Fe-cladded MgB2 conductors have been prepared by the powder-in-tube method using mechanically alloyed nanocrystalline Mg+2B powder mixtures consisting of the constituents Mg, B, and MgB2 as precursor. Despite low Tc values of about 31 K, maximum critical current densities (Jc) of 22 and 7 kA/cm2 in external magnetic fields of 7.5 and 10 T, respectively, are achieved at 4.2 K. These excellent values can mainly be attributed to the very fine-grained microstructure of the superconducting phase. Higher values have only been reported for tapes doped with SiC nanoparticles. The irreversibility fields Hirr of these tapes are 9.5 and 4.2 T at 10 and 20 K, respectively.

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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.

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Instead of commercially available MgB 2 powder, we have used partially reacted powder prepared by mechanical alloying. This precursor powder consists of grains with a size of only a few nanometres and contains reacted MgB 2 , and also the starting material Mg and B, and is, therefore, more reactive than fully reacted commercial powders. Using copper as a sheath material, tapes were prepared by the usual powder-in-tube process. After annealing at relatively low temperatures (770-870 K) in inert atmosphere, the tapes have good superconducting properties. Magnetically we have measured a critical current density of 400 kA cm −2 at 4.2 K.