Superconductivity in porous MgB2

Porous magnesium diboride samples have been prepared by the heat treatment of a pressed mixture of Mg and MgB2 powders. It was found that linked superconducting structure is formed down to the minimum normalized density γc=d/d0≅0.16 (percolation threshold), where d is the density of MgB2 averaged over the sample, d0=2.62 g/cm3 is the X-ray density. Lattice parameters and critical temperature of the porous sample decrease with increasing porosity (decreasing γ) and Tc2≅32 K is minimal at γc. The grain boundaries in the porous samples are transparent for the current and Jc∼3×105 A/cm2 in self field at T=20 K in the samples with γ∼0.24.