Germanium-based pinning dopants for MgB2 bulk superconductors

Effects of the spherically shaped Ge and the rod-like carbon-coated Ge on the superconducting properties of MgB2 were investigated. Pure Ge and carbon-coated Ge nano-powders were synthesized under the different amount of CH4 (0 to 5 kPa) by using DC thermal plasma method. When the CH4 was added similar to 100 nm sized Ge with a spherical shape changed to rod-like morphology with a diameter of similar to 30-70 nm and a length of similar to 400-500 nm. Also it was confirmed that thin carbon layers of a few nanometers were formed along the rod length and the agglomerated carbons were found on the edges of rods. Pure spherical Ge and Ge/C rods were mixed and milled with Mg & B precursor to form the doped MgB2 bulk samples by the solid-state reaction method. Almost no change of T-c was noticed for the pure Ge-added MgB2, whereas T-c was found to decrease with the Ge/C-added MgB2 samples. It was found that the pure spherical Ge showed to have a negative effect on the flux pinning of MgB2. However, Ge/C rods can enhance the flux pinning property of J(c) due to the coated carbon on Ge rods.