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  c = d/d o  0.16 (percolation threshold), where d is the density of MgB 2 averaged over the sample, d o = 2.62 g/cm 3 is the X-ray density. Lattice parameters and critical temperature of the porous sample decrease with increasing porosity (decreasing ) and T c2  32K is minimal at  c . The grain boundaries in the porous samples are transparent for the current and J c~3 . 105 A/cm 2 in self field at T= 20K in the samples with ~ 0.24.

Using a magnetic compression method a 2.3 mm diameter superconducting MgB 2 rod was synthesized in a stainless steel tube. After the compression the MgB 2 density inside the tube reached d = 2.45 g cm −3 , i.e. very near to the theoretical value 2.62 g cm −3 . At T = 4.2 K and B = 0.5 T the quench current reached 870 A. The critical current density versus temperature was determined from electric and magnetic measurements.

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Successful replacement of B by C in the series MgB 2-x C x for values of x upto 0.3 are reported. Resistivity and ac susceptibility measurements have been carried out in the samples. Solubility of carbon, inferred from the observed change in the lattice parameter with carbon content indicates that carbon substitutes upto x=0.30 into the MgB 2 lattice. The superconducting transition temperature, T C measured both by zero resistivity and the onset of the diamagnetic signal shows a systematic decrease with increase in carbon content upto x=0.30, beyond which the volume fraction decreases drastically. The temperature dependence of resistivity in the normal state fits to the Bloch-Gruneisen formula for all the carbon compositions studied. The Debye temperatures, θ D , extracted from the fit is seen to decrease with carbon content from 900K to 525K, whereas the electron-phonon interaction parameter, λ, obtained from the McMillan equation using the measured T C and θ D , is seen to increase monotonically from 0.8 in MgB 2 to 0.9 in the x=0.50 sample. The ratio of the resistivities between 300K and 40K versus T C is seen to follow the Testardi correlation for the C substituted samples. The decrease in T C is argued to mainly arise due to large decrease in θ D with C concentration and a decrease in the hole density of states at N(E F).