Investigation of Ag-Sheathed Multi-Filamentary Bi₂Sr₂Cacu₂O[8-X] Superconducting Round Wires Processed with Overpressure, for High Field Magnets

• Other Authors: Matras, Maxime (authoraut)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Materials science; Engineering
• Online Access: http://purl.flvc.org/fsu/fd/FSU_FA2016_Matras_fsu_0071E_13550

The high temperature Ag-Mg-sheathed Bi₂Sr₂Cacu₂O[subscript x] (2212) multi-filamentary superconducting round wire shows very promising electromagnetic properties with an isotropic behavior in field, a very high irreversibility field at 4.2 K, and progressive decrease in critical current density (J[subscript c]) with increasing magnetic field, which make 2212 one of the three main candidates with ReBCO and 2223 tapes, for high field magnets. In addition 2212 wires have the advantage of being made into round conductors with an infinite variety of cross-sections, which is ideal for winding and cabling technology for any type of applications. The strong limitation of 2212 round wire comes from its low J[subscript c] compared to the other HTS conductors. This limitation is the consequence of the wire manufacture process that traps 30 vol% of gas-filled porosity in the filaments of 2212 powder, which agglomerate during the thermal processing of the wire into filament-size bubbles. The bubbles severely limit the flow of supercurrent along the filaments and generate 2212 leakage. Those issues are drastically stopping the progression of 2212 round wires toward high field magnet applications. We added the overpressure (OP) process during the heat treatment on 2212 round wires to compress the wire at high temperature with up to 100 atm, and reduce the volume of the filaments porosity. This was a major breakthrough as we increased J[subscript c] from ~500 A/mm2 to more than 4000 A/mm2 at 4.2 K, 5 T, which made 2212 round wire a very competitive candidate for high field magnets. The OP reduced the porosity volume near zero and significantly increased 2212 connectivity. The densification of 2212 round wires is the key to reach high J[subscript c] and was studied as a function of time and temperature to optimize the densification process. We found that the wire diameter decreases by 3.8 ± 0.3% after full heat treatment at 50 atm OP and by 3.30 ± 0.07% at 820 °C, below the melting point of 2212 powder. The densification is homogeneous and does not change the filament shape before melting. We also compared the microstructure and J[subscript c] of 2212 round wires densified before and densified after melting the 2212 precursor powder. We found that densifying the wire after melting decreases IC by 77 % compared to the usual densification before melting because of filament deformation and filament-size non-superconducting secondary phases w showing that densification before melting is required to densify the filaments homogeneously giving wires with high phase uniformity and high J[subscript c]. The standard heat treatment of 2212 round wire was designed and optimized at 1 atm pressure before the filament-size bubbles that limit IC were significantly reduced in size using the OP processing. We studied the OP HT of 2212 round wires with a statistical design of experiment (DOE) with the goal of increasing IC, reducing processing time, and understanding what are the critical parameters in OP processing 2212 round wires. We studied the effect of the maximum temperature (Tmax), the time 2212 is in the melt state (tmelt), the cooling rate to grow 2212 grains (RF), and the post annealing time (tA) cooling from 840 °C to 830 °C on the microstructure and electromagnetic properties of 2212 round wires. The DOE analysis gives strongly predictive equations and shows that RF and tmelt statistically affect IC significantly and must be minimized to increase IC, and that Tmax has a small to no statistical effect on IC for cooling rates RF ≤ 3 ˚C/h. Using the equations we shortened the OP HT time by more than a factor of 2, from 88 h to 41 h for similar IC as the STD OP HT. The decrease in diameter during the OP HT is an issue for solenoid 2212 magnets. The diameter shrinkage of conductor occurs at high temperature and the winding pack becomes loose and may deform under its own weight, which reduces the predictability of the properties and field homogeneity of the magnet. We introduced a multi-step process to predensify 2212 wires with OP below the melting point of the 2212 precursor powder, before winding the coil, to minimize the wire diameter shrinkage during the OP processing of the solenoid magnet. We report that 2212 wire diameter was decreased by 3.0 % before melting the 2212 powder and the dense wire was wound into coils as small as 10 mm diameter. After OP heat treatment of the coils, no 2212 leakage was observed for coil diameter as small as 20 mm as well as no IC degradation occurred because of wire predensification. === A Dissertation submitted to the Department of Materials Science and Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Fall Semester 2016. === November 17, 2016. === Bi₂Sr₂Cacu₂Ox (2212) round wires, heat treatment optimization, high temperature superconductor, Overpressure processing === Includes bibliographical references. === Vincent Salters, University Representative; Geoffrey Strouse, Committee Member; Kenneth Hanson, Committee Member.