The Inverstigation Of Flux Avalanche And Mechanical Behavior In High Temperature Superconducting Materials

Due to its excellent current carrying capacity and high critical field,high temperature superconductors have attracted wide attention since they were discovered,and have widely applied prospects in the fields of medical technology,nuclear fusion devices and so on.The working environment of superconducting materials is extremely low temperature,high current and strong magnetic field.Thermal stability and mechanical characteristics in extreme environments have become the key factors,which restrict the safe operation of superconducting structures.In extremely low temperature environment,the motion of flux vortices in superconductors maybe lead to the thermomagnetic instability.For example,the flux jumps usually cause the sharp change of the distributions of magnetic field and current in superconductors,accompanied by apparent jump of temperature,which will threaten the stability of superconducting materials and structures.Under the high field,the superconductors will withstand extremely high electromagnetic body force and thermal stresses,which may also result in permanent damage of the material,and thus it can directly threaten the safety of the superconducting structure.This paper aims at the thermal stability and mechanical characteristics of superconducting materials.Based on the Fast Fourier Transform method,the magnetic flux avalanche behavior in high temperature superconducting thin films is studied,the mechanical response in superconducting thin film under external field is discussed,and the fracture problems of Bi2212 superconducting composite wires are also analyzed under electromagnetic force.Firstly,the thesis introduces the basic equations which describe the characteristics of the electromagnetic field in superconductors,and gives the calculation process about the Fast Fourier Transform method to calculate the magnetic-thermal coupling equations.On this basis,the flux avalanche behaviors in thin films with non-uniform critical current,internal cracks and multi-films are investigated,respectively.The effects of magnetic field ramp rate,environment temperature,magnitude of external magnetic field and notch on flux avalanche are considered,and the avalanche in uniform and non-uniform films are compared.For films containing the internal cracks,we analyze the relationship between the flux avalanche and crack's location,angle and length.The results of multi-films show that the current and magnetic field in different films can interact with each other.Therefore,the magnetic flux avalanche is easier to occur on the adjacent sides of the two films.Secondly,the mechanical response in the film under external magnetic field is studied.Based on the Fast Fourier Transform method,the current and magnetic field distributions in the superconductor can be obtained,and thus the electromagnetic body force distribution can also be obtained.Combining with the Finite Element method with the electromagnetic force in the superconducting film,the stress of circular and square films in the descending and ascending fields is studied.The influences of some factors such as notch,inclusion and substrate on stress distributions of the films are analyzed.During the magnetic field decreasing process,the tensile and compressive stresses both exist in the film,which may cause the film damage.In addition,the thermal stress during the magnetic flux avalanche is studied.At the initial stage of flux avalanche,the thermal stress in the film is only compressive stress.When the flux avalanche occurs completely,tensile stress appears in the film.Meanwhile,the stress is mainly concentrated in the avalanche area.Finally,the double cantilever beam bridge model is established to study the fracture behavior in the Bi2212 wire.We have established different models,such as symmetric model,asymmetric model and four-layer beam structure model to study the fracture behavior in Bi2212 wires.The position and size of bubbles,and interlayer of the beam are considered.The ratio will change the mechanical properties and current-carrying capacity of the wire.The numerical results show that the position of bridge has different effects on strain and strain energy release rate of double cantilever beam.In summary,the thesis studies the flux avalanche behavior and mechanical response of high temperature superconducting materials,which provides a good theoretical basis for the safety operation of superconductors in extremely low temperature and high current.The numerical result has a certain value for the design and safe operation of superconducting electronic devices and structures.