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