Sugar Coating of Boron Powder for Efficient Carbon Doping of MgB2 with Enhanced Current‐Carrying Performance

The second boom in superconductivity during the last two decades has been powered up by the discovery of MgB2 having the transition temperature (Tc) of 39 K [1] as well as a strong market potential for applications. The strongest impact on the development of this superconductor has been made by the discovery of the significant enhancement of the critical current density (Jc) as a function of the applied magnetic field (Ba) that results from SiC nanodoping. [2] Since then, a large variety of dopants have been attempted in order to achieve or improve on the attained enhancement. In the vast majority of these doping works the nanodopants have been introduced via a solid-state reaction; however, an important challenge is achieving homogeneity between a small amount of nanoadditives and matrix materials, because any dry mixing poses the major problem of nanoparticle agglomeration. In this Communication, we have chosen sugar (C6H12O6) as the dopant. This is the most readily available carbohydrate that enables liquid homogeneous mixing. We have demonstrated that sugar doping resulted in an effective substitution of carbon for boron in MgB2, so that a significant enhancement of the Jc performance over the entire applied field range is readily achievable. This method is applicable to the fabrication of a wide range of carbon-based compounds and composites. The MgB2 superconductor has a strong potential for various applications because of its high Tc, strong connectivity between the grains, and low anisotropy. One major problem is posed by the relatively weak pinning in the pure material, which leads to rather rapid degradation of Jc as a function of Ba. Different approaches, such as irradiation and chemical doping, have been used to enhance the pinning strength in MgB2. . Chemical doping is a desirable, relatively easy, and cheap method to introduce pinning sites into the superconductor. Nanometer-size SiC has been found to be the most effective dopant for Jc enhancement. [2,5,6,15,20–22] The carbon-based materials introduce the strongest enhancement of Jc(Ba) performance in MgB2, owing to the fact that carbon can be incorporated into the MgB2 crystal lattice by replacing boron. This substitution results in the enhancement of chargecarrier scattering occurring on C-substituted sites and Mgvacancies in two energy bands discovered in this material. This scattering has been shown to be responsible for the considerable upper critical field increase. Generally, nanometer-sized particles are necessary to ensure a homogeneous doping procedure, which in our case should provide a strong enhancement of the pinning force. However, regardless of how well mixing, grinding, milling, or ultrasonic dispersion is carried out, the doping process is usually impeded by the formation of large agglomerates of particles. In addition, the precursor materials are commonly in their passivated state, which can further degrade the reactivity and doping quality. Another problem for sustainable industrial applications is that nanometer-sized powders are expensive. Herein, we introduce a new approach to simple molecular mixing that ensures i) homogeneity, ii) strong reactivity with a maximum reaction surface, iii) fresh and clean interfaces between the atomic-scale dopant layer (C) and matrix components (B and Mg), and iv) rules out the necessity of nanometer-scale additives. This is achieved by “wet” mixing of a sugar solution with a boron powder and successive drying of the slurry to achieve homogeneous doping, resulting in Jc(Ba) enhancements comparable to the best results achieved by SiC nanoscale doping. Scanning electron microscopy (SEM) images of the raw and the sugar-coated boron powders show no difference. Both powders have ball-shaped particles down to a few nanometers in size. In contrast with pure MgB2 formation upon heat treatment, the sugar-added powder first undergoes the decomposition of sugar to, most likely, water and fresh highly reactive carbon above 100 °C: C6H12O6 → 6H2O + 6C. Concurrently, the following process may also occur: C6H12O6 → 6CO + 6H2. However, the first route is more likely to occur, as can be concluded from the obtained X-ray diffraction (XRD) results. XRD pattern analysis of the MgB2 samples indicates that the sugar slightly affects the phase constituents. Figure 1a shows XRD patterns of wet premixed MgB2 samples with different levels of C6H12O6 content. The samples mainly consist of an MgB2 phase, with some MgO phase as the main impurity even for the sample with no sugar addition (carbon substitution level (x) equal to 0). This is probably a remnant effect of hydrotreatment and/or the presence of water in the sugar. Starting from 5 % sugar addition (x = 0.08), the C6H12O6-doped samples show two peaks that belong to Mg2C3. For the x = 0.03 doping level sample only one small Mg2C3 peak is observed, as a result of the low amount of sugar added. Surprisingly, the C O M M U N IC A IO N