Y. Yanagi

Construction of a 2–5 T class superconducting magnetic field generator with use of an Sm123 bulk superconductor and its application to high-magnetic field demanding devices

Abstract Superconducting permanent magnet systems generating magnetic fields of 2–5 T have been constructed for the first time in the world by c -axis oriented Sm–Ba–Cu–O bulk superconductors fabricated through the melt processing as trapped field magnets. The trapped magnetic fields of 3.8 and 6.7 T have been produced on the bulk surface at 30 K by the pulsed field magnetization (PFM) and field cooling (FC) methods, respectively. The magnetic field in the open space outside the vacuum vessel exceeds 2.0 and 3.2 T when magnetized by the PFM and FC modes, respectively. It was found that the iteratively magnetizing pulsed-field operation with reducing amplitudes (IMRA method) is very effective in magnetizing high J c bulk superconductors. This implies the superconducting permanent magnets can be used as high magnetic field generators in various practical applications.

Critical current density and mechanical strength of YBa2Cu3O7-δ superconducting composites containing Zr, Ag and Y2BaCuO5 dispersions by melt-processing

Abstract The high- T c YBa 2 Cu 3 O 7−δ composite superconductors containing Zr, Ag and Y 2 BaCuO 5 have been fabricated by melt processing. Both the critical current density J M c and the flexural strength α max of these samples were measured. The value of J M c increased initially for all inclusions added. The microstructure analyses revealed that the Y 2 BaCuO 5 , Ag and BaZrO 3 were finely dispersed in the matrix. We believe that the fine dispersion of these inclusions is most likely responsible for an increase in J M c . The largest value of J M c we obtained is 64 800 A/cm 2 on the sample containing 50 mol.% Y 2 BaCuO 5 . In contrast, the value of α max was limited below 60 MPa due to the presence of mechanically weak grain boundaries. The addition of Ag to YBa 2 Cu 3 O 7−δ was effective in increasing both α max and J M c .

Preparation of melt-textured NdBa2Cu3Oy bulk with Nd4Ba2Cu2O10 addition

Abstract The NdBa 2 Cu 3 O y (Nd123) bulk superconductor, to which Nd 4 Ba 2 Cu 2 O 10 (Nd422) particles were intentionally added, was prepared through the so-called MMTG process in Ar (99% purity) flowing atmosphere at an ambient pressure. The quasi-single crystal thus grown was about 1 cm × 1 cm × 1 cm in dimension. It turned out that the Nd422 particles were uniformly distributed in the Nd123-phase matrix in a fashion similar to the distribution of the intentionally added Y211 particles in the Y123 phase matrix. The superconducting transition temperature T c for the sample subjected to post-annealing in oxygen atmosphere was 94 K. The critical current density J c was determined to be 45 000 A/cm 2 at 77 K and 1 T, when the field was applied parallel to the c -axis of the sample. To the best of our knowledge, the J c value is the highest and the size of the quasi-single crystal is the largest in the melt-textured Nd123 bulk superconductors so far reported.

Pulsed field magnetization of a 36?mm diameter single-domain Sm?Ba?Cu?O bulk superconductor at?30, 35?and 77 K

A c-axis oriented single-domain Sm–Ba–Cu–O bulk superconductor 36 mm in diameter was magnetized by the pulsed field magnetization (PFM) method at 30, 35 and 77 K. The trapped field distributions after applying pulsed fields with different amplitudes were measured by scanning a Hall sensor 0.5 mm above the surface of the sample. We also measured the time evolution of magnetic fields during the PFM by using an oscilloscope connected to two Hall sensors mounted on the bulk superconductor surface. Fluxes are found to penetrate into the bulk superconductor and to escape from it by choosing passes through the direction inclined at 45° to growth sector boundaries (GSBs) of the sample. At 35 K, the temperature rise of the sample caused by heat generation due to flux motion becomes more substantial than that at 77 K. Thus, flux jumps occurred through the passes and assisted magnetic fluxes to reach rather easily the centre of the bulk superconductor. As a result, the magnetic field necessary for PFM is lower than that to fully magnetize the sample by means of the static zero-field-cooling magnetization method. The optimized multi-PFM with reducing amplitudes, which was specifically referred to as the IMRA technique, turned out to be very effective in achieving excellent trapped field characteristics by PFM at low temperatures. We could achieve a maximum trapped field of 3.6 T together with a well conical trapped field distribution at 30 K.

