K. Nakaoka

Recent Progress on R&D of Advanced TFA-MOD Process for Coated Conductors

The recent progress in R&D of TFA-MOD process for the coated conductors is reviewed. On the effort towards fabricating high Ic tape, a new process to prevent crack formation in the thick film, which is the intermediate annealing between calcination and crystallization steps, was developed. Additionally, control of the composition from the 123 stoichiometric ratio, especially for Ba-poor composition, is found to be effective to improve Jc values of the films. Consequently, the extremely high Ic value of 692 A was obtained. On the other hand, a reel-to-reel system has been applied to fabricate long tapes. The high Ic value of 250 A/cm-w in a 4 m long tape and the reasonable long tape of 25 m with 100 A/cm-w were obtained. Additionally, it was newly found that the humidity control to be low PH 2 O in the coating room is effective to suppressing the degradation of the Jc characterization.

Development of TFA-MOD Process for Coated Conductors in Japan

Progresses in the development of the TFA-MOD process for the coated conductors in Japan are reviewed. In the fundamental approaches, some important advantages of Ba-poor starting solution were found such as for obtaining higher Jc, shorter reaction time and wider process temperature window. The high Ic value of 735 A/cm-w@77 K was attained. Additionally, introduction of artificial pinning centers by Sm-substitution and Zr-addition to the Ba-poor films was successfully investigated. The films showed a high minimum Ic value of 42 A/cm-w at 77 K, 1 T. Furthermore, isotropic Ic properties in the angular dependence of applied magnetic fields was realized by the uniform distribution of nanoprticles of Zr-compound. On the other hand, two different process of MOD including batch and reel-to-reel (RTR) types have been developed as an engineering process for low cost and long tape production with high superconducting performances at a high rate. In the batch type, long tape synthesis process could be developed by the optimization of the growth conditions in a large furnace. A 500 m long tape was heat-treated and a high Ic value of 310 A/cm-w in a 250 m tape was confirmed. Concerning the RTR type, a high production rate system was developed after confirmation of the ability of the continuous process in a 90 m long tape. A multi-turn with a vertical gas flow system was developed for this purpose. This system was applied to the fabrication of superconducting films on the lower cost buffer layers including a high deposition rate IBAD-MgO layer. Currently, a 5 m long tape with 250 A/cm-w was obtained at the production rate of 3 m/h using a part of the multi-turn system, which is equivalent to 15 m/h in the entire use of the furnace.

Effect of Solution Composition on Superconducting Properties of YBCO Derived From TFA-MOD Process

The crystallization process of trifluoroacetate-metal-organic deposition derived YBa₂Cu₃O_7-y (YBCO) crystals has been investigated to understand the influence of the starting solution composition on the crystal growth process. The starting solutions with molar ratios of Y: Ba: Cu = 1.0: 2.0: 3.0 (stoichiometry) and 1.0:1.5:3.0 (Ba-deficient) were examined. The highest <i>I</i>_c values were achieved at 100 min holding time of the conversion step for 0.8- μm-thick YBCO films, which were independent of the starting solutions. The degradation of <i>I</i>_c performance was observed only in the films prepared from the stoichiometric solution with longer holding time than 100 min. The longer holding times for disappearing of the X-ray diffraction peak of BaF₂ derived from the stoichiometric solution is required compared to that from the Ba-deficient solution. On the other hand, the epitaxial growth rate measured from cross-sectional scanning electron microscopic observation for the YBCO films showed no difference between each other. X-ray diffraction analysis on the YBCO films fabricated from stoichiometric starting solution suggested that BaCuO₂ and/or Y₂O₃ phases formed during the solid-state reaction process. The degradation of <i>I</i>_c performance might be caused by the formation of BaCuO₂ phases.

Transmission electron microscopic studies on growth mechanism of YBa 2 Cu 3 O 7− y films formed by advanced trifluoroacetates metalorganic deposition process

Microstructure evolution of YBa 2 Cu 3 O 7− y (YBCO) films during the two heat-treatments in the advanced trifluoroacetates metalorganic deposition (TFA-MOD) process has been investigated by means of transmission electron microscopy. In the calcination process, precursor films including nanopores were formed through the shrinkage of the film after a remarkable increase of the thickness due to the thermal decomposition of metalorganic salts in the starting solution. During the crystallization process, the densification and shrinkage of the film occurred after agglomeration of nanopores and coarsening of unreacted phase particles such as Y 2 Cu 2 O 5 , CuO, and Ba–O–F in the precursor films. The YBCO films were then epitaxially grown with the remaining unreacted phase particles in the film, finally pores were generated again by a reaction of these unreacted particles to form YBCO accompanied by the volume reduction. It is important to control the densification of precursor films and coarsening of the unreacted phase particles in the crystallization process, to fabricate YBCO final films with fine crystallinity and high critical current values.

