Tetsuji Honjo

Fabrication and growth mechanism of YBCO coated conductors by TFA-MOD process

Abstract In the development for the coated conductors, the trifluoroacetates-metal organic deposition (TFA-MOD) process using the multi-coating method was applied to form thicker YBa 2 Cu 3 O y (YBCO) films on the CeO 2 buffered IBAD(Zr 2 Gd 2 O 7 )/Hastelloy tape. As a result, in the 5 times coated film with 1.38 μm in thickness, the J c and I c values achieved to be 1.5 MA/cm 2 and 210 A, respectively. Then, we fabricated a 1 m long YBCO tape by the continuous reel-to-reel system. As a result, we confirmed the tape to be homogeneous by XRD analysis. Additionally, we report the theoretical analysis of YBCO growth during post-annealing in the TFA-MOD process considering both the diffusion in the gas boundary layer and the growth kinetics at the precursor/YBCO interface. This model reveals a basic idea of the growth mechanism to define the steady state growth rate, and could explain the experimental results. In addition, it predicts that the growth rate could be estimated from the partial pressure of the water vapor of the inlet gas.

Fabrication processing of Y123 coated conductors by MOD-TFA method

Abstract The metal organic deposition process of YBa 2 Cu 3 O 7− X (Y123) using metal trifluoroacetate precursors is considered to be a strong candidate not only for high J c but also for low cost processing due to its a non-vacuum process. In this work, in order to obtain high I c films, thicker Y123 film processing on LaAlO 3 was investigated using the multi-coating method. In the case of thicker films, it was found that the low partial pressure of H 2 O, P (H 2 O) for the second heat treatment is effective to maintain the entire epitaxial growth for the thick films. Consequently, increasing of I c was realized from single to triple coating films. Further, in order to apply the technique to fabrication of coated conductors on metal, the bi-axially textured buffer layers is necessary to achieve a high in-plane alignment of the superconducting layer. Subsequently, the thicker film processing was applied to the CeO 2 /IBAD-YSZ/Hastelloy substrates using the multi-coating method. The triple coating Y123 film with 1 μm in thickness on the metal substrate shows the J c value of 1.6 MA/cm 2 at 77.3 K in self-fields and the I c performance of 153 A/cm-width.

Growth mechanism of YBCO films in metal organic deposition method using trifluoroacetates

We report the theoretical analysis of YBCO growth during post annealing in the TFA-MOD process considering both the diffusion in the boundary layer and the growth kinetics at the precursor/YBCO interface. As a result, we could obtain the analytical solution of the growth rate of YBCO. Subsequently, the unknown parameters in this solution were evaluated by the experimental measurements of the growth rate. The experimental results of the growth rate showed that it was independent of the film thickness and proportional to the square root of the water vapor pressure. These results suggested that the mass transfer in the gas boundary layer limited the growth rate. This model reveals a basic idea of the mechanism to determine the steady state growth rate, and could explain the experimental results. In addition, it predicts that the growth rate can be estimated from the water vapor pressure of the inlet gas. Consequently, it was confirmed that this growth model is effective for understanding of the growth kinetics in the TFA-MOD process.

Growth mechanism of Y123 film by MOD-TFA process

Abstract In order to clarify the growth mechanism of Y123 film in the metal organic deposition (MOD) process using TFA, transmission electron microscopy (TEM) observation of the quenched samples and measurement of the growth rate under several different conditions were carried out. TEM observation showed that the Y 2 Cu 2 O 5 , BaF 2 and CuO are converted into Y123 by release of HF with supplying H 2 O at the reaction interface. The MOD-TFA process using the multi-coating method was applied to form thicker Y123 films on LaAlO 3 substrates, and the growth mechanism for the Y123 crystallization was investigated. In order to evaluate the growth rate, the electrical resistance of the precursor films during the crystallization was measured by the DC four-probe method. It was observed that the thickness of the Y123 linearly increases with increasing annealing time. This result suggests that the growth rate is limited by the HF diffusion in the boundary layer and/or growing interface kinetics. According to the results from the gas flow rate dependence of the growth rate, it was suggested that the growth of Y123 films in this process might be limited by both the diffusion in boundary layer and the interface kinetics. Then, the growth model, which includes the two limiting systems, was developed. This model reveals a basic idea of the mechanism to determine the steady state growth rate.

