# Presentation: 323101

## NSF Nugget for FY06 Advanced 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)

## Advanced Characterization of MgB2 by Transmission Electron MicroscopyWork 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 -band loacted on the Mg planes,  consistent with Gurevich’s interband-mixing theory of high Hc2 in MgB2.

**MgO phase in a SiC-doped MgB****2**** tape **

- STEM EDS maps showing Si, Mg, and O distributions

- STEM EELS profiles across the same particle showing the B, C, and O distributions

- Z-contrast STEM image of a foreign particle in the MgB2 tape.

**Notes:**
The new superconductor Magnesium Diboride (MgB2) promises advances in magnet technology for MRI, fusion energy, and particle accelerators, as well as aerospace applications.  The performance envelope of MgB2 is limited by the critical temperature (Tc) and the upper critical magnetic field (Hc2).  Work by our FRG, in collaboration with XX Xi at Penn State and Chang-Beom Eom at Wisconsin, has shown that dramatically increased Hc2 is possible by correct C doping, potentially opening up new realms of applications beyond those possible with any Nb-base superconductor.  However, the nanoscale mechanism by which properties are enhanced is not at all clear.  We believe interstitial doping with carbon, rather than purely substitution of B by C is the cause.  We are performing detailed microstructural analysis to get to the bottom of the problem

We have used advanced transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and various electron spectroscopies to characterize the nanoscale atomic structure and composition of our samples. These extremely powerful techniques can in some cases allows us to “see” individual atoms.

Our work revealed beneficial distortions of the usually regular atomic lattice.  According to interband-mixing theory of Hc2 developed by another member of the FRG (Gurevich), that distortion and other features we observed could be the origin of the exceptional properties of these samples.

## Advanced Characterization of MgB2 by Transmission Electron MicroscopyP. M. Voyles, University of Wisconsin - Madison, DMR-0514592

- 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.

**Advanced Characterization of MgB****2**** by Transmission Electron Microscopy**P. M. Voyles, University of Wisconsin - Madison, DMR-0514592

- Ye Zhu, graduate student, at the TEM

**Notes:**
Paul Voyles and his student Ye Zhu provide detailed structural and analytical information on samples made by graduate student Ben Senkowicz