Mg(B,O)2 precipitation in MgB2

TL;DR

This study investigates Mg(B,O)2 precipitates in MgB2, revealing their size, shape, composition, and effects on flux pinning, with implications for superconductor performance.

Contribution

It provides detailed characterization of Mg(B,O)2 precipitates in MgB2, including their structure, composition, and transformation, advancing understanding of flux pinning mechanisms.

Findings

Precipitates range from 5 nm to 100 nm in size.

Smaller precipitates contain Mg, B, and O; larger ones mainly Mg and O.

Precipitates transform into MgO after high-temperature exposure.

Abstract

MgB2 samples prepared by solid-state reaction were investigated using high-resolution transmission electron microscopy (HREM), X-ray energy-dispersive spectroscopy (EDX), electron energy-loss spectroscopy (EELS), and energy-filtered imaging. Large amounts of coherent precipitates with a size range from about 5 nm up to about 100 nm were found in the MgB2 crystallite matrices. The precipitates are of different shapes including sphere, ellipsoid, and faceted polyhedron depending on the size of the precipitates. EDX and EELS analyses confirm that smaller precipitates contain magnesium, boron and oxygen while larger faceted precipitates contain mainly magnesium and oxygen, implying that the oxygen content increases with precipitate size. HREM and electron diffraction investigations found that the precipitates have the same crystal lattice structure as that of MgB2 but with various composition modulations depending on the composition of the precipitates. The precipitates transform to the MgO phase after long exposure to residual oxygen in flowing Ar gas at high temperatures. The effect of the precipitates in different size ranges on flux pinning is discussed.