(Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223), the leading high temperature superconductor with Tc of 110K, has been demonstrated to be the most technically viable present materials for superconducting application to electric power transmission lines, fault current limiters, transformers, electromagnets and motors. UW-Madison has been studying this key material since it was first discovered in 1988. The University of Wisconsin, the home of ASC before the centerís move to the Magnet Lab and Florida State University in 2006, was a founding member in 1991 of a national collaboration called the Wire Development Group (WDG) consisting of American Superconductor Corporation (AMSC), three national laboratories, Argonne National Laboratory (ANL), Los Alamos National Laboratory (LANL), Oak Ridge National Laboratory (ORNL) and UW-Madison. WDG has conducted closely coordinated, highly integrated, multidisciplinary research aimed at understanding and addressing the critical performance issues in the Bi-2223 conductor for electric power applications.
Bi-2223 wires and tapes are made by the powder-in-tube technique. This process normally consists of two heat treatment steps separated by an intermediate rolling step. Industries are producing kilometer-long Bi-2223 wires. UW-Madison has developed and applied novel measurement and characterization methodologies to elucidate supercurrent transport within the Ag/Bi-2223 composite conductor and to shed new light on pathways for improvement of the superconducting performance of Bi-2223. Our goals are
- to explore novel processing strategies, such as overpressure processing
- to characterize in great detail the progression of chemical, physical and microstructural states that occur through the process
- to establish key microstructure-property-process relationships through which critical performance issues are identified and then addressed. Our current research focuses are overpressure processing and micrometer scale characterization
Overpressure Processing (OP)
The most serious current limiting mechanisms in the highest performance Bi-2223 tape are porosity and cracks. These are an unwanted consequence of the thermo-mechanical processing scheme presently used to make Bi-2223 conductors. We believe the key to eliminating porosity and cracking in Bi-2223 is to apply isostatic pressure during thermal processing. OP processing uses a mixture of inert gas to apply a compressive stress and O2 to control the thermodynamics and it may simplify heat treatment to a single step. Samples OP processed at 150 ATM have higher critical current density and higher core density and fewer microcracks than tape processed at 1 ATM, which suggests better connectivity in OP processed tape than in 1 ATM-processed tapes. Current work is aimed at developing new processing schemes and improving critical current density Jc by optimizing the OP heat treatment to form 2223 and improve the microstructure within the tape.
LEFT: Low temperature scanning laser microscopy (LTSLM) image for the same section of filament. The image was taken at 90K. The bright regions indicate a larger voltage response due to the "hot-spot" effect induced by the laser-beam. Such non-uniform voltage responses, which are caused by defect distribution, provide useful information for identifying and investigating material defect that directly block the transport current.
RIGHT: 3D SEM image for a section of filament extracted from a 19-filament Bi-2223 tape with the Jc (77K, self-field) of 60 kA/cm2.
It is quite clear from our studies that the properties of Bi-2223 conductors are essentially constant at all length scales from about 100 µm to beyond 200 m. This constancy shows that the leading industries have effectively eliminated large-scale manufacturing-induced current-limiting mechanisms. Further improvements will only come by identifying and understanding the sub-100µm-scale current limiting mechanisms. In order to establish how spatially variable the critical current density is in the different sorts of microstructure, and to establish unambiguously how much higher critical current density (Jc) is in the better regions, several techniques are being applied to the micrometer scale characterization:
- Magneto-optical reconstruction of the spatial distribution of the local critical current density.
- Single filament evaluations by coupling laser cutting and focused ion beam cutting with in-field transport measurement and magneto-optical examination.
- Low temperature scanning laser probing of local voltage dissipation.
- Scanning Hall probe microscopy.
- Microstructure and composition analysis by electron microscopy.
Low temperature scanning laser microscopy (LTSLM) has a resolution of about 1 µm, while magneto-optical current reconstruction has a resolution of 5 µm. We are systematically examining the local critical current density in variously processed Bi-2223 tapes so as to better understand the role that poor connectivity and porosity play in limiting the transport current density. We find that local Jc (77K, 0T ) values exceed 200 kA/cm2 in monocore tape with transport Jc (77K, 0T ) of 40 kA/cm2.
Thorough-process study of factors controlling the critical current density of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox tapes
The attainable Jc of Bi-2223 is determined by the electrical connectivity of each filament which is itself an uncertain compromise between minimizing the significant porosity produced by the Bi-2223 formation reaction and the extensive crack network that porosity reduction by intermediate rolling produces. Better control of microstructure through process will allow for substantial performance improvements in Bi-2223 composites.
Overpressure processing of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox tapes
Heat treatments of monocore and multifilament Ag-sheathed 2223 tape were carried out using overpressure (OP) processing in a static and a flow OP system between 125 and 180 bar. Mass density and critical current density were improved in overpressure processed Ag-sheathed Bi-2223 tapes.
Development of 2201 intergrowths during melt processing Bi2212/Ag conductors
This paper shows that the number of 2201 half-cell intergrowths in the 2212 structure is influenced by the cooling rate during melt processing, with faster rates leading to more 2201 intergrowths. The fraction of 2201 intergrowths is determined quantitatively by analyzing the shape of 2212 (00l) x-ray diffraction lines. A model is proposed to account for the growth of these 2201 intergrowths.
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