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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]

As the “low activation” superconductor, Mg 11 B 2 has a potential application in superconducting coils for fusion reactor. In present work, the sintering process and critical current density of low activation Mg 11 B 2 superconductors were systemically studied from low temperature to high temperature. It was found that the J c and H irr values of Mg 11 B 2 bulks in present work are both obviously higher than that of those samples prepared in previous studies. Furthermore, the low-temperature sintered samples exhibit better J c performance at high fields than the high-temperature sintered samples, due to strong grain boundaries pinning. On the other hand, the high-temperature sintered samples have higher J c at low fields compared to low-temperature sintered samples, mainly owing to their better crystallinity and grain connectivity. The highest J c value (2.20 ×10 5 A cm −2 at 20 K, self-field) is obtained in the Mg 11 B 2 sample sintered at 850 °C for 45 min. [ABSTRACT FROM AUTHOR]

Highlights: [•] An enhancement of J c was observed in MgB2 samples containing CNTs prepared with fine Mg powders. [•] The so-called fishtail effect was observed in both sets of CNT-containing polycrystalline samples. [•] We fitted the data to each sample by the model used in (RE)BCO system. [ABSTRACT FROM AUTHOR]

Abstract: Based on the thermal analysis, microstructure observation and superconductivity measurement, the phase formation and superconductive properties of Cu doped MgB2 samples sintered with milled B powder were investigated. The minor Cu addition could prompt the solid–solid reaction between Mg and milled B and the formation of MgB2 phase is almost completed before Mg melting. The MgB2 grains near the local Mg–Cu liquid could grow faster than others during the phase formation of MgB2 before Mg melting. Furthermore, abnormal growth of these preferentially formed MgB2 grains could occur after Mg melting. Hence, the MgB2 grains in the Cu-doped samples are larger and less homogeneous than that in the undoped sample sintered with milled B powder. As a result, the pinning of grain boundaries is reduced obviously and the J c becomes more sensitive to the applied magnetic fields in the Cu-doped sample sintered with milled B powder. On the other hand, the accelerated solid–solid reaction between Mg and B would accompany with less residual Mg and thus the volatility of Mg is weaker after melting, which result in a higher density and fewer voids in the Cu-doped samples and thus enhance the grain connectivity between MgB2, finally lead to the improvement of J c at low fields. [Copyright &y& Elsevier]