Control of Nanophase Defects during Ceramic Melt Processing of Bi2Sr2CaCu2O8+x High Tc Superconductors
T. Haugan a, W. Wong-Ng a, L. P. Cook a, H. J. Brown b, L. Swartzendruber b
aMaterials 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.
bMaterials 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
To improve the critical current density of Bi2Sr2CaCu2O8+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 Bi2Sr2CaCu2O8+x thick 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)14Cu24O41 phase defects did not significantly increase Jc, 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 Jc(0T, 30K).