Study On The Delamination And Current-Carrying Degradation Of HTS Coated Conductors And Coils

With the excellent current-carrying capacity and high upper critical magnetic field,second-generation high temperature superconducting(2G HTS)coated conductors has a broad application prospect in the electromagnetic equipment,such as superconducting cables,high field magnets,nuclear magnetic resonance magnets and generators.Due to the special lattice orientation of the ceramic oxide high-temperature superconductor,the 2G HTS tape is manufactured as a multilayer composite structure.However,at the extremely low temperature and strong magnetic field,the coated conductor tape is subject to the effects of thermal stress,electromagnetic force and mechanical load,and it is very easy for the delamination to occur.The delamination occurs in or near the superconducting layer due to the inherent brittleness of the ceramic oxide,causing serious degradation of the current-carrying performance,which directly threatens the safe and stable operation of the superconducting electromagnetic equipment.Therefore,focusing on the delamination of 2G HTS coated conductors,the inter-layer strength of superconducting tape,the delamination evolution of stacked superconducting tapes under electromagnetic load,the delamination and current-carrying degradation degree of superconducting coils after cooling are investigated in this paper.Firstly,a peel model for 2G HTS coated conductor is established based on the bilinear traction-separation constitutive.The delamination evolution of the superconducting tape under concentrated force is analyzed,and the peel strength of the superconducting tape is obtained.Under the condition of the elastic deformation of the stabilization layer,the peel model is compared with the theoretical analysis.The interface fracture characteristics,delamination evolution modes and plastic deformation degree of the stabilization layer are analyzed in detail.Furthermore,the effects of elastic-plastic deformation,peel angle,fracture energy release rate,delamination tensile strength and thickness of stabilization layer on peel strength are discussed in detail.In addition,the finite element analysis model of thermal residual stress for the complex lamination process of the 2G HTS tape is established,and the variation of thermal residual stress with heat treatment temperature is analyzed.Finally,the effects of thermal residual stress and substrate thickness on the peel strength is simulated.The results show that the peel stress only acts on the crack tip and damages the superconducting tape easily.The peel strength is the result of the combination of the elastic-plastic deformation of the stabilization layer and the interfacial fracture energy.The thermal residual stress reduces the peel strength by 20%.The peel model also reveals that the reduction of the thickness of the substrate layer will not reduce the delamination strength of the superconducting tape,and the optimal thickness of the substrate layer is 20-30 ?m.Secondly,a delamination model of stacked superconducting tapes is established based on the T-A method for electromagnetic field and the cohesive zoom model.For a simple superconducting thin film-matrix system with the injected current,the shear stress at the superconducting layer/substrate interface is analyzed and compared with the results of a theoretical model.The current density,magnetic field and electromagnetic force distributions of a single superconducting tape and a stack of superconducting tapes under the effect of transport current and applied magnetic field are studied.The change of interface stress is analyzed under the conditions of complete constraint and partial constraint.The simulation results show that when a single superconducting tape and a stack of superconducting tapes transport current under the condition of self-field,the electromagnetic force is small and the effect on interface fracture can be ignored.Under the action of parallel magnetic field,the electromagnetic force in the superconducting tape can reach the magnitude of the delamination strength,which may cause the phenomenon of delamination.The number of layers and the angle of applied magnetic field have significant influence on the damage of the stacked superconducting tapes.Finally,at the level of magnetic coil,an analysis model of thermal-mechanicalelectric-magnetic field is established.The delamination and the degradation of currentcarrying capacity of the epoxy-impregnated superconducting magnetic coil at extremely low temperature are analyzed.The temperature variation and its distribution of the superconducting coil in the liquid nitrogen pool are given by solving the Fourier heat conduction equation.At the same time,based on the elastic theory and the mixed-mode traction-separation constitutive,the distribution of the thermal stress inside the superconducting coil and the interfacial stress of the cohesive layer are analyzed.In addition,the steady-state electromagnetic field distribution of the superconducting coil is solved by using the T-A method,and the effect of delamination on the critical current density and value of n of the superconducting tape is considered.At last,the E-I characteristic curve of the superconducting coil is obtained.Based on the critical current and n,the degradation degree of the superconducting coil with the number of turns is analyzed.The simulation results show that the radial thermal stress locally exceeds the electrical delamination strength of the superconducting tape,resulting in the degradation of the critical current density and n of the superconducting coil.The higher turns number,the greater the internal thermal stress,and the higher the current-carrying degradation of the superconducting coil.The current degradation of the superconducting coil can be reduced by adjusting the thickness of the insulation tape and epoxy resin.