Search

Abstract: We have investigated the influence of partial pressure of water vapor [P(H2O)] in the crystallization process on the superconducting properties of YBa2Cu3O7− y (YBCO) films fabricated by a trifluoroacetate–metal organic deposition (TFA–MOD) method. The starting solution with different compositions of Y: Ba:Cu=1.0:2.0:3.0 and 1.0:1.5:2.0 were studied. The critical current density (J c) values of YBCO films fabricated from Y:Ba:Cu=1.0:2.0:3.0 starting solution significantly increase (1.71→2.55MA/cm2) with increasing P(H2O) from 12.3 to 47.4kPa. In the YBCO films fabricated from Y:Ba:Cu=1.0:1.5:2.0 starting solution, high J c values of over 2.5MA/cm2 were recognized in a wide range of P(H2O) (12.3–47.4kPa). One of the reason for J c improvement is the suppression of coarsening of the secondary phases grains such as Y2Cu2O5 and CuO due to increase in growth rate of YBCO layer in the crystallization process. The J c values of all YBCO films decreased as P(H2O) increased up to 70.1kPa. This degradation of J c values may be caused by difference of crystal growth mechanism in high growth rate. [ABSTRACT FROM AUTHOR]

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 Jc 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 Jc properties were investigated to realize higher Jc. 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 Jc 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. [ABSTRACT FROM AUTHOR]

Abstract: For production of low cost coated conductor tapes, the TFA-MOD (metal organic deposition using trifluoroacetates) process is one of the promising methods and textured NiW alloy substrates are expected to be a candidate for their substrates. However, the superconducting properties obtained on the NiW substrates are not high enough due to lack of understandings on YBCO grain growth on the NiW. In this work, MOD growth conditions such as molar ratio (Ba/Y) of the starting solution, the crystallization temperature and P (O2) (O2 partial pressure) especially on the NiW substrates were studied. Compositional effects of the starting solution on I c values have a trend having the highest value at Ba/Y=1.5, which is similar to those on IBAD substrates, though the I c values of YBCO on NiW are lower. One of the differences between YBCO layers on NiW and IBAD could be explained by the significant difference in a-axis oriented grain growth. The higher temperature and/or the lower PO2 were required to suppress the nucleation for a-axis oriented grain growth on the NiW. 1m-long RTR production exhibited the end-to-end I c value of 221A/cmw, which could be improved further by the optimization of the process conditions. [Copyright &y& Elsevier]

Abstract: On the research and development of a reel-to-reel TFA-MOD (Metal Organic Deposition using Trifluoro-acetates) process, a present status is reviewed and its future strategy is addressed. As a base of the study, the 90m long tape with uniform Ic distribution of the 300A/cm-width level was obtained on the CeO2 buffered IBAD-Gd2Zr2O7/Hastelloy C276 substrate. The tape has the 56m region with the end-to-end Ic value of 250A, which corresponds to maximum product of Ic × L of 14,000Am. Based on the results, several directions on R&D have been studied such as “higher Ic ”, “higher Ic –B”, “higher production rate both in coating/calcinations and crystallization steps” and lower cost buffer/substrate”. Then, an extremely high Ic value of 735A/cm-width was achieved in a short tape by the compositional control (e.g. Ba-deficient), in the starting solutions. On the efforts for achieving higher Ic –B properties, high Ic values of 115 and 35A/cm-width under the magnetic fields of 1 and 3T were obtained by the RE mixture of Y and Gd in REBCO, addition of Zr and a growth rate control process. On the other hand, the production rate for the coating/calcinations process was improved by development of new starting solutions, which uses F-free Y salt instead of TFA salt of Y. The high Jc value of 1.9MA/cm2 was confirmed using the precursor films fabricated at a high traveling rate of 10m/h. Concerning a higher rate in the crystallization step, the multi-turning system with a vertical gas flow system was developed. The validity of the concept was confirmed using 2-turn parts of the furnace. The high Ic value of 250A/cm-width was realized in the 5m tape crystallized with a traveling rate of 3m/h, which is equivalent to 15m/h for usage of entire area of the furnace of 10-turns. Furthermore, in order to achieve the lower cost, the architecture of the coated conductor with a low cost buffer/substrate system has been developed. An IBAD buffered substrate using IBAD-MgO layer (CeO2/LMO/IBAD-MgO/Hastelloy C276) was developed and a high production rate of 24m/h was realized for IBAD-MgO layer using a small ion gun system with the area of 6×22cm2. The grain texturing of the substrate was reached the Δϕ value of 4° in the CeO2 layer. This substrate was applied to the above mentioned multi-turning crystallization furnace for TFA-MOD process. Then, a 5m long tape with 260A/cm-width (@77K. s.f.) was achieved. According to the TFA-MOD process in the above achievements, the prospects of each issue for the future stage were independently confirmed. Consequently, R&D combining the above-mentioned achievements for longer tapes are expected in the next stage. [Copyright &y& Elsevier]

