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

Wenbin Qiu,Yao Lu,Dipak Patel,Zongqing Ma,Vladimir Luzin,Shi Xue Dou,Md Shahriar Al Hossain,Hyunseock Jie,Mehmet Somer,Mohammed Shahabuddin,Arnaud Devred,Jung Ho Kim

doi:10.1038/srep36660

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.

Highlights

It should be noted that our best monofilament Mg11B2 wire shows comparable transport Jc performance to the multifilament wire fabricated by the National Institute for Fusion Science (NIFS)[7]
This result is considered as a big breakthrough, and it strongly supports the feasibility of replacing commercial NbTi by high-performance Mg11B2 wires in highly radioactive fusion reactors
The effects of sintering temperature on the superconducting performance and morphology of Mg11B2 monofilament wires made from isotopically pure boron powder were investigated in this work

Summary

Experimental Details

The standard in-situ powder-in-tube (PIT) procedure was applied to all the samples. After mixing the precursor powders, the mixture was tightly packed into Nb/Monel tubes with 10 mm outer diameter and 6 mm inner diameter. The composite wire was swaged and drawn to a final outer diameter of 1.08 mm. The fabricated Mg11B2 wires were sintered at different temperatures ranging from 700 °C, 750 °C, 770 °C, and 800 °C for 60 min (ramp rate: 5 °C/min) under high purity flowing argon gas. The transport critical current (Ic) measurements were carried out by using an American Magnetics superconducting magnet with DC current (with the upper limit of the current source 200 A) under possible magnetic field up to 15 T, with the standard four-probe method and the criterion of 1 μV/cm. In case of XRD, SEM, and PPMS measurements, the outer sheaths of the Mg11B2/Nb/Monel wires were removed for better data accuracy

Results and Discussion

Conclusions

Author Contributions

Additional Information