New Processing and Characterization Approaches for Achieving Full Performance of High Temperature Superconducting Tapes of (Bi,Pb)2Sr2Ca2Cu3Ox

Hellstrom, E.E. ; Larbalestier, D.C.

University of Wisconsin--Madison

关键词: Partial Pressure;    Superconducting Wires;    Superconductivity;    Capacity;    Oxygen;   

DOI  :  10.2172/877697
RP-ID  :  DOE/GO/11052-F
RP-ID  :  FC36-01GO11052
RP-ID  :  877697

美国|英语

来源: UNT Digital Library

【 摘 要 】

The thrust of this research was to identify and understand current limiting mechanisms (CLMs) that limit the current carrying capacity of (Bi,Pb)2Sr2Ca2Cu3Ox (2223) in Ag-sheathed wire. Our program concentrated on developing new methods to identify CLMs at the micrometer scale and new processing techniques to eliminate CLMs. All of the DOE Superconductivity Partnership Initiative (SPI) programs are using 2223 wire, so increasing the critical current density (Jc) in the wire can improve the technical performance of the demonstration projects, and at the same time it can decrease the cost of the wire. The important cost metric for superconducting wire is $/kAm, so increasing Jc, which is in the denominator, decreases the wire cost. The obvious CLMs were micrometer size obstacles in the 2223 ceramic that block current flow, including: misaligned grains, cracks, pores, and nonsuperconducting phases. Pores and cracks - regions where there is no superconductor or the grains are not physically connected to one another ? cannot carry supercurrent, so they were the first CLMs we tried to eliminate with improved processing. Prior to the contract, we had started investigating overpressure (OP) processing with Williams at ORNL to heal cracks and remove pores. OP processing, which is a variant of hot isostatic pressing (HIP), uses an Ar/O2 gas mixture to apply a high pressure (up to 200 atm) to compress the sample and to set the oxygen partial pressure (pO2) to form 2223. Williams had a static pressure system we used to demonstrate that OP processing healed cracks and densified the wire, but the static system limited the processing parameters we could investigate. We proposed building a new gas-flow OP system to expand the experimental capabilities and to investigate new processing routes using the gas-flow OP system. Using the gas-flow OP system, we established new world records in 2003 for Jc and Ic. These records were finally matched by Sumitomo Electric Company in early 2006. The finest scale at which we could probe the local electromagnetic properties of a sample was about 100 m at the beginning of the contract. This was done by attaching voltage taps (10 m diameter wires) about 100 m apart on the 2223 conductor, and measuring the local I-V characteristics between each set of voltage taps. However, the largest CLMs were 2-3 times smaller than this length scale, and most CLMs were even much smaller. The original proposal was to investigate new methods to identify specific regions in samples that contained CLMs from their electromagnetic response, then to examine these regions of the sample using microstructural techniques to identify the CLM. We extended the use of magneto-optic (MO) imaging and magneto-optic current reconstruction (MOCR) and began developing a low-temperature laser scanning microscope (LTLSM) to show local current flow and local current dissipation, respectively, with a resolution of ~5 m. With MOCR we were able to show that local Jc in small regions of OP processed 2223 wire was as high as 300 kA/cm2 at 77K, which was 5-6 times higher than the average Jc measured across the whole sample.