Applied pressure-dependent anisotropic grain connectivity in shock consolidated MgB2 samples

Three different cylindrical MgB2 bulk samples were prepared by the underwater shock consolidation method in which shock waves of several GPa, generated by detonation of explosives, were applied to a metallic cylinder containing commercially available MgB2 powders with no additives. Resistivity anisotropy of the samples increased with shock pressure. The highest- and medium-pressure applied samples had finite resistivities in the radial direction for the whole temperature range down to 12 K, whereas their axial and azimuthal resistivities dropped to zero at 32–35 K. By contrast, the lowest-pressure applied sample was approximately isotropic with a normal-state resistivity of ∼40 μΩ cm, an onset temperature of ∼38.5 K, and a transition width of ∼4.5 K. These extremely anisotropic properties would have resulted from the distortion of grain boundaries and grain cores, caused by the shock pressures and their repeated bouncing.