Study On The Connection Process And Electromechanical Properties Of YGdBCO High Temperature Superconducting Tape Joints

Due to the high critical current density and upper critical magnetic field and excellent mechanical properties in the temperature range of liquid nitrogen,YGdBCO coated conductors,that is,second-generation high-temperature superconducting tapes,have broad application prospects in the fields of power,transportation,medical and military.Among the many applications of superconducting materials,superconducting magnets are currently the largest engineering applications.In order to meet the operational requirements of different types of superconducting magnets,such as different magnetic field shapes and field strengths and closed-loop continuous current mode.Joint manufacturing is an inevitable key technology in the research and development process.This paper focuses on the lead-free soldering technology of stable and repeated YGdBCO low resistance joint,and a new connection technology based on nano-silver sintering were proposed.The influence of welding table parallelism,roughness and pressure control uniformity on the welding process were strengthened using self-built program-controlled welding equipment.The effects of solder type,loading pressure,pressurization speed and lap length on the critical current Ic(77 K,self-field),resistance and n value of YGdBCO joints were systematically studied,and brazing process with high stability and repeatability were obtained.It has a positive effect on the practical application of YGdBCO tape.The results show that with Sn Bi solder and optimized soldering technology,a low-resistance joint comparable to the original tape Ic can be obtained repeatedly at pressure of 12.5 MPa and pressurization speed of 50 N/s.The length of the joint is 25 cm,and the resistance of the joint is 4.35?5.58 n?(resistivity43.5?55.8 n?·cm~2).It is found that the solder spreads evenly on the surface of the tape,and the thickness of the solder and the resistance of the joint are gradually reduced as the pressure increases observed by SEM and light microscope.The resistance of the joint gradually decreases with the increase of the pressurization speed and the length of the joint.The minimum resistance of the joint is obtained under the condition of 50 N/s pressurization speed and 25 cm length of the joint,but the length of the joint has almost no effect on the critical current and n value of the joint.Based on the optimized brazing process,a series of low-resistance joints were fabricated and their mechanical properties were studied.The axial tensile test results in the liquid nitrogen temperature zone show that the axial tensile strength of the joint part is equivalent to that of the non-joint part,213 N and 212 N,respectively.The delamination strength test found that the delamination strength of the joint and the original tape at room temperature is less than that at the liquid nitrogen temperature,and the delamination strength of the joint is smaller.In this paper,the joint technology based on nano-silver low-temperature sintering is proposed,and the sintering temperature zone of nano-silver particles is preliminarily determined by thermogravimetric analysis.The influence of sintering temperature and pressure on the electrical properties of the original tape were explored.YGdBCO joints were made by two pressure methods of self-made mold and powder tablet press.77 K.The electrical resistivity of the joint prepared by self-made molds under at the 77K,self-field is 52.9 n?·cm~2,and the electrical resistivity of the joint prepared by the powder tablet press is at the 77K,self-field 44.2 n?·cm~2.The nano-silver layer of the joint pressurized by the powder tablet press is sintered denser,with lower porosity and better connectivity under the connection condition of sintering temperature of 200?and pressure of 40 MPa observed by light microscope.Compared with the welding technology,the way of pressurized by the powder tablet press does not introduce solder that increases the resistance of the joint,and the low-temperature sintering of nano-silver was used to directly connect the silver layer of the tape.The method is simple and easy,and it is beneficial to further reduce the joint resistance,but the connection process needs to be further optimized,and the sintering mechanism needs to be studied in depth.