Reseach On The Growth And Control Of Buffer Layers Of YBCO Superconductor Wire

The second generation high temperature superconductor wires, based on YBa2Cu3O7-δ(YBCO), have excellent electrical performaces and extensive applications prospects in electricity tranmission, power storage, high field magnet and other fields. In order to obtain high performance YBCO superconductor wires, the buffer layers are necessary to transfer the biaxial texture and prevent the inter-diffusion of oxygen and metal atoms. However, there still exist some challenges, such as large angle grain boundaries(GB) in YBCO films, new buffer structures and cost reduction. In this research, based on the rolling-assisted biaxially textured substrates(RABi TS) technique, the texture and surface morphology of the buffer layers were systematically studied, and the influence of microstucture of buffer layer on the current carrying capacity of superconducter films and its internal mechanism were also investigated. In more detail:Double-sided CeO2/YSZ/Y2O3 films were simultaneously deposited on both sides of Ni-5at.%W(NiW) substrates by reel-to-reel direct-current(DC) magnetron reactive sputtering. The deposition parameters including water vapor pressure, substrate temperature and sputtering power were systematically studied. X-ray diffraction(XRD) analysis showed the FWHM values of out-of-plane and in-plane of the best Y2O3 sample were 1.5o and 4.8o, respectively. Atomic force microscope(AFM) results revealed a smooth, dense and crack-free surface morphology and the root mean square surface roughness(Rrms) was less than 2 nm. Two models of thin film growth were proposed to explain the mechanism of the improvement of Y2O3 out-of-plane texture. The theoretical models of depostion rate and atom diffusion rate were used to interpret the phenomena of the surface morphology evolvement of the CeO2 films. And therefore, smooth and dense cap layers with Rrms of less than 3 nm were controllably achieved.YBCO filmswith different GB misorientaiton angles distributions were adjusted by different buffer layers, and a percolation model was proposed to explain the trend of Ic and GB misorientaiton angles.The YBCO films were deposited on the buffer layers with different surface morphologies to study the influence of surface mophology on the current carrying capacity of YBCO. The dislocation densities in the films were measured and calculated quantificationally by high resolution XRD. The relationship between the surface morphology of buffer layer and dislocation density was systematically investigated. According to the above research, double-sided YBCO films grown in a controlled manner on CeO2/YSZ/Y2O3 buffered Ni-W tapes by sputtering exhibited a critical current of 500 A/cm-width.Alternating current(AC) magnetron reactive sputtering was used to depostied double-sided Y2O3 seed layer, which provides an effective approach to eliminate the target poisoning and improve a more stable, and fast sputtering mode. The deposition parameters such as AC voltage, sputtering pressure were systematically studied. The preparation efficiency of the Y2O3 layer enhanced from 5 m/h to 15 m/h, even up to 50 m/h, which is very important for the industrialization of YBCO superconductor wires. YSZ barrier layer and CeO2 cap layer deposited on such the Y2O3 seed layer, proved that AC magnetron reactive sputtering have the advantages of fast deposition of high quality buffer layers.To simplify buffer preparation process for cost reduction, and improve the engineering practicability of YBCO superconductor wires, a novel buffer architecture, i.e., conductive SrRuO3(SRO)/TiN, has been developed on biaxially textured NiW substrate using pulsed laser deposition. The structure and surface morphology of the TiN films were noticeably affected by the substrate temperature and pulse repetition rate. XRD analysis confirmed that the biaxial textures were transferred from Ti N seed layer to SRO cap layer and YBCO film with excellent out-of-plane and in-plane textures, the Rrms of SRO layer smaller than 5nm. The I-V curves of all conductive buffer layer showed a good conductivity, and the R-T curves showed the resistivity of 2×10-5 Ω?cm near the transition temperature.