Superconductivity in porous MgB 2

Abstract

Porous magnesium diboride samples have been prepared by the heat treatment of a pressed mixture of Mg and MgB 2 powders. It was found that linked superconducting structure is formed down to the minimum normalized density γ c= d/ d 0≅0.16 (percolation threshold), where d is the density of MgB 2 averaged over the sample, d 0=2.62 g/cm 3 is the X-ray density. Lattice parameters and critical temperature of the porous sample decrease with increasing porosity (decreasing γ) and T c2≅32 K is minimal at γ c. The grain boundaries in the porous samples are transparent for the current and J c∼3×10 5 A/cm 2 in self field at T=20 K in the samples with γ∼0.24.