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 gamma= d/do= 0.16 (percolation threshold), where d is the density of MgB2 averaged over the sample, do= 2.62 g/cm3 is the X-ray density. Lattice parameters and critical temperature of the porous sample decrease with increasing porosity (decreasing gamma) and Tc2= 32K is minimal at gamma= 0.16. The grain boundaries in the porous samples are transparent for the current and Jc~3.105A/cm2 in self field at T= 20K in the samples with gamma~ 0.24. 
V.A.Grinenko (1), E.P.Krasnoperov (1), V.A.Stoliarov (1), A.A.Bush (2)   B.P.Mikhajlov (3) ((1) RRC Kurchatov Institute, Mos...