Scientific Instrument Development And Its Application Research On High Temperature Superconducting Materials' Properties Under Extremely Low Temperature-electro-mechanical-magnetic Multi-fields

High temperature superconducting materials,with high critical transition temperature,high critical magnetic field and superior current-carrying capability in high magnetic field,have a wide application prospects,such as in electrical power systems,high magnetic fields research,transportation and other domains.In above applications,superconducting materials are operated at low temperature,high magnetic field and large current complex environments,inevitably.Thus,the thermal stress and/or the electromagnetic force will cause deformation of superconducting materials,thereby reducing the critical current of the material,which will bring great potential safety hazard in superconducting devices.So the research of the critical current characteristics of high temperature superconducting materials under extremely low temperature,electro-mechanical-magnetic multi-fields environment have become the key scientific problems to optimization of material preparation process and further promote the practical application.In addition,limited by the producing length of high temperature superconductor,joint has become the key component in application,and thus to carry out preparation technology and mechanical-electrical properties experimental study of joint under multi-fields have great importance in scientific research and engineering application.Focus on the two problems mentioned above,the thesis main achievements as the following:Firstly,a critical current measurement instrument(first generation)under the electro-mechanical-magnetic multi-fields,which is directly cooled by the liquid nitrogen has been constructed by ourselves.This apparatus can be used to study the multi-fields characters of the critical current and its current carrying stability of high temperature superconducting materials,with high measurement precision.And then experimental measured the multi-fields characters of the YBCO coated conductor(YBCO CC)and the first generation tape of 2 2 2 3 10-xBi Sr Ca Cu O(Bi-2223),respectively.And some important parameters of those materials,such as critical current under self-field at 77 K,the electro-mechanical-magnetic multi-fields properties,irreversible strain,current-carrying stability,were obtained.Secondly,based on the first generation instrument,a set of test fixture which can be used to study the change of the critical current of superconducting material caused by transverse compression was designed.The transverse compression characteristics of YBCO CC was tested,and the degradation of the critical current was explained by n value.Meanwhile,a device for the inter layer delamination strength of the superconducting material under multi-fields was developed.The experiments about mechanical delamination strength of the YBCO CC at both of room temperature and liquid nitrogen temperature,and also mechanical-magnetic,electromechanical characteristics of the YBCO CC at 77 K,were carried out.And the analysis of three-dimensional finite element method was adopted to reveal the delamination position and mechanism of YBCO CC.Then,put forward a kind of new superconducting joint welding process,using this new process,a contact resistance of 8.35n? was obtained at the welding length of 8cm,which is the lowest value in the literature.Meanwhile,the experimental results show that the welded joints have superior mechanical and electrical properties,and which similar to the original tape.Then,the influence of the key parameters on the performance of the joint,such as the welding pre-pressure and welding length,were systematically studied,and then the optimal process parameters were obtained.Also,the degradation of the critical current caused by lateral compression and/or delamination stress were studied,and the results of the experimental data show that the essential reason of the critical current degradation is the concentrating stress caused by YBCO superconductor joint's edge effect,which damaged the YBCO layer.Based on the results above,the corresponding strengthening scheme was proposed and its validity were proved by experiments.Finally,after successfully solved the technical problems of transmission large current into the confined space and Joule heat by a new high temperature superconducting binary current lead,a visualization instrument(second generation)to investigate the electromechanical properties of high temperature superconducting tapes under multi-fields has been constructed,which is directly cooled by G-M cryocooler.The equipment can fulfill mechanical and electromagnetic properties test of superconducting material,in ultra-high vacuum,small space,and which is capable of realizing continuous temperature control within 6.59 K to 300 K,the maximum test current is 600 A,the maximum tensile strain is 20%,under perpendicular magnetic field 0~5T.Meanwhile,combination the magneto optical system realized the in-situ and real-time observation of the internal damage of the superconducting material and its influence on the critical current in the process of deformation.Finally,using the device,the multi-fields performances of YBCO CC and Bi-2223 tape were tested,and the rationality of the theoretical prediction model was analyzed according to the experimental data.