Microstructure and impurity dependence in mechanically alloyed nanocrystallineMgB2 superconductors

Abstract

In order to improve the intrinsic properties ofMgB2 superconductors, the application of mechanical alloying (MA) of elemental Mg and B powdersis a very promising fabrication technique. The enhancement of the upper critical fieldHc2 and the irreversibilityfield Hirr as well as of thecritical current density Jc shows the potential of this preparation route. Nevertheless, a better understandingof the MA process would allow further optimization of its parameters forMgB2 preparation.The coaction of the grain refinement of the starting substances Mg and B with the chemical reactionforming MgB2 by mechanical fracturing, cold-welding and solid-state-reaction of the powder particlesleads to a complex behaviour of the whole system. Additionally, the introduction ofoxygen from the working atmosphere and the incorporation of W, C and Coimpurities stemming from the milling tools has a strong influence. Hence, twoopposed processes are taking place which lead—with the milling time as the onlyparameter—in the beginning to an improvement of the superconducting properties ofMgB2. This can be attributed to the grain refinement resulting in a higher reactivity and, therefore, anoptimal grain connectivity and a high density of grain boundaries in hot pressed nanocrystallineMgB2 bulks, which is due to clean surfaces and a larger surface area of the particles. Incontrast, for milling times longer than 50 h this excellent performance degradesrapidly. 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 associatedwith an enrichment of the impurities (mainly oxygen) to a maximum content ofabout 4.5 at% for the longest milling time, causes a porous microstructure withreduced grain connectivity. These results allow us to achieve an optimumMgB2 microstructure by applying appropriate mechanical alloying conditions, i.e. a mediumprocessing time of 50 h.