1 research outputs found
Microstructure and Structural Defects in MgB2 Superconductor
We report a detailed study of the microstructure and defects in sintered
polycrystalline MgB2. Both TEM and x-ray data reveal that MgO is the major
second-phase in our bulk samples. Although MgB2 and MgO have different crystal
symmetries, being P6/mmm and Fm-3m, respectively, their stacking sequence of Mg
and B (or O) and lattice spacings in certain crystallographic orientations are
very similar. The size of MgO varies from 10~500nm, and its mismatch with the
MgB2 matrix can be a source for dislocations. Dislocations in MgB2 often have a
Burgers vector of . 1/3 and 1/3 partial dislocations and their
associated stacking faults were also observed. Since both dislocations and
stacking faults are located in the (001) basal plane, flux pinning anisotropy
is expected. Diffuse scattering analysis suggests that the correlation length
along the c-axis for defect-free basal planes is about 50nm. (001) twist
grain-boundaries, formed by rotations along the c-axis, are major grain
boundaries in MgB2 as a result of the out-of-plane weak bonding between Mg and
B atoms. An excess of Mg was observed in some grain boundaries. High-resolution
nano-probe EELS reveals that there is a difference in near edge structure of
the boron K-edge acquired from grain boundaries and grain interiors. The change
at the edge threshold may be suggestive of variation of the hole concentration
that would significantly alter boundary superconductivity.Comment: 20 pages and 12 figure