Takamitsu Higuchi

Melt processing for obtaining NdBa2Cu3Oy superconductors with high Tc and large Jc

A reduced oxygen atmosphere during melt processing turned out to be critical for the fabrication of NdBa2Cu3Oy (Nd123) superconductors possessing high superconducting transition temperature (Tc) with a sharp transition and large critical current density (Jc) at 77 K. In a dc magnetization measurement, Nd123 superconductors melt processed in flowing a mixture gas of 1% O2 in Ar exhibited the Tc of about 95 K and the transition width of 1.5 K with the applied field of 10 Oe. A four‐probe measurement showed the zero resistive transition Tc (R=0) of about 95 K. An anomalous peak effect in the magnetization hysteresis (M‐H) loops was commonly observed and lead to large magnetic Jc of 2×104 A/cm2 at 77 K and 2 T for the applied field H parallel to the c axis of a sample (H∥c). This achievement is attributable to a preferential formation of high Tc phase (x<0.1) among the Nd1+xBa2−xCu3Oy solid solutions in a reduced oxygen atmosphere.

Melt-processed light rare earth element-Ba-Cu-O

Unlike Y123 which forms only a stoichiometric compound, the light rare earth elements (LREs: La, Nd, Sm, Eu, Gd) form a solid solution . The presence of such solid solution caused a depression in the superconducting transition temperatures , particularly for La123, Nd123 and Sm123 when they are melt processed in air. Recently, we have found that the of these LRE123 superconductors can greatly be enhanced when they are melt processed in a reduced oxygen atmosphere. Furthermore, values of these superconductors were larger than that of a good quality Y123 superconductor in high magnetic fields at 77 K. In this article, on the basis of our study over the last several years, the melt processes for LRE - Ba - Cu - O are described, the microstructural and superconducting properties of the superconductors are reviewed and the flux pinning mechanism is also discussed.

Flux Pinning in Melt-Grown NdBa2Cu3Oy and SmBa2Cu3Oy Superconductors

REBa 2 Cu 3 O y superconductors with rare earth (RE) ions with large radii (RE: La, Nd, Sm) exhibit relatively low T, due to the presence of RE-Ba solid solution. We have found that this solid solution can be suppressed if these superconductors are melt processed in a reduced oxygen atmosphere. We have also found that critical current densities of these superconductors are higher than those of melt processed YBa 2 Cu 3 O y with fine Y 2 BaCuO 5 inclusions in a high field region. The irreversibility line was also shifted toward the higher H-T region. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba 1-x , RE x ) 2 Cu 3 O y phase in a good superconductive matrix

Enhanced Tc and Strong Flux Pinning in Melt-Processed NdBa2Cu3Oy Superconductors.

The effects of various oxygen partial pressures (P O2) during melt processing on the superconducting properties of fabricated NdBa2Cu3Oy (Nd123) superconductors have been systematically investigated. Unlike in air or O2, samples melt-processed in reduced oxygen atmospheres exhibited unprecedentedly high T cs of 95–96 K with a sharp superconducting transition. Especially, for a sample melt-processed in 0.1% O2 in Ar, the magnetization hysteresis ( M-H ) loop was kept open up to 7 T at 77 K and for the field parallel to the c-axis of the sample (H//c), implying that very effective flux pinning can be achieved in high magnetic fields.

Analysis of Pinning in NdBa2Cu3O7-δ Superconductors

Volume pinning forces, F p , are measured in a wide temperature (5 ≤ T ≤ 92 K) and field range (0 ≤ μ0 H a ≤ 9 T) on different NdBa2Cu3O7-δ samples. Above 60 K a good scaling of F p , versus the reduced field, h = H a /H irr can be established. The scaled pinning curves are compared to several theoretical predictions. Experimental evidence for strong pinning at extended (Vpin ~ ξ2 d) defects is given. These defects are ascribed to spatial composition fluctuations found in light rare earth superconductors, providing a scatter of the transition temperature, T c . Such a pinning echanism is especially important for applications of high-T c superconductors operating at 77 K.

