Study On Superconducting MgB₂ Bulks

MgB2 bulks,using magnesium powder and boron powder as starting materials,were successfully synthesized by conventional pressureless sintering (PLS) and sparkplasma sintering (SPS) methods, and their microstructure and superconductivity werestudied. The component, density and microstructure of the samples prepared bydifferent methods were also compared. In this paper, we presented a systematic study of the fabrication processes ofMgB2 and focused on discussing the effects of the fabrication parameters on theproperties of the samples including the powder mixing time, the sintering temperature,the sintering time as well as the ratio of starting materials. The experiment shows thatMgB2 bulks with single component were fabricated at 750℃ for 3 hours in theflowing argon atmosphere at the ambient pressure when the ratio of Mg powder and Bpowder is 1:2≤Mg:B≤1.2:2. It was found that MgB2 bulks expanded with a differentexpansion coefficient in three-dimension after sintering. The expansion coefficientwas found to be about 4.0％. The purity of MgB2 bulks prepared by conventionalmethod is higher than that of MgB2 bulks prepared by SPS method, but their densityis low. A further experiment shows that increasing pressure in pressing process canincrease the density of the samples and make it reach 47% of the theoretical density,however, it is far from the theoretical value. This paper also presented a primary study on the synthesis of MgB2 bulks usingSPS method, and the sintering parameters including the isolated material, thetemperature and the heating time were discussed. The results revealed that SPSmethod can make fast sintering of MgB2 bulks. The density of the bulks made by thismethod was greatly increased, which is near 81% of theoretical density compared toconventional method. Meanwhile the SEM micrographs indicate that MgB2 bulksprepared by SPS method have denser microstructure, less pores and flatter transectcompared with the bulks prepared by PLS method. At last, the superconducting transition temperature of the samples synthesized bytwo methods were measured, and the results showed that their Tc were about 39 K.