Noriki Hayashi

Effect of high-temperature heat-treatment on the pinning properties of melt-textured Nd123

Abstract We have investigated the effect of high-temperature annealing on the superconducting transition and the flux pinning properties of Nd123 melt-textured in 1% O 2 Ar. After the high-temperature annealing, a decrease in J c was observed in most samples even though no appreciable change was observed in T c . TEM observations and EDX analysis show that the spatial variation of the Nd/Ba ratio almost disappears after high-temperature annealing. Therefore, the observed change in J c - B properties can be understood by a reduction in the field-induced pinning due to homogenization of Nd/Ba substitution regions. These results strongly support the idea that the secondary peak effect and high pinning performance in Nd123 can be ascribed to the presence of Nd/Ba spatial variation.

Effects of oxygen content on the superconducting properties of melt-textured LRE123 superconductors

Abstract The effects of oxygen content on the superconducting transition temperatures, T c , and magnetic properties of melt-textured LRE123 (LRE  Nd, Sm) superconductors were studied. The oxygen content could be varied by changing the annealing temperature, T a , in that the oxygen deficiency, δ, monotonically decreased with decreasing the T a from 600°C to 300°C. A further decrease of T a to 275°C showed no appreciable change. In both systems the highest T c was obtained for the samples annealed at 300°C. Magnetic properties were also strongly dependent on the oxygen content, showing that the control of oxygen content is critical for achieving good pinning properties.

Fabrication of large single-domain Sm123 superconductors by OCMG method

Abstract Melt processing of Sm-Ba-Cu-O bulk superconductors was studied to find the optimum conditions for fabricating a large single-domain superconductor. First, the holding time at the maximum temperature for the partial melting is important for the fabrication of a uniform bulk, especially for a large diameter. When the holding time at the maximum temperature during partial melting was not enough, two separated regions were observed, which can be diminished by sufficient holding time. Next we studied the optimum conditions for grain growth, varying the temperature gradient and cooling rate. The processing condition with which we have succeeded in growing a single-domain SmBa 2 Cu 3 O y (Sm123 hereafter) with dimensions of 35 φ × 15 mm 2 is a temperature gradient of 7.5 °C cm −1 with a slow cooling rate of 0.33 °C h −1 .

Comparative study of magnetic fields trapped in YBa2Cu3Oy and SmBa2Cu3Oy, superconducting bulks

Abstract We have measured the trapped field distribution in plates of melt processed YBa 2 Cu 3 O y (Y123) and SmBa 2 Cu 3 O y , (Sm123) superconductors that have different field dependence of the critical current density ( J c ). Sm123 materials were fabricated by the oxygen-controlled melt-growth (OCMG) process and exhibited the peak effect. The trapped fields were measured by scanning a Hall probe on the sample surface. Measurements were performed after a field of 3.5 T was applied in the field-cooled process. On the basis of the critical state model, the magnetic field distribution was calculated taking account of the field dependence of the critical current density. The calculated values were in good agreement with the measured data. We have also estimated trapped magnetic fields for Y123 and Sm123 when the grain size is enlarged, which showed that OCMG-processed Sm123 bulk superconductors have greater potential as trapped field magnets.

Fabrication of a high current superconductor in Y-system by excimer laser ablation

A hundred pieces of thin film Y1Ba2Cu3Ox(YBCO) superconducting tape, each of which was 1.1meter long, were fabricated using KrF excimer laser deposition. Almost all the tapes had Jc values more than 1.0×104 A/cm2. The best data showed the Jc of 4.5×104A/cm2 and the Ic of 4.3A at 77.3K for 1.1m voltage tap spacing. The sixty tapes were spirally wound in six layers on a stainless steel former to assemble a high current conductor. The Jc of 1.8×104A/cm2 and the Ic of 103A were achieved for 1.0m in length of the conductor. With the Jc improvement, process conditions of the YBCO deposition were refined. The Jc of 1.0×105A/cm2 is obtained for a short sample. Morphological features of the high Jc film are also discussed.

