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.