Investigations On The Thermal Stability And Mechanical Response In The Superconductors

The superconductors have wide application prospects in the fields of energy,communications,biomedicine and transportation due to their excellent electromagnetic properties.However,they usually operate in extreme environments such as low temperature,high magnetic field and so on.Their engineering applications have long faced two major challenges,and one is that the energy loss of the superconductor will cause the temperature rise of its local location,which have a strong impact on thermal stability.The second one is that the mechanical deformation of the superconductor not only affects the current-carrying capacity,but also may cause mechanical failure and damage of the superconducting devices.Therefore,studying the thermal stability and mechanical properties is the basis for ensuring the safe and stable operation of various superconducting devices.The thesis mainly studies the thermal stability and mechanical response of superconductors for the real operating environment based on theoretical analysis and numerical modelling.Firstly,we focus on flux jump in the bulk superconductor during the magnetization,and study the mechanical behavior of superconductor during the flux jump.The electro-magnetic-thermal-mechanical model of MgB₂ cylindrical superconductor is built to simulate the change of temperature and electromagnetic bodyforce during dynamic loading of external magnetic field.The stress and strain during the flux jump are analyzed.The results indicate that strain jump is consistent with temperature jump.However,the peak of stress jump has a difference with the temperature jump.Besides,the strain jump caused by temperature change is lager than the strain jump caused by electromagnetic force.Secondly,the termination resistance of YBCO tape can change the current distribution in the stacked tapes,thus it can also affect the quench characteristics.A two-dimensional quench model of four stacked tapes is established by coupling the circuit equation,magnetic field equation and heat conduction equation.The effects of termination resistance on the quench behaviors of stacked tapes are analyzed with taking into account different transport current loading,size and location of the heater.The mechanical response of the stacked tapes is also studied in the quench process.Then,the mechanical response of the MgB₂ coils are studied under the combined action from winding to cooling and Lorentz force.In order to accurately analyze the mechanical behaviors of the coil which is wound after heat treatment,it is necessary to consider the strain caused by winding,cooling and Lorentz force.The results indicate that the mandrel material will affect the stress and strain in the winding process.Thermal strain caused by the cooling progress dominates the total strain,and the material and size of the mandrel have a larger impact on thermal strain.The Lorentz force has a little contribution for the mechanical deformation of the coil in low magnetic field.Finally,the contact force between the triplets in the CICC conductor is derived by combining the thin rod theory and the two-scale method,and the value of constriction resistance between the triplets is given.The contact between the triplets is simplified as a series of point contacts.After estimation through the Hertz contact and AF contact models,the contact between micro-bulges is treated as completely plastic deformation,and the contact radius of the contact point is obtained.The value of constriction resistance between the triplets is obtained based on the contact force.The numerical results show that the surface roughness and the material properties of the strands are the key factors which affect constriction resistance and film resistance.In addition,different cabling patterns in the preparation process of CICC conductor will affect the value of constriction resistance.