Bulk superconducting MgB 2 samples, 20 mm in diameter, were prepared by hot-pressing of ball-milled Mg and B powders using fine-grained boron powders. High maximum trapped fields of B 0 = 5.4 T were obtained at 12 K in one of the investigated trapped field magnets (height 8 mm) at the centre of the bulk surface. Investigating the temperature dependence of the trapped field for short MgB 2 samples (height ≤ 1.6 mm), trapped fields of up to B 0 = 3.2 T at 15 K were achieved. These high trapped fields are related to extremely high critical current densities of up to 10 6 A cm −2 at 15 K, indicating strong pinning due to nanocrystalline MgB 2 grains. Expected trapped field data for long trapped field magnets prepared from the available MgB 2 material are estimated.

The influence of the quality of boron precursor powder on the microstructure and superconducting properties of MgB2 bulk samples and tapes was investigated. The nominal purity specified by the suppliers considers only metallic impurities and is not sufficient for the characterization of the boron precursor powder. Oxygen impurities and the grain size of the B precursor powder were found to affect Tc and the microstructure of the MgB2 tapes. The microstructure was investigated by SEM and TEM. Grains in the boron precursor powders were either nanocrystalline or crystalline, with grain sizes varying between 110 and 500 nm. MgB2 precursor powder was prepared by mechanical alloying, which resulted in a small, 20–60 nm, MgB2 grain size of bulk samples. Bulk samples showed the highest MgB2 phase fraction and a critical current density of 4.7 × 104 A cm−2 (at 20 K, 1 T) if boron precursor powder with small grain size and small fraction of metallic impurities was used. Such powder also yielded compact tapes and required lower annealing temperatures for the MgB2 phase formation. The typical critical current densities of the tapes were 5.0 × 104 A cm−2 (at 20 K, 3 T) and were significantly better than those of samples reported recently. These results underline the importance of mechanical alloying for enhancing the critical current density of MgB2 tapes. Summarizing, the phase content, the density and the superconducting properties of MgB2 bulk and tapes depend on the choice of boron precursor powder.

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.