Study On The Doping To Improve Grain Connectivity And Flux Pinning Of MgB₂ Superconductor

Since the discovery of superconductivity in MgB₂ in 2001, the interface morphology and electric property have been studied in many types. However, as a promising candidate for high field magnet use, the critical current density of MgB₂ is still insufficient. So the focus of the research is to find out a way of improving the grain connectivity and increasing pinning centers to improve the critical current density in high magnetic field.Since doping is a traditional way of improving MgB₂ properties. the doping of Cu and Al were used in the experiments reported here. However, the doping mechanisms of Al and Cu are different. The mechanism of Cu doping is reacting with Mg to generate second phase MgCu₂, which can improve the grain connectivity and form pinning centers to improve the critical current density in high magnetic field. However, the mechanism of Al doping is substitution of Mg in MgB₂ crystal structure, which brought about crystal distortion that is an effective pinning centers.The grain connectivity has relationship with the fabrication process, so the fabrication of MgB₂ powder and bulk were studied with different parameters. According to the fabrication of Cu addition MgB₂ powder, by means of changing the ratio of Cu doping, sintering temperature, holding time, and surface area of stainless steel mesh, two sintering parameters with good intercrystalline connectivity were obtained, namely: Mg: B: Cu = 1.1:2:0.1, 600°C +5h and 750°C +2h, an additional 10% Mg increased the flow of molten Mg and made up the volatilization when sintering.According to the fabrication of Al addition to MgB₂ powder, the different percents of Al and Mg were studied. The Al content causes dramatic increase in lattice distortion, and Mg content influences the phase composition. A series of experiments showed than a ratio of Mg: B: Al = 1:2:0.1, heat treated under argon (Ar) gas at 750°C+2h gave good parameters.In the fabrication of bulk MgB₂, the mass and density decreased and the height of compacted powder pellets increased, which resulted from Mg volatilization and the stress release. The grain boundary of Cu addition in the bulk was serious destroyed at high temperature. While at 750°C a good phase purity and intercrystalline connectivity was obtained. According to Al substitution within a bulk sample, a higher sinter temperature can bring about impurities, although the connectivity is good. If an ex situ Al substitution is used for bulk samples, then Al is found to partly enter the MgB₂ crystal structure, other exists in the form of an Al-Mg alloy.Considering the different doping mechanisms of Al and Cu, the co-doping of Al and Cu was applied to the MgB₂ fabrication. However, when the Al and Cu all exist in samples, impurity phases appear, which are preliminary determined as Al₂Mg and Al₄Cu₆. The presence of these impurities will certainly affect the electrical properties of MgB₂. However, if the size of impurity can be made smaller than coherence length of MgB₂, they would be available pinning centers.According to the T_c measurement of co-doping, the T_c is mainly affected by the Al relative percent, while in the near T_c regions, the appearance of second transition phenomenon is the combination of Al and Cu doping effect, which result from the vortex state protect superconducting state from suddenly disappear.