Critical processing parameters for bulk and wire high temperature superconductors for applications (invited) (abstract)

Abstract

To make true large applications of high temperature superconductors (HTS’s), one requires not just a large critical current density (Jc) and high transition temperature (Tc), but also large total current (Ic is the JcA, where A is the cross sectional area) and large total current length (Icl where l is the length) of an HTS which can be further shaped into a usable form, e.g., disk, bar, cylinder, or wire. The large anisotropy, short coherence length, and large penetration depth of HTS’s pose serious challenges to efforts to enhance the Jc, Ic, or Icl of HTS’s. Melt-texturing technique by slow cooling through the peritectic temperature has proven to be effective in removing large-angle grain boundaries detrimental to Jc in HTS samples of small size; but many problems remain. The short coherence length reduces the pinning potential for magnetic fluxoids and also renders ineffective the nonsuperconducting dispersion in the HTS matrix to enhance pinning through the conventional metallurgical method. Defects of atomic scale acting as pinning centers to raise Jc have been obtained through irradiation by high energy particles and in situ chemical decomposition. However, in most of these studies Jc was determined only by magnetic measurements, not by transport techniques. We have therefore carried out a systematic study on the melt-texturing of HTS’s in both bulk and wire forms. We have identified major processing parameters in reducing the microcracks, increasing the dimensions of the bulk HTS, controlling the grain alignment, improving the chemical homogeneity, enhancing the processing rate, and controlling the epitaxial growth of HTS’s in an Ag sheath. Effects of irradiation by various types of particles of different energies on Jc determined both by magnetic and transport techniques have also been examined. These results will be presented and discussed, following a brief review on the current status of bulk and wire processing of HTS’s.