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

Abstract

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 ( ). 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 . However, in tests of 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, increasedby a factor exceeding 2. The deposits compared had the same size and number density. Neither deposits significantly affected critical temperature ( ). 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.Index Terms-Ineffective pinning centers, second phase deposits, single perovskite, sub-micron sized precipitates.

The Puzzle of Two Different Sub-Micrometer Tungsten-Rich Deposits in Bulk Ybco: One Acts as Pinning Centers and the Other Does Not

“…In an earlier study of uranium-doped, Pt-free YBCO, similar surprising results of B TRAP were found [8]. U-doped, Pt-free melt-textured YBCO samples contained profuse sub-micrometer sized deposits of a U-Y-Ba-O compound.…”

“…T C is defined as the midpoint of the temperature transition. In all samples measured, the T C is determined to be between 90 K and 91 K. The estimated error in our T C measurement is ± 0.5 K. With the samples being large, the value obtained for T C is rather typical for bulk YBCO superconductors [8][9]. The T C transition of all samples appears to be sharp.…”

“…This is easily seen in FIGURE 3 where all the elements increase in atomic % with increasing size, or increasing W atomic % except Cu, which instead decreases. It is widely observed in melt-textured YBCO superconductor that large deposits (up to several hundred micrometers in size) of a compound rich in Ba and Cu is a byproduct of the texturing process, e.g., [1][2][3][4][5][6][8][9]. A similar analysis (not shown) of the (W +Pt)-doped samples also concluded that Cu is not a component of the W-Pt-Y-Ba-O particles.…”

A pinning puzzle: two similar, non-superconducting chemical deposits in YBCO—one pins, the other does not

“…These are Pt-free deposits, and are the main subject of this report. As described in detail later in this report, the Ptfree deposits were studied and found to be of composition [12]:…”