The Employment Of Reversible Reaction "MgB₂?MgB₄" In The Ex-situ Sintering Of MgB₂ Superconductor And Itssuperconducting Properties Research

MgB₂ is the intermetallic compound with a relatively high superconducting transition temperature of 39 K which was discovered at 2001.MgB₂ superconductor has been applied to superconducting transformers,particle accelerators and windings after the development about ten years.According to the different precursor powder,there are two preparation methods of MgB₂ superconductor,in-situ and ex-situ.These two methods have their own advantages and disadvantages.Advantages of in-situ method are holding better intergranular connectivity and stronger intergranular coupling which are critical factors for the improvement of critical current density of MgB₂,as well as an easy way to introduce pining centers.Disadvantage of this method is the low density of MgB₂ because of the existent of many pores resulting from Mg migration which can restrict the critical current density.The advantages of ex-situ in this method can contribute to high density of MgB₂.However,this method can not achieve high grains connectivity which is not good for the improvement of critical current.Hence,improving the critical current density of ex-situ sintering MgB₂ superconductor is the hot topic in superconductor field at present,and is also importantly and meaningfully to meet the practical need.In this paper,I have researched the MgB₂ bulks and wires by ex-situ,and employed the reversible reaction to promote the mass migration and achieve the densification(high temperature is 900 ^o C and MgB₂ will be decomposed to MgB₄ and Mg at this temperature,low temperature is 650 ^oC and MgB₂ will be composed by MgB₄ and Mg at this temperature).Researches show that the amount of decomposed MgB₂ increased as the sintered time prolonged during higher temperature process and more MgB₂ can be composed during lower temperature process.According to the microstructure observation,this reversible reaction can contribute to the migration of mass in these samples,condense these samples and enlarge the grains scale,meanwhile consume the rest of MgB₄ which are benefit to the enhancement of intergrain connectivity.The scale of new-born boundary is around 10 to 15 nm,meanwhile the scale of the MgB₄ grains surrounded by new-born boundary is around5 to 10 nm in the samples after two-step sintering,which can act as effective pining centers and the enhancement critical current density at high field.The amount of the rest of MgB₄ in the wires sample through phase analysis is not matching the sintering time.So it can be speculated that the reverse reaction between MgB₄ and Mg with the result of MgB₂ is also occured during the high temperature sintering process.The higher temperature is more helpful for decomposed reaction and the lower temperature is more helpful for the composed reaction.