Improvement of Critical Current Density and Flux Trapping in Bulk High-Tc Superconductors

International audience; The present chapter describes an overview of flux trapping with enhancement of the critical current density (Jc) of a melt-growth large domain (RE)Ba2Cu3O7-d, where RE is a light rare earth ions such as Y, Gd or Sm. These high-Tc superconductor bulks have attracted much interest for a variety of magnet applications, since high density and large volume materials potentially provide an intensified magnetic flux trapping, thanks to the optimized distribution of pinning centres. The melt growth process and material processing to introduce well-defined flux pinning properties are overviewed. As a first step, we summarize an effort to achieve a growth of homogeneous large grains with the second phase RE211 in the RE123/Ag matrix. RE-Ba-Cu-O material has a short coherence length and a large anisotropy, and thus any high-angle grain boundary acts as a weak link and seriously reduces the critical current density [1, 2]. In engineering applications, high texture and c-axis-orientated single grains/domains are required. Large-sized, high-performance RE-Ba-Cu-O single grains are now commercially available. The trapped flux density (Btrap) due to flux pinning or associated superconducting currents flowing persistently in a RE-Ba-Cu-O grain is expressed in a simple model, such as:Btrap = Aµ0Jcr,where A is a geometrical constant, µ0 is the permeability of the vacuum and r is the radius of the grain [1]. There are two approaches to enhancing the trapped flux of the grain. One is to enhance the critical current density and the other is to increase the radial dimension of the crystals. Increasing the dimension requires the formation of homogeneous grain growth, and the enhancement of the critical current density is encouraged with the improvement of flux pinning properties.