| High temperature superconductors(HTS)have received extensive attentions due to their outstanding performances in high energy physics and high magnetic fields.However,the HTS tapes are inevitably subjected to mechanical and magnetic forces in the processes of fabrication,heat treatment,cooling,coil winding,magnet operation,etc,which always induce the obvious variations of superconducting properties and even lead to irreversible degradation of superconducting critical properties.The engineering application of HTS tapes therefore is seriously limited.These problems are directly related to the the mechanism of mechanical deformation effect on multi-field performances and practical engineering applications of superconductors,which has become a popular issue in the field of superconducting science and technology application.In this thesis,the mechanical behaviors of HTS tapes and their influences on the superconducting critical characteristics are systematically studied from both macro and micro aspects by comprehensive utlize of quantitative numerical simulation,theoretical modeling and experimental testing methods.Firstly,in order to study the stress and strain states of YBCO coated composite superconducting tapes,an efficient 3D/2D mixed-dimensional finite element(FE)numerical model is developed by taking consideration of the structural characteristics.The analyses of the elastic-plastic mechanical behaviors during the whole process from the fabrication to external load are realized.The calculated results show the constituent layers of YBCO HTS tapes experience strong thermal residual stresses.Particularly,there is compressive residual stress in YBCO layer due to the mismatch of coefficients of thermal expansion.The simulations of uniaxial tension and bending show the constituent layers of YBCO HTS tapes including the silver protective layer,the copper coated layers,the Hastelloy alloy substrate,the YBCO superconducting layer and the buffer layer achieve the yield limit successively,and the calculated results are in good agreement with the experimental results.Further more,another efficient 3D/2D mixed-dimensional FE numerical model for delamination failure of YBCO coated composite superconducting tapes is established based on the cohesive zone model.Using the model the quantitative simulation of the mechanical behaviors of the YBCO HTS tapes under the transverse tensile load can be realized.The calculated results show when the delamination occurs there are two stresses concentration regions,one is located at loaded edges,and the other is located at crack tip.In addition,the results indicate that the location of the delamination is related to the interface strength.Both the the profile of transverse tensile loading surface and the initial defects of YBCO HTS tape have significant influence on the delamination strength.The 3D/2D mixed-dimensional models perform sufficient accuracy and much higher computational efficiency than the full 3D model.Thirdly,the experimental and theoretical studies on the critical current degradation of Bi-based HTS tapes under three kinds of basic mechanical deformation modes: uniaxial tension,bending and torsion,are carried out.The critical current characteristics under these three kinds of basic deformation modes are studied throuth self-designed cryogenic mechanical-electro coupling superconducting test system.The observations show there are critical strain values for each deformation mode,when the strain is smaller than corresponding critical value the critical current varies with an reversible process and no obvious degradation occurs,while the internal strain of the material is greater than the corresponding critical value,the critical current degrades significantly with an irreversible process.The degradation of critical current in Bi-2223 tapes under tension strain is much greater compared to those of bending and torsion strain.Additionally,From the perspective of the experimental observation of the current degradation phenomena and the mechanism of the damage of superconducting filaments in the cross section of the composite superconducting tapes,a general phenomenological degradation model based on the damage theory of brittle fiber reinforced metal matrix composites and Weibull's distribution function is developed,which can explain the critical current degradation behaviors with uniaxial tension and compression,bending and torsion three kinds of deformation modes.There show quite good agreements between the theoretical predictions and experimental observations on degradation of the critical current of Bi-2223 superconducting tapes.And it is expected to be applied to the coupled deformation modes of superconducting tapes.Finally,in order to reveal the mechanism of the mechanical deformation effect on the superconducting critical properties from a microscopic point of view,the mechanism of deformation effect on superconducting properties and critical temperature under hydrostatic pressure is carried out with specific components YBCO HTS-YBa2Cu3O6.95.The variation of microstructure and microscopic parameters dependence of critical state of YBCO HTS is studied based on the first-principles calculation,combined with bond valence sum(BVS)calculation and hole concentration analysis.Numerical simulation results show that the apex oxygen which connects the CuO2 plane and the Cu-O chain shifts towards CuO2 plane with the increase the pressure,and buckling angle of CuO2 plane,Cu(2)-O(2,3)-Cu(2),is directly related to the superconducting critical temperature.The hole concentration in CuO2 plane increases with the increase of applied pressure,which can be reduced as a self-doping process and results influence on superconducting properties.Based on the characterization of these microscopic mechanisms,a modified critical temperature model is developed by mean of self-doping process,and the asymmetric relationship between critical temperature and pressure is revealed.The predicted results exhibit good agreements with the experimental data in the literature. |
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