No abstract

In this study, nanosize Silicon and Carbon (Si+C) were reacted with MgB2 in order to enhance the critical current density. The polycrystalline bulks were synthesized by the direct in situ reaction method and their phase formation, crystal structure, and superconducting properties were evaluated. The enhanced relative peak intensity of Mg2Si and MgB4 indicates the formation of a large volume fraction of these two phases with increasing (Si+C) additions. The a-axis lattice parameter shrinks significantly while c-axis increases slightly. The estimated C doping level at B site increases, leading to a degradation of the superconducting transition temperature with increasing (Si+C) additions. By a reaction with (Si+C), the field dependence of critical current density is shown to enhance at both 5 K and 20 K.

Instead of reacting B and Mg powders, MgB 4 was used as precursor to react with Mg to obtain the polycrystalline samples of nominal composition Mg x B 2 (x ¼ 1.2, 1.5, and 1.7). Upon increasing the sintering temperature to 750 8C, the reaction between MgB 4 and Mg was thermodynamically intensified leading to the formation of larger MgB 2 weight fraction as estimated from X-ray diffraction (XRD) data using the Rietveld method. Despite no further improvement to the weight fraction of this phase by increasing the excess nominal Mg, it was shown that the critical current density (J c ) was enhanced by almost one order of magnitude. At 5 K, 2 T, J c for the sample Mg 1.5 B 2 is estimated to be 4.5 Â 10 5 A cm À2 . The enhancement of J c is attributed to the collective effect of improved grain connectivity and flux pinning by unreacted Mg. Incorporation of nano-SiC for reaction resulted in a more gradual drop of J c with applied magnetic field. Hence, J c can be tailored to meet the range of fields intended for various applications via optimization of excess nominal Mg and dopant additions.