TEM Characterization of Microstructure and Chemistry in Magnesium Diboride Superconductor
MgB2 is a superconductor with promising properties, but to meet application requirements, the properties of MgB2 have to be improved through microstructure modifications: chemical doping, introduction of precipitates, and atomic-scale control of defects. The small length scale of these modifications means that structure and chemistry characterization using the transmission electron microscope (TEM) are required to better understand the correlation between MgB2?s superconducting properties and its microstructure. We have used TEM-based techniques to characterize microstructures of pure MgB2 bulks, SiC-doped MgB2 tapes, pure and C-doped MgB2 thin films prepared by hybrid physical-chemical vapor deposition (HPCVD) method with different carbon precursors, degraded pure HPCVD thin films, and O-doped MgB2 thin films prepared by molecular beam epitaxiy. Characterization results and effects of microstructure on superconducting properties are discussed for each material. We have found that in pure MgB2 the main contaminating current-blocking phases are MgO grains and amorphous MgO + boron + BxOy phases. Both second phase nanosegregations and grain boundaries are effective pinning defects in MgB2, which can enhance Jc(H) at high fields. However, second phases also act as current-blocking phases that reduce the measured Jc, which suggests that MgB2 samples with small grain size and without any boundary second phases are desirable to achieve higher Jc. Extensive studies have been made to understand the Hc2 enhancement mechanisms in MgB2. Both carbon dopant and grain boundaries in MgB2 can cause more electron scattering that increases Hc2. Degradation in MgB2 leads to the formation of the amorphous boundary phase which also enhances Hc2 by introducing electron scattering. The over 60 T Hc2 ||(0 K) obtained in C-doped HPCVD MgB2 films is caused by the twoband nature and strong ?-band scattering of MgB2. We believe that the strain fields generated from c-axis tilt boundaries and coherent MgO nano-platelets are responsible for the anomalous ?-band scattering.