Investigation On Critical Current And Mechanical Characteristics Of HTS Quasi-Isotropic Strands In Low Temperature

In recent decade years,significant progress has been made on critical current,anisotropy and other properties on the second generation(2G)high temperature superconducting(HTS)coated conductor(REBCO CC)with tape configuration.The realization of the large-scale commercial production and coming price collapse greatly promote the application of the REBCO CC in HTS cable,superconducting transformer,superconducting fault current limiter,magnet coil and other electric power devices.In the past five years,high capacity conductors made of REBCO CC/tape have been put forward for power applications,such as twisted stacked-tape cable,compact conductor on round core cable,Roebel assembled coated conductor,cable in conduit conductor and quasi-isotropic HTS strand,which are predicted to have potential application in large-scale HTS electrical devices and magnets.This thesis mainly investigates the electromagnetic and mechanical characteristics of HTS Quasi-Isotropic strands made from REBCO CCs.The structural feature of the strand was analyzed and the manufacturing process of the strand is described,the strand samples were fabricated with different metal sheathes.Besides,the terminal treatment method is described.When the strands are bent,the REBCO CCs within the strand are subjected to bending with different angles.The strain of the REBCO CCs at different bending angles so called out-of-plane bending is analyzed theoretically.The general strain expression of different bending angles and different bending radii is deduced.A series of molds are suggested and designed to bend the REBCO CCs at different angles and radii.Characteristics of REBCO CC bending at different angles were experimentally studied in liquid nitrogen.Based on the geometrical structure of the strand,critical current at 77 K,self field is calculated by using self-consistent model.Experimental results in liquid nitrogen verifies the rationality of the calculation.The anisotropy of the critical current in liquid nitrogen is calculated shown that the strand is quasi-isotropic.Then,critical current at 4.2 K,different external magnetic fields is systematically analyzed by using the same model.However,the anisotropy of critical current is obvious under the strong magnetic field of liquid helium temperature.Tensile properties,bending properties and twisting properties are studied.First,the critical tensile stress of superconducting REBCO CCs and strands was measured experimentally.The strain distribution of the strands in bending and twisting is then calculated and analyzed.The every point strain of the strand is simulated by numerical simulation.The strain of the strand under both twisting and bending conditions is studied theoretically,the coupling strain of them is analyzed emphatically.Based on the relationship between the strain and the critical current,the dependences of critical current on bending radius and different twist pitch were performed.Finally,the strand was heat treated to improve the bending characteristics,and the experimental results show that the heat treatment can effectively improve the bending properties of the strands.