NSF Nugget for FY06 Advanced Characterization of MgB 2 by Transmission Electron Microscopy

1. NSF Nugget for FY06Advanced Characterization of MgB2 by Transmission Electron Microscopy Sept. 2006 FRG on Two Gap Superconductivity Wisconsin-Penn State- Arizona State-Puerto Rico-Mayaguez collaboration DMR-0514592 PIs David Larbalestier, Eric Hellstrom and Paul Voyles (UW), Zi-Kui Liu (Penn State), Nate Newman and John Rowell (Arizona State), and Oswald Uwakweh (U. Puerto Rico Mayaguez) Additional collaborations with Xiaoxing Xi (Penn State ) and Akiyoshi Matsumoto (NIMS, Japan)

2. 10 B 8 6 Si B Normalized Counts (a.u.) 4 2 0 0.8 C 0.6 0.4 C Normalized Counts (a.u.) 0.2 0.0 O 0.4 Mg O 0.3 0.2 Normalized Counts (a.u.) 0.1 0.0 0 20 40 60 Probe Position (nm) c MgB MgB 2 2 SiC SiC Dark-field TEM (left) and HRTEM (right) images of the C-doped MgB2 thin films, showing increasing disorder away from the substrate and nanodomains with disorder in the a-b planes near the film surface. Advanced Characterization of MgB2 by Transmission Electron MicroscopyWork in group of P. M. Voyles, University of Wisconsin DMR-0514592 Recent work at UW (Visiting NIMS young sabbatical scientist with Larbalestier) has reported record high upper critical field (Hc2) of ~40T in MgB2 tapes doped with SiC and thin films doped with C, a value about 20% higher than the prior record from just C doping in the bulk. In order to elucidate the materials science of this advance, we used advanced TEM and STEM to characterize the microstructure and microchemistry of these record samples(Matsumoto et al. to appear APL 2006). We found that Hc2 is enhanced by lattice distortion when MgB2 is formed at low temperatures, and by the presence of second phases involving Si, C, and O introduced with the SiC powder. High-resolution microscopy of even higher Hc2 (>60T) Penn State MgB2 thin films doped with C of the type recently demonstrated by former Larbalestier PhD student Valeria Braccini shows much nanoscale structural inhomogeneity too. Near the substrate, the film is epitaxial and well-ordered, but further away the order decays into very fine nanodomains with disordered a-b planes. It appears that these structural distortions enhance scattering in the p-band loacted on the Mg planes, consistent with Gurevich’s interband-mixing theory of high Hc2 in MgB2. MgO phase in a SiC-doped MgB2 tape Z-contrast STEM image of a foreign particle in the MgB2 tape. STEM EELS profiles across the same particle showing the B, C, and O distributions STEM EDS maps showing Si, Mg, and O distributions

3. Advanced Characterization of MgB2 by Transmission Electron MicroscopyP. M. Voyles, University of Wisconsin - Madison,DMR-0514592 Education: One graduate research assistant (Ye Zhu) was supported by this NSF award. Ye collaborated with Akiyoshi Matsumoto of NIMS (Visiting Scientist on sabbatical from NIMS, Tsukuba Japan with David Larbalestier) and Xiaoxing Xi of PSU on TEM characterization. These results are featured on the web site tem.msae.wisc.edu. Data collected in this research will be added to the Electron Microscopy Database (EMdb) at tem.msae.wisc.edu/emdb/. The EMdb distributes full bit-depth, full resolution TEM data to encourage the teaching of quantitative techniques in electron microscopy. MgB2 will provide examples of a hexagonal crystal system and of epitaxial thin films. Societal Impact: The ability to transport electric current without resistance and therefore without dissipating electricity as heat is of paramount importance for electrical transport and electro-magnet technologies. This work could enable a new generation of magnets for MRI, fusion energy, and particle accelerators. Ye Zhu, graduate student, at the TEM