Crystallization mechanism of Nd1+xBa2-xCu3O7-y and YBa2Cu3O7-y films deposited by metalorganic deposition method using trifluoroacetates

YBa 2 Cu 3 O 7 - y (Y123) and Nd 1 + x Ba 2 - x Cu 3 O 7 - y (Nd123) films were deposited by the metalorganic deposition method, and the growth mechanism of these films was investigated by high-resolution transmission electron microscopy and energy dispersive x-ray spectroscopy. The Y123 and Nd123 films were prepared by spin-coating LaAlO 3 (001) and SrTiO 3 (STO) (001) substrates, respectively, with solutions including trifluoroacetates. Then, the samples were heat treated at 673 K in a humid O 2 gas flow to form amorphous precursor films. Finally, the precursor films were heated at higher temperatures for 0-30 min in a humid Ar/O 2 gas flow and cooled rapidly from those annealing temperatures. It was found that CuO crystals with a size of 10-20 nm are segregated in the Y123 and Ndl23 amorphous precursor films. In the Y123 quenched film prepared by cooling the precursor film rapidly after the heat-treatment at 1048 K for 30 min, a polycrystalline film including Y 2 CU 2 O 5 , BaF 2 , and CuO crystals was found to be generated on the c-axis-oriented Y123 film. In contrast, in the Nd123 quenched films, (Nd,Ba) 2 CuO 4 (Nd201) phase was found to be formed first on the surface of the STO substrate. In conclusion, the c-axis-oriented Y123 film is formed by diffusion and reaction of Y 2 Cu 2 O 5 , BaF 2 , and CuO crystals, and the Nd201 phase reacts with BaF 2 and CuO crystals in a humid atmosphere to form a c-axis-oriented Nd123 film.

Deposition of CeO2/YSZ buffer layer on Hastelloy substrates for MOD process of YBa2Cu3O7−x film

Abstract Trifluoroacetate metalorganic deposition (TFA-MOD) process is expected as a low cost process for mass production of coated conductors because it is a non-vacuum process. In order to apply the technique to fabrication of coated conductors, suitable buffer layers have to be considered to achieve a high orientation of superconducting layer and prevention of the reaction with metal substrate. The combination of CeO 2 on IBAD-YSZ is considered as an effective buffer for TFA-MOD process expecting to satisfy a high acid resistivity and high crystal grain alignment. The CeO 2 buffer layer was deposited on IBAD-YSZ/Hastelloy substrates by RF magnetron sputtering. From XRD analysis, the CeO 2 buffer layer showed very good in-plane alignment on YSZ-IBAD buffer layer. In a holding time of 1 h, the suitable maximum heat treatment temperature was found to be from 750°C to 775°C for TFA-Y123 on metal substrate. The J c – B property of Y123 on CeO 2 /YSZ/Hastelloy shows the J c values of 1.4 MA/cm 2 at 77.3 K, 0 T and more than 10 5 A/cm 2 at 77.3 K, 2 T. The high performance under high magnetic field was confirmed.

High-Ic YBCO coated conductors by metal organic deposition method using trifluoroacetates

For the development of the YBa/sub 2/Cu/sub 3/O/sub y/ (YBCO) coated conductors, processing of the thicker film and long tape have been investigated using the metal organic deposition (MOD) method with the trifluoroacetates solution. About 1 micro-meter thick MOD film was realized by the control of the P/sub H2O/ condition during annealing for the formation of the YBCO layer. The improvement was explained by the competition between the nucleation of the undesirable grains and the epitaxial growth up to the surface. By applying the suitable condition to the metallic substrate system, high critical current density (Jc) and critical current corresponding to 1 cm wide tape (Ic/sup */) were realized as to be 1.6 MA/cm/sup 2/ and 153 A/cm-w in the film with 1 micrometer thickness, respectively. For the fundamental investigation of long tape processing, influence of the gas flow direction during annealing for the YBCO formation was investigated. Although nonuniformities in the X-ray diffraction (XRD) intensities and Jc values were recognized in the film grown under the gas flow parallel to the long direction of the tape, both of them was improved by changing the gas flow direction to the transverse one.

