Baorong Ni

Critical Current Characteristics in Superconducting Y-Ba-Cu-O Prepared by the Melt Process

Critical current densities in superconducting Y-Ba-Cu-O specimens with different sizes of 211 particles prepared by the melt process were measured under various magnetic fields and temperatures. Discussion is given on the flux pinning properties by 211 particles and background pinning centers. At temperatures above 60 K, 211 particles are expected to be dominant pinning centers. A rapid degradation of the critical current density with elevating temperature at low temperatures below 25 K seems to be attributed to weak links in specimens.

AC Inductive Measurement of Intergrain and Intragrain Currents in High-Tc Oxide Superconductors

Bulk intergrain current and closed intragrain current in sintered Y-Ba-Cu-O superconductors were measured at 77 K by using an ac inductive method. These currents can be separated because of the large difference in penetrating rates of the magnetic flux into specimens with respect to the ac field amplitude. The obtained intergrain current density agreed approximately with critical current density measured resistively. The closed intragrain current density amounted to 4.3×108 A/m2 at B=0.5 T and decreased gradually with increasing magnetic field. The present results show that the ac inductive measurement is one of the available nondestructive methods to characterize sintered oxide specimens.

Estimate of Attainable Critical Current Density in Superconducting YBa2Cu3O7-δ

Attainable critical current density in oxide superconductor YBa2Cu3O7-δ is estimated by an ordinary analyzing method in flux pinning phenomena with superconducting parameters obtained from a magnetization measurement. The pinning mechanisms considered here are an anisotropy in the upper critical field and an electron scattering effect for grain boundaries and twinning planes. While the elementary pinning force of these planar defects is small due to a small thermodynamic critical field at 77 K, a critical current density comparable to that of Nb-Ti at 4.2 K is expected to be attained by increasing the pin concentration.

Effect of reversible fluxoid motion on AC susceptibility of high temperature superconductors

Abstract It has been found in measurements of AC susceptibility versus temperature for high temperature superconductors that, with decreasing AC magnetic field amplitude, the peak value of the imaginary part, χ decreases remarkably accompanied by a shift of its position to higher temperatures, while the height of the transition of the real part, χ does not change appreciably. This behavior contradicts the prediction by the critical state model in which the flux pinning phenomenon is assumed to be completely irreversible. It is shown that this anomalous magnetic behavior originates from reversible fluxoid motion in pinning potentials. The AC susceptibility is calculated on the basis of the Campbell model for the reversible fluxoid motion. The result obtainained warns us against incorrect use of the critical state model for analysis of experimental data.

IMPROVEMENT OF CRITICAL CURRENT DENSITY IN YBA2CU3O6+X SUPERCONDUCTOR BY SN ADDITION

When SnO2 was added to Y-123 (YBa2Cu3O6+x) powder and sintered at 1223 K for 86.4 ks, the densification was enhanced remarkably, and the density reached 99% of the theoretical value for the 2-mol%-SnO2-added specimen, while the average grain size became smaller with Sn addition. Fine spherical BaSnO3 precipitates were observed to be distributed in the grains and at grain boundaries. The AC magnetization measurements indicated that Sn addition causes intragrain Jcm to increase at 77 K in the magnetic field between 0.02 and 0.12 T. The Jcm exceeded 106 A/cm2 at 0.04 T for the 3-mol%-SnO2-added specimen.

Critical Transport Current Density in Sintered Oxide Superconductors with High Critical Temperature

Percolative critical transport current density in sintered Y–Ba–Cu–O superconductors is estimated by taking into account the effect of anisotropies in the upper critical field and the pinning structure. This value is much smaller than the estimate for the optimized situation, where misorientation between the magnetic field and anisotropic pinning structures does not occurs. As the volume fraction of the superconducting region becomes small, the critical transport current density decreases drastically due to a decrease in the percolation probability of the current. To improve the transport property, it is necessary to align the orientation of grains or to introduce isotropic pinning centers, in addition to increasing the superconducting volume fraction.

Flux creep in sintered superconducting Y-Ba-Cu-O

The flux creep rate in sintered superconducting Y-Ba-Cu-O was measured at 77 K and in fields of up to 0.1 T. The pinning potential mainly caused by flux pinning inside grains was 0.25 eV at B=50 mT and decreased monotonically with increasing magnetic field. The obtained results can be explained by the theoretical model with observed intragrain current density. This theoretical model suggests that the flux creep at high fields is notable even if a superconducting wire with the critical current density of 2×1010 A/m2 at B=5 T is realized.

History effect of critical current density and weak links in superconducting BiPbSrCaCuO tape wires

Abstract Critical current density and its history effect in Bi-based high Tc superconducting tapes were measured using an a.c. inductive method with various magnetic fields and temperatures. A rapid decrease in the critical current density with increasing temperature and the history effect were observed in the same low temperature region between 4.2 and 30 K. This suggests that the two phenomena are caused by low temperature weak links in the specimens. A weak link region is considered to lose superconductivity at temperatures higher than 30 K. Above 30 K, a strongly bonded area retains superconductivity and the Jc changes slowly with increasing temperature, showing no history effect.

Critical current densities in superconducting Y-Ba-Cu-O prepared by the quench-and-melt growth technique

The critical current characteristic of a Y-Ba-Cu-O specimen prepared by the quench-and-melt growth (QMG) technique was investigated by the ac-inductive method. It was found that there were two kinds of currents. The bulk critical current density is 1.0×108 A/m2 at 77 K and 0.12 T. This value is comparable to a previous estimate from dc-magnetization measurements. The local current density amounts to 3.2×1010 A/m2 under the same conditions. Appearance of two kinds of currents seems to be caused by nonsuperconducting second-phase layers formed along the direction of grain growth, which disturb uniform current.

Irreversibility line in superconducting Bi-2223 tape wires with silver sheath

Abstract The irreversibility line was measured for three silver-sheathed Bi-2223 tape wires with different critical current densities. It was found that the irreversibility line was shifted to a higher temperature region according to an increasing critical current density. The temperature dependence of the irreversibility line was divided into two regions. In the lower temperature region the irreversibility line strongly depends on the temperature, while the dependence at higher temperatures is weaker and is similar to that observed for Y-Ba-Cu-O. Such a difference is considered to be due to different pinning mechanisms. Nonsuperconducting particles observed microscopically are expected to be responsible for a pinning interaction, contributing to the enhancement of the irreversibility line at high temperatures. This possibility is clarified by comparison of the observed irreversibility line with theoretical prediction.