High-pressure synthesis of pure and doped superconducting MgB2compounds

Superconducting properties of bulk, dense, pure MgB 2 and doped (Mg 1− x A x )B 2 samples with A = Na, Ca, Cu, Ag, Zn and Al were studied for compositional ranges 0 < x ≤ 0.20. The effects on pinning properties and critical current were investigated, particularly for A = aluminium. The samples were sintered and/or synthesized at high pressure–high temperature in a cubic multi-anvil press (typically 3.5–6 GPa, 900–1000 °C). They were characterized by x-ray diffraction, scanning electron microscopy and their superconducting properties were investigated by ac susceptibility, magnetization (VSM and SQUID) and transport measurements under a magnetic field. Only Al really substitutes 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 MgB 2 samples with these doping elements added: T c , critical current j c , H irr and H c2 . For Al, the effect on H c2 remains small, and the irreversibility line does not move, thus not improving the critical current of the Al-doped MgB 2 samples.