A new type of pinning center in melt grown Nd123 and Sm123

Abstract REBa2Cu3Oy (RE: Nd, Sm) superconductors exhibit relatively high Jc values even at 77K when melt-processed in a reduced oxygen atmosphere. The Jc values of these superconductors are higher than those of melt processed YBa2Cu3Oy with fine Y2BaCuO5 inclusions in a high field region. The irreversibility field with H∥c axis was dramatically increased to 8T at 77K. We believe that flux pinning in these superconductors is ascribable to a finely distributed RE(Ba1−x, REx)2Cu3Oy phase in a good superconductive matrix.

The trapped field of YBCO bulk superconducting magnets

Abstract We have measured trapped field distributions by scanning a Hall probe on various rectangular YBCO bulk superconductors and studied the effects of field dependence of Jc bulk thickness and bulk size on the trapped field. The trapped field could be increased by increasing the bulk thickness and size. However, a saturation was observed for the maximum thickness. We also calculated the distribution of trapped field taking account of the field dependence of Jc. Experimental trapped field distributions were in good agreement with our calculated values when we used the Jc-B curves obtained from a d.c. magnetization measurement for samples.

Preliminary study of a superconducting bulk magnet for the Maglev train

One of the prospective applications of high critical temperature superconductors is a superconducting magnet for the magnetically levitated (Maglev) train. Development shows that RE (rare earth) BaCuO and LRE (light rare-earth) BaCuO superconductors prepared by melt processes have a high critical current density at 77 K and high magnetic fields. LRE-Ba-Cu-O bulk superconductors melt-processed in a reduced oxygen atmosphere, named oxygen-controlled-melt-growth (OCMG) process, are very promising for high field application as a superconducting permanent magnet with liquid nitrogen refrigeration. Compared to good quality melt-grown REBaCuO bulks, LREBaCuO bulks exhibit larger critical current densities in high magnetic fields and much improved irreversibility field at 77 K, implying that more effective flux pinning can be realized in a commercially feasible way. In this study, we discuss the possibility of a superconducting bulk magnet for a Maglev train. A preliminary design of the bulk magnet and also melt processing for REBaCuO and LREBaCuO bulk superconductors and their characteristic superconducting properties are presented.

Melt processing for obtaining REBa/sub 2/Cu/sub 3/O/sub y/ superconductors (RE=Nd, Sm) with high T/sub c/ and large J/sub c/

An oxygen-controlled melt growth (OCMG) process has been employed for NdBa/sub 2/Cu/sub 3/O/sub y/ (Nd123) and SmBa/sub 2/Cu/sub 3/O/sub y/ (Sm123) superconductors for a comparative study. Both Nd123 and Sm123 samples melt-grown in reduced oxygen atmospheres exhibited an enhanced superconducting transition temperature (T/sub c/) with a sharp transition and large critical current density (J/sub c/) in a high field region at 77 K and for the applied field parallel to the c-axis of samples (H/c). For Nd123, a preferential formation of high T/sub c/ phase in a low oxygen partial pressure (P(O/sub 2/)) could be qualitatively described considering the thermodynamic stability boundaries of the Nd/sub 1+x/Ba/sub 2-x/Cu/sub 3/O/sub y/ solid solutions on the LOG[P(O/sub 2/)] vs. 1/T (K/sup 9/-1) phase diagram. A long-term oxygen annealing study for an OCMG-processed Nd123 sample revealed that the annealing temperature should be as low as 300/spl deg/C.<<ETX>>

Low magnetic relaxation in a single crystal and a melt processed sample

Induced current densities and magnetic relaxation rates were measured over a wide temperature and field range on a single crystal and a melt processed sample of using a SQUID magnetometer. The temperature dependence of the magnetic relaxation rate shows a pronounced minimum at elevated temperatures. This leads to very high values of the effective pinning energy and for the single crystal and the melt processed sample respectively, for an applied magnetic field . Furthermore, these samples show a well developed fishtail effect, leading to a current density . This demonstrates that the fishtail effect is closely related to the low relaxation rates observed here.