Study On Low Temperature Friction Properties Of Superconducting Materials

The International Thermonuclear Experimental Reactor(ITER)Program is one of the most influential and far-reaching international scientific research cooperation projects in China,aiming to verify the possibility of magnetically constrained controlled nuclear fusion reactors.The implementation principle of the device is to generate a strong magnetic field by a Tokamak magnet device wound by a CICC conductor(NbTi or Nb3 Sn wound tubular cable)to restrain plasma,thereby achieving a controlled nuclear fusion reaction.Among them,the superconducting magnet is subjected to the pre-tightening force existing in the coil and the lorentz force generated by the strong magnetic field during the operation of the superconducting magnet.When the superconducting coil is subjected to ten thousands times of cyclic compressive stress,the strands are mechanically deformed to generate friction and abrasion.However,the heat capacity of the magnet at 4.2 K is only 1/4000 at normal temperature,and the small relative sliding between the strands generates a heat source sufficient to make the magnet quench.Therefore,the friction characteristics of the superconducting strand and Cu wire in the low temperature medium become key scientific issues of stable operation in large structure.In addition,the micron-sized NbTi filament is the main part of the NbTi/Cu superconducting composite wire.The design,preparation and surface wear of the material seriously affect the electrical performance degradation of the superconducting material,and the low temperature of the micro-scale material is carried out.The research working on mechanics and friction characteristics have important scientific significance and engineering application value.The research in this paper will have an important significance for understanding the low-temperature friction characteristics of practical superconducting materials,and can provide a powerful theoretical and experimental basis for the large-scale application of superconducting materials.In this paper,the experimental and numerical calculations are carried out for the development of low-temperature friction testing device for strand structure materials,the friction characteristics of micro-scale NbTi materials,the friction characteristics of NbTi/Cu superconducting strands and the friction characteristics of YBCO high-temperature superconducting materials.The main results achieved in this paper are as follows:Firstly,a set of high-precision friction characteristics testing system for micro-scale strand materials in normal and low temperature environments was developed.The system can test the tribological properties of micro-scale strand materials in different medias,positive pressures and sliding rates.The displacement and resolution of force are 0.01 mN and 0.03 um,respectively.Aiming at the friction performance test requirements of NbTi/Cu superconducting strands,a low-temperature friction test device for small-angle contact strand materials was developed,which can test the cyclic friction characteristics of strands.The results showed that the device meets the requirement of low temperature friction performance test of strand material.Secondly,The friction characteristics of micro-scale NbTi superconducting materials were studied.Firstly,NbTi core wire and micron-sized copper fiber were tested by self-built fiber material low temperature mechanical property testing device.The results show that the elastic modulus,fracture strength and yield strength of the material in the low temperature medium are significantly enhanced,and non-contact is adopted.The laser roughness meter measures the surface of the copper tube in the normal temperature and low temperature medium respectively,and finds that the surface roughness of the material in the low temperature medium is significantly enhanced.Secondly,the NbTi core wire is in different mediums(water,air and liquid nitrogen solution).A detailed comparative study of the friction characteristics reveals that the friction of the NbTi core wire in liquid nitrogen is significantly higher than that in air and water,and has a significant stick-slip phenomenon,and the presence of wettability in the aqueous medium can significantly reduce the friction.In addition,the micro-scale NbTi core wire in air and water has obvious scale effect,but not in liquid nitrogen.Finally,the friction of NbTi core wire in different media is improved with time by improved spring-slider model.The numerical calculation of the variation curve is in agreement with the experimental results,and the key factors affecting the low-temperature friction characteristics such as modulus and thickness And the wettability of the studies were carried out,and the results showed that the surface roughness of the material modulus curve changes such that the frictional force in the low-temperature medium strands of significant stick-slip effects.Again,The friction characteristics of NbTi/Cu superconducting strands were investigated.Firstly,the experimental and numerical analysis of the mechanism of stick-slip phenomenon in NbTi/Cu superconducting strands in low temperature medium was carried out.The results show that the roughness of the surface of the material mainly affects the fluctuation amplitude of the friction curve without changing the slope of the stick-slip.The modulus of the material mainly affects the slope of the stick-slip during the friction process,but does not change the fluctuation amplitude of the friction curve.Secondly,a systematic experimental study on the cyclic friction characteristics of the material is carried out,and the roughness of the surface of the material is found along with the number of cycles.Increasing and decreasing gradually,in addition,the frictional amplitude of the strands in the air and liquid nitrogen medium varies with the number of cycles,and then a new method for determining the wear cycle using the frictional vibration amplitude in the low temperature medium is proposed.Finally,Influencing factors such as current carrying,sliding rate and positive pressure during the relative sliding process of superconducting strands and copper tubes are studied separately..Finally,The friction characteristics of YBCO high temperature superconducting materials(bulk and film)were studied.Firstly,the YBCO superconducting block was tested in room temperature and liquid nitrogen by small angle contact friction tester.It was found that YBCO superconducting block has good low temperature friction performance,can effectively inhibit the occurrence of stick-slip phenomenon,and when superconducting block When the superconducting state is applied,the external magnetic field is applied,and the frictional force is increased due to the viscous force generated by the flux vortex motion.Secondly,it is found that the frictional properties of the material in the low temperature medium are mainly determined by the wetting property when the weight of the weight is small.When the weight of the weight is large,it is determined by the properties of the surface of the material;then the macroscopic friction behavior of the YBCO superconducting film is experimentally studied by Rtec multi-functional friction and wear tester,and the results show that the film has good wear resistance;Finally,the ultra-high vacuum variable temperature scanning probe/electron microscope was used to explore the microscopic friction properties of YBCO superconducting thin films.It was found that the friction of superconducting thin films increased with the decrease of temperature.