Defect structure of ultrafine MgB2 nanoparticles.

Defect structure of MgB₂ bulk and ultrafine particles, synthesized by solid state reaction route, have been investigated mainly by the aid of X-band electron paramagnetic resonance spectrometer. Two different amorphous Boron (B) precursors were used for the synthesis of MgB₂, namely, boron 95 (purity 95%-97%, <1.5 µm) and nanoboron (purity >98.5%, <250 nm), which revealed bulk and nanosized MgB₂, respectively. Scanning and transmission electron microscopy analysis demonstrate uniform and ultrafine morphology for nanosized MgB₂ in comparison with bulk MgB₂. Powder X-ray diffraction data show that the concentration of the by-product MgO is significantly reduced when nanoboron is employed as precursor. It is observed that a significant average particle size reduction for MgB₂ can be achieved only by using B particles of micron or nano size. The origin and the role of defect centers were also investigated and the results proved that at nanoscale MgB₂ material contains Mg vacancies. Such vacancies influence the connectivity and the conductivity properties which are crucial for the superconductivity applications. [ABSTRACT FROM AUTHOR]