Abstract: YBa2Cu3O y (YBCO) films grown by starting solutions with Ba-poor (Y:Ba:Cu=1:1.5:3, Ba/Y=1.5) have been reported to perform higher J c than that for YBCO films with stoichiometric one due to smaller and less pores in YBCO films by metal organic deposition (MOD) using tri-fluoroacetates (TFA). However, we have reported that a lot of non-reacted particles such as Y and Cu oxides were remained for the YBCO film surface grown by the precursors using Ba-poor solution. In this study, influences of YBCO growth rate on microstructures of the YBCO film were discussed to control microstructures of these second phase particles. As a result, microstructures of the YBCO film depended strongly on the YBCO growth rate, residues of Y and Cu oxides were dispersed randomly throughout the YBCO film grown at higher growth rate. Consequently, it was succeeded to control the segregated phases as seen in the previous YBCO film with Ba/Y=1.5 at the film surface into the YBCO film. J c of the YBCO film grown at higher rate was a value almost equal to that grown at previous growth rate. [Copyright &y& Elsevier]

Abstract: YBa2Cu3O7− δ (YBCO) films were grown by using precursor films with barium concentration gradient in film thickness by an advanced metal organic deposition process using trifluoroacetates (TFA-MOD). We have reported previously that a lot of non-reacted particles such as Y- and Cu-oxides were remained for the YBCO film surface grown by the precursors using a starting solution with barium-poor (cation ratio as Y:Ba:Cu=1:1.5:3). Then, the barium concentration was increased in the film surface to complete the reaction among these Y and Cu residues and Ba and to increase the Y123 growth thickness for realizing higher J c performance. Transmission electron microscopy (TEM) observation showed the increase of film thickness of YBCO grown by the precursors with barium concentration gradient in film thickness, indicating that the reaction between these Y and Cu-oxides and Ba proceeded to form Y123 phase. Consequently, higher J c was obtained for the YBCO film by the precursors mentioned above than that of the YBCO film by the precursors with a constant concentration. Effects of barium concentration gradient in the precursors on the superconducting properties and microstructures in the YBCO film were discussed. [Copyright &y& Elsevier]

Abstract: YBa2Cu3O y (YBCO) coated conductors were fabricated by trifluoroacetates (TFA)–metal organic deposition (MOD) previously using a single reel-to-reel (RTR) crystallization furnace with a gas flow system parallel to the tape surface which could realize only a low production rate. In this study, we developed a new multi-turn (MT) RTR furnace with a vertical gas flow system to increase the production rate of the crystallization step and attained the uniform reaction in the tapes of the multi-turn system. We fabricated YBCO films with different conditions partial pressure of water vapor, the total pressure and the gas flow volume using the MT-RTR furnace with the vertical gas flow system, in order to investigate the influence of the processing parameters on the growth rate and the superconducting properties of the YBCO films. It was found that the growth rate of the YBCO phase film increased under the conditions of the high gas flow rate, the low total pressure and the high water vapor partial pressure. As a result, 21 times higher J c value was attained at 17 times faster growth rate than that without controlling the processing parameters in the crystallization step. This is thought to be due to the suppression of coarsening of Y2Cu2O5 and CuO particles in the precursor film. Consequently, the high I c,end-to-end value of 250 A was achieved in a 5m long tape fabricated at the production rate of 3m/h using the only two lanes of the entire MT-RTR crystallization furnace with 10 lanes. [Copyright &y& Elsevier]

