Nanocoating of particles for optimal doping and universal enhancement of current-carrying ability in "organic" MgB2-xCx superconductors

The universal approach of liquid mixing has been found to produce “organic” MgB2−xCx superconductors with structural and supercurrent-carrying properties unattainable by other techniques. We show by transmission electron microscopy combined with energy dispersive spectroscopy that these unique properties are enabled through coating of boron particles with a nanolayer of carbon. The subsequent reaction between carbon-encapsulated boron and magnesium results in enhanced carbon content x in the crystal lattice, as determined by x-ray diffraction pattern analysis. Among various carbon-containing compounds investigated, polycarbosilane has been found to be the optimal dopant for MgB2 superconductor, producing the largest critical current density due to the formation of wide range of nanodefects with pinning properties within the MgB2 crystal lattice, as well as due to minimal formation of current-blocking layers around the grains.