Magnesium diboride (MgB2), first found to be superconducting in 2001, has great potential for technology applications thanks to its high critical temperature of 39 K, low cost and simple chemical composition. However, the low upper critical field, Hc2, and low critical current density, Jc, of the undoped/clean bulk MgB2 is holding it back.

In recent years, researchers have shown that it is possible to improve the superconducting properties of MgB2 by doping it with carbon in various forms, such as silicon carbide, carbon nanotubes and pure carbon itself. Now, Herrmann and co-workers have shown that pre-milled carbon with an enhanced reactivity is a very effective dopant too.

The team, which did its work in the framework of the European HIPERMAG project, found that it was able to increase MgB2's Hc2 by carbon doping up to 11.4 T at 20 K for x=0.221 in MgB2-xCx. According to the researchers, this improvement is thanks to the changed electron-scattering properties of the formed MgB2-xCx phase. Using these carbon-doped powders for MgB2 tapes prepared by powder-in-tube technology resulted in the highest reported Jc value so far for this system (10 kA/cm2 at 14.3 T and 4.2 K). An encouraging Jc of 10 kA/cm2 at 5 T and 20 K also shows the advantage of MgB2 compared to other commercial low-temperature superconductors.

Andries den Ouden and Marc Dhallé, coordinators of HIPERMAG told "MgB2 has rapidly developed into an economical technical superconductor for medium magnetic fields, comparable to NbTi. Since its higher Tc value allows you to reduce system and maintenance costs in important commercial applications such as MRI or NMR, we will certainly see an increasing role for the material. Moreover, the exciting results of our colleagues in Dresden illustrate how the performance limits of MgB2 conductors are still evolving, facilitating a breakthrough in areas where existing superconductors only have a marginal impact."

The researchers made their carbon-doped MgB2 using magnesium powder, amorphous boron powder and highly pure graphite powder. Next, they mechanically alloyed the components together in a planetary ball mill with tungsten carbide vials and balls under an argon atmosphere – very effective in comparison to other techniques. Bulk samples were obtained by hot pressing the powders under vacuum at 700 °C for 10 minutes using an applied pressure of 640 MPa.

The team now hopes to further investigate the influence of carbon on the properties of MgB2 and optimise its mechanical alloying technique. "But there might also be another dopant waiting to be discovered in the periodic table," muses Herrmann.

The work was reported in Supercond. Sci. Technol..