Characteristics of the Switching in a Superconducting Y–Ba–Cu–O Thin Film by Laser Irradiation

Radiation-induced switching of a Y-Ba-Cu-O thin film sample is studied using a 1.06 µm pulsed Q-switched Nd:YAG laser with a duration of 10 ns. The results of the switching from the super- to normal conducting states in our sample caused by laser irradiation are attributed to a bolometric effect, i.e., heating of the sample by radiation. The response time of the switching is less than 30 ns and it almost agrees with the relaxation time of the thermal conduction in the film. Using a simple model assuming uniform distribution of energy through the film thickness, the specific heat of the film is also estimated and a clear jump in the specific heat at the critical temperature is shown.

Comparative study of pulsed field magnetization for Y–Ba–Cu–O and Sm–Ba–Cu–O

Abstract We have performed a comparative study of the magnetization behavior with applying pulse fields for Y–Ba–Cu–O and Sm–Ba–Cu–O, which exhibit the different field dependence of critical current densities (Jc’s). Time variation of the field was monitored with the Hall probe sensor which is attached to the sample surface by varying the maximum field values from 0 to 2 T. In a low field region, the trapped magnetic field of Sm–Ba–Cu–O was slightly smaller than that of Y–Ba–Cu–O, however, it exceeded the value of Y–Ba–Cu–O in a high field region. The results reflect the Jc–B characteristic of Sm–Ba–Cu–O, in which the Jc values show the secondary peak effect and high field Jc values are higher than those of Y–Ba–Cu–O.

Grain Enlargement in OCMG-Processed RE-Ba-Cu-O Superconductors and their Properties

REBa2Cu3Oy (RE=Nd,Sm) bulk superconductors were fabricated by the oxygen — controlled- melt -growth (OCMG) method, using a reduced oxygen partial pressure during melt growth. We investigated the grain enlargement and the properties of OCMG-processed REBa2Cu3Oy superconductors fabricated in a tube furnace with temperature gradient. From observations with a polarized optical microscope, it was revealed that the largest grain size with this process was 10×10 mm The dc magnetization measurement showed the Tc values from 94 K to 95 K in both NdBa2Cu3Oy and SmBa2Cu3Oy. The magnetic Jc for Ndl23 and Sml23 samples were 2.1×l04 A/cm2 and 9.7×103 A/cm2 at 2 Tesla and 77 K, respectively.

A.C. magnetization characteristics of Nd123 bulk superconductors

Abstract The a.c. magnetization characteristics of NdBa 2 Cu 3 O y (Nd123) bulk superconductors were investigated using a.c. susceptometry. For the present study, an OCMG-processed Nd123 bulk sample (OCMG-Nd123; T c = 93.9 K ) and a flux-grown Nd123 single crystal (Nd123SC; T c = 93.2 K ) were used. An MPMG-processed Y123 bulk sample (MPMG-Y123; T c = 91.6 K ) was also studied for comparison. While the fundamental responses are basically the same for all samples, the third harmonic responses by sweeping the bias field, x 3 ( H dc ), show an interesting feature in that the x 3 ( H dc ) curves for Nd123 samples indirectly reflect the fishtail peak of their d.c. magnetization hysteresis ( M-H ) curves. The boundary lines of Nd123 samples, determined from both the peak temperature ( T p ) of the x 1 ″( T ) maxima and the offset of the x 3 ( H dc ), are commonly located at higher field-temperature region relative to that of the Y123. Using the scaling relationship of H ∞(1 − t ) n , where t = T / T c , all the boundary lines from the x 3 ( H dc ) and the x 1 ″( T ) maxima could be best fit with a power n of ~ 1.2 and ~ 1.4, respectively.

Penetration Dynamics of RE-Ba-Cu-O Superconductors on Pulsed Field Magnetization

It is known that melt processed RE-Ba-Cu-O (RE: rare earth elements) superconductors can trap magnetic field over 1T at liquid nitrogen temperature (77K). The trapped field is dependent on the critical current density (J c ). J c -B characteristic of oxygen-controlled-melt-growth (OCMG) processed Sm-Ba-Cu-O superconductors are different from that of Y-Ba-Cu-O. Therefore, we investigated the J c effect on pulsed field magnetization at various maximum applied field values of 0 – 2T for Sm-Ba-Cu-O grown by the OCMG method.