Fine structure of a bulk MgB2 superconductor after deformation and heat treatment

Abstract

The structure of the MgB₂ superconductor subjected to high-temperature restoration annealing after cold deformation under high pressure in a Toroid chamber or Bridgman anvils has been investigated by transmission electron microscopy. It has been shown that after postdeformation annealing at 950°C the average size of crystallites in the matrix phase increases 5–10 times compared to the deformed state, reaching ~50–150 nm, as well as the critical current density increases by a factor of three (up to 6.7 × 10⁴ A/cm², 30 K) compared to the initial state. It has been found that the MgO phase and the higher magnesium borides are present in the form of dispersed precipitates 10–70 nm in size.