Limitations for the trapped field in large grain YBCO superconductors

Abstract

The actual limitations for the trapped field in YBa2Cu3O7−δ (YBCO) monoliths are discussed. The influence of the sample geometry and of the critical current density on the trapped field is investigated by numerical calculations. The field dependence of the critical current density strongly influences the trapped field. A nonlinear relationship between the sample size, the critical current density and the resulting trapped field is derived. The maximum achievable trapped field in YBCO at 77 K is found to be around 2.5 T. This limit is obtained for reasonable geometries and high but realistic critical current densities. Such high fields have not been reached experimentally so far, due to non-optimized flux pinning and material inhomogeneities. These inhomogeneities can be directly assessed by the magnetoscan technique, and their influence is discussed. Significant differences between the a- and the c-growth sectors were found. Limitations due to cracks and non-superconducting inclusions (e.g. 211 particles) are estimated and found to be candidates for variations of Jc on a millimetre length scale, as observed in experiments.

Flux pinning in Al doped TSMG YBCO bulk superconductors

“…a/coxygenation macrocracks are observed (figures 6(a)) after SO, but not after HPO ( figure 6(b)). Due to the higher effective cross section, the overall or effective J c is significantly increased [28]. However, these cracks do not influence the intrinsic J c , which is influenced by artificial pinning centres or oxygen vacancies.…”

“…The a/c-oxygenation macrocracks are formed during oxygenation by the tensile stresses in the oxygenated layer [27] and are perpendicular to the direction of the current flow. They directly reduce the effective cross section and consequently the measured critical current density [28].…”

The influence of post-growth thermal treatments on the critical current density of TSMG YBCO bulk superconductors

“…The oxygenation cracks parallel to the a/c-plane have a more serious influence on the superconducting properties as they reduce the effective cross-section of the sample. Simple analysis done by Eisterer [3] showed that the reduction of intrinsic critical current density, J c0 , for supercurrent flowing along the {001}plane is proportional to the l/λ parameter…”

Time Dependent Changes in Ag Doped YBCO Superconductors

“…Significant improvement of pinning properties of Ag-doped YBCO bulk superconductors can be reached by HPO [8] (Fig. 2b) which eliminates the formation of oxygenation cracks and increases the effective cross-section of the samples [9].…”