Search

The un-doped Mg 11 B 2 and Cu-doped Mg 11 B 2 bulks using 11 B as a boron precursor were fabricated by solid-state reaction and sintered at different temperature in present work. By analyzing the sintering process, it was found that 11 B original powder is more reactive and can react with Mg severely even at low temperature before Mg melting, which leads to the formation of refined Mg 11 B 2 grains. Consequently, the critical current density of Mg 11 B 2 sample prepared in this work is higher than that of natural MgB 2 . Furthermore, it was found that proper Cu addition could obviously accelerate the reaction between Mg and 11 B and promote the formation of Mg 11 B 2 due to the appearance of Mg–Cu liquid at low sintering temperature before Mg melting. On the other hand, Cu addition also introduced more MgCu 2 impurity, leading to a relatively lower critical current density on the whole. Interestingly, the critical current density of the Cu doped sample sintered at 800 °C is surprisingly enhanced at the low field, compared with that of the corresponding un-doped one. This is due to the peritectic reaction which generated small-sized MgCu 2 occurred that at 800 °C, providing MgCu 2 pinning centers well-distributed in the MgB 2 matrix. [ABSTRACT FROM AUTHOR]

Sn added bulk FeSe 0.5 Te 0.5 samples are synthesized through two-step solid state reaction route. It is found that Sn addition can significantly enhance the transformation from the hexagonal phase to the superconducting tetragonal phase during the sintering process of FeSe 0.5 Te 0.5 samples due to the appearance of liquid Sn at low temperature. With increase of superconducting phase proportion as well as improvement of its crystallinity, 5 wt% Sn addition dramatically enhances the zero resistivity temperature ( T c 0 ) by 3 K accompanying with a bit improvement in the onset temperature of superconducting transition ( T c on ). From this view, our work suggests a new prospective that how to search proper dopant to further improving superconductivity in 11 type Fe based superconductors. [ABSTRACT FROM AUTHOR]

Highlights: [•] Glycine doped MgB2 can be made at low temperature by Cu-activated sintering. [•] We obtained excellent J c in glycine-doped MgB2 sintered at 600°C for 15h. [•] Cu and glycine have synergistic effects on improvement of J c. [•] We illustrated the precipitation morphologies of secondary phase MgCu2 and MgO phases. [ABSTRACT FROM AUTHOR]

Abstract: The minor Cu addition (<3at%) could apparently reduce the amount of MgO impurity in the in situ sintered MgB2 samples, accompanying with the improvement in the critical transition temperature (T c) from 36.4K for the undoped sample to 38.6K for the samples with 3at% Cu addition. The decrease of MgO impurity within Cu-doped MgB2 samples can be attributed to the following factors: (i) the formation of local Mg–Cu liquid at about 485°C wrapped the neighboring Mg particles and thus protected them from the oxidation before the melting point of Mg; (ii) when the sintering temperature increased above the melting point of Mg, the minor Cu addition could also decrease the vapor pressure of Mg liquid and then reduce the volatilization of Mg liquid, which can depress the reaction between gaseous Mg and O2 impurity in the protective Ar gas forming MgO impurity. Our results provide a new route to govern the oxidation of Mg during the in situ sintering of pure or doped MgB2 samples by altering the minor amount of Cu addition. [Copyright &y& Elsevier]

Abstract: MgB2 nanoparticles were synthesized by a one-step reactive sintering method. The sample was heated from room temperature to 994K, and then directly cooled down at a rate of 40K/min. The results of X-ray diffraction indicate that MgB2 superconductors with high quality were successfully prepared, and only a few impurities were detected on it. The sample consisted of two distinguishable structures by the TEM observation: nanoparticles and single crystals, which is a result of nonequilibrium conversion under the direct cooling condition. The nanoparticles with a diameter of about 10–20nm are considered to be at the onset for the formation of MgB2 phase. Transition temperature (T c) and critical current density (J c) of the compound structures were determined to be 38.5K and 1.8×105 Acm−2 by means of SQUID measurement, which indicate good superconducting properties. The inter-diffusion and dislocation incorporation mechanisms for the formation and growth of MgB2 nanoparticles are also proposed, and the further growth will be accelerated due to an adequate holding time at 994K. [Copyright &y& Elsevier]

Highlights: [•] Sn expedites the transformation from Fe7Se8 to β-FeSe. [•] β-FeSe textured grains and their better connection with each other were observed. [•] The Tc of the Sn doped FeSe0.93 samples is not depressed compared to undoped one. [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]