Preparation And Properties Of FeSe Based Superconducting Wires

Iron-based superconducting materials are a new superconducting material system in the field of superconductivity after copper oxide superconducting materials.Iron-based superconductors contain ferromagnetic elements and have extremely high upper critical fields,which makes them very interesting for the study of the origin of superconducting.On the other hand,the FeSe-based superconducting material of“11”system has become a research hotspot as practical superconductor for its simple structure,no toxic elements and low raw material cost among the iron-based superconducting materials.However,there is a big gap between the current carrying performance of the wires of this system and its intrinsic performance.The main reason is that the wire core density is low and the grain boundary connectivity is poor.In this paper,FeSe0.5Te0.5 in FeSe-based superconducting materials is taken as the research object.By optimizing the heat treatment process of FeSe0.5Te0.5 superconducting bulk,the effects of different heat treatment processes on the phase composition,microstructure and superconductivity of the system are systematically analyzed.On this basis,the FeSe0.5Te0.5 superconducting wire was prepared by powder tube loading method.The influences of different rising and cooling rates on the phase composition,microstructure and superconducting properties of the core wire were explored.The fast heating and fast cooling process was developed to solve the problem of Se evaporation during heat treatment of the core wire of FeSe-based superconducting wire and improve the grain boundary connectivity of the core wire.FeSe0.5Te0.5 superconducting wires with high current carrying performance were prepared with the introduction of new techniques in order to obtain the wire with high superconducting phase content.The research contents are as follows:For the complex phase composition of Fe-Se-Te system in the process of phase formation,this paper adopts the solid-phase sintering and annealing process to prepare FeSe0.5Te0.5 polycrystalline material.Through systematic optimization of the heat treatment process,it is determined that the sintering process at 900℃ for 48 h,and "annealing at 500℃ for 3 h in the cooling stage,which can increase the content of superconducting phase.The obtained critical current density,Jc of FeSe0.5Te0.5 bulk at 5 K reached 2.2×104 A/cm2.Then,the Ca element doping of FeSe0.5Te0.5 superconducting poly crystals at the Fe site was studied.According to the stoichiometric molar ratio of elements,the dependence on magnetic field of samples with doping content of 0.01 was weakened under high field conditions.When the temperature is 5 K and the magnetic field is 6 T,the Jc value is 2.3 ×103 A/cm2,which is 2.6 times higher than that of the undoped sample.To solve the problem of selenium volatilization of FeSe0.5Te0.5 wires during heat treatment,in this paper,the in-situ FeSe0.5Te0.5 wires were prepared by powder tube loading method,and the method of rapid rise and drop temperature during heat treatment was explored.Under the condition of heat treatment at 800℃ for 12 h,the rapid heat-quenching process can effectively reduce the residual Fe content on the basis of obtaining the superconducting tetragonal βFeSe0.5Te0.5 phase,and reduce the Se element loss in the heat treatment process to a certain extent.The critical transition temperature Tc of obtained wire was 14.5 K.In order to solve the problem of selenium volatilization during the heat treatment process of FeSe0.5Te0.5 wires,rapid heating/quenching and transformation(RHQT)was performed for the first time on FeSe0.5Te0.5 wires in this paper.This method could further improve the rate of temperature rise and temperature drop in the process of heat treatment.It was found that RHQT has a positive effect on avoiding the volatilization of Se element.When the heating current intensity was 90 A,the critical transition temperature Tc of the sample reached 14.4 K.