Superconducting Joints Between Iron-based Superconducting Tapes

Iron-based superconductors were considered to be the second high-temperature superconducting series following the copper oxide superconductor.This new family of superconductors has caused great interest for high-field applications due to their unique properties such as high transition temperature,ultrahigh upper critical field and low anisotropy.The fabrication of superconducting joints is an essential part of the future application of iron-based superconductors.In this paper,a method for jointing Sr-122 superconducting tapes was prepared.By the study of the preparation process of Sr-122 superconducting tapes and the joining process of other superconductors,a suitable process of fabricating superconducting joints between Sr-122 superconducting tapes was proposed.The critical current ratio（CCR）of the joint fabricated by this way was 66.7%at 4.2 K and 10 T,the resistance estimated from the I-V curve was below 10^-9?.We analyzed the interaction between the superconducting core and the air.The result shows that the process of fabricating joints can be operated without protective atmosphere environment.The etching liquid of the silver sheath was studied,and a mixed solution of hydrogen peroxide and ammonia（30wt%ammonia:30wt%hydrogen peroxide=5:2 in volume）was chosen.Additionally,we presented the fabrication process of the superconducting windows.Finally,considering the characteristic of iron-based superconductors,we chose the lap joint as the jointing method of iron-based superconductors.The prepared superconducting joints were heat-treated under normal pressure and hot pressure,respectively.The superconducting properties between normal pressure sintering joints and hot pressure sintering were studied.The result indicates that hot pressure sintering can effectively reduce the loss of elements in the joint area of the joint sample and improve the superconducting properties of the joint sample.There are still obvious macroscopic cracks perpendicular to the current direction in the connection area of hot-pressed sample,the occurrence of such cracks is directly related to the pressure during hot pressing.The transport properties of iron-based superconducting joints are affected mainly by the superconducting phases and microstructures in the connection area.In this study,three kinds of cracks were observed:longitudinal macro-cracks,transverse macro-cracks and micro-cracks.The macro-cracks are mainly in the tapes which were connected;while the micro-cracks are mainly distributed between the tapes which were connected.Transverse macro-cracks are more harmful to the transport performances of tapes,compared to the longitudinal ones.Meanwhile,the micro-cracks will block the current transport from one tape to another tape for the joint.Therefore,theses cracks should be avoided in the joint section.In the other side,the loss of element K in the joint area should also be avoided.Longer pressing time can obviously prevent the element K from losing,compared with the shorter pressing time.That’s because the loss of potassium element was happened at high temperature,thus extending pressing time can eliminate this problem.The highest CCR of 63.3%（at 4.2 K in 10 T）is obtained for the iron-based joint sample pressed at 2.76 MPa with pressing time of 4.5 h.