Influence of high-pressure deformation and annealing on the structure and properties of a bulk MgB^sub 2^ superconductor

A synthesized MgB2 superconductor has been investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and by the measurements of the superconducting characteristics and microhardness after cold high-pressure deformation in a Toroid chamber and in Bridgman anvils and subsequent high-temperature annealing. A nanocrystalline structure is formed in the superconductor after high-pressure treatment, but internal cracks appear, and the critical current density decreases strongly. The annealing leads to a coarsening of the structure and to an increase in the critical current density up to 5.8-6.7 × 104 Ðn/Ñm2, which is more than three times greater than that in the initial state.