Fabrication And Enhanced Critical Current Density Of Iron-based Superconducting Wire And Tape

The iron-based superconductor?IBS?family has simulated unprecedented interests due to its rich physical properties and promising application potential since it was discovered.Its potential application prospects are coming from its highlighting advantages,such as high superconducting transition temperature?T_c?,high upper critical field(H_c2),low H_c2 anisotropy and so on.Similarly to other promising superconductors,the most important prerequisite for the application of the IBS family is obtaining long wire/tape with hundreds of kilometer in length.Moreover,among the IBS family,122-and 11-type superconductors are recognized as the ones with simple structures,easy preparation,bulk superconductivity and the most promising application potential.Due to these outstanding advantages together with taking the outstanding progresses of their coated conductors,we take Ba_1-xK_xFe₂As₂ and FeSe_1-xTe_x which have been proved to be quite promising in application respectively from 122-and 11-type superconductors as our research objects,and do the fabrication of their wires\tapes and systematical studies in this thesis.In chapter 1,we emphatically introduce the superconductivity,the discovery of high-temperature superconductor?HTS?and its studies,and the discovery of IBS family and the current research of its superconducting wires/tapes.It is well known that the IBS is hard and brittle,which leads to the severely difficulty of performing plastic deformation on it without damaging the superconductivity.Hence,we prefer to use traditional powder-in-tube?PIT?method to fabricate its wires/tapes.In chapter 2,we describe the PIT method and analytical methods.So far,multi-nary superconducting wires/tapes are mainly fabricated with the ex-situ PIT method,which can effectively reduce the pores inside the superconducting core.Moreover,the precursor plays the key role in the ex-situ PIT method.Therefore,in chapter 3,we detailed introduce the preparation of FeSe_0.5Te_0.5 precursor with high quality by a rapid melting technique,and Ba_0.6K_0.4Fe₂As₂ precursor by the high-energy ball-milling or the high-energy ball-milling aided PIT method,which provide the precursors for the next fabrication of wires/tapes.In chapter 4,to help fill the gap of the study of 11-type ex-situ PIT superconducting tape and promote the progress of practical research of 11-type superconductor,we successfully process the high-quality FeSe_0.5Te_0.5 precursor by the rapid melting technique mentioned in chapter 3 to tapes,which is also the first attempt all over the world to the best of our knowledge.Moreover,the c-axis orientation of FeSe_0.5Te_0.5grains for our tapes is confirmed by the XRD and SEM analyses.Also,the obtained T_c can be up to about 15.7 K.The self-field magnetic J_c at 5 K reaches 1×10⁴ A/cm²which is higher than the one of wires of the same period.Also,these tapes get high H_c2 and are almost isotropic.All these indicate its promising application potential at high field.Similarly,in order to enhance the current J_c of 11-type wire and promote its research progress,we firstly fabricate the Cu/Nb sheathed ex-situ PIT round-wires based on the high-quality FeSe_0.5Te_0.5 precursor by the rapid melting technique?mentioned in chapter 3?in chapter 5.According to our results,the FeSe_0.5Te_0.5 cores react with the Nb sheaths when heat-treated at a high temperature or at a low temperature for a long time,but we still respectively observe 1.6×10⁴ and1.5×10³ A/cm² at 0.5 and 0.75 T,which is the highest one among the FeSe_1-xTe_x single core wires reported so far.On the other hand,in chapter 6,we employ the Ag sheath which has been proved to be benign to 122-type superconductor,and apply the precursor by the high-energy ball-milling?SPEX high-energy ball-milling?mentioned in chapter 3 to wires/tapes.Compared to previous reported wires,the transport J_c of our samples is significantly enhanced.Currently,the high-J_c 122-type superconducting wires mainly come from hot-isostatic-pressing?HIP?.However,HIP requires special equipment with high operational costs and limited efficient,and it can be available for lad-used short-samples only.Thus,in chapter 7,we fabricate round wires using the precursor by the high-energy ball-milling aided PIT method presented in chapter 3.Then,we study the effect of biaxial cold pressure densification?BCPD?on the as-fabricated wires,because the BCPD is easily to be gotten and simply to be handled,cheap,and can be applied to scalable production in long wires.We conclude that the BCPD densified the core without degrading the superconductivity.More importantly,the J_css of BCPDed samples are enhanced,such as the transport J_c of a typical pressed sample with 1.0 GPa researches 1.14×10⁵ A/cm²?4.2 K,2 T?.This high value refreshes the record of the transport J_c in 122-type superconducting wires to the best of our knowledge.Even,it is higher than the best result from HIPed samples.All these suggest the huge potential of BCPD in enhancing the superconductivity of IBS-type wires.Finally,in chapter 8,we extensively target Ba_1-xK_xBiO₃ which is another HTS and holds promising application potential as well.We fabricate the first Ba_1-xK_xBiO₃ wire around the world by the ex-situ PIT method and then study the effect of precursor and heat-treatment condition on its superconductivity.Chapter 9 is a summary of the whole research contents.