Eric E. Hellstrom - US grants

This research focuses on one of the most critical and difficult issues in the field of high temperature superconductors; the role of grain boundaries as current limiting structures in bulk and thin film materials. This research will provide a detailed study of weak links, grain boundary structure and composition as well as surface properties of high temperature superconductors. The weak link investigations focus on single grain boundaries in polycrystalline bulk materials, and will employ thinned samples for which the critical current can be probed locally and the grain boundaries to their structure and composition. Complementary surface characterization work using synchrotron radiation techniques will also be carried out. This research is potentially of great strategic importance and could provide enabling technology for commercial applications of bulk high temperature superconducting materials.

This award supports Professor Eric Hellstrom and a junior colleague and graduate student of the University of Wisconsin to collaborate in materials research with Dr. P Majewski and his group at the Institute for Non-Metallic Inorganic Materials of the University of Stuttgart. The objective of their research is to increase the flux pinning in bismuth-based high temperature superconductors. Their approach will be to create flux pinning centers in silver sheathed bismuth-based tape by manipulating the chemistry of the bismuth-based superconductor and varying the thermal processing. The collaboration brings together the expertise of the U.S. group in the scientific issues involved in fabricating, processing and characterizing long lengths of silver-sheathed high temperature superconductors, with the detailed knowledge of phase relations in the bismuth-based system generated at the University of Stuttgart. Increased flux pinning will raise the current density that these superconductors can carry at thirty degrees Kelvin to a level that will greatly expand the applications in which they can be used. The complementary expertise of the U.S. and German groups creates a uniquely qualified collaboration to pursue this valuable objective.

***NON-TECHNICAL ABSTRACT***
Superconductors are essentially frictionless conductors of electricity. Thus, they avoid the heating effects that occur even in very high-conductivity copper wires. High magnetic field devices, impossible to make with copper, therefore become a possibility. Unfortunately superconductivity occurs only at rather low temperatures. There was great excitement when MgB2, an apparently simple superconductor made from inexpensive raw materials, was recently discovered to be superconducting at twice the temperature of any other simple superconductor. The electronic characteristics of MgB2 provide unique opportunities to explore the possibility of improving the material's properties to make them superior to those of any present superconductor. This Focused Research Group (FRG) award provides support for an inter-institutional collaboration between groups at the U. of Wisconsin-Madison (UW), Arizona State U. (ASU), Pennsylvania State U. (PSU), and the U. of Puerto Rico-Mayaguez (UPRM). The project seeks to understand how to make MgB2 attractive for applications. The team has already demonstrated that MgB2 remains superconducting in higher magnetic fields than materials based on Nb. More than 99% of all current superconducting magnets are made from Nb based materials. Superconductivity is vital to many aspects of technology, especially to Magnetic Resonance Imaging (MRI). General Electric, one of the world's largest manufacturers of MRI machines will collaborate on this study. This effort will be implemented by research carried out by graduate research students and amplified by collaboration with the developing materials science program at UPRM and by outreach at the K-12 level to Native American and Hispanic communities in Arizona. The project is supported by the Condensed Matter Physics, Ceramics, and MRSEC programs in the Division of Materials Research, as well as by the Office of Multidisciplinary Activities.


***TECHNICAL ABSTRACT***
Magnesium diboride is a hexagonal layered compound recently found to have a 40K superconducting transition temperature, almost twice as high as any other electron-phonon superconductor. Even more interesting is that MgB2 contains two distinct superconducting gaps that are only weakly coupled to each other. The larger sigma gap is formed by in-plane sigma boron bonds, whereas the smaller pi gap results from pi boron bonds between the Mg and B planes. This inter-institutional Focused Research Group (FRG), consisting of groups at the U. of Wisconsin-Madison (UW), Arizona State U. (ASU), Pennsylvania State U. (PSU), and the U. of Puerto Rico-Mayaguez (UPRM), will address fundamental physics and materials science issues of MgB2 alloys, concentrating on bulk-form samples and damage studies that have great potential for MgB2 technology. The project seeks to understand how the upper critical field is affected by scattering in and perhaps between the sigma and pi bands of MgB2 and how the scattering changes as MgB2 is alloyed or ion irradiated. Bulk form samples will be the primary thrust of the studies at UW and UPRM; transmission electron microscopy will be performed at UW. Researchers at PSU will concentrate on modeling of the alloying process. Ion irradiation and connectivity effects will be the focus of research at ASU. The broader impacts are both technological and educational. The superconducting magnet user community is excited by recent demonstrations that Hc2 of alloyed MgB2 can exceed Hc2 of the Nb-based superconductors, from which virtually all superconducting magnets are presently made. US industry and national laboratories, as well as international academic collaborators, will work with the FRG to explore the full potential of MgB2 for cryocooled magnets in the 10-30K range, as well as ultra high-field magnets beyond the reach of any Nb-based material. Outreach collaborations in research through a recently started UW-UPRM NSF-PREM at UPRM will be further developed and K-12 outreach will start at ASU to Hispanic and Native American communities in Arizona. The project is supported by several programs in the Division of Materials Research, as well as by the Office of Multidisciplinary Activities.