Details

Autor(en) / Beteiligte

• Barua, Shaon

Titel

Critical Current Distributions in Multifilamentary Bi2Sr2CaCu2O8+x Round Wires

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2024

Link zum Volltext

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• Bi2Sr2CaCu2O8+x (Bi – 2212) is a promising candidate for ultra-high field magnets (>25 T) because of its round wire geometry, twisted multifilamentary architecture, and macroscopically isotropic electromagnetic properties. However, in composite superconductors, the local critical current may vary due to variable vortex pinning interactions and variations in filament connectivity arising from filament shape variation and grain-to-grain connectivity variations, particularly in high-temperature superconductors. The critical current density (Jc) in Bi – 2212 round wires is high [Jc(30 T, 4.2 K) = 4600 A/mm2 ], but can fluctuate by 30–40% between wires, due to a complex interplay of powder quality, filament uniformity, and the conditions of overpressure heat treatment (OP-HT), especially the highest temperature of the OPHT. We here report on an extensive study of the critical current distributions in ∼1.5 m long samples of wire manufactured in km lengths by Bruker-Oxford Superconducting Technology (Bruker-OST) using d 2V /dI2 analyses of the V –I curves measured on ITER-like barrels. The transitions are well-fitted by Gaussian distributions characterized by their relative standard deviations (σ/µ).This dissertation details experiments characterizing the influence of OP-HT parameters, filament separation, powder quality, and oxygen doping on the supercurrent path connectivity. We show that the highest Jc values occur for minimum time-in-the-melt (tmelt) and lower maximum temperature (Tmax) during OP-HT and they also correspond to significantly lower σ/µ values. We attribute the degradation of properties that occurs with higher Tmax to decreasing filament connectivity associated with deteriorating texture due to filament merging during the melt step of the OP-HT. Having obtained systematic results on the variation of σ/µ as a function of heat treatment conditions, we then applied this analysis to short sections cut from both ends of test magnets wound with about 200 m of wire, to understand the extent of systematic variations in the 1 km length wires being delivered by Bruker-OST. We found that recent Engi-Mat fine-particle powder wires have significantly higher Jc and significantly lower σ/µ than found in earlier Bruker-OST wires made with milled Nexans granulate powder. We find it interesting that (i) there are clear trends that Jc rises as σ/µ declines and (ii) that the σ/µ values are only about twice as high as fully optimized Nb – 47 wt.% Ti wires, which unlike Bi – 2212 are isotropic, fully connected and with only one single value of irreversibility field (Hirr). Furthermore, our investigation into the long-length uniformity of a 10 kg Bi – 2212 billet reveals that a kilometer-length conductor is macroscopically uniform with a σ/µ ratio almost independent of position along the ∼1.2 km conductor. We attribute this to good fabrication practice by Bruker-OST and by the averaging effects of ∼1000 filaments that render the complex critical current path functionally uniform along the wire length, in spite of many local percolation paths within each filament.Finally, we report on the nature of the current path after de-oxygenation of an initially overdoped wire in its highest Jc state. We find that the current path did not change significantly by deoxygenation, at least at the relatively high and broad range of electric field (E) accessed by d 2V /dI2 . However, gradual de-oxygenation reveals two separate current paths that become evident in transition temperature (Tc) measurements. We attribute these two transitions either to dissipation due to flux flow within the ab-planes and also to flux flow at grain boundaries. Within a model that ascribes high Jc in polycrystalline Bi – 2212 to its strong biaxial texture, we see the current-limiting obstacles as either occurring at grain boundaries obstructing ab-plane supercurrent or to the c-axis current flow required to transfer supercurrent from grain to grain when the ab-plane paths are blocked. The very special microstructure of Bi – 2212 in which long [100]-oriented grains stack with other well-oriented [100] grains confers a large grain boundary (GB) area for c-axis current flow. A plausible interpretation of our de-oxygenation experiments is that this c-axis transport is markedly degraded by underdoping that drives the c-axis from the SNS or even SS’S state when overdoped into a strongly Josephson-coupled SIS state, with degraded c-axis Ic. These experiments show that much can be learned about current flow in complex Bi – 2212 wires even though we believe that many independent characteristics control the current flow. We are hopeful that having such analytical tools for identifying the distribution of critical currents and the way that the dominant dissipation mechanisms change with doping will enable further Jc enhancement in what are already very high Jc conductors.