The 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 x =  0 to 0.20. The effects upon pinning properties and critical current were investigated; particularly for Al. The samples were sintered and/or synthesized at high pressures and high temperatures in a cubic multi-anvil press (3.5 to 6GPa, 900 to 1000C ). They were characterized by X-ray diffraction and scanning electron microscopy, and their superconducting properties were investigated by performing alternating-current susceptibility, magnetization (VSM and SQUID) and transport measurements in a magnetic field. Only Al really substituted at the Mg site. The other elements formed secondary phases, with B or Mg, which did not act as pinning centers. No positive effect was observed upon the superconducting properties (TC, JC, Hirr, HC2) of bulk MgB2 samples with these doping elements added. For Al, the effect upon HC2 remained small, and the irreversibility line did not move; thus not improving the critical current of the Al-doped MgB2 samples.

High-Pressure Synthesis of Pure and Doped Superconducting MgB2 Compounds. P.Toulemonde, N.Musolino, R.Flükiger: Superconductor Science and Technology, 2003, 16, 231-6