Abstract: YBa2Cu3O7− δ (YBCO) films were grown on LaAlO3 substrates by a metal organic deposition using trifluoroacetates (TFA-MOD). In this study, effects of calcination condition such as temperature on microstructures and J c properties for the YBCO films were studied. The TFA starting solution was coated on a substrate by a spin-coating method. Then, a two-step heat treatment was applied to the coated films. In the calcination step, the coated film was calcined to form a homogeneous amorphous precursor by increasing the temperature which was varied from 300°C to 500°C at a constant heating rate of 2°C/min. The prepared precursor films were crystallized at a constant condition. As a result, J c value depended on the calcination temperature, YBCO film calcined at 400°C showed the highest J c of 2.5MA/cm2. J c’s for the YBCO films calcined at 300°C and 350°C were 1.9MA/cm2 and 2.0MA/cm2, respectively. On the other hand, YBCO film calcined at 500°C showed the lowest J c of 1.4MA/cm2. Cross-sectional TEM images showed that the YBCO film calcined at 400°C had less pores and small size of pores in the crystallized film randomly throughout the film. Large pores were observed in the YBCO film calcined at 500°C. Also, these large pores were existed near substrate in the YBCO film. It is considered that the differences of these pore formation mechanisms are the main cause of the difference of J c characteristic. [Copyright &y& Elsevier]

Abstract: The importance of the β-FeSi2 bulk single crystals has increased not only to investigate the intrinsic properties of β-FeSi2 but also to use it as substrate of β-FeSi2 thin films for optical devices. Though single crystals of β-FeSi2 are grown by chemical vapor transport (CVT) method, most of those crystals are needle-like and widths of those flat surfaces are 0.5 mm or less. In order to understand the mechanism of the growth process of β-FeSi2 by the CVT method and to obtain the conditions for large size crystal growth, we have carried out in-situ observations of the crystal growth by using a transparent electric furnace. Based on the experimental data, we have proposed the most likely reaction process, FeI2(g)+2SiI4(g)→FeSi2(s)+5I2(g), and we found that the crystal growth progresses under the environment where the FeI2 gas is insufficient compared with a suitable ratio of FeI2/SiI4. Then, to raise the partial pressure of FeI2 gas, the composition ratio of Fe to Si for the source material was increased and we have obtained the plate-type β-FeSi2 crystals that exceeded a few square millimeters in size. [Copyright &y& Elsevier]

Abstract: The crystal growth mechanism of TFA (trifluoroacetates)–MOD (metal organic deposition) derived YBa2Cu3Oy has been investigated to understand the process for higher production rates of the conversion process. YBCO films were prepared by TFA–MOD on CeO2/Gd2Zr2O7/Hastelloy C276 substrates. The growth rates of YBCO derived from Y:Ba:Cu=1:2:3 and 1:1.5:3 starting solutions were investigated by XRD and TEM analyses. YBCO growth proceeds in two steps of the epitaxial one from the substrate and solid state reaction. The overall growth rate estimated from the residual amounts of BaF2 with time measured by XRD is proportional to a square root of P(H2O). The trend was independent of the composition of starting solutions, however, the growth rate obtained from the 1:1.5:3 starting solutions was high as twice as that of 1:2:3, which could not be explained by the composition of BaF2 included in the precursor films. On the other hand, the growth rate measured from the thickness of the YBCO quenched film at the same process time showed no difference between the samples of 1:2:3 and 1:1.5:3. The epitaxial growth rate of 1:1.5:3 was also the same as the overall growth rate of that, which means there was no solid state reaction to form YBCO after the epitaxial growth. The YBCO growth mechanism was found to be as follows; YBCO crystals nucleate at the surface of the substrate and epitaxially grow into the precursor by layer-by-layer by a manner with trapping unreacted particles. The amounts of YBCO and the unreacted particles trapped in the YBCO film are independent of the composition of the starting solution in this step. Unreacted particles react with each other to form YBCO and pores by solid state reaction as long as there is BaF2 left in the film. The Ba-poor starting solution gives little BaF2 left in the film and so the solid state reaction is completed within a short time, resulting in the fast overall growth rate. [Copyright &y& Elsevier]