Studies On Temperature Characteristics And Temperature Rise Measurement Scheme Of High-temperature Superconducting Bulk

High-temperature superconducting（HTS）material has unique characteristics of strong magnetic flux pinning,and the HTS pinned maglev technology utilizes this characteristic to realize the stable levitation and guidance of vehicles above the permanent magnet guideway（PMG）.The HTS maglev vehicle has technical advantages in principle and is one of the important directions for the development of high-speed maglev trains in China in the future.The onboard HTS bulk is the core of the HTS magnetic levitation system,and temperature is an important factor affecting the levitation and guidance performance of the HTS bulk.Therefore,this thesis focused on the temperature of the bulk and explored the relationship between the temperature and the levitation and guidance properties of the YBCO and Gd BCO bulks.Then,a scheme that can detect the severe TR of bulks was proposed,which provides an alternative way for the condition monitoring and early warning of the severe TR of the superconductor in the future HTS high-speed magnetic levitation device.Firstly,a test platform for magnetic characteristics at low temperatures was built based on the conduction cooling method,which can test the levitation and guidance performance of the HTS superconductor at different temperatures.The YBCO and Gd BCO bulks,which are widely used in the field of HTS magnetic levitation,were selected as experimental samples to explore the differences in levitation and guidance performance between the two bulks in the temperature range（60～77 K）lower than the temperature of liquid nitrogen（LN₂）.The results show that the levitation force and hysteresis effect of the levitation force can be improved by decreasing the temperature,but the effect is no longer obvious with the further decrease of temperature.When the temperature is lower than that of the LN₂,the levitation and guidance performance of YBCO is better than that of Gd BCO.Secondly,the differences in levitation and guidance performance between YBCO and Gd BCO in the temperature range（77～87 K）higher than the temperature of（LN₂）were explored.The results show that the levitation performance of Gd BCO is better than YBCO when the temperature is higher than that of LN₂.With the increase of temperature,the levitation and guidance performance of HTS bulks decreases,for YBCO bulk,the temperature inflection points at which the levitation and guidance performance decrease significantly are both around 82 K;for Gd BCO bulk,the temperature inflection point where the guidance performance decreases significantly is also about 82 K,but the temperature inflection point where the levitation performance decline dangerously lags behind YBCO bulk.Moreover,in the temperature range from the LN₂ temperature to the critical temperature,the reliability and stability of Gd BCO are better,that is,when the temperature inside the bulks increases,especially when the temperature rises violently（>82 K）,the mechanical properties of Gd BCO are more stable.Finally,aiming at the engineering application scenario of HTS maglev vehicles,the HTS coated conductors were introduced into the onboard levitation unit.Based on YBCO bulks,a variety of HTS coated conductor-bulk hybrid levitation configurations were designed by stacking and winding.Based on the violently fluctuating magnetic field background,the TR characteristics of hybrid levitation configurations under different magnetic field fluctuation frequencies were investigated,moreover,a nondestructive scheme was proposed to measure the severe TR of bulk.For the hybrid levitation configurations composed of single-seed YBCO bulk and coated conductor stacks,the TR of the stacks is larger and the response speed is faster when the bulk has a large TR.In practical application,if the corresponding safety measures are taken in time before the TR of the coated conductors to prevent the overall severe TR of the bulk,the irreparable loss can be avoided.This study will provide data reference for nondestructive measurement of TR in bulk and early warning of severe TR in the bulk.