. Essentially, most of the processes use solid-solid diffusion between Nb and solid bronze in the final heat treatment for forming Nb3Sn. We, however, succeeded in manufacturing a Nb3Sn superconductor using solid Nb and liquid Sn-Cu alloy.1

Preparation of Bi-Sr-Ca-Cu-O Superconducting Fibers by Laser Pedestal Growth Method

2212 BiSrCaCuO superconducting fibers were prepared by the laser pedestal growth method. The influence of fiber diameter and growing speed on microstructures and superconducting properties was investigated. Temperature gradient and growing speed were important factors in obtaining highly aligned micro-structures. The temperature gradient increases as the fiber diameter decreases. So highly aligned structures were obtained when the fiber diameter was small and the growing speed was low. When the nominal diameter of grown fiber was 0.5mm and the growing speed was 30mm/h, Jc of 17,500A/cm2(77.3K, 0T) was obtained. In the case of small fiber diameter, aligned crystal structures and relatively high Jc’s were maintained in spite of the high growing speed.

Development of High Tc Superconducting Wire by Powder Method

Ag-sheathed high Tc superconducting wires of YBa2Cu3O7-x and Bi0.8Pb0.2SrCaCU1.5Ox were developed. Improvements in the density and the orientation of crystals brought about by press processing have been investigated, and by adopting adequate processing procedures the critical current densities were increased up to 4140A/ cm2 and 4400A/ cm2 for Ag-sheathed YBa2Cu3O7-x and Bi0.8Pb0.2SrCaCu1.5Ox superconductors, respectively.

Elementary Pinning Force of Grain Boundaries in Supereonducting V3Ga Tapes

The elementary pinning force of grain boundaries was estimated for superconducting V3Ga tapes prepared by the surface diffusion method on pure and zirconium-doped vanadium tapes. It is 1.6-3.7×10-4 N/m for pure vanadium tapes and 3.8-6.2×10-4 N/m for zirconium-doped tapes. These values are much larger than the theoretical values by Zerweck, by Yetter et al. and by Welch based on the electron-scattering mechanism. Although Thunebergs theory givesa value comparable to the observed results, it is incompatible with the observed increase in the elementary pinning force with the addition of zirconium.

Transport properties and in-plane alignment of YBaCuO films

Abstract We report on the structure difference in YBaCuO films deposited on MgO(001) and mirror polished polycrystalline YSZ substrates. The critical current density of the film on polycrystalline YSZ is two orders lower than that on MgO(001). To improve this J c value, the substrate with relief structure was employed. This relief modification of substrate surface was found to be effective in controlling the in-plane alignment of the film grains deposited on it.

Microstructure and Jc of BiSrCaCuO Superconducting Fibers Prepared by Laser Pedestal Growth Method

BiSrCaCuO Superconducting fibers were prepared by laser pedestal growth method using a focused CO2 laser. The relationships among microstructure, Jc-B, and post-growth treatments, were investigated using the fibers prepared at 30mm/h of growing speed. As-grown structures were mainly consisted of aligned 0112 phase and 2201 phase. Aligned 2212 superconducting phase was created along the alignment of 0112 phase after post-annealing. When samples were annealed under tensile force, structures observed by optical microscope became dense and nearly single 2212 phase even at short time annealing, but non-superconducting phases that are sub-micron order size, stacking faults were observed by transmission microscope. Non-superconducting phases were increased when the starting composition was changed from 2212 to 0112. Jc-B characteristics were improved by combination of tension annealing and the change of starting composition.

Properties and Application of High Ic Bi Based Oxide Superconductors Prepared by Laser Pedestal Growth Method

Bi2Sr2Ca1Cu2Ox Superconductors were prepared by laser pedestal growth method. Annealing conditions were investigated to increase Jc and the diameter dependence of Ic was studied to increase Ic. The best annealing condition was 840° C*more than 500 hours. When the diameter was 3.0mm, Ic=500A was obtained. Also, electric current was applied to a sample in liquid helium or with the one electrode of a sample in liquid helium and another over liquid helium as a current lead situation. Oxide superconductor will be thought to be useful for current lead on a superconducting magnet in liquid helium. The possibility of application for it was investigated. In liquid helium, 1000A current could be applied without quenching. But in current lead situation, there was increasing of the generated voltage at the upper electrode with more than 450A current.

Superconducting Y-Ba-Cu-O Tapes Fabricated by Excimer Laser Deposition

Thin film Y-Ba-Cu-O superconducting tapes with the length up to 20cm were successfully prepared using a continuous laser deposition technique. Sintered yittria-stabilized zirconia tapes were used as flexible substrates. The as-grown Y-Ba-Cu-O films were highly c-axis oriented over the full length of the tape. The critical current density(Jc) at 77.3K of 3.4×104A/cm2 was obtained for 15cm voltage tap spacing. The highest value of local Jcs was 6.8×104A/cm2 (1cm spacing). The critical current(Ic) of 3.5A was achieved for 15cm voltage tap spacing. The crystal orientations and the morphological feature of the thin film tape are discussed in connection with the Jc values. The effect of bending strain on the Jc were also determined.

Y-Ba-Cu-O Films Grown on Flexible Polycrystalline Substrates by Excimer Laser Ablation

We report the results of a study on as-grown Y-Ba-Cu-0 films deposited on flexible and random oriented polycrystalline yittria-stabilized zirconia substrates using excimer laser ablation technique. Zero resistance Tc’s of 88–90 K are reproduciblly obtained. The best Jc(77.3K,0Tesla) is 2.9x104 A/cm2, which is one of the highest reported for superconducting films on any polycrystalline substrate. Films are c-axis oriented, while the in-plane orientation has been found to be random. This random orientation in the a-b plane is thought to be related to the existence of large angle grain boundaries which block the super-current. We have also examined the flexibility of our samples. Je value is never degraded up to the compressive strain of 0.2%.