Microstructure and impurity dependence in mechanically alloyed nanocrystalline MgB₂superconductors

Abstract

In order to improve the intrinsic properties of MgB2 superconductors, the application of mechanical alloying (MA) of elemental Mg and B powders is a very promising fabrication technique. The enhancement of the upper critical field Hc2 and the irreversibility field Hirr as well as of the critical current density Jc shows the potential of this preparation route. Nevertheless, a better understanding of the MA process would allow further optimization of its parameters for MgB2 preparation. The coaction of the grain refinement of the starting substances Mg and B with the chemical reaction forming MgB2 by mechanical fracturing, cold-welding and solid-state-reaction of the powder particles leads to a complex behaviour of the whole system. Additionally, the introduction of oxygen from the working atmosphere and the incorporation of W, C and Co impurities stemming from the milling tools has a strong influence. Hence, two opposed processes are taking place which lead—with the milling time as the only parameter—in the beginning to an improvement of the superconducting properties of MgB2. This can be attributed to the grain refinement resulting in a higher reactivity and, therefore, an optimal grain connectivity and a high density of grain boundaries in hot pressed nanocrystalline MgB2 bulks, which is due to clean surfaces and a larger surface area of the particles. In contrast, for milling times longer than 50 h this excellent performance degrades rapidly. The saturation of the grain refinement at a final coherent scattering length, which is regarded as a minimal bound for the grain size of about 10 nm associated with an enrichment of the impurities (mainly oxygen) to a maximum content of about 4.5 at% for the longest milling time, causes a porous microstructure with reduced grain connectivity. These results allow us to achieve an optimum MgB2 microstructure by applying appropriate mechanical alloying conditions, i.e. a medium processing time of 50 h.

Complete flux jumping in nano-structured MgB2 superconductors prepared by mechanical alloying

“…Mechanical alloying technique allows one to prepare materials at ambient temperature in a solid state reaction, which normally form only during heat treatment. Hence Mg loss due to evaporation will be avoided and oxidation of the constituents can be minimized [7,8].…”

X-ray Micro-Tomography as a New and Powerful Tool for Characterization of MgB2 Superconductor

“…Both, high density and reduced grain size, as already noted above, are very important features for maximization of the properties in MgB 2 . Also, doping with various elements or compounds into MgB 2 has been found to enhance the critical current properties [25][26][27]. In this respect, best results were obtained by using nano-SiC [28], SiC whiskers, nanometer Si/N/C [29] and B 4 C [30], that showed a positive influence on irreversibility field (H irr ) and critical current density J c under magnetic fields.…”

High Trapped Fields in C-doped MgB2 Bulk Superconductors Fabricated by Infiltration and Growth Process

“…It is well understood that the performance of MgB 2 can be enhanced by maximising the density of grain boundaries, which form effective pinning sites in the microstructure 17–21 . This has been achieved by employing nano-scale Mg and B precursor powders for the synthesis of MgB 2 , which results ultimately in the formation of fine-grained MgB 2 .…”