MOCVDResearch On Superconducting Layers Of YBCO Tapes

Due to their high irreversible magnetic field, large current carrying capacity, excellent mechanic performance, and potentially low cost, the second generation YBa2Cu3O7-δ(YBCO) high temperature superconducting tapes are of wide application prospects in electric field. At present, the researches on YBCO tapes mainly focus on further improving the performance and reducing the production cost, to which the key is the fabrication of YBCO superconducting layer. Therefore, this dissertation has applied the home-made metal organic chemical vapor deposition(MOCVD) system to perform the fabrication researches on YBCO films and the mainly applied substrates are the ionbeam assisted deposition(IBAD)-MgO templates. The details are as the following:(1) The atomization nozzle and gas showerhead of home-designed MOCVD system have been optimized. And a new substrate heating method that the substrate tape is heated by the joule effect after applying an electrical current through the tape itself has been proposed. Compared to the resistant wire heating method, this new heating method is of simpler structure, easier maintenance and higher energy efficiency, and especially suitable for reel-to-reel fabrication of double-sided YBCO tapes. Meanwhile, the new heating method could yield 2-3 times increase in utilization efficiency of metal organics(MOs) and 2-4 times increase in deposition rate of YBCO film, thus greatly cut the production cost of YBCO film.The metal organics(MOs) were synthetized by a new method, and their evaporation properties and shelf life were studied. The experiments revealed that the home-made MOs not only meet the requirements on fabrication of high quality YBCO films, but also reduced the production cost. Right now, the cost of home-made MOs in laboratory was about 10 yuan per gram, which was less than a tenth of the unit price of MOs from foreign companies.(2) The effects of substrate temperature, heating method, metal atom ratio in precursor and the total pressure on the YBCO films have been studied separately. The underheated substrate would induce a-axis growth, while the overheated substrate would result in the decomposition of YBCO films and formation of impurities. Compared to that in the resistant wire heating, the tape surface temperature in the new heating method is deeply affected by the tape structure and the thickness of deposited YBCO film. The yttrium(Y) ratio in precursor mainly affected the morphology and superconducting performance rather than the textures of YBCO films, while the total chamber pressure would affect them all. As the Y ratio was 1.17, the YBCO film was smooth and dense, and the critical current density at 77 K and 0 T(Jc 77 K, 0T) achieved the maximum of 2.57 MA/cm2(700 nm-thick). As the total pressure increased from 230 to 880 Pa, the film deposition rate increased. And the deposition rate achieved the maximum of ~1 μm/min under the total pressure of 880 Pa. When the total pressure was 560 Pa, the deposition rate was about 750 nm/min and the YBCO film texture were better, consequently yielded the maximum Jc 77 K, 0T of 1.6 MA/cm2(1.25 μm-thick).A 30 m-long single-sided YBCO tapes with 500 nm-thick YBCO films has been fabricated under the optimal conditions. The out-of-plane and in-plane textures along the tape length were uniform, and the corresponding full width at half maximums(FWHMs) were around 1.5o and 3.3o, respectively. The Jc 77 K, 0T of the tape was 2.8-3.2 MA/cm2, corresponding to the critical current(Ic) of 140 A/cm-width.(3) The effects of gadolinium(Gd) substitution for Y and zirconium(Zr) addition on the YBCO films have been investigated separately. The substitution of Gd for Y could restrain the formation of CuYO2 impurities and strengthen the flux pinning effects, but induced a-axis growth of YBCO grains. As Gd content was 50%, the Jc 77 K, 0T achieved the maximum of 2.8 MA/cm2, yielding the maximum flux pinning force. As Zr addition amount was 2.83 mol%, the Jc 77 K, 0T of YBCO film achieved the maximum of 2.8 MA/cm2. The Zr0.06Gd0.6Y0.6Ba2Cu3O7-δ(ZrGdYBCO) films with co-doping of Gd and Zr have been successfully fabricated under the optimal conditions, and yielded the world’s smallest FWHMs of 0.9o and 1.9o for out-of-plane and in-plane textures, respectively. And the 200 nm-thick Zr GdYBCO film achieved the world’s highest Jc 77 K, 0T of 7 MA/cm2.(4) The double-sided YBCO tapes based on the double-side CeO2/YSZ/Y2O3(YYC) templates and IBAD-MgO templates have been successfully fabricated by MOCVD for the first time. The double-sided YBCO tape based on YYC template was of Jc 77 K, 0T of 0.4 MA/cm2 and 1.0 MA/cm2, and critical transition temperature(Tc) of 87.72 K and 88.09 K for each side. The double-sided YBCO tape based on IBAD-MgO template was of Tc of 91.2 K and Jc 77 K, 0T of 3.2 MA/cm2 for both sides, yielding the total Ic of 320 A/cm-width. And this research laid the foundation for fabrication of double-sided YBCO superconducting tapes with higher performance.