Vol.7 No.4 2015

Research paper : Preparation of superconducting films by metal organic deposition (T. MANABE et al.)−244−Synthesiology - English edition Vol.7 No.4 (2015) it reacted with BaCO3 in the prefired film. We obtained Tc = 90 K only when the yttria-stabilized zirconia sintered compact with low reactivity was used as the substrate, but the film was polycrystalline, and the Jc at liquid nitrogen temperature (77 K) was low (~1000 A/cm2).[16]4.2 Development of a low-temperature process and achievement of high JcThe development of a low-temperature process using low-oxygen pressure was the most important point in achieving Goal I-2.Since the superconductivity is lost when the high-temperature oxide superconductor is deprived of oxygen, it was conventionally fired in oxygen. The authors obtained the hint from the study by Kishio et al.,[17] and considered that the valence control of functional oxides that contain transition metals such as YBCO was important, and heat treatment must be done by controlling the oxygen partial pressure (pO2) and temperature (T). Therefore thermal analysis was conducted by changing the pO2 for the powder obtained by thermal decomposition (or prefiring) of the coating solution. As a result of x-ray diffraction of the product, it became apparent that the production temperature of YBCO could be decreased by 100 °C or more by using low oxygen pressure.[18]In the heat treatment at maximum temperature of around 700 °C, the reaction between the YBCO and a lattice-matched single-crystal substrate such as SrTiO3 could be sufficiently suppressed, and the YBCO film was formed on the SrTiO3 substrate. To improve the uniformity and reproducibility of the thickness of the film product, the solution was applied using a spin coater[19] and prefiring was done at 500 °C in an ambient atmosphere.Next we succeeded in decreasing the temperature by about 200 °C from the maximum temperature of the conventional heat process by optimizing the oxygen partial pressure and the heating rate for the final heat treatment of the prefired film (development of the low-temperature process). Figure 6 shows the schematic representation of the stable range of YBCO and copper oxides (Cu2O-CuO) on the Ellingham diagram, with the logarithm of oxygen partial pressure (pO2) and the reciprocal of temperature (1/T) as the two axes (orientation will be discussed in the next chapter).[20] Here, the conventional heat process in oxygen corresponded to Route I-1, while the low-temperature process to Route I-2. Since low-oxygen pressure was used in Route I-2, the non-superconductor YBa2Cu3O6 with less oxygen was produced in the final heat treatment, but by switching to 1 atm oxygen after the final heat treatment and allowing the oxygen to be incorporated into the crystal during cooling, it converted to superconductor YBa2Cu3O7. Moreover, as an amazing finding at the time, the YBCO film manufactured in Route I-2 grew epitaxially on the substrate even though it started from the solution, and a Jc of 1,000,000 A (=1MA) /cm2, which is equivalent to that of the YBCO film made by the gas-phase method was achieved at 77 K. Hence, Goal I-2 was achieved.[21][22]5 Deposition of a large-area YBCO film with high JcIn this chapter, setting the “success of epitaxial film formation, achievement of high Jc” of Scenario I-2 as core technology, the outline up to the realization of high- Jc large-area film by Scenario II as shown in Fig. 5 is explained.The YBCO film prepared on SrTiO3 substrate in chapter 4 was of a small size of 5 mm × 10 mm. Due to the reactivity of the substrate and the film as well as due to lattice mismatch, it was difficult to concurrently achieve the deposition on a large-area sapphire (single-crystal alumina) substrate that was desirable for FCL, as there was strong demand and desire to test the performance of the large-area YBCO film as soon as possible. Therefore, it was decided that, as shown in Scenario II in Fig. 5, while attempting to primarily increase the surface area of the YBCO film on the lattice-matched substrate, II-1, the manufacture of a buffer layer on the sapphire and tuning of YBCO deposition was conducted concurrently, II-2, and the enlargement of superconductor/buffer /sapphire layer was done afterwards, II-3.5.1 Achievement of large-area deposition on the lattice-matched substrateFor the achievement of Goal II-1, the selection of optimal heating rate in the low-temperature process was the main issue.When the surface area of the lattice-matched substrate was increased, Jc tended to decrease compared to the smaller Fig. 6 Orientation and reaction of the YBCO film in the Ellingham diagramYBCO decompositionⅠ -2Ⅰ -1BaCeO3formationYBa2Cu3O6a-axis orientationc-axis orientationCu2OYBa2Cu3O7400600800Temperature (℃)CuO101102103104105(O2) (Pa)p


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