Study On The Fabrication Of YBCO Superconducting Film Using Low Fluorine Solution

Due to the special physical properties,YBa2Cu3O7-x(YBCO film),the second-generation high temperature superconductor,had important applications in the field of power and electronic devices.The chemical solution deposition(CSD)route with advantages of costing and technology was one of the main methods for the fabrication of YBCO film.However,there still has been some problems in the preparation of CSD-film.It mainly involved the unascertained fluorine content in the low-fluorine solution,the "thickness effect" in the fabrication of thick film,the poor superconducting properties in the high magnetic field,the nonuniform distribution of metal elements during the pyrolysis process and poor film quality,etc.As far as these problems were concerned,this paper mainly focused on the following works.First,in order to study the mechanism of fluorine content,six kinds of low-fluorine YBCO solutions with F/Ba= 1-4.5 were synthesized and used for the fabrication of single-layer YBCO film(about 230 nm)and the micron-sized YBCO thick film(1.2 ?m),respectively,which were deposited on LaAlO3(LAO)substrates.The film properties and the phase transformation mechanism under different temperatures and time were investigated systematically.The results showed that the BaC03 phase was easily formed due to the lack of fluorine in F/Ba<2 solutions,which is adverse to the fabrication of high-quality YBCO superconducting film.The excess fluorine content leaded to the formation of Ba1-xYxF2+x(BYF)intermediate phase,which would be residual in the stage of YBCO grains crystallization,to which the superconductivity of the micron-sized YBCO thick film was very sensitive.As a result,F/Ba>2 solutions were not suitable for the fabrication of micron-sized YBCO thick films.However,it had a tolerance to the excess fluorine content in the solutions for the single-layer YBCO film due to small thickness,thus F/Ba=2?4.5 solutions would be used for the single-layer YBCO thin film.According to entirely obtained results,owing to no BYF and BaCO3 residual in the stage of YBCO grains crystallization,the F/Ba=2 YBCO solution with 10.3%fluorine content was best for the fabrication of high-performance YBCO film.Further,based on the above fluorine content investigation,a new method,namely the layer by layer overlaying,was used to fabricate micron-sized YBCO thick films.The results indicated that this new method could slow down the decrease of superconducting properties and go against the "thickness effect" effectively when the single-layer thickness was properly controlled.The BaTiO3(BTO)nano-dots with different forms were fabricated on the LAO substrates by adjusting solution concentrations(0.01 mol/L,0.025 mol/L,0.05 mol/L).The flux pinning ability and superconducting property of YBCO films decorated by different BTO nano-dots were studied respectively.The results displayed that BTO nano-dots derived from 0.025 mol/L solution had a proper density and mixed-distribution of different heights,which was tended to produce a significantly enhanced flux pining force in the YBCO film.As a result,the Jc value of as-decorated YBCO film reached about 8.42 MA/cm2 at 77 K and 0 T.And its flux pining force was higher than others during 0?2.5 T,the max flux pining force(Fp max)was 9.58 GN/m3at 0.75 T.Next,a deep UV irradiation route was put forward to replace the conventional pyrolysis step in the fabrication of YBCO superconducting films,in which the precursor amorphous film with uniform distribution of metal ions was obtained after the decomposition and oxidation of organics by high-energy photons and oxidants.The resulting YBCO films after crystallization pyrolysis had traits of high-Jc value,densification without pores,high-texture and high-crystallization.The Jc value of 100 min-UV film was about 7.95 MA/cm2 at 77 K and 0 T.Further,a water-vapor assisted deep UV irradiation method was proposed to further reduce the content of residual carbon in the precursor UV film due to the formation of hydroxyl radical(*OH).It would significantly promote the oxidization and decomposition of residual organics so as to improve the surface quality and properties of YBCO films.The Jc value of 30 min-water-UV film reached about 8.4 MA/cm2 at 77 K and 0 T,which was 1 time higher than 30 min-UV film and larger than 100 min-UV film.Finally,three kinds of buffer layers(LZO,LZO/YCO and/LZO/CeO2)with good(l00)-orientation and low surface roughness were deposited on Ni-5%W metal substrates using CSD method,and the YBCO films were deposited on them via CSD method subsequently.The results showed that the crystallinity,texture orientation and superconducting properties of YBCO films deposited on LZO/YCO and LZO/CeO2 buffer layers were much superior to that on LZO buffer layer.The Jc values of them were both near to 0.5 MA/cm2 at 77 K and 0 T,and superconducting transition temperature(Tc)value was about 90 K.However,the Jc values of the YBCO film deposited on LZO/CeO2 buffer layer were of more uniform distribution compared with that on LZO/YCO buffer layer due to the better film quality and orientation.