A high density CaKFe 4 As 4 bulk sample was successfully synthesized using a spark plasma sintering (SPS) technique. The density of the synthesized sample was 5.02 g cm −3 , corresponding to 96.2% of the theoretical density of CaKFe 4 As 4 . Moreover, a reasonably high Vickers hardness of 1 GPa was measured. The electrical resistivity of the SPS bulk sample was as low as approximately 600 µΩ cm at 300 K, which is smaller than that of the ordinary sintered polycrystalline sample by nearly one order of magnitude, and exhibited a sharp superconducting transition with the transition width (∆T c ) less than 2 K, indicating an improved grain connectivity. The critical current density of the SPS bulk sample calculated from the magnetization hysteresis loops (magnetic J c ) reached 18 kA cm −2 at 4.2 K under 5 T, which is the highest among the iron-based superconductor polycrystalline samples reported thus far.
Vanadium based alloys are usually produced by alumino thermic reduction of V 2 O 5 . In order to use it in hydrogen storage alloys, contained impurities such as aluminum and oxygen have to be removed by electron beam purification, increasing the alloy cost by one order. In this work, we have developed an electro slag remelting purification method for removing the impurities with low cost. In this electro slag remelting process, a crude vanadium consumable electrode made by alumino thermic reduction and flux of CaF 2 CaO Al 2 O 3 are used. By applying high current, the vanadium electrode is partially melted, the contained impurity such as aluminum oxide is removed to the flux and a purified molten metal is dropped to bottom.Removal ratio of the aluminum oxide is in proportional to the CaO content in the flux. Then, remained oxygen in the metal is removed by remelting with rare earth metal as a reducing agent. The alloy prepared by this process shows the same hydrogen storage capacity as that prepared using high purity metal. The electro slag remelting purification method is suitable for mass production with reasonable cost.
The effects of adding low-melting point metals (Pb, Sn, In and Ge) to polycrystalline CaKFe 4 As 4 (CaK1144) superconductors were investigated. We found that Sn is the most stable with CaK1144 among these low-melting point metals. Synthesis conditions as well as the doping quantity of Sn were optimized, and phase stability, microstructures and superconducting properties were investigated. At the optimized sintering temperature of 700 • C, Sn was effectively solidified in grain boundaries (GBs) of the CaK1144 bulk samples and secondary phase formations were not serious in optimized samples. As the Sn amount increased, filling of Sn in the GBs increased up to 30 wt% with homogeneous dispersion. The magnetic field dependence of critical-current density (J c ) was significantly enhanced by 30 wt% Sn addition and decreased with further increase of Sn addition. The filling up of 1144 grain boundaries by the optimal amount of Sn enhanced the J c from 11.7 kA cm −2 to 21.7 kA cm −2 in a self-field and at 4.2 K. Sn addition was effective both to suppress the impurity phases as well as increase J c of CaK1144.
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