Superconducting properties and microstructure of MgB2 wires synthesized with a low-temperature diffusion process
Abstract:The compound MgB2 has a higher Tc value (39 K) and shorter decay time for induced radioactivity than Nb-based superconducting materials such as Nb–Ti, Nb3Sn and Nb3Al. However, the Jc value of MgB2 wire is lower than Nb–Ti alloy superconducting wire, and further Jc improvement is required for use in an advanced fusion reactor application. In order to improve Jc properties, we tried to investigate a reconsideration of the heat treatment condition through comparisons with low-temperature and long-time treatment, so-called 'low-temperature diffusion'. We confirmed that there was no reduction in Tc values compared with conventional processes when heat treated at low temperatures and for longer times (450 °C for 200 h). Furthermore, core Jc values when heat treated at 450 °C increased with extended heat treatment time, and then the maximum core Jc value was found to be over 1000 A mm-2 at 4.2 K under 5 T for the 200 h treatment sample and its value was higher than for conventional samples with SIC addition. From the results of SEM observation, 100 nm grain-sized MgB2 microcrystals were densely formed in the wire core and these nanograin sizes of the MgB2 phase might affect the Jc improvement in the low and middle magnetic field regions. On the other hand, we tried to fabricate 100 m classed mono-cored Ta sheathed wire samples having stabilized Cu in order to investigate the workability of the Ta-sheathed sample for possible use in an advanced fusion reactor application. We found that the wire deformation of 100 m classed wire without breaking was successful for this sample, and this result proved the good workability and possibility of fusion application of metal Ta sheathed wire.
Document Type: Research Article
Publication date: December 1, 2007