Optimization of Process Parameters for Calcination in TFA-MOD Method

We have investigated the influence of the H2O vapour inlet temperature in the calcination on both the microstructure and superconducting properties of YBa2Cu3O7-x, (YBCO) films fabricated by the TFA-MOD method, from the viewpoint of the process control in the reel-to-reel system. Low critical current (Jc) values were obtained in the films prepared by introducing the H2O vapour at temperatures less than 200degC. XRD measurements and cross-sectional TEM observations suggest that the YBCO films calcined with the H2O vapour inlet temperature below 200degC revealed a random orientation without the epitaxial growth of the YBCO phase grains. Consequently, it is necessary to introduce H2O vapour from temperatures higher than 250degC in calcination for fabrication of high performance YBCO tapes. In reel-to-reel system, it is important that the humidity management in a low temperature range in the furnace.

Progress in R&D for YBCO Coated Conductors by TFA-MOD Processing

Recent progress in the research and development of the TFA-MOD processing for the coated conductors is reviewed. For the higher Ic performance, process conditions such as heating rate and water vapor inlet temperature in the calcination step and heating rate and water vapor pressure in the crystallization step have been optimized to obtain YBCO films with good crystallinity. Consequently, a high Ic value of 431A/cm-w was achieved by the combination of the above optimized conditions with the use of highly textured CeO2 cap layered substrates. Forwards the effort for long tape fabrication with high production rate, the multi-turn system for coating and calcination step was developed. Additionally, the reel-to-reel system for the crystallization step was developed for the longer tape, and a 25m long YBCO tape with a reasonable end-to-end Ic value of 100A, which corresponds to 2500Am as a product of IcxL, was obtained by the above continuous system. For the higher production rate in the crystallization step, it was confirmed that optimization of the processing parameters such as high water partial pressure, high diffusion constants due to low total pressure, and a high gas flow rate affects the YBCO growth rate.

Effects of the Grain-Boundary Angle of the Buffer Layer on the In-Field Critical Current Density in (Y 0.77Gd0.23)Ba2Cu3Oy + BaZrO3 Nanoparticle Coated Conductors

We fabricated Y_0.77Gd_0.23Ba₂Cu₃O<italic>_y</italic> [(Y,Gd)BCO] and BaZrO₃ (BZO) nanoparticle doped (Y,Gd)BCO ((Y,Gd)BCO+BZO) films on CeO₂ buffered metallic substrates [ = coated conductor (CC)] by using trifluoroacetate metal organic deposition (TFA-MOD). In this paper, in order to investigate the influence of grain-boundary angles of CeO₂ buffer layer (<italic>Δφ</italic>_CeO2) on crystallinity and superconducting properties of (Y,Gd)BCO, the TFA-MOD films are fabricated on metallic substrates with various <italic>Δφ</italic>_CeO2. The <italic>Δφ</italic> of the superconducting layer decreases with decreasing <italic>Δφ</italic>_CeO2 in both CCs. The self-field critical current density (<italic>J_c</italic>^s.f.) of (Y,Gd)BCO+BZO CC with <italic>Δφ</italic>_CeO2 = 2.0° is 5.8 MA/cm² at 77 K, which is 1.5 times higher than that of CCs with <italic>Δφ</italic>_CeO2 = 4.0°. Moreover, the (Y,Gd)BCO+BZO CC with <italic>Δφ</italic>_CeO2 = 2.0° showed the minimum critical current density (<italic>J_c</italic>_,min) of 0.81 MA/cm² at 77 K and 3 T. Thus, the improvement of the grain boundaries by using the improved buffer layer has an important role in the improvement of <italic>J_c</italic> in TFA-MOD REBa₂Cu₃O<italic> _y</italic> CCs.

Crystal Growth of Ba Concentration Controlled YBCO Films by TFA-MOD Process

YBCO films grown by the starting solution with Ba-poor (cation ratio as Y:Ba:Cu = 1:1.5:3) have been reported to have higher J c than that for YBCO films with stoichiometric composition due to smaller and less pores in TFA-MOD YBCO. In this study, Ba concentration in the Ba-poor YBCO films was controlled in film thickness direction, and influences of the Ba concentration gradient on microstructures and J c properties were investigated to realize higher J c. Precursor films with Ba-poor composition and with Ba-rich composition in the thickening direction were prepared and then the films were crystallized. Increasing the Ba concentration in thickening direction led to higher J c than that of Ba-poor YBCO. Cross sectional TEM observations showed that the effective YBCO thickness increased for YBCO film with higher Ba concentration in comparison to the previous Ba-poor YBCO film.

Crystal grains alignment of SmBCO film by advanced TFA-MOD method

We grew SmBa2Cu3O7-y (SmBCO) films on LaAlO3 (LAO) single crystalline substrates by an advanced TFA-MOD method and discussed the effects of process conditions such as oxygen partial pressure in the crystallization step on the crystal grains alignment of SmBCO film. Oxygen partial pressure affected strongly on the orientation of SmBCO film. Formation of a-axis crystal grains of the SmBCO was suppressed, and, at the same time, c-axis crystal grains of the SmBCO increased with decreasing the oxygen concentration.