Vol.7 No.4 2015
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Research paper : Preparation of superconducting films by metal organic deposition (T. MANABE et al.)−242−Synthesiology - English edition Vol.7 No.4 (2015) 2) Problems of the conventional method(1)The gas phase method requires the simultaneous control of the processes of gas production and deposition on the substrate, and therefore, controlling the composition and achieving large surface areas are difficult. Also, since high vacuum and high voltage are necessary, expensive facilities and a large amount of power are required, thus making the process costly and energy consuming.(2)As powder or gel formed by drying sol is fired, the liquid phase method results in a polycrystalline, non-oriented film with low performance.It is possible to obtain an epitaxial film with high Jc by the gas phase method, but it is expensive and may generate non-uniformity in a large surface area. Conventionally, the maximum size of a YBCO film manufactured by the gas phase method is: 20 cm in diameter (with a non-deposited area in the center) and 10 cm × 20 cm (substrate transfer in one direction) by co-evaporation;[10] 7 cm × 20 cm by PLD;[12] and 7 cm in diameter by sputtering.[13] On the other hand, the liquid phase method enables the production of a low-cost, uniform, large-area film, but it will be polycrystalline, non-oriented, and have low Jc.3.2 Scenario to achieve the goalIn this study, R&D was conducted using a scenario divided into the following two stages to achieve the goal to fulfill the product requirement extracted in chapter 2.I.Demonstration of YBCO thin film manufacturing and achievement of high JcII.Deposition of a high-Jc large-area YBCO filmWhen discussing the research of the superconducting film deposition by the MOD method in chronological order, initially only Scenario I was the goal of development. There was fierce international competition to develop the superconducting film deposition technology by a solution method immediately after the discovery of the YBCO superconductor. The authors were able to demonstrate the zero resistance of the YBCO film ahead of other research institutions and were able to file the patent. Immediately after the discovery of the YBCO superconductor, development in Josephson elements for thin film application and superconducting wire rods, coils, magnets, and others in the thick film application were discussed, but the achievement of high Jc (>1 000,000 A/cm2) was required in all these applications. Figure 4 shows the diagram of the research scenario at this point.When the firing temperature is high, a chemical reaction occurs at the interface between the YBCO film and the substrate, and therefore a low-temperature process was developed to inhibit this interface reaction. Then, a lattice-matched substrate that became available due to low-processing temperature was used to increase the orientation capability of the YBCO film, epitaxial film was formed unexpectedly even though it was through a solution method, and high Jc was obtained. The outline of this process will be discussed in the next chapter.When it became apparent that the epitaxial YBCO film could be manufactured in Scenario I, the talks began of power deregulation and large-volume interconnection of distributed power supply sites. Since the superconducting film FCL became hopeful in strengthening the durability of the electrical devices for high-volume interconnection of dispersed power at low cost, the core technology for “epitaxial YBCO deposition and achievement of high Jc” obtained in Scenario I was expanded to set Scenario II. However, many difficulties were predicted in manufacturing the large-area YBCO film with high Jc all at once, and the R&D was done concurrently to achieve the goals of II-1 and II-2 as follows. Ultimately, Goal II-3 would be achieved to fulfill the product requirement extracted in chapter 2.II-1 Achievement of large-area YBCO deposition on lattice-matched substratesII-2 Multilayer deposition of buffer and superconducting layers on sapphire (lattice-mismatched) substratesII-3 Manufacture of a large-area film with superconducting/buffer/sapphire multilayersThis scenario is shown in Fig. 5 and the outline will be explained in chapter 5. Table 1 shows the outlines of the elemental technologies that were necessary to achieve the goals in Scenario I and II for manufacturing the superconducting film, and the elemental technologies that played a major role in achieving the goals are framed by thick lines.Fig. 4 Scenario I for the manufacture of high-Jc superconducting film by MOD UseGoalJosephson elementWire rod, coil, magnetLengthening/thickeningDiscovery of high-temperature superconductorMODEpitaxial deposition, achievement of high Jc Ⅰ-2MOD, realization of high Tc Ⅰ-1

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