| Superconducting magnets occupy small volume,can save a lot of energy,and produce high magnetic field,so they have strong practical value and are widely used in industrial production,medicine,scientific research and other fields.As an electric insulation material,epoxy resin is used to encapsulate superconducting magnets.It is capable of limiting the movement of superconducting coils under electromagnetic force and it also can provide mechanical strength,conduct heat,etc.It is very important for stable operation of superconducting magnets.However,there are some practical problems for encapsulated low and high temperature superconducting magnets.Epoxy resin has low strength and poor toughness,which lead to cracks.It has poor adhesion property,which leads to debonding.Also,the thermal conductivity of epoxy resin at low temperature is extremely low.Therefore,Nb3Sn and NbTi low temperature superconducting magnets are easy to quench.After a quench,a lot of manpower and material resources are needed to reconstruct the superconducting environment,and even the superconducting coils may be damaged.For high temperature superconducting magnets such as YBa2Cu30x(YBCO),the main problem is that YBCO tapes have multilayer structure and they are prone to performance degradation caused by epoxy resin,limiting seriously the engineering applications of high temperature superconducting magnets.Therefore,there is an urgent need for study on the effect mechanism of epoxies on the performance of YBCO superconducting magnets and solving method of the performance degradation.In order to solve the quench problem of low temperature superconducting magnet caused by epoxy resin,this thesis prepared epoxies with low viscosity,good mechanical properties and high glass transition temperature for Nb3Sn superconducting magnet by two ways of formulation design and filling two-dimensional boron nitride nanosheets.By adding surface modified high thermal conductivity fillers,epoxy composites with high thermal conductivity and high adhesion property at low temperature were prepared for NbTi superconducting magnets.In order to solve the problem of performance degradation of YBCO high temperature superconducting magnets,the performance effects of epoxy encapsulation on YBCO tapes under two scenarios of thermal cycles and quenches were studied.Mechanism of the performance degradation was revealed by microstructure observation.Thermal stress distribution was calculated using a model.A method was proposed to slove the performance degradation of YBCO superconducting magnets.The main research contents and conclusions are as follows:(1)In order to solve the low strength problem of epoxy resin,key properties of four current epoxy resins used for superconducting magnets were tested firstly in this thesis.The epoxy resin with best comprehensive performance has 7.6%fracture strain and 60.5 MPa tensile strength.On this basis,a variety of new epoxy formulations were designed by selecting resin matrix with low viscosity,and selecting curing agents,diluent and bonding agent for improving properties.The effects of various components on the properties of epoxy resin were studied.A epoxy with good toughness,tensile strength improving greatly to 83.1 MPa,high glass transition temperature and low viscosity was prepared by adding two curing agents at same time.The new formulation can reduce the generation of cracks and thus reduce the likelihood of magnets quench.It can be used for Nb3Sn low temperature superconducting magnets.(2)In order to prepare epoxy resin with better toughness,two-dimensional boron nitride nanosheet with high strength and elastic modulus,large specific surface area,high thermal conductivity and excellent electrical insulation property was selected as toughening filler.By ultrasonic treatment,planetary centrifugal mixing and adding fumed Al2O3 filler,problems of agglomeration and deposition of the boron nitride nanosheets were solved.Epoxy composites with nano fillers were successfully prepared and the fillers were uniform dispersion.Test results showed that adding 1 wt%fumed fillers fully promoted dispersion of the nanosheets.Compared to boron nitride fillers with other shapes,the nanosheet had better toughening effect.Fracture strain increased by 48.7%compared to the epoxy matrix when content of the nanosheet was 0.5 wt%.Microstructure observation revealed that the main toughening mechanism was crack deflection at low content.At high content,the toughening mechanisms were generation and growth of microcracks,and cavity effect.The nanocomposite with good toughness and comprehensive properties can be used for Nb3Sn low temperature superconducting magnets to solve the problem of generating cracks of epoxy resin.(3)For improving the thermal conductivity of epoxy resin,this thesis successfully prepared epoxy composites filled with boron nitride single-crystal micro platelets by planetary centrifugal mixing method.Surface modification method of dopamine was used to improve bonding strength of the interface between fillers and epoxy matrix.The composite with 20 vol%boron nitride micro platelets increased the thermal conductivity from 0.24 W/(m·K)of pure epoxy to 1.17 W/(m·K)at room temperature,and the composite was 1.92 times the thermal conductivity of pure epoxy at 100K.Lewis-Nielsen model was modified to predict thermal conductivity of the composites accurately.The errors were reduced from 29.5%,40.5%and 31.6%of original model to 10.7%,14.3%and 10.4%of modified model for the composites with 5 vol%,10 vol%and 20 vol%fillers,respectively.Shear strength of the composite filled with 20 vol%surface modified fillers increased from 4.3 MPa of pure epoxy to 7.6 MPa.The composite with high thermal conductivity and high bonding performance can solve the problems of difficulty to conduct heat and debonding of the epoxies at low temperature.Therefore,it can be used for NbTi low temperature superconducting magnets.(4)In order to solve the performance degradation problem of YBCO high temperature superconducting magnets,the effects of epoxies on the performance of YBCO superconducting tapes were tested.The maximum allowable temperature during quenches was over 690 K for the tape without epoxy resin.For the tape with unfilled epoxy resin,critical current degraded close to 45%after the third thermal cycle.Although thermal cycles had little effect on performance of the tape with filled epoxy,serious damage occurred during the quenches,and the maximum allowable temperature was less than 150 K.The microstructure analysis and stress calculation model revealed that the performance degradation of the tapes was mainly due to delamination caused by concentration of shear and peeling stresses at the edges of tape,and melt of the tape caused by high local temperature.In addition,a method of coating mold release on surface of the tape was proposed and experimental results showed that the method was able to solve the performance degradation problem of YBCO superconducting tapes. |
PDF Full Text Request