Sub-Micron Non-Superconducting Deposits That Fail to Act as Pinning Centers in Textured YBCO Superconductor

摘要

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 $(J_{C})$. 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 $sim$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 $J_{C}$. However, in tests of $J_{C}$ 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 $({rm U}+{rm Pt})$-doped YBCO, increased $J_{C}$ by a factor exceeding 2. The deposits compared had the same size and number density. Neither deposits significantly affected critical temperature $(T_{C})$. 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.