Search

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: [•] Histidine is an effective dopant for improving the critical current density. [•] The dominating pinning effects are provided by nano-scale MgO pinning centers. [•] MgO performed stronger pinning effects than defects caused by C substitution. [•] We clarified how to select the amino acid for doping. [Copyright &y& Elsevier]

Highlights: [•] Residual Mg in low-temperature sintered MgB2 improves grain connectivity. [•] Excellent Jc was obtained in the prepared sample with excess Mg. [•] Residual Mg makes the sintering microstructure more homogeneous and dense. [Copyright &y& Elsevier]

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: The influence of different heating rates, ranging from 5 to 30Kmin−1, on the microstructure and superconducting of the MgB2 bulk was investigated. No obvious variation in the grain size was found for the samples heated from 5Kmin−1 to 20Kmin−1 except for the changes in morphologies. Moreover, the grain refinement was obtained under the heating rate of 30Kmin−1. The critical current density (J c) suggested that the 5Kmin−1 sample had the best performance in high field. Here, the differential thermal analysis (DTA) was employed to analyze the kinetics of MgB2 phase formation with the different heating rates. The results showed that the large amount of MgB2 formed at low temperature, which lead to compact structures under the slow heating rate. The fast heating rate would promote the evaporation of Mg at high temperature, which was considered to generate the vacancy and impurities in the sample. [Copyright &y& Elsevier]

Abstract: The minor Cu (<3at%) addition could significantly reduce the amount of MgO impurity in the in situ sintered MgB2 samples and thus improve the critical current density (J c) at low field and the critical transition temperature (T c). The formation of local Mg–Cu liquid at about 485°C could wrap the neighboring Mg particles and thus protect them from the oxidation during the sintering process. Our results provide a possible route to further improve the superconductive properties of pure or doped MgB2 samples through decreasing the amount of MgO impurity with minor Cu addition. [Copyright &y& Elsevier]

Abstract: A direct synthesis route from Mg and B powders to crystalline MgB2 nanowires was developed by reactive sintering: the sample was heated from room temperature to 994K at a rate of 20K/min and then cooled down at a rate of 40K/min, according to the non-equilibrium conversion reaction theory. The prepared MgB2 nanowires exhibit a uniform diameter of 50nm. Both SEM and SQUID studies on the obtained nanowires suggest it has a high quality and a critical transition temperature comparable with that of an ideal one. A presumable mechanism for the process in the formation MgB2 nanowires is also proposed, through which the formation of this special structure is considered to be connected with the preferable growth in several fixed crystal facets by assisting with liquid Mg melt. [Copyright &y& Elsevier]

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]

Extending the holding time and slightly excessive Mg addition and Mg-Cu co-addition are introduced to the MgB 2 superconductors prepared by ex-situ sintering method aiming to improve their grain connectivity and J c. The influence of which on superconducting performance and microstructure of ex-situ sintered MgB 2 was also systemically studied. The A F value of Mg-added samples is obviously improved, especially the A F value of the Mg-Cu co-addition sample has reached the level of the in-situ sintering samples. The J c of two samples with Mg addition is improved by comparing to others within the entire whole field. [ABSTRACT FROM AUTHOR]