Mitsuru Sawamura

Trapped field of YBa2Cu3O7 QMG bulk magnets

Trapped field distributions of quench melt grown (QMG) bulk magnets were measured in several external fields and temperatures. QMG bulk superconducting material was made by a modified QMG process. The size of the QMG magnets was 46 mm in diameter and 15 mm in thickness. The QMG bulk magnets were magnitized by application and reduction of an external magnetic field from 3.5 T after cooling at several temperatures. The field distributions were measured by scanning a Hall probe or aligned Hall probes over the magnet surface. Maximum trapped fields at 77, 63, 50, 20 and 10 K were 1.08, 2.72, 3.07, 3.27 and 3.26 T, respectively. From these distribution shapes, it appears that maximum fields at temperatures lower than 50 K were limited by the applied external field of 3.5 T. At 77 K and high external field, 4-fold symmetry was observed in the field distribution. Flux jumps were observed at 10 K, although mechanical damage was not observed during these experiments. From these results it was found that the QMG bulk magnets can trap at least 3 T steadily at a temperature higher than 20 K and lower than 50 K.

Magnetic properties of Ag-added Gd–Ba–Cu–O superconductors

Abstract In order to obtain helpful guidelines for designing high- T c bulk superconductor magnet systems, the gap, temperature and time dependence of the trapped field for Y–Ba–Cu–O and Gd–Ba–Cu–O/Ag bulk superconductors were measured. For the gap dependence, we compared the experimental results and the analytical solution on the condition that J c is constant for the disk-shaped bulk superconductor, and we found that the analytical solution corresponds well with the experimental results. The peak value of the trapped field reached 4.6 T at 63 K for the Gd–Ba–Cu–O bulk superconductor, and the flux creep rate at 63 K was slower than that at 70 and 77 K. This indicates that cooling down to at least 63 K creates superior conditions for the bulk magnet in respect of the increasing trapped field and its stability.

Mechanical properties of Gd123 bulk superconductor at liquid nitrogen temperature

In order to investigate the mechanical properties of the Gd123 single grain bulk superconductor at low temperature, bending tests were carried out. The bending fracture strength at LNT (liquid nitrogen temperature) scattered from 83 to 120 MPa was higher than that at RT, from 62 to 86 MPa. However, the Youngs modulus at LNT scattered from 109 to 123 GPa was comparable to that at RT, from 110 to 129 GPa. The Weibull coefficient for the bending strength at LNT, 6.5, was almost identical with that at RT, 6.9. The endurance limit for the repeated pulsating bending up to the5 /spl times/ 10/sup 4/ cycles at LNT, 70 MPa, was about 70% of the average value of the monotonic bending strength.

The magnetic levitation forces for single-grain bulks of Y-Ba-Cu-O and LRE-Ba-Cu-O/Ag with LRE = Sm, Gd

We have studied the distributions of the trapped field and the levitation forces for various large c-axis-oriented single-grain samples of LRE–Ba–Cu–O/Ag with LRE = Sm,Gd and Y–Ba–Cu–O prepared by the melt-process in air and a reduced oxygen atmosphere. Although the peak values of the trapped field vary from 1.0 to 1.5 T, the maximum values of the levitation force indicate 20 kgf for all the samples. Numerical analyses of the levitation force of the samples with various macro-Jc have been performed, and the results we obtained have been able to give a consistent explanation to our experimental results described above.

A hybrid-type bulk magnet combining Sm-Ba-Cu-O and Y-Ba-Cu-O superconductors

Abstract We have studied a bulk magnet combining Sm–Ba–Cu–O and Y–Ba–Cu–O superconductors for generating a higher magnetic field. The hybrid-type bulk magnet consists of disk-shaped Sm–Ba–Cu–O and ring-shaped Y–Ba–Cu–O bulk superconductors, thus has the advantageous properties of both of these superconductors. The peak value of the trapped field for the hybrid-type bulk magnet was larger than the simple sum of the Y-type trapped field and the Sm-type trapped field. This effect can be explained by considering the peak effect of Jc(B) for the Sm–Ba–Cu–O superconductor.

