Current and field distribution in high temperature superconductors

The manufacture of wires from HTS materials containing copper-oxide planes is difficult because their physical and electrical properties are highly anisotropic. The electrical connectivity depends on the nearest-neighbour grain alignment and although a high degree of grain texture is achieved through processing, the tape microstructure is generally far from uniform, with weak links and porosity also complicating the picture. In order to optimise the processing, the microstructural features common to good tapes must be identified, requiring knowledge of the local properties. The preferential path taken by transport current is determined by the properties of the local microstructure and as such can be used to measure the variation in quality across the tape cross-section. By measuring the self-field profile generated by a current-carrying tape, it is possible to extract the associated current distribution. I have designed and built a Scanning Hall Probe Microscope to measure the normal field distribution above superconductor tapes carrying DC currents, operating at liquid nitrogen temperature and zero applied magnetic field. It has a spatial resolution of 50*50 μm and a field sensitivity of 5 μT, and can scan over a distance of 6 mm. The current extraction is performed by means of a deconvolution procedure based on Legendre functions. This allows a nondestructive, non-invasive method of evaluating the effects of the processing on the tapes - especially when correlated with transport and magnetisation measurement data. Conductors fabricated from Bi2Sr2Ca2Cu3O10, Bi2Sr2CaCu2O8 and (Tl0.78Bi0.22)(Sr0.8Ba0.2)2Ca2Cu3Ox, have been investigated. I have confirmed the reports that in Bi-2223/Ag mono-core conductors produced by the oxide-powder-in-tube (OPIT) technique, the current flows predominantly at the edges of the tape, where the grains are long and well-aligned. This is in contrast to Bi-2212 ribbons, where the better microstructure formed by the melt-texturing technique gives excellent electrical connectivity and the current distribution follows that predicted for a homogenous Type II superconductor. Tl-1223 OPIT tapes have been found to show granular characteristics, with a uniform current distribution and poor performance in low magnetic fields. Current sharing between filaments in multi-core Bi-2223/Ag OPIT tapes has been investigated. The exact configuration of the filaments has been shown to influence the current flow at low DC currents. This is due to enhanced coupling in one configuration, where the tape acts more like a mono-core than a collection of individual filaments. (author)