J. Matsuda

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.

High magnetic field properties of critical current density in Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// coated conductor fabricated by improved TFA-MOD process

Current transport characteristics in a Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) coated conductor fabricated by a trifluoroacetates-metal organic deposition (TFA-MOD) process were investigated over a wide range of temperature and magnetic field up to 25 T. Improved TFA-MOD process was successfully introduced for the better property of critical current density, J/sub c/, at high magnetic fields. The J/sub c/ and critical current, I/sub c/, values of multi-coated film with 1.2 /spl mu/m thickness and 1 cm width were 2.1 MA/cm/sup 2/ and 251 A at 77 K in self-field. Additionally, the superior J/sub c/ properties remained even at high magnetic fields over 20 T and lower temperature, e.g. J/sub c/ value at 30 K in 25 T was 1.0 MA/cm/sup 2/. The J/sub c/ value was about 2.5 times higher than those of previous TFA-MOD process at wide range of magnetic field. Moreover, the statistical distribution of J/sub c/ in the conductor was also estimated within a framework of the percolation model. The uniformity in the YBCO coated conductor was improved by optimizing the TFA-MOD process.

Growth mechanism of YBCO film by TFA-MOD process

Abstract The growth rate expression in the TFA-MOD process for fabrication of coated conductors was revised according to the measurement of the growth rate using a long tape. The P (H 2 O) distribution along the gas flow-direction was calculated by the advection diffusion model. The above two outputs were combined to predict the minimum annealing time for complete reaction in the sample tape with a finite width. The prediction from the model was in good agreement with the experimental results.

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.

High Production Rate Process of YBCO Coated Conductors by TFA-MOD Process

Influence of process conditions including water vapor partial pressure, gas flow rate and total annealing pressure on YBa2Cu3O7-y (YBCO) film growth has been investigated in order to clarify the YBCO growth mechanism and to increase the growth rate. YBCO films have been fabricated on a LaAlO3 substrate by metalorganic deposition (MOD) process using tri- fluoroacetates (TFA) as a solute source. It was found that the processing parameters such as high water partial pressure, high diffusion constants due to low total pressure, and high gas flow rate affect the YBCO growth rate. Furthermore, it was empirically suggested that the growth rate of the YBCO film was in proportion to both the square root of water partial pressure and gas flow rate and was in inverse proportion to the one sixth power of the total pressure. The YBCO growth rate of about 2 times as fast as the previous condition was achieved with high performance by optimizing the process parameters.

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.

High Magnetic Field Properties of Critical Current Density in

Current transport characteristics in a Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) coated conductor fabricated by a trifluoroacetates-metal organic deposition (TFA-MOD) process were investigated over a wide range of temperature and magnetic field up to 25 T. Improved TFA-MOD process was successfully introduced for the better property of critical current density, J/sub c/, at high magnetic fields. The J/sub c/ and critical current, I/sub c/, values of multi-coated film with 1.2 /spl mu/m thickness and 1 cm width were 2.1 MA/cm/sup 2/ and 251 A at 77 K in self-field. Additionally, the superior J/sub c/ properties remained even at high magnetic fields over 20 T and lower temperature, e.g. J/sub c/ value at 30 K in 25 T was 1.0 MA/cm/sup 2/. The J/sub c/ value was about 2.5 times higher than those of previous TFA-MOD process at wide range of magnetic field. Moreover, the statistical distribution of J/sub c/ in the conductor was also estimated within a framework of the percolation model. The uniformity in the YBCO coated conductor was improved by optimizing the TFA-MOD process.

rm Y_1rm Ba_2rm Cu_3rm O_7-delta

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.

Coated Conductor Fabricated by Improved TFA-MOD Process

We have investigated the effects of the heating rate in the crystallization process on Ic values and microstructures of YBa2Cu3O7-5 (YBCO) films, which were fabricated by the advanced metalorganic deposition(MOD) method using trifluoroacetates. As a result, it was found that the slow heating rate less than 2degC/min in the crystallization process increases the volume of randomly oriented YBCO crystals, which results in low Ic value of the YBCO film. Quenched samples were also prepared by heating at the various heating rates and cooling rapidly during the crystallization process. TEM observations of these quenched samples revealed that unreacted phase particles such as CuO, Y2Cu2O5 and Ba-O-F crystallize and coarsen to large crystals before the nucleation and growth of YBCO crystals in the case of slow heating in the crystallization process. We found that it is important to control the sizes and distributions of the unreacted phase particles during the crystallization process as well as those in the calcination process, in order to fabricate the YBCO coated conductor with high Ic.