High-pressure synthesis of pure and doped superconducting MgB₂compounds

Abstract

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 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, 900-1000 °C). 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 MgB 2 samples added with these doping elements : T c , j c critical current, 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.

Enhancement of critical current density in low level Al-doped MgB2

“…Nevertheless, our results compare favourably with previous studies of low level Al doped (~5 at.%) samples, where the doping caused sharp reductions of J c over the temperature range 5 -35 K [9,15]. Most importantly, the possibility now exists to tune the intraband scattering rates by selectively doping both the B and Mg sites.…”

“…Improved performance through alloying rather than changes to the pinning mechanism was corroborated from curves of the reduced flux pinning force ( Toulemonde et al 9 Slusky et al 11 Xu et al 10 (a) …”

“…It appears that only light C substitution of B can be tolerated before T c drops sharply [7]. Extensive research into substitutional chemistry has shown that only a few elements can substitute onto the Mg site [8] (Al [8][9][10][11][12], Li [13], Cu [14]). However, so far, there have been no reports of improved superconducting properties with doping on the Mg site [11,15,16].…”

Superhard Superconductive Composite Materials Obtained by High-Pressure-High-Temperature Sintering

“…Many investigations were devoted to the effect of the conditions of MgB 2 producing and treatement at high pressures and temperatures on the superconducting properties with various dopants (Jung et al, 2001;Prikhna et al, 2002;Pachla et al, 2003;Tampieri et al, 2004;Toulemonde et al, 2003;Zhao et al, 2003;Dou et al, 2003). It is of interest to obtain a superconducting composite material in which diamond or cubic boron nitride are used as the superhard components and MgB 2 i s u s e d a s t h e superconducting component.…”

Superconducting Properties of MgB<SUB>2</SUB> Particle Impregnated with Mg-Based Alloys

“…Especially J c improvement in MgB 2 has been one of major issues of research; a well known problem is that J c drops rapidly with increasing magnetic field due to its poor flux pinning. Improvements in J c (enhancements of flux pinning) were achieved by various approaches; doping by chemical compounds, 7) substitution of Mg and/or B by other elements, 8,9) high-pressure application during process, 10) various heat-treatment, 11) etc. Theoretical studies for variations of the band structure by element substitution haven been also carried out.…”