Improvement in the transport critical current density and microstructure of isotopic Mg11B2 monofilament wires by optimizing the sintering temperature

Abstract

Superconducting wires are widely used in fabricating magnetic coils in fusion reactors. In consideration of the stability of 11B against neutron irradiation and lower induced radio-activation properties, MgB2 superconductor with 11B serving as boron source is an alternative candidate to be used in fusion reactor with severe irradiation environment. In present work, a batch of monofilament isotopic Mg11B2 wires with amorphous 11B powder as precursor were fabricated using powder-in-tube (PIT) process at different sintering temperature, and the evolution of their microstructure and corresponding superconducting properties was systemically investigated. Accordingly, the best transport critical current density (Jc) = 2 × 104 A/cm2 was obtained at 4.2 K and 5 T, which is even comparable to multi-filament Mg11B2 isotope wires reported in other work. Surprisingly, transport Jc vanished in our wire which was heat-treated at excessively high temperature (800 °C). Combined with microstructure observation, it was found that lots of big interconnected microcracks and voids that can isolate the MgB2 grains formed in this whole sample, resulting in significant deterioration in inter-grain connectivity. The results can be a constructive guide in fabricating Mg11B2 wires to be used as magnet coils in fusion reactor systems such as ITER-type tokamak magnet.

Evaluation of isotopic boron (11B) for the fabrication of low activation Mg11B2 superconductor for next generation fusion magnets

“…The undoped low activation Mg 11 B 2 wires were recently manufactured by using the isotopic 11 B (P) powder. 49 The J c value of this isotopic wire showed 2 × 10 4 A cm −2 at 4.2 K and 5 T which is relatively lower than the J c value (10 5 A cm −2 at 4.2 K and 5 T) for the natural boron based high activation undoped MgB 2 wire reported in the ref. 43. In the comparison with the best examples of the high activation MgB 2 superconductors, our low activation isotopic Mg 11 B 2 -wires lack their performance in terms of J c ; however, several ways of the performance improvements are envisaged (especially low-and high-field J c ).…”

“…This performance is the best J c reported so far in in situ processed wires. The undoped low activation Mg 11 B 2 wires were recently manufactured by using the isotopic 11 B (P) powder 49 . The J c value of this isotopic wire showed 2 × 10 4 A cm −2 at 4.2 K and 5 T which is relatively lower than the J c value (10 5 A cm −2 at 4.2 K and 5 T) for the natural boron based high activation undoped MgB 2 wire reported in the ref.…”

“…However, the critical current density (J c ) is of the most importance in the performance characteristics of our Mg 11 B 2 wires. In our previous study, 48,49,50 we synthesized Mg 11 B 2 using the same 11 B (P) powder and systematically studied the sintering process and critical current density of low activation Mg 11 B 2 superconductors in bulk and wire form. It is of interest to compare the achieved J c in our samples with the best reported results.…”

Evaluation of residual stress and texture in isotope based Mg11B2superconductor using neutron diffraction

“…60,61 In fact, it was previously reported in our research results that the wire sintered at 800 C did not show a transport J c , even though the sample was fully reacted with a high Mg 11 B 2 superconducting phase fraction. 62 On the other hand, the wire sintered at 750 C has a superior transport critical current density, J c ¼ 2 Â 10 4 A cm À2 at 4.2 K and 5 T compared with the multilament wire manufactured by the National Institute for Fusion Science (NIFS). 62 Therefore, based on the above discussion, the cracks and the pores have a detrimental inuence on J c through fracturing of the intergrain connections, while, at the same time, these defects act as stress relief factors in the Mg 11 B 2 wire sintered at 800 C. The superconducting transition temperature (T c ) was observed at a temperature of 36.5 K and 36.9 K, for the samples sintered at 700 C and 800 C, respectively.…”

“…62 Therefore, based on the above discussion, the cracks and the pores have a detrimental inuence on J c through fracturing of the intergrain connections, while, at the same time, these defects act as stress relief factors in the Mg 11 B 2 wire sintered at 800 C. The superconducting transition temperature (T c ) was observed at a temperature of 36.5 K and 36.9 K, for the samples sintered at 700 C and 800 C, respectively. 62 Compared to the reported results, e.g. 39.2 K in, 63 the lower T c in the present samples is most likely due to the presence and complex interactions of different types of MgB 2 lattice defects, such as lattice strains/stresses, poor crystallinity, the presence of point defects and defects with higher dimensions, and issues with chemical purity and phase purity (e.g.…”