Control of Nanophase Defects During Ceramic Melt Processing of Bi2Sr2CaCu2O8+x High Tc Superconductors
T J. Haugan, Winnie Wong-Ng, Lawrence P. Cook, H J. Brown, L Swartzendruber
To improve the critical current density of Bi2Sr2CaCu2O8+x high temperature superconductors (HTS) in high magnetic field applications (> 1 T from 20 K to 50 K), it is necessary to introduce a high density (approximately equal to} 5 per 100 nm in a linear direction) of nanosize (approximately equal to} 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 Φ 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 K to 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 mm to 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 mu)m to 50 υm 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).
, Wong-Ng, W.
, Cook, L.
, Brown, H.
and Swartzendruber, L.
Control of Nanophase Defects During Ceramic Melt Processing of Bi2Sr2CaCu2O8+x High Tc Superconductors, Electronic Publication, [online], None
(Accessed December 8, 2023)