Interfacial reactions and oxygen distribution in MgB₂wires in Fe, stainless steel and Nb sheaths

Abstract

Superconducting MgB2 wires have been made by powder-in-tube (PIT) processing of commercial MgB2 powder in Fe, stainless steel and composite metallic sheaths using annealing temperatures from 850 to 1100 °C. Technologically interesting Jc values around 106 A cm−2 in the self-field at 4.2 K have been achieved. However, as the annealing temperature is increased, substantial chemical reactions have been observed at the MgB2/sheath interface. We report on a detailed study using electron probe microanalysis of the reaction products in the interfacial region and the distribution of oxygen in the core. Iron and chromium borides are formed at the core/sheath interface in Fe and stainless steel sheaths respectively, with consequent depletion of B from the superconducting core. A similar effect is caused by the formation of a solid solution of B in Nb sheaths. Milling the starting powder before wire manufacture results in a very significant increase in the oxygen content of the core, which probably explains the relatively poor performance of these wires.

Improved critical current densities in B4C doped MgB2based wires

“…This is in contrast to B 4 C doped wires, where an enhancement of J c with the reaction temperature is observed, in agreement with the results on bulk samples [8]. The highest value measured so far for a wire with In our case, a deterioration of the transport properties of the conductors was observed at 900°C, which is attributed to reaction with the Fe matrix [13,14,15]. It is important to note that the used powders were mixed with an excess of B with respect to the stoichiometric ratio, thus probably compensating the B losses due to the interaction with the sheath.…”

Comparison on Effects of ${\rm B}_{4}{\rm C}$, ${\rm Al}_{2}{\rm O}_{3}$, and SiC Doping on Performance of ${\rm MgB}_{2}$ Conductors

“…Previously, Nb sheath has been found to react with ex situ at of 950 [31] but to have an inert nature at 650 [32]. Therefore, the possible diffusion between the sheath and core of the samples with different is studied with Carl Zeiss EVO 40 SEM EDX.…”

Anisotropy of<tex>$rm J_rm c$</tex>in Ex Situ<tex>$rm MgB_2/rm Fe$</tex>Monofilamentary Tapes

“…MgO tends to be homogeneously distributed within the filament, while the content is higher at the center of the filaments. Near the Fe sheath the excess B has reacted with Fe, forming a thin layer at the interface, identified as the intermetallic compound [7], [12]. No substantial difference has been found among the …”