Optimization of Phase Formation and Superconducting Properties in MgB2 Prepared by Phase Transformation from MgB4

In this work, polycrystalline MgB4 was prepared, and then the presence of MgO in the powder was reduced by acid leaching. To synthesize MgB2, the MgB4 powder was reacted with Mg at temperatures from 650A degrees C to 950A degrees C for 4 h and 8 h. Using the Rietveld method, the MgB2 phase was estimated to be in greater proportion over the sintering temperature range of 650A degrees C to 750A degrees C, above which it decreased rapidly, accompanied by the formation of more of the MgB4 phase. Scanning electron microscopy showed that the samples have reduced porosity compared with those synthesized by the direct reaction of (Mg + 2B). Upon increasing the sintering temperature, the superconducting transition temperature decreased, which is attributed to lattice distortion. The results of this work demonstrate that control of heat treatment is essential to optimize the weight fraction of the MgB2 phase and thereby increase the critical current density, J (c). The value of the magnetic J (c) (5 K, 1 T) for the sample sintered at 750A degrees C for 4 h is estimated to be 1.00 x 10(5) A/cm(2).