Effect Of Rapid Heating/Quenching Treatment And Doping Methods On The Critical Current Density Of MgB₂

Magnesium diboride?MgB₂?is the binary alloy compound with the highest superconducting transition temperature?T_c=39 K?,and the MgB₂ superconductor has a wide energy gap,no grain boundary weak connection,and extremely short electroacoustic action time,which makes it have great application potential in the preparation of superconducting devices.Therefore,MgB₂ is of great significance in the research of superconducting mechanism and the application development of superconducting devices.The MgB₂ wire with high-performance kilometer-scale is the premise of its practical application,therefore,the high-performance kilometer-scale MgB₂ long-line preparation is the research focus and difficulty in this field.In this paper,the first set of rapid heating/quenching?RHQ?equipment designed and developed by our group is used to explore the low-cost,high-performance,practical and easy-to-promote MgB₂ wire preparation process.Further,the chemical doping method was studied to improve the superconducting properties of MgB₂bulk materials.The main contents and main conclusions reached in the paper are as follows:?1?Compared with the MgB₂ wire which is directly phased in the annealing furnace,the MgB₂ wire treated by the RHQ method has finer grain characteristics,and its microstructure exhibits a fiber shape,indicating that the RHQ method has a fine grain effect on the MgB₂ wire,enhancing its grain boundary pinning force.?2?The RHQ method for processing MgB₂ wire has the advantages of refining the grain-enhanced pinning mechanism and rapid production for industrialization,however,the extremely rapid quenching process results in a large amount of raw materials not forming phase to hinder the connectivity.We post-annealed the RHQ sample which was treated a trace speed of 120 mm/s,295 A,and the sample was optimally annealed at 580?,the critical current density of the wire reached 2.14×10⁴ A/cm² A/cm²@20 K,1 T.For the 180mm/s,385 A sample,680°C is the optimized annealing condition,and the current carrying capacity is 2.23×10⁴ A/cm²@20 K,1 T.The post-annealing process makes the MgB₂superconducting phase newly formed by unreacted Mg and B,which fills the gap between the fibrous structures,enhances the grain connectivity of MgB₂ and improves the current-carrying performance.?3?A solid phase reaction method was applied to the doping of NaCl in MgB₂superconductor by hot pressing technique.It was found that the c value in MgB₂ lattice parameter can be effectively reduced by doping.Microscopic analysis shows that the connectivity of MgB₂ grains is enhanced,thereby increasing the critical current density at high fields.MgB₂ doped with 5 at%NaCl has the highest critical current density and the strongest magnetic pinning ability.It has a J_c of 6.8×10⁵ A/cm² at 20 K,2 T,which is about20%higher than the performance of undoped-sample.?4?In the study of Ca powder doped MgB₂ bulk material,it is found that CaB₆ can be formed by Ca doping with different Ca metal doping components.The XRD results indicate that the c value in MgB₂ lattice parameter has a certain change,which effectively increases the critical field H_c22 on MgB₂ bulk,indicating that Ca can promote its superconducting properties as a doping element of MgB₂.