S.I. Yoo

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.

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.

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>>

Critical currents of OCMG processed Nd123 crystals

Abstract Critical current densities of Nd123 crystals flux grown by the oxygen-controlled-melt-growth (OCMG) process have been studied with DC magnetization measurements. The superconducting transition temperature ( T c ) of the Nd123 crystal flux grown in 0.1%O 2 Ar gas was 94 K with the applied field of 10 Oe parallel to the c -axis. The sample was found to be homogeneous without weakly linked regions through magneto-optical measurements. DC magnetization-hysteresis curves measured with a SQUID magnetometer exhibited a peak effect at various temperatures above 70 K. The peak effect is attributable to field-induced pinning by finely distributed low- T c phases. The sample exhibited high critical current density ( J c ) valuesof 4 × 10 4 A/cm 2 in 2 T and over 1 × 10 4 A/cm 2 in 4 T at 77 K for fields parallel to the c -axis.

Confirmation of Ba-rich Nd1+xBa2-xCu3O7-δ solid solutions

Unlike Y-Ba-Cu-O, the Nd-Ba-Cu-O system forms a Nd1+xBa2-xCu3O7-δ-type solid solution (Nd123ss). Recently, it was found that Nd123ss with x<0 can be stabilized at high temperatures under a low oxygen partial pressure, P(O2). In the present study, we further confirmed the presence of a Ba-rich Nd123ss single phase through the investigation of the x dependence of lattice parameters and the P(O2) dependence of the peritectic decomposition temperatures in Nd123ss samples with various x values.

Effects of deposition rate and thickness on the properties of YBCO films deposited by pulsed laser deposition

YBCO films with various thicknesses from 100 nm to 1.6 µm were deposited on single crystal SrTiO3 substrates by pulsed laser deposition (PLD). The effects of thickness and deposition rate—by means of controlling the pulsed laser frequency—on the critical current density (Jc) were studied. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to examine the orientation, crystallization and surface quality. The amount of a-axis YBCO component evaluated from the ratio of XRD chi-scan intensities of the a-axis and c-axis for the YBCO (102) plane increased as the YBCO film became thicker. SEM was used to analyse the surface of YBCO film, and it was shown that the surface of YBCO film became rougher with increasing thickness. There were many large singular outgrowths and networks of outgrowths on the surface of the YBCO films with thickness greater than 0.4 µm. The increased amount of a-axis YBCO component and the coarse microstructure of the thick YBCO film caused degradation of Jc with increasing thickness.

Scaling of pinning forces in NdBa2Cu3O7−δ superconductors

The scaling of the volume pinning forces, Fp=jc×Ba, is performed versus the reduced fields bf=Ba/Bfp (where Ba is the applied field, and Bfp denotes the peak position of Fp,max) and h=Ha/Hirr (where Hirr denotes the irreversibility field) for two different NdBa2Cu3O7−δ superconductors, a single crystal and a melt-processed sample. In both cases, an excellent natural scaling is found to hold. Based on these good scaling results, we extend the scaling analysis towards lower temperatures. There, the scaling parameters are no longer directly accessible within our experimental field range, but can be deduced from the Fp(Ba) curves normalized using fitting values of Bfp and Fp,max so that these overlap with the high temperature curves. It is shown that such a procedure allows to obtain reasonably good estimates of Fp,max and Hirr, which leads in turn to an estimation of the irreversibility line well down to 0 temperature. At low temperatures, deviations from the analytical dependencies appear which are due to i...

Studies of the Nd1 +xBa2-xCu3Oy solid solutions

It is known that the amount of Nd ions substituting for a Ba site in the Nd1+xBa2−xCu3Oy solid solution (Nd123ss) tends to decrease with either decreasing oxygen partial pressure PO2 or increasing temperature, however, the precise solubility limit of Nd123ss has not been obtained. In the present study, we carefully studied the solubility limit of Nd123ss under low PO2 and for the first time could show the presence of x<0, i.e. the substitution of Ba for the Nd site.