A new method for multiseeding RE?Ba?Cu?O superconductor

We have developed a new multiseeding method, the MUSLE (MUlti-Seeded seamLEss bulk) technique, for alleviating a problem of the conventional multiseeding method, i.e.?the existence of excluded non-superconducting phases (liquid phases and segregated RE211 phases) at the grain boundaries orientated by the multiple seeds in the bulk superconductor. The basic concept of this technique is that the precursor is composed of two or more RE?Ba?Cu?O layers with different peritectic temperatures. In order to study the superconducting prospects for MUSLE bulk, RE?Ba?Cu?O bulk superconductors of various sizes up to 100?mm in diameter were produced by the MUSLE technique. All trapped field distributions for the RE?Ba?Cu?O bulk superconductors had a single peak. For the 46?mm diameter MUSLE bulk, Jc(B) across the multiseed-orientated grain boundary was comparable to that in the grain. The misorientation angle of the c-axis between the multiseed-orientated grains was approximately 3??7? in this sample, whereas each Jc(B) for the specimens across the multiseed-orientated grain boundary has the same level. We also estimated the macro-Jc flow in bulk superconductors through the peak values of the trapped fields. The macro-Jc for the MUSLE bulk was larger than that for the single-seeded bulk at sizes greater than 65?mm diameter.

Enlargement of bulk superconductors by the MUSLE technique

Abstract We have previously reported on the multi-seeded seamless bulk (MUSLE) technique, a new multi-seeding method for producing RE–Ba–Cu–O bulk superconductors. In order to study the superconducting prospects for MUSLE bulk, RE–Ba–Cu–O bulk superconductors of various sizes up to 100 mm in diameter were produced by the MUSLE technique. All trapped field distributions for the RE–Ba–Cu–O bulk superconductors had a single peak. For the 46 mm diameter MUSLE bulk, the J c ( B ) across the grain boundary was comparable to that in the grain. We also estimated the macro- J c flow in the bulk superconductors through the peak values of the trapped fields. The macro- J c for the MUSLE bulk was larger than that for the single seeded bulk at sizes greater than 65 mm in diameter.

Properties of a few hundred A class Y–Ba–Cu–O bulk current leads usable in magnetic fields

Abstract In order to investigate the properties of high-temperature Y–Ba–Cu–O bulk current leads usable in magnetic fields, a prototype having a 250 A current capacity at 77.3 K and 0.5 T has been fabricated using melt-processed bulk superconductors, QMG. The critical current was larger than 800 A at an applied field of 0 T and larger than 500 A at 0.5 T. When a current of 250 A was continuously applied for 10 min at 0.5 T, no change was observed in the voltage between the current lead ends. When the current lead was exposed to a repetition of thermal cycles between the liquid nitrogen temperature and room temperature, the transport properties did not change in the experimental range. The amount of heat leak through the current lead can be estimated to be about 0.095 W when the high-end temperature is 77.3 K and the low-end temperature is 4.2 K.

Behavior of Basal Plane Dislocations in Hexagonal Silicon Carbide Single Crystals Grown by Physical Vapor Transport

The behavior of basal plane dislocations in hexagonal silicon carbide (SiC) single crystals grown by physical vapor transport (PVT) was investigated by defect selective etching and transmission electron microscopy (TEM). Oval-shaped etch pits on the etched vicinal (0001)Si surface due to basal plane dislocations were densely distributed around hollow-core threading screw dislocations (micropipes) and formed etch pit arrays perpendicular to the off-cut direction, indicative of the multiplication of basal plane dislocations around micropipes during crystal growth or post-growth cooling. Arrays of oval-shaped etch pits were also observed in the vicinity of small hexagonal etch pit rows due to threading edge dislocation walls, i.e., low angle grain boundaries (LAGBs). They were asymmetrically distributed across LAGBs, and TEM revealed that threading edge dislocations constituting LAGBs trapped basal plane glide dislocations. The interaction between basal plane dislocations and threading screw and edge dislocations extending along the c-axis in SiC crystals was modeled, and the characteristic behavior of basal plane dislocations in hexagonal SiC single crystals was discussed.

New Multi-seeding Method of RE-Ba-Cu-O Superconductor

Abstract We have developed a new multi-seeding method, multi-seeded seamless bulk (MUSLE) technique, for improving a problem of the conventional multi-seeding method, i.e. the existence of the excluded non-supercondcting phases (liquid phases and segregated RE211 phases) at the grain boundaries in the bulk superconductor. It is the basic concept of this technique that the precursor is composed of two or more RE–Ba–Cu–O layers with different peritectic temperatures. Thereby, the directions of the crystal growth from the seed crystals can be controlled. RE–Ba–Cu–O superconductors were fabricated by the MUSLE technique with four seeds. The trapped field distribution obtained at 77 K showed a single peak with a value of 0.9 T, indicating that the MUSLE technique is effective in eliminating the excluded phases formed at the grain boundaries in the multi-seeded bulk superconductor. We suggest that the elimination of the excluded phases can be explained by the seed crystal proximity effect.