High critical current YBCO thick films by TFA-MOD process

Abstract As a method of the fabrication processes of YBa 2 Cu 3 O 7− x (YBCO), the metalorganic deposition (MOD) process using metal trifluoroacetete (TFA) is considered to be a strong candidate due to its low cost fabrication process for coated conductors with high J c . In our previous work, a triple coated film with 1 μm in thickness was fabricated on a CeO 2 /IBAD–YSZ layer buffered Hastelloy substrate by optimizing the condition of heat treatments such as P H 2 O in the multi-coating method [Physica C 378–381 (2002) 1013]. The J c value of 1.6 MA/cm 2 (77 K in self-field) in this film patterned 100 μm width and the I c * value of 153 A/cm-width at 77 K in self-field were achieved. In order to obtain a thicker film with high overall I c * for 1 cm width, the influence of the heat treatment conditions of P H 2 O , P O 2 , and the temperature in the MOD process was investigated. Subsequently, a 5 times coated film was obtained on a CeO 2 /IBAD–Zr 2 Gd 2 O 7 layer buffered Hastelloy substrate by optimizing the conditions of heating and dip coating. As a result, the overall transport I c value was improved to 210 A and J c value of 1.53 MA/cm 2 was obtained (77 K in self-field).

In-plane textured oxide buffer layer for the TFA-MOD method on 〈100〉{001} Ni tapes using MOD process

We developed a low cost process, the so-called all-MOD process, whereby we used MOD to deposit both YBCO and oxide buffer layers on {100}(001) Ni tapes. Highly in-plane textured and crack-free Ce-Gd-O (CGO) buffer layers on the cube-textured Ni tapes were achieved by the MOD process. Highly in-plane textured YBCO films on the CGO-buffered Ni tapes were also observed. Long tapes were made by applying a 1 m long CGO buffer layer on the Ni tapes and firing in a batch-type furnace. The CGO films were made by a continuous bead coating process and firing under an Ar-H 2 atmosphere. The 1 m CGO-on-Ni tape was highly in-plane textured. R 1 ( 2 0 0 ) and FWHM of phi-scan values were 88-96% and 7.0-8.5°, respectively. We determined the degree of (n00)-orientation: R l ( n 0 0 ) [≡I ( n 0 0 ) /{I ( n 0 0 ) + I ( m m m ) } x 100(%)], where I ( n 0 0 ) and J ( m m m peaks represent the X-ray θ-2θ scan intensities. These results indicate that buffered Ni tapes made by the MOD process are useful for TFA-MOD fabrication of long-length YBCO coated conductors.

Processing for long YBCO coated conductors by TFA-MOD process

Abstract The metal organic deposition (MOD) process of YBa 2 Cu 3 O 7− X (YBCO) using metal trifluoroacetate (TFA) precursors is considered to be a strong candidate as a low cost fabrication process in coated conductors since the TFA-MOD process is a non vacuum process and can provide high J c films. In order to develop long tape conductors using this process, the influence of the gas flow direction in the heat treatment was investigated. The J c value decreases toward to leeward side in the case of the parallel gas flow condition. On the other hand, in the case of the perpendicular gas flow condition. The J c value reveals uniformity in all over the tape and the high J c level is maintained. In the case of long tape production, it was found that the perpendicular gas flow to the tape length is effective to fabricate the uniform films. It could minimize the influence of consumption of H 2 O and generation of HF gas in the up-stream area to the leeward region. The J c and I c properties and these distribution of the YBCO tape fabricated by this method on CeO 2 /IBAD-Gd 2 Zr 2 O 7 /Hastelloy is presented. Furthermore, a large scale equipment for the continues long tape process was developed based on the above findings. Consequently we have fabricated a 1 m long Y123 tape conductor by a reel to reel system. Even in the first trial, the uniform reaction for the entire length was recognized.