Increasing dissipation-free supercurrent has been the primary issue for practical application of superconducting wires. For magnesium diboride, MgB 2 , carbon is known to be the most effective dopant to enhance high-field properties. However, the critical role of carbon remains elusive, and also low-field critical current density has not been improved. Here, we have undertaken malic acid doping of MgB 2 and find that the microscopic origin for the enhancement of high-field properties is due to boron vacancies and associated stacking faults, as observed by high-resolution transmission electron microscopy and electron energy loss spectroscopy. The carbon from the malic acid almost uniformly encapsulates boron, preventing boron agglomeration and reducing porosity, as observed by three-dimensional X-ray tomography. The critical current density either exceeds or matches that of niobium titanium at 4.2 K. Our findings provide atomic-level insights, which could pave the way to further enhancement of the critical current density of MgB 2 up to the theoretical limit.

Carbon‐encapsulated crystalline boron nanopowder and coarse magnesium powder are used as inexpensive tailored starting materials for the fabrication of high‐performance MgB2 superconducting wire. A low sintering temperature leads to a high critical current density, as a result of nanometer‐sized boron powder, surface oxidation preclusion by carbon encapsulation, and grain alignment by elongated magnesium coarse powder.

Experimental AspectsIn the experimental apparatus shown in Fig. 2 Fig. 3(a).Argon or oxygen could be blown on the melt surface.Gases were controlled by mass flow controller and introduced from the nozz]e as illustrated in Fig. 3(b).The melting parameters are given in Table 3. 3. (Fig. 4(a)) and 70~/ o (Fig. 4(b)) of the button melt samples was melted during each experlment (3.8 and 6.2kW, respectively). Higher power gave more molten silicon. Figure 5 showscarbon Electron beamfurnace for button melting. Preliminary Experiment