Fabrication and characterisation of high temperature superconducting bulk YBCO

The processing environment required for the manufacture of large, mono-domain specimens of bulk YBCO via the seeded melt textured growth technique has been developed. As a result of this it has been possible to examine in detail the growth morphology and crystal structure of samples prepared in this way. A novel powder preparation technique involving the use of a high temperature plasma spray was employed to produce homogeneous YBCO powder suitable for melt texturing. It was found that the well mixed, un-reacted starting powder would produce material with a fine distribution of secondary phase inclusions. Melt textured samples were produced from this powder using differing thermal cycles in an attempt to ascertain the optimum conditions for producing the desired microstructure. In an attempt to improve the flux pinning properties of single grain Y123 undoped and samples doped with lithium underwent irradiation with thermal neutrons. Magnetic measurements were then used to illustrate the enhancement due to this process. In order to improve the electrical properties of Sm123 it is desirable to be able to control the amount of Sm/Ba substitution. Already grown samples of Sm123 were processed in a reduced oxygen atmosphere in a manner that was shown to control the degree of Sm/Ba substitution. For many potential application large, shaped samples of YBCO are required. Due to the properties of the material this is very difficult. Using a liquid phase infiltration method single grain samples of YBCO have been re-joined without the introduction of a grain boundary. R(T) and I-V dependencies close to Tc and in zero magnetic field were obtained from single grain samples of YBCO in the nV region. For T < Tc, the lower parts of the resulting curves were interpreted in the frame of the J-M model of current induced unbinding of thermally generated vortex/anti-vortex pairs, allowing us to obtain the temperature dependence of the characteristic current, lcl(T), close to Tc. In addition, the current value for a given temperature above which the J-M model becomes inadequate has been measured and interpreted. Direct transport measurements for melt textured YBCO in an applied magnetic field have been taken. The angular dependence of the superconducting properties have been discussed in the context of sample morphology. (author)