Effect of Nanostructural Inhomogeneities on the Superconducting Characteristics of <inline-formula> <tex-math notation="TeX">$\hbox{MgB}_{2}$</tex-math></inline-formula> With Enhanced Grain Connectivity

Abstract

MgB 2 -based bulk materials synthesized under 30 MPa-2 GPa pressure demonstrated an enhanced connectivity A F ∼ 50%−98%, as compared to most other reports (below 50%), and a shielding fraction of 75%-100%. The materials inhibit high critical current densities j c , but no strict correlation was found between A F and j c . On the other side, we found correlations between j c and the distribution of nanostructural inhomogeneities such as impurity oxygen in the form of oxygen-enriched Mg-B-O nanolayers or nanoinclusions and its solution in the MgB 2 matrix and as well with the amount and size of higher magnesium borides MgB x , x ≥ 4 inclusions. The distribution of boron-and oxygenenriched (as compared to stoichiometric MgB 2 ) inhomogeneities affects the type of pinning and, thus, j c , and to some extent, it can be regulated by synthesis temperatures and pressures and by Ti addition. Auger spectroscopy, scanning electron microscopy, and X-ray with Rietveld refinement were used for structural study.

Size reduction of boron particles by high-power ultrasound for optimization of bulk MgB2

“…Some of the effective pinning centers used are non-superconducting inclusions, defects, grain boundaries, voids etc. Such pinning centers can be MgB 4 [28,29], MgO [30,31], metals [32,33], metal oxides [34,35], grain refiners, rare-earth elements [36], dislocations, defects created by irradiation [37], carbon doping [38][39][40][41][42][43][44][45] etc.…”

Bulk MgB2 Superconducting Materials: Technology, Properties, and Applications

“…The superconducting characteristics of MgB 2 materials depend on many factors and their combination. Very deep and comprehensive studies of the synthesis process of MgB 2 -based materials, the correlation between material structure and superconducting characteristics, and the manufacturing technology have been performed by the authors of [ 7 , 8 , 9 , 10 , 11 , 16 , 19 , 73 , 76 , 82 , 84 , 85 , 98 , 99 , 101 , 102 , 103 , 104 , 105 , 106 , 107 , 108 , 109 , 110 , 111 , 112 , 113 , 114 , 115 , 116 , 117 , 118 , 119 , 120 , 121 , 122 , 123 , 124 , 125 , 126 , 127 , 128 , 129 , 130 ]. These correlations were comprehensively studied for materials prepared using initial powders of MgB 2 and stoichiometric Mg:2B mixtures (typical characteristics are given in Table 1 ) at manufacturing temperatures in the range of 600–1100 °C under different pressure conditions using the methods noted above.…”

“…The Mg-B-O inclusions with a somewhat smaller amount of oxygen (points 1, 2 in Figure 15 c) than in the inclusions (points 5, 6 in Figure 15 c) are observed near the grain boundary, inside of the Ti-containing grain. Magnesium defuses into titanium more intensively than boron (compare points 3, 4 and points 5, 6 in Figure 15 c and Table 6 ) [ 113 ]. Magnesium and oxygen diffuse deeper into the Ti grain ( Figure 15 ) than boron, and this could be an explanation for the redistributions of boron and oxygen in MgB 2 , and possibly the reason for the higher magnesium boride grains formation.…”

“…( c , d )—Schema of MgB 2 -based material structures synthesized at low temperature of 800 °C ( e ) and high temperature of 1050 °C ( f ) [ 85 ]. ( e , f )—X-ray patterns of samples shown in ( a , b ) [ 113 ].…”

“…X-ray patterns of these materials ( e , f ). ( c , d ) show the places where Ti is absent [ 103 , 113 ].…”