Investigation On Thermodynamic Properties Of Superconducting Magnet New Packaging Materials And 3D Printed YBCO Bulk Materials

Superconducting magnet is the basis for the development of many large-scale frontier scientific and engineering devices such as particle accelerators,nuclear magnetic imaging,thermonuclear fusion reactors,etc.It is a high-tech field that developed countries in the world are competing to seize,and is a major national demand.In the process of developing superconducting magnets,besides the nonlinear constitutive relation of the interaction of critical temperature,critical magnetic field and critical current,the local mechanical deformation caused by extremely low temperature and strong electromagnetic field often leads to quench of superconducting materials.The formation and spread of a large amount of heat dissipation brings huge potential safety hazards to superconducting devices in service.For example,in 2008,the giant superconducting magnet of the European Hadron Collider exploded due to the rapid vaporization and expansion of liquid helium caused by the heat dissipation accompanying the quench process,resulting in the shutdown for 14 months and the economic loss of 60 million US dollars.Therefore,in the practical process of superconducting materials,the research on the thermodynamic characteristics of superconducting materials under extreme environments has become one of the most challenging basic topics in the development of superconducting magnets.In this doctoral dissertation,the thermodynamic characteristics of superconducting magnet packaging materials are studied firstly.A whole new idea of depositing zirconium tungstate(Zr W₂O₈)material with negative thermal expansion effect on graphene substrate with high thermal conductivity is proposed.Thus,the preparation and characterization of ultra-light composite materials with higher negative thermal expansion coefficient has been realized.Based on this proposal,the 3D printing preparation technology of superconducting bulks,which was first realized by the superconducting mechanics research team of Lanzhou University,was adopted.Through physical doping and structural optimized design,the thermal conductivity of3D printed superconducting bulks were significantly improved.The main results are as follows:(1)A new type of superconducting magnet packaging material with giant negative thermal expansion was developed.A new method of one-step hydrothermal synthesis was proposed to realize the preparation of ultra-light porous negative thermal expansion material with Zr W₂O₈ negative thermal expansion material as doping and graphene porous structure as skeleton.The density of this material is only 4.25g/cm~3,and its negative thermal expansion coefficient can reach 4.97 times that of pure Zr W₂O₈material,and the maximum is-44.7×10^-6K^-1.During the experimental measurement of the thermal expansion coefficient of this composite material,it is found that the thermal deformation of this composite material will increase with temperature's rising.However,when the sample in the measurement process is treated at constant temperature the thermal deformation will suddenly jump,and then even if the temperature is changed,the sample will not continue to produce deformation,which is a new phenomenon.SEM in-situ microscopic characterization and quasi-static uniaxial compression experiment at room temperature of the experimental samples reveal that the main reason for this new phenomenon is the thermal stress buckling of graphene sheets inside the three-dimensional structure.Subsequently,a mechanical model describing the buckling of laminated structures is established,and the calculated results are qualitatively consistent with the experimental measurements,which proves the reliability of the model.(2)A new method to improve the thermal conductivity of superconducting bulk materials prepared by 3D printing is proposed.By doping high thermal conductivity nanoparticles into the precursor printing paste of DIW 3D printing and customizing rheological and molding,high quality nano doping method was achieved.Therefore,the thermal conductivity of YBCO high temperature superconducting material was increased by 2.1 times at most.Then,a theoretical model of thermal conductivity of variable truss structure induced by thermal expansion is established.On one hand,the thermal conductivity of various microstructures,such as triangular,rectangular and hexagonal structures is given.On the other hand,the variation law of thermal conductivity of 3D printed YBCO superconducting material during temperature change is calculated,and the results are in good agreement with the experiment.On this basis,macro YBCO samples with microstructure units with high thermal conductivity are prepared by 3D printing method.The results show that the thermal conductivity of 3D printed YBCO superconducting bulk materials can be increased by 1.47 times at most by this optimized design method.(3)The AC loss of 3D printed YBCO superconducting gyroscope is significantly reduced by structural optimization.Numerical models for calculating AC loss of superconducting gyroscopes with different shapes are established.The results show that AC loss of superconducting gyroscopes with elliptical holes can be reduced by 58.49%compared with superconducting gyroscopes with circular holes under the same conditions.Subsequently,the superconducting gyroscope with elliptical holes was fabricated by DIW 3D printing technology.Experimental results show that the suspension rotation time of superconducting gyroscope with circular hole which is 120s can be significantly increased to 162s.In order to quantitatively describe the difference of levitation performance of superconducting gyroscopes with different masses,the concept of mass levitation time ratio is defined.It can be found that the levitation quality ratio of YBCO superconducting gyros with optimized structure is about 1.15 times that of traditional 3D printed superconducting gyroscope.