Research On Fabrication And Application Of High-performance REBCO Bulks For HTS Maglev

High-temperature superconducting(HTS)Maglev has become a research hotspot in Maglev field for its unique advantages of static levitation,passive stability,and non-inherent magnetic resistance on the travel direction.The successful commissioning of the world’s first high-speed HTS Maglev engineering sample vehicle and test line has demonstrated broad application prospect of HTS Maglev.Meanwhile,the levitation system is the key to producing levitation and guidance force for HTS Maglev vehicle and it is mainly composed of the permanent magnet guideway and the low-temperature Dewar with the REBCO bulks encapsulated inside.Due to the complicated fabricating process and high technical threshold of producing REBCO bulks,the imported bulks are limited in supply while with expensive prices.Under the dual background of escalating international struggle situation and increasing domestic application demand for REBCO bulks,it is urgent to explore a mature,independent and controllable technical route for fabricating the high-performance REBCO bulks for HTS Maglev.In this thesis,a series of studies had been done and some achievements had been made.Firstly,the batch fabrication process of high phase purity REBCO precursor powder was studied.The feasibility of a dry mixing technology was proposed and experimentally verified,and the optimal parameter scheme was determined to achieve uniform mixing of primary raw materials.The research also investigated and defined the important parameters of solid-phase sintering for fabricating high phase purity RE123 and RE211 powders,also discovered an efficient grinding method of high hardness RE123 polycrystal.The result was verified that the phase purity of self-made precursor powder reached the international advanced level.This thesis determined a series of optimization technologies and processes,which compared with the traditional method,greatly improved the fabricating efficiency of REBCO precursor powder and the target of batch fabrication had been achieved.Secondly,the technology of fabricating high-performance REBCO bulks by the Topseeding Melt Texture Growth method was studied.The study determined a temperature program suitable for the growth of YBCO bulks in various sizes also designed and confirmed a slow cooling rate optimization scheme.The results showed that adding a liquid buffer layer at the bottom of the precursor pellet can effectively enlarge the temperature window of crystal growth,alleviate the problems of liquid loss and size shrinkage and contribute to reducing the grain size of Y211 particles.Meanwhile,Sm BCO bulks and its seeding crystals were successfully fabricated,and the difference in thermal stability between Sm BCO bulk seed and Nd BCO film seed was compared in the study.Also,High-performance YBCO bulks were successfully fabricated by self-made Sm BCO seeds and their magnetic levitation performance reached an advanced level in China through experimental measurement.Finally,the correlation between the density of YBCO bulk and magnetic levitation performance had been studied.A large number of YBCO bulk samples were measured over the permanent magnet guideway to study their magnetic levitation forces and relaxation.It was found that the density of YBCO bulk,as a physical quantity reflecting the microstructure of the material and irrelevant to the size and shape,is closely related to the magnetic levitation force.For the YBCO bulks fabricated by the same method and process,the higher density means the higher magnetic levitation force per unit area and the lower relaxation rate.