For 10-15 years, the Powder-In-Tube (PIT) process has been one of the leading manufacturing methods for producing the highest critical current density (Jc) Nb₃Sn wires for at small effective filament diameter (deff), both required for future applications in high energy physics. Since Nb₃Sn first became commercially available in the 1960's, non-Cu Jc values have steadily climbed until a plateau was reached at about 3,000~A/mm (12~T, 4.2~K) in the year 2000. Comprehensive analysis of recent wires suggests that both PIT and the other high Jc wire design, Rod Restack Process (RRP), have yet to achieve their maximum potential as high Jc conductors. Currently, PIT wires obtain a maximum Jc(12~T, 4.2~K) of about 2700~A/mm2 and do so by converting up to 60% of the non-Cu cross section into superconducting Nb₃Sn. However, about a quarter of this volume fraction is made of large grains of Nb₃Sn which are too large or otherwise disconnected to carry current in transport, wasting both real estate, as well as Sn and Nb that would be better used to make the desired small grain A15. The most recent RRP wires typically achieve Jc values of around 3000~A/mm2 by also converting about 60% of the non-Cu cross section into A15, however nearly all of that has the desired small grain morphology with high vortex pinning, ideal for current transport. Studies at the Applied Superconductivity Center have shown that one route to improvement for both wires may be in controlling the formation of intermediate phases which form before Nb₃Sn . An intermetallic Nb-Cu-Sn, commonly referred to as Nausite, is considered responsible for the formation of the undesirable large grains. We studied the phase evolution in PIT Nb₃Sn from the starting powder mixture at room temperature up to 690°C to better understand what role Nausite ((Nb0.75Cu0.25)Sn₂) actually plays in forming large grain A15 with the goal of preventing its formation and making better use of the Sn available to form the desired small grain A15 morphology. After heat treatment, all wires were imaged in an SEM and then processed through digital image analysis software, extracting area fractions of each phase and their morphology. For heat treatments which showed interesting metallographic results, additional measurements were made by transport, resistivity, magnetization, and/or heat capacity to develop a complete picture of how the microstructure affects critical wire properties. Based on these results, novel heat treatments were developed and demonstrated our ability to reduce the undesired large grain A15 while simultaneously producing more current-carrying small grain A15, increasing the ratio of small:large grain A15 from 3.0 to 3.8. Another possible path to improvement is to reduce the non-uniform deformation incurred during wire fabrication. A PIT Nb₃Sn wire begins as a mono-filament consisting of a thick Nb7.5wt%Ta tube clad in high-purity Cu, inscribed with a Cu sleeve, and filled with a Sn-rich NbSn₂ powder. The external Cu cladding will later provide a low resistance normal conducting path around superconducting filaments, a necessity for magnet stability. The final wire diameter is between 0.7-1.25~mm with 156 or 192 filaments, whose diameters are 33-50~μm, organized into 6-7 concentric rings. Through advanced digital image analysis software, we can extract geometric information which describes how the wire and filaments deform from their nominally circular shape, becoming elliptical or otherwise having non-uniform deformation which can be detrimental to wire properties. We found that the non-uniform deformation incurred during wire fabrication can degrade the wire performance. The most severe effect is caused by the different deformation rates of the Nb-Ta tube compared to its powder core, which leads to the Sn-rich core drifting from the center of the Nb-Ta tube, leading to an uneven A15 reaction front. This is referred to here as 'centroid drift'. In PIT wires, the diffusion barrier must be consumed to form A15 while still leaving a thin, protective annulus behind to protect the Cu. Centroid drift then causes a large inefficiency as it creates a thick and thin side of diffusion barrier, the thin side limiting the reaction if Sn leaks are to be prevented, while the thick side becomes wasted Nb-Ta. Up to 30% of the final non-Cu cross-section remains as unused diffusion barrier. When Sn leaks out of the filaments it increases the resistivity of the Cu stabilizer, lowering the Residual Resistance Ratio (RRR). A high RRR is required for magnet stability, and such Sn leaks can be detrimental to magnet performance. In addition, we found that filaments farther from the center of the wire tend to be those with highest centroid drift, and they are also the most susceptible to leaking Sn. Moreover, we observed that in leaks severe enough to produce a Kirkendall void, the A15 volume is also reduced. By comparison, RRP type wires manage a similar reaction with less than 10% residual barrier in the non-Cu cross section. Recently, Bruker EAS, the manufacturer of PIT Nb₃Sn wires, developed a new wire design which added a bundle barrier around the filament pack to contain Sn leaks and maintain a high RRR, as well as increasing the Sn content in the powder core to produce more A15. We believe that by improving deformation properties and optimizing new heat treatments to account for the higher Sn content, Jc can be substantially enhanced while maintaining RRR at small filament diameters. / A Dissertation submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Spring Semester 2018. / April 16, 2018. / CERN, magnets, nb3sn, powder in tube, superconductivity, wires / Includes bibliographical references. / David C. Larbalestier, Professor Directing Thesis; Jennifer Proffitt, University Representative; Fumitake Kametani, Committee Member; Emmanuel Collins, Committee Member; Chiara Tarantini, Committee Member; Peter J. Lee, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_653504 |
| Contributors | Segal, Christopher B. (author), Larbalestier, D. (professor directing thesis), Kametani, Fumitake (committee member), Collins, E. (committee member), Tarantini, C. (committee member), Lee, Peter J. (committee member), Florida State University (degree granting institution), College of Engineering (degree granting college), Department of Mechanical Engineering (degree granting departmentdgg) |
| Publisher | Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text, doctoral thesis |
| Format | 1 online resource (153 pages), computer, application/pdf |
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