High-pressure synthesis of pure and doped superconducting MgB₂compounds

Abstract

Superconducting properties of bulk, dense, pure MgB2 and doped (Mg1-xAx)B2\nsamples with A = Na, Ca, Cu, Ag, Zn and Al were studied for compositional\nranges 0 < x < 0.20. The effect on pinning properties and critical current were\ninvestigated, particularly for A = aluminium. The samples were sintered and/or\nsynthesised at high pressure - high temperature in a cubic multi-anvils press\n(typically : 3.5-6 GPa, 900-1000C). They were characterised by x-ray\ndiffraction, scanning electron microscopy and their superconducting properties\nwere investigated by a.c. susceptibility, magnetisation (VSM and SQUID) and\ntransport measurements under magnetic field. Only Al substitutes really on the\nMg site. The other elements form secondary phases with B or Mg which do not act\nas pinning centres. No positive effect is observed on the superconducting\nproperties of the bulk MgB2 samples added with these doping elements : Tc, jc\ncritical current, Hirr and Hc2. For Al, the effect on Hc2 remains small, and\nthe irreversibility line does not move, thus not improving the critical current\nof the Al-doped MgB2 samples.\n