Utilization of Second Phase Defects to Improve the Flux-Pinning of Bi-2212/Ag Tapes

Control of Nanophase Defects during Ceramic Melt Processing of Bi₂Sr₂CaCu₂O_8+x High T_c Superconductors

T. Haugan ^a, W. Wong-Ng ^a, L. P. Cook ^a, H. J. Brown ^b, L. Swartzendruber ^b

^{a Materials Science and Engineering Laboratory, Division of Ceramics, National Institute of Standards and Technology, 100 Bureau Dr. Stop 8520, Gaithersburg, MD 20899-8520 U.S.A.

^{b Materials Science and Engineering Laboratory, Division of Metallurgy, National Institute of Standards and Technology, 100 Bureau Dr. Stop 8550, Gaithersburg, MD 20899-8550 U.S.A}}

Abstract

To improve the critical current density of Bi₂Sr₂CaCu₂O_8+x high temperature superconductors (HTS) in high magnetic field applications (> 1 T from 20-50 K), it is necessary to introduce a high density (» 5 per 100 nm in a linear direction) of nanosize (» 3-20 nm) defects directly into the superconductor. The addition of non-superconducting or 'flux pinning' defects pins the magnetic field that enters the type II superconductor in interacting vortex structures (individual F _o = 2.07 x 10^-11 T•cm²). Our approach to this problem is to introduce nanophase defects which have minimal chemical interaction with the superconductor phase. A complication of using this method, however, is that nanosize defects can quickly coarsen during the melt-growth methods typically used for processing HTS materials. Therefore a challenge is to add nanophase defects that do not coarsen or otherwise affect the processing of the superconductor. This paper shows initial results obtained with addition of 5% to 25% volume fraction defects (nanosize Al₂O₃, nanosize Au, submicrometer (Sr,Ca)₁₄Cu₂₄O₄₁, and carbon nanotubes) to Bi₂Sr₂CaCu₂O_8+xthick films processed on Ag foils. Addition of nanosize defect particles in general improved flux-pinning properties from 20-50 K, and greatly inhibited secondary Sr-Ca-Cu-O defect formations. Nanosize Al₂O₃ was observed to react with the Bi:Sr:Ca:Cu:O matrix after melting was initiated (> 870 ° C), to form solid-solution (Sr,Ca)₃Al₂O₆ phase with Sr:Ca ratio varying less than 10% of the Sr:Ca ratio of the Bi:Sr:Ca:Cu:O precursor matrix. The (Sr,Ca)₃Al₂O₆ phase formed as submicrometer particles, however some coarsening to 1-5 mm size was noted for longer melt processing times (or higher temperatures). Addition of (Sr,Ca)₁₄Cu₂₄O₄₁phase defects did not significantly increase J_c, and particles were observed to coarsen quickly to 2-50 mm size. Addition of nanophase Au particles caused a significant problem for processing by completely suppressing 2212 c-axis oriented textured growth, however Au particles were not observed to coarsen. Addition of carbon nanotube defects produced submicrometer defects, however reduced c-axis oriented texture growth, altered phase assemblages, and reduced transport J_c(0T, 30K).