Sub-Micron Non-Superconducting Deposits That Fail to Act as Pinning Centers in Textured YBCO Superconductor
A conundrum is observed where one type of small, non-superconducting particles fails to provide pinning, in textured YBCO superconductor, while other non-superconducting particles of comparable size and number density, pin very well. Pinning centers in high temperature superconductors (HTS) have been shown to greatly enhance critical current density (JC)- An experiment was performed in which large grain YBCO doped with uranium (U), but no platinum (Pt), was produced. Profuse deposits of a U-Y-Ba-O compound were formed during texturing. These U-rich deposits have an almost spherical morphology with an average diameter of ~300 nm and are uniformly distributed inside the YBCO. Conventional wisdom dictates that the U-Y-Ba-O sub-micron deposits should act as pinning centers and increase JC .However, in tests of JC via trapped magnetic flux density at 77 K, no increase was measured. This result is especially surprising because chemically similar and morphologically indistinguishable deposits of U-Pt-Y-Ba-O, found in (U + Pt)-doped YBCO, increased JC by a factor exceeding 2. The deposits compared had the same size and number density. Neither deposits significantly affected critical temperature (TC). One difference between these two compounds is that the non-pinning U-Y-Ba-O deposits have a single perovskite structure, while the U-Pt-Y-Ba-O pinning centers are a double perovskite. Data are presented on the remarkably similar morphology and strikingly different ability to trap magnetic flux density of the U-Y-Ba-O and U-Pt-Y-Ba-O deposits.