Intergranular Critical Currents Research Articles

Processing and grain boundary effects in Nd/sub 1-x/Ba/sub 2-x/Cu/sub 3/O/sub 7+d/ superconductors

Nd/sub 1-x/Ba/sub 2-x/Cu/sub 3/O/sub 7+d/ samples have been prepared with sharp superconducting transitions, with careful attention given to the details of sample preparation. Two causes for broad samples have been observed. The first is sample inhomogeneity, which is more difficult to eliminate than in the YBa/sub 2/Cu/sub 3/O/sub 7/ case, and the second appears to be associated with site disorder within a homogeneous sample. Very reasonable intragranular critical currents have been obtained in a sample with a small amount of Nd substituted on the Ba sites, but it is not clear if the Nd plays a direct role in flux pinning. Intergranular critical currents are still quite low due to the large grain size in the high-temperature-processed material and the Nd-deficient layer on the grain boundaries.

Novel magnetometer and the interpretation of low field magnetic hysteresis of high Tc superconductors

A novel electronic integrating magnetometer, based on a computer controlled phase sensitive detector, has been developed to investigate magnetic hysteresis in high temperature superconductors associated with intergranular and intragranular critical currents. The high sensitivity of the magnetometer (≳ 10 −6 electromagnetic units (emu)) enables accurate measurements to be made on mg superconducting samples in low fields of up to 50 mT. A computer fitting procedure based on a modified Bean model with a field dependent critical current density gives values for the magnetic filling factor, the strength of the zero-field critical current density and the scaling field for a given assumed field dependence of critical current. Magnetization measurements on various ceramic YBCO samples, including wires of various diameters, yield values for bulk critical currents in good agreement with transport measurements.

Dissipation in granular high-temperature superconductors

The current-voltage characteristics of bulk-sintered YBa 2Cu 3O 7 have been studied in detail. The onset of dissipation indicates that an intergranular critical current can be defined. The magnitudes of the resistance of a single grain boundary and of its critical current are estimated. These estimates lead to values of the r n i c(0) product for the intergranular weak-links that are of the order of 100 μV, well within the wide range of r n i c(0) product measured on bi-crystal grain boundaries by Dimos et al., but two to three orders of magnitude less than that expected from “classical” models of superconducting weak-links.

Non-contact determination of critical current densities in a superconducting YBa 2Cu 3O y cylinder

The determination of the critical current density ( j c) of a superconducting ceramic is a vital part of the assessment of its suitability for electrical and electronic applications. However, in bulk materials where the current carrying capacity of a test specimen is typically several amps, great care is required in the preparation of low resistance current contacts if heating effects are to be eliminated. Several groups have therefore developed non-contact techniques for the determination of j c which rely on the measurement of the hysteretic magnetisation loop of a material in an external applied magnetic field. However, one difficulty with such measurements is the need to identify separately the contribution to the magnetisation due to intra-granular currents (i.e. circulating currents which do not cross grain boundaries) from that due to inter-granular currents (i.e., bulk currents which flow between grains); the critical current densities in the former being typically several orders of magnitude larger than the latter. For the assessment of the transport properties of bulk materials only the intergranular critical currents are of practical importance. Here we present an extension of the basic technique which reduces the effects of intra-granular magnetisation and eliminates the effects of the dependence of the critical current density on the external magnetic field.

Ultra-low field magnetic studies of ceramic superconductors

Measurements of the magnetisatisation of multi-phase weakly-superconductingYBCO ceramics are presented at the individual flux quantum level. The magnetisation has been measured from 1-40K on a number of bulk and ring-shaped samples. Such measurements provide interesting information about the temperture and field-dependence of the intergranular critical currents and of the dynamics of the flux transitions which ultimately determine the magnetic properties of more strongly superconducting material.