Research On Preparation And Uniformity Of MOCVD-derived YBCO Tapes

Due to the advantages of high irreversible magnetic field,large current carrying capacity and excellent mechanical strength,the second generation(YBa₂Cu₃O_7-?)YBCO high temperature superconducting tapes are of great application potential and value in the electric field.At present,the optimization of performance,the improvement of preparation efficiency and the reduction of cost about the YBCO superconducting tapes are the research focuses around the world.In this dissertation,the home-designed metal organic chemical vapor deposition?MOCVD?system has been improved,and the high efficiency and low-cost preparation of high-quality Y?Gd?BCO superconducting films on the IBAD-MgO templates have been researched.The details are as follows:1?The MOCVD system has been optimized.Firstly,the evaporator of MOCVD system was redesigned to improve the evaporation efficiency of metal organic sources and extend the flow time of metal organic sources in the evaporator.Thus,the vapor of metal organic sources could flow out from the evaporator steadily.Secondly,the self-heating device was also improved to keep good electrical contact between electrode rod and metal tape.Moreover,in order to ensure that the current flowing through each electrode is far lower than the total current,the number of electrode rods was also increased.Therefore,the self-heating stability could be guaranteed.At last,the condensing unit of MOCVD system was redesigned.The effective condensing area was greatly increased by using multi-cavity metal condensing filters instead of copper plates.The metal organic sources and metal oxides could be fully condensed to ensure the cleanliness of vacuum pipeline and the stability of mechanical pumps.2?The effects of heating temperature,metal organic sources ratio,heating method,multilayer structure and Zr addition on the preparation of Y?Gd?BCO superconducting layer have been researched.The research showed that the high proportion of Y?Gd?source in the precursor would lead to the formation of needle-like impurities on the surface of Y?Gd?BCO films.The low heating current would lead to the formation of a-axis oriented grains,while the high heating current would lead to the formation of Y-Cu-O and other non-superconducting impurities.The deviation of Cu/Ba ratio in the precursor would result in the formation of non-superconducting impurities.Based on the self-heating technology,the Y?Gd?BCO thick film has been prepared by adopting the multilayer structure.Through adjusting the heating current of every layer,the shortcoming of insufficient growth temperature of thick Y?Gd?BCO film could be overcome.Moreover,the feasibility of SmBCO/Y?Gd?BCO structure has been verified,which can provide a good scheme for the preparation of multilayer superconducting thick films.The nano-size pinning centers were formed in the Y?Gd?BCO films by adopting excessive Y?Gd?and10%Zr addition,which could improve the superconductivity of Y?Gd?BCO films.Concretely,based on the buffer tapes used in the experiments,the Y?Gd?BCO film with the flat and dense scaly morphology was prepared at the heating current of 26.8 A and the Cu/Ba ratio of 1.1,whose full width at half maximum?FWHM?values of out-of-plane and in-plane were 1.3°and 2.5°respectively.And the critical current density?J_c?at77 K and 0 T could reach 7.0 MA/cm².Due to the difference of the thickness of metal tape and buffer layers,the Y?Gd?BCO film with the smooth and dense surface was prepared at the heating current of 27.0 A and the?Gd,Y?/Ba ratio of 0.6,and its microwave surface resistance?R_s?at 77 K and 10 GHz could reach 0.581 m?.Thus,the1?m thick Y?Gd?BCO multilayer film with the critical current?I_c?of 328 A/cm at 77 K and 0 T was prepared based on the optimized process conditions.Finally,the nano-size pinning centers were formed in the Y?Gd?BCO films by adopting 10%Zr addition.The out-of-plane and in-plane FWHM values of 10%Zr-doped Y?Gd?BCO film were 0.93°and 2.0°respectively,and the I_c at 77 K and 0 T could reach 431 A/cm.3?The high-efficiency and low-cost preparation of Y?Gd?BCO films have been researched.Firstly,the distance between the metal tape and the shower head was reduced based on the self-heating method.Therefore,the deposition rate of Y?Gd?BCO films could be increased to 1?m/min by adjusting the preparation process.What's more,the Y?Gd?BCO films prepared at the deposition rate of 1?m/min had good crystal quality,biaxial texture,surface morphology and superconducting performance,which greatly improved the preparation efficiency of high-quality superconducting tapes.Moreover,based on the self-heating technology,a novel narrow channel reaction chamber was designed.The one-side deposition rate of Y?Gd?BCO films could reach 1?m/min and the utilization rate of metal organic sources could be increased to 31%,which provided a good scheme for the high-efficiency and low-cost preparation of Y?Gd?BCO films.4?The preparation uniformity and stability of Y?Gd?BCO films have been studied.Firstly,the effects of the slit shower head,the multi-aperture shower head and the narrow channel reaction chamber on the preparation uniformity of superconducting films were studied.When the distance between metal tape and shower head was less than 10 mm,the deposition rates of Y?Gd?BCO films which were prepared based on the 1 mm wide slit shower head were different in transverse direction,which resulted in the difference of structure and performance of superconducting films.The results showed that the multi-aperture shower head or narrow channel reaction chamber instead of the slit shower head could be used in the home-made MOCVD system to ensure the transverse uniformity of Y?Gd?BCO films.Then,based on the optimized MOCVD system and the IBAD-MgO template,one hundred meters Y?Gd?BCO superconducting tape with good uniformity was prepared.The FWHM values of out-of-plane and in-plane were 1.1°-1.3°and 2.6°-3.1°respectively,and the J_c at 77 K and 0 T was 4.0-4.4 MA/cm².Moreover,based on the double-sided IBAD-MgO template,the double-sided Y?Gd?BCO films were successfully prepared by MOCVD process.The I_c values of the double-sided Y?Gd?BCO films at 77 K and 0 T were 280 A/cm and 329 A/cm respectively,and the total I_c was 609 A/cm.Compared with the single-sided superconducting tape,the performance of doubled-sided Y?Gd?BCO tape could be greatly improved.