Mg ( B,O ) 2 precipitation in MgB2

Abstract

MgB 2 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 MgB 2 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 MgB 2 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.

Superconductivity in porous MgB2

“…The existence and precipitation of oxygen in MgB 2 have been observed experimentally [14]. However, many MgB 2 samples containing MgO precipitates, which indicate that an oxygen source is available during growth, did not cause a significant decrease in T c [15].…”

The doping effect of multiwall carbon nanotube on MgB2∕Fe superconductor wire

“…The very fine-grained nanocrystalline microstructure of the superconducting phase seems to be responsible for these excellent J c values. 31 The temperature dependence of H c2 and H irr for short and long CNT doped MgB 2 sintered at 900°C is shown in Fig. 7.…”

Microstructure and high critical current of powder-in-tube MgB2

“…Another source of pinning are intra-grain defects. We have recently shown 15,16 that bulk MgB 2 prepared by solid-state reaction contains a large amount of intra-grain Mg(B,O) 2 precipitates coherent with the MgB 2 matrix, with sizes ranging from 5 nm to 100 nm, which are very well suited to act as pinning centers. A second and equally important requirement for wire and tape optimization is a better understanding of more macroscopic aspects such as the influence of the porosity, the formation of MgO phase, the presence of microcracks, and the weak link behavior after deformation.…”