Influence of bending and heat treatment under high isostatic pressure on synthesis reaction and transport critical current density in isotopic IMD Mg11B2 'wind and react' coils

Abstract

We show for the first time the results for small coils, which were wound with unreacted magnesium diboride (MgB2) wires with nano-amorphous isotopic boron (11B) by using combined internal magnesium (Mg) diffusion (IMD) and hot isostatic pressing (HIP) techniques. The small coils with a diameter of 20 mm or 10 mm were annealed under low (0.1 MPa) and high (1.1 GPa) isostatic pressure. The 10 mm coils annealed under high isostatic pressure of 1.1 GPa had significantly increased critical temperature (Tc), irreversible magnetic field (Birr) and transport critical current density (Jtc), with significantly accelerated Mg diffusion and improved homogeneity of the Mg11B2 material. Moreover, our findings show for the first time that the bending of the unreacted IMD Mg11B2 wires significantly affects the synthesis reaction under the low and high isostatic pressure. Our findings also show for the first time that diffusion of Mg during heat treatment under high isostatic pressure is completely different than during heating under low isostatic pressure. Mg diffusion under low isostatic pressure annealing is mainly dependent on the heat treatment temperature, bending diameter and 11B layer density. This Mg diffusion is related to the simultaneous formation of the superconducting phase. However, the diffusion of Mg under high isostatic pressure heat treatment is mainly dependent on the isostatic pressure, 11B layer density and Mg state (liquid or solid). Our results indicate that liquid Mg is first pushed into the 11B layer by high isostatic pressure and then a superconducting phase is formed.