Mechanism of the formation and specific features of the structure of massive samples of compound MgB2

Mechanisms of the formation of a bulk superconducting phase MgB2 at high temperatures

“…The experimental data obtained in our work allowed us to suggest that one of the main mechanisms of the formation of the MgB 2 phase at 1000°C is the liquid mechanism [1], namely: solid boron dissolves in large "reservoirs" of liquid magnesium, and, after reaching the composition close to stoichiometry, the formation of MgB 2 crystals begins to occur (see also the phase diagram in [2,6] tion. Characteristically, the sizes of these regions are close to the sizes of magnesium flakes.…”

“…The micro hardness of loose regions cannot be determined, because the imprint of the indenter is not fixed. As was noted in the Introduction, this "dendrites like" struc ture of two fractions (phases) is formed according to the liquid mechanism [1]. We assume that such a clear demonstration of this mechanism and the formation of two pronounced fractions of the MgB 2 phase 1 mm became possible owing to the use of large magnesium flakes (50-200 μm), which at a high temperature form large "reservoirs" of the liquid phase.…”

“…In [1], we showed that annealing of pressed pellets from magnesium flakes (with sizes of ~50-200 μm) and a boron powder (~1-10 μm) of the composition Mg : B = 1 : 2 at 1000°C for 2 h in an argon atmosphere (10 atm) actually leads to the formation of two frac tions of the MgB 2 phase, the microstructures of which differ significantly. One fraction forms very dense regions consisting of closely spaced crystallites of the MgB 2 phase, whereas the fraction of the second type contains loose regions (MgB 2 crystallites are poorly connected to each other).…”

Effect of deformation with Bridgman anvils on the structure, hardness, and critical current of a massive MgB2-based sample

“…It was shown in our previous study [1] that the annealing of tablets at 1000°С for 2 h (at an argon pres sure of 1 MPa) that were prepared from magnesium flakes and metallic boron powder (1-10 µm in particle size) taken in the proportion Mg : B = 1 : 2, results in the formation of two MgB 2 components that differ substantially in the microstructure and chemical com position. One of these components is highly dense and characterized by a microhardness of ~20 GPa that is close to the theoretical value, with optimum magne sium and boron contents, and low oxygen content.…”