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The improved critical current density was obtained for ex-situ MgB 2 bulks initially sintered at 900 °C for 2 h and then at 650 °C for 1 h. It was found that local liquid Mg phase sintering is dominant during first high-temperature sintering process, which results in the formation of MgB 2 aggregates with excellent grain connectivity. The second low-temperature sintering process not only can consume part of the residual MgB 4 but also enlarge the aggregates zones. Consequently, The MgB 2 grain connectivity in sintered samples is enhanced and responsible for the improved critical current density. [ABSTRACT FROM AUTHOR]

Self-sintered ex-situ MgB 2 polycrystalline bulks have experienced a two-step sintering process, initially at 900 °C for 0–20 min and then at 650 °C for 1 h. MgB 2 was decomposed to MgB 4 and Mg at 900 °C and composed again at 650 °C from MgB 4 and Mg. The reversible reaction promotes the material migration, and thus eliminates pores and enhances the connectivity between the grains. The critical current density ( J c ) is significantly improved due to both improved grain connectivity and the additive pinning centers such as MgB 4 and new-born boundaries. This two-step sintering process can be a promising method to fabricate high-performance ex-situ MgB 2 bulks and wires. [ABSTRACT FROM AUTHOR]

High performance Cu doped MgB 2 bulks were prepared by an in-situ method with carbon-coated amorphous boron as precursor. It was found that the usage of carbon-coated boron in present work leads to the formation of uniformly refined MgB 2 grains, as well as a high level of homogeneous carbon doping in the MgB 2 samples, which significantly enhance the J c in both Cu doped and undoped bulks compared to MgB 2 bulks with normal amorphous boron precursor. Moreover, minor Cu can service as activator, and thus facilitates the growth of MgB 2 grains and improves crystallinity and grain connectivity, which can bring about the excellent critical current density ( J c ) at self fields and low fields (the best values are 7 × 10 5 A/cm 2 at self fields, and 1 × 10 5 A/cm 2 at 2 T, 20 K, respectively). Simultaneously, minor Cu addition can reduce the amount of MgO impurity significantly, also contributing to the improvement of J c at low fields. Our work suggests that Cu-activated sintering combined with employment of carbon-coated amorphous boron as precursor could be a promising technique to produce practical MgB 2 bulks or wires with excellent J c on an industrial scale. [ABSTRACT FROM AUTHOR]