Investigations On Magnetization And Mechanical Behaviors Of High Temperature Superconductor And Array

Since the discovery of high temperature superconductors(HTSs),they have attracted much attention in the fields of condensed matter physics and material sciences due to their characteristics of high critical temperature,large critical current density and strong trapped field.Various electromagnetic and magnetic devices using HTSs as core components have produced huge economic benefits and tremendously promoted social advance.Under electromagnetic environment,the HTSs always suffer huge electromagnetic body force.The body force may lead to the extension of the interior crack,which finally engenders fracture and threatens the mechanical stability of the whole structure.In this context,this dissertation mainly focuses on the electromagnetic properties and mechanical behaviors of several superconducting structures.Firstly,based on the minimum magnetic energy method and critical state model,numerical simulations of magnetizations of arrays composed of several coaxial superconducting rings in uniform magnetic field are completed.The influences of arrangement mode,gap dimension,number of rings and critical current density on magnetizations are discussed.The results show that when the gap is small,there is relatively strong electromagnetic coupling effect between the superconducting rings.So the capacity of resisting the external magnetic field of the superconductor is weakened,and the whole structure is more easily magnetized in external field.Besides,when several superconducting rings are radically arranged or when the radial gap is widened,a larger saturated magnetization and trapped field can be obtained.Increasing critical current density nearby the external wall of superconducting ring has the same effect.When the external field decreases from the maximum value,tensile and compressive body force coexist in the ring body and cracks easily appear.Subsequently,the same method is employed to numerically analyze the influence of geometrical parameters on the levitation force for several axial maglev systems composed of superconducting rings and permanent magnet rings.The result indicates that the levitation force is proportional to the size of SC or PM,and the maximum levitation force is correlated with the ratio of the external radius of superconducting ring array to that of permanent magnet ring array.When the ratio is about 1,the system gets its maximum levitation force.Finally,the distribution of induced currents and flux line of typical radical maglev systems are presented.It is found that there is repulsive interaction between adjacent permanent magnet rings which have opposite magnetization direction.This interaction leads to no induced current appearing in special region of superconducting ring and the radical repulsive force has an inhomogeneous distribution along the ring.Some reasonable arrangements are provided for the future design of similar maglev systems,such as magnetic bearings.Secondly,the magnetization and electromagnetic mechanical properties of high temperature superconducting strip with cavity defect in different magnetic field are numerically analyzed.The distribution of the induced currents in strips in vertical magnetic field is obtained by using minimum magnetic energy variation method.The dependence of currents,magnetostriction and stress distribution on dimension and location of cavity defect is presented.The results indicate the dimension and location of cavity defect affect the value and direction of magnetization.If the superconducting strip has a geometrically asymmetric cross section,the induced currents distribute asymmetrically in strip and horizontal component of magnetization appears.Due to stress concentration,maximum tensile stress appears in the vicinity of the defect edge in different stages of magnetization.As for the superconducting strip in oblique magnetic field,the changes of value and direction of magnetization in different oblique angles are presented,too.The analysis results show the induced current in oblique magnetic field has a non-monotonic penetration in superconductor.Due to uneven distribution of electromagnet forces in strip,the strip suffers a torque which is correlated with the angle of external magnetic field.Thirdly,minimum magnetic energy functional method is applied to analyze the features of induced current and magnetic field distribution in a strip with transport current in external field.Depending on the relative sizes of current and magnetic field,induced current,magnetic induction and electromagnetic body force in different loading process are presented.The features of internal stress and strain distributions are obtained by taking finite element method.These results suggest that the magnetic fluxes distribute asymmetrically at both sides of superconducting strip,and the stress in strip is different when boundary constriction condition is different.For the superconducting strip carrying direct current in alternating magnetic field,the induced current penetrates strip monotonously and the magnetization procedure is irreversible.For the superconducting strip carrying alternating current in alternating magnetic field,the induced current has a non-monotonic penetration when the current and external magnetic field is large.Finally,the magnetization of different arrays composed of high temperature superconducting strips in an increasing oblique external field and the dependence of arrangement mode,gap dimension,and number of strips on magnetization are presented.The results indicate that magnetization of array increases to a saturated value as the external field increases to maximum.The movement of external magnetic flux and penetration procedure of induced current in superconducting strips show non-monotonic feature.The saturated magnetization of isolated array is independent of gap between strips or amount of strips.For two arrays with same parameters in an oblique field,the value of saturated magnetization of isolated array is smaller than the connected one.