Application of 60 mmφ superconducting bulk magnet to magnetron sputtering

Abstract We constructed the planar magnetron sputtering apparatus using a c -axis oriented single-domain Sm123 bulk superconductor with 60 mm in diameter as a very powerful magnet in place of an ordinary Nd–Fe–B magnet. A high magnetic field of 4.2 T at the surface of the superconductor coupled with a high target voltage of maximum 6 kV enabled us to discharge even at pressure of 1xa0×xa010 −3 Pa. A target-to-substrate distance of 300 mm was successfully employed under low pressures of 10 −2 –10 −3 Pa to make the deposition of almost contamination-free films feasible. The simulation software (JMAG) was used to optimize the magnetic circuit configurations. The simulations could reproduce well the distribution of the magnetic field above the target measured by a three-axial Hall sensor. The discharging characteristics of Cu, Ni and Fe targets in the pressure range over 10 −1 –10 −3 Pa were studied under different target voltages. The deposition rates of 0.063 nm/s (or 38 A/min) and 0.013 nm/s (or 8 A/min) were achieved for Cu and Fe targets with 3 mm in thickness, respectively, under the Ar pressure of 6.6xa0×xa010 −2 Pa (or 4.9xa0×xa010 −4 Torr).

Performance of the magnetron sputtering apparatus equipped with 60 mm superconducting bulk magnet

The performance of the planar magnetron sputtering apparatus with the use of a c-axis oriented single-domain Sm123 bulk superconductor with 60 mm in diameter is discussed. A high magnetic field of 4.2 T at the surface of the superconductor coupled with a high electric field of maximum 6 kV enabled us to discharge even at the Ar gas pressure of 1 × 10−3 Pa. A target-to-substrate distance could be extended to 30 mm under low pressures of 10−2–10−3 Pa to allow the deposition with an incidence angle normal to a substrate. The discharging characteristics of Cu, Ni, Fe, Al and carbon targets in the pressure range over 10−1 to 10−3 Pa were studied under different target voltages. The deposition rates of 0.063 nm s−1 (or 38 A min−1) and 0.013 nm s−1 (or 8 A min−1) were achieved for Cu and Fe targets with 3 mm in thickness, respectively, under the Ar pressure of 6.6 × 10−2 Pa.

Improvement of the microstructure of melt-processed Sm-based superconductors

Abstract We report on the effect of the atmosphere during the decomposition of the precursors of Sm–Ba–Cu–O bulk superconductors. It was found that the formation of voids was greatly suppressed by two methods, melting in an oxygen gas atmosphere or in vacuum. For both methods, the atmosphere during crystallization was switched to an Ar gas flowing one. The samples prepared by these methods exhibited high superconducting transition temperatures with sharp transition widths. However, the zero-field critical current density was lower than that of the reference sample, which was prepared by decomposing the precursor in an Ar gas flowing atmosphere.

Magnetron sputtering activated by a 60 mm diameter superconducting bulk magnet

A magnetized c-axis oriented single-domain Sm123 bulk superconductor of 60xa0mm in diameter was employed in place of a conventional Nd–Fe–B permanent magnet in a planar-type magnetron sputtering apparatus and its sputtering performance was studied by measuring the Ar gas pressure dependence of deposition rate for elemental targets Cu, Ni, Fe, C and Al of different thicknesses. The value of on the surface of the Cu target of 3xa0mm thickness reached 0.63xa0T in comparison with 0.05xa0T for the Nd–Fe–B magnet in conventional magnetron sputtering. The use of such a high value enabled us to carry out sputtering down to the extremely low pressure of 1 × 10−3xa0Pa, which is two orders lower than in a conventional procedure.

Melt processing of (rare earth)-Ba-Cu-O bulk superconductors

Several research efforts on improving the performance of melt-processed bulk superconductors are reported. First, we extended our study on the Nd–Eu–Gd (NEG) system by exploring the effect of ball milling the secondary phase powders of the precursor, and observed an increase in the critical current density (Jc) as well as in the trapped field. We also studied the effect of changing the ratio of the rare earth elements of the secondary phase powders of the NEG system. The samples prepared with a larger amount of Gd were found to trap a larger magnetic field without fracturing. On the other hand, the samples with a nominally Nd-rich composition showed a somewhat lower critical temperature. We also studied the effect of doping Zn into DyBCO, and found a favourable influence in terms of Jc and trapped field. Calculations of the magnetic flux density above a field-trapped superconductor indicated that increasing the domain size is very effective to improve the performance of the superconductor even if the larger sample cannot be prepared with the same quality as a smaller one. Therefore, we studied the melt-processing of SmBCO superconductors that are 60 mm in diameter. It was crucially important to release a sufficient amount of oxygen from the melt for the preparation of a homogeneous sample, and it is demonstrated that the magnetic flux usable in applications significantly increased on enlarging the domain size.

A 3 T magnetic field generator using melt-processed bulk superconductors as trapped field magnets and its applications

Abstract An intense magnetic field generator yielding 3.15 T in the open space between the magnetic poles has been constructed by using a pair of melt-processed bulk superconductors as trapped field magnets. The field was measured in a 2 mm gap between the magnetic poles set face-to-face after the pulsed-field magnetization “IMRA” method. This field generator is composed of Sm-based 123 compounds, vacuum pumps, pulsed-field coils and GM refrigerators with compressors. The system can be used in various applications. We investigated, for instance, the application to a high gradient magnetic separation system. It was found that the alpha hematite fine particles mixed in the flowing water was completely removed by this technique which was operated in the field of 1.7 T in the gap of 20 mm.