  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.
