A combination of grain refinement of the starting substances, Mg and B, with chemical reaction to form MgB2 by mechanical fracturing, cold-welding and solid-state-reaction of the powder particles led to a complex behavior of the whole system. In addition, the introduction of O from the atmosphere and the incorporation of W, C and Co impurities from milling had a strong influence. Hence, 2 opposed processes took place which led, with milling time as the only parameter, to an initial improvement of the superconducting properties. This could be attributed to grain refinement, resulting in a higher reactivity and therefore an optimum grain connectivity and a high density of grain boundaries in hot-pressed nanocrystalline bulk MgB2; due to clean surfaces and a larger surface area of the particles. For milling times longer than 50h, this excellent performance degraded rapidly. Saturation of the grain refinement at a final coherent scattering length, which was regarded as being a minimum bound for the grain size of about 10nm which was associated with enrichment of the impurities (mainly O) to a maximum content of about 4.5at% at the longest milling times. This produced a porous microstructure, with reduced grain connectivity. The results permitted an optimum MgB2 microstructure to be obtained by applying appropriate mechanical alloying conditions; such as an intermediate processing time of 50h.

Microstructure and Impurity Dependence in Mechanically Alloyed Nanocrystalline MgB2 Superconductors. O.Perner, J.Eckert, W.Hässler, C.Fischer, K.H.Müller, G.Fuchs, B.Holzapfel, L.Schultz: Superconductor Science and Technology, 2004, 17, 1148-53