Study On The Fabrication And Mechanical Properties Of YBCO Films And Coated Conductors By Doping With Graphene Oxide

Superconducting materials,based on high critical transition temperature,zero resistance,Meissner effect and Josephson effect,have widely application prospects in electrical and electronics engineering.For example,YBCO high temperature superconducting?HTS?films have been used in various scientific instruments such as microwave devices,infrared radiation detectors and weak magnetic field measurement,and these scientific instruments have obtained important application value.However,the intrinsic brittleness,strong anisotropic behavior and degradation of critical transition temperature and critical current caused by mechanical deformation seriously affect and restrict the further large-scale application of YBCO superconducting material.For this reason,a new concept of coated conductor is put forward,that is,the brittle YBCO material is deposited on flexible metal substrates,which greatly enlarges the application space of YBCO material.It has been commercially fabricated and gradually applied in superconducting cables,motors and magnets and so on.For YBCO coated conductors?CCs?,delamination strength and axial irreversible strain?i.e.,the critical current cannot recover to the initial value after unload?are the core parameters.It is found that the main failure mode of YBCO CCs is interlaminar delamination.Therefore,how to effectively enhance interlaminar delamination strength has become a key issue.In addition,the traditional enhancement of irreversible strain is introducing a protective layer or increasing the thickness of the protective layer,but the enhancement is limited.This Ph.D.thesis attempts to introduce graphene with excellent mechanical properties and current carrying capacity into the fabrication of YBCO,which improves the critical current density of YBCO under magnetic field and decreases the anisotropy.At the same time,controlling the growth temperature of the Ag layer during the fabrication greatly improves the interlaminar delamination strength of CCs.Finally,the effects of graphene doped and the mechanics of substrate on the overall mechanical properties of YBCO CCs are discussed,the main contents are as follows:Firstly,the effect of GO doped on the superconductivity of YBCO is studied.GO doped YBCO films are prepared.On this basis,the critical temperature of GO doped YBCO films is measured.It reveals that GO doped can modulate the critical temperature of YBCO films.TEM images show that GO evenly distributes as carbon nanoparticles in YBCO.The critical current density of GO doped YBCO films at different temperature-magnetic intensity-magnetic angles is measured.The results show that carbon nanoparticles below 35 K can act as effective pinning centers to increase the pinning force,thereby increasing critical current density and reducing the anisotropic behavior.Secondly,the effects of thermal stress and compressive strain on the critical current density are studied.The full field surface curvature of YBCO films is measured by using a coherent gradient sensing?CGS?system and a vacuum cryogenic cooling system.The stresses of YBCO film surface are obtained by combining the stress-curvature constitutive model of the film-substrate system with non-uniform temperature distribution.The results show that the thermal stresses of the film surface are non-uniform.Compared with 280 K,the radial stress and hoop stress is larger than the interfacial shear stress at 60 K.During the heating process,the radial and hoop stresses decrease with the increase of temperature,and the stresses decrease rapidly in the early stage of heating.It is also found that GO doped has little effect on the thermal stress of YBCO films.Then,the variation of critical current density of YBCO thin films with compressive strain is obtained by using the self-developed liquid nitrogen cooling-compression-current testing system.Thirdly,the fixture suitable for the preparation of short sample CCs is designed.YBCO CCs are prepared by DC sputtering,magnetron sputtering and electroplating.The optimum preparation parameters of YBCO CCs are obtained:the optimum growth temperature of YBCO on LaMnO₃ buffer layer is 720 ^oC,the critical current density is 1.2 MA/cm2;when the temperature is 100 ^oC-200 ^oC,the grain distribution of Ag layer is uniform and the structure is compact,Ag and YBCO bond well.When the distance between the cathode and anode is 9 cm,6 mm*100 mm*3 CCs,the electroplating current is 1 A and time is 30 min.Then,the surface of the electroplating copper is smooth,the microstructure is dense,and the thickness is about 40?m.At77K and self-field ambient,the critical current density of CC is 1.2 MA/cm² which is equivalent to the commercial product of SuperPower Company.Finally,the effects of delamination strength and axial tension deformation of self-fabricated superconducting CCs on critical current are studied.Using self-designed welding device and existing delamination strength testing device,the delamination strength of CCs at room temperature and liquid nitrogen temperature are tested.The results show that the average delamination strength of superconducting CCs fabricated with the optimum parameters can reach 50 MPa at room temperature,while the delamination strength of SuperPower CCs is only 30 MPa.Through Weibull cumulative density function analysis,the reliability of the instrument and method in this thesis is verified.The effects of cutting,YBCO layer thickness,Cu layer thickness and graphene oxide doping on the the degradation process of critical current under axial tension are studied.Based on the micro-morphology of each layer and the degradation law of critical current,it is found that the irreversible strain of CCs depends on the elastic strain limit of the Hastelloy alloy substrate.A new idea to improve the irreversible strain of CCs by improving the elastic strain limit of Hastelloy alloy is proposed and preliminary experimental exploration is carried out.