Vol.7 No.4 2015
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Research paper : Preparation of superconducting films by metal organic deposition (T. MANABE et al.)−245−Synthesiology - English edition Vol.7 No.4 (2015) substrate even under the same heat treatment conditions. Initially, we did not know the reason for this, but referring to the Ellingham diagram in Fig. 6, it was revealed that there were areas in which the YBCO films readily become c-axis oriented (orientation that the superconducting current easily flows) with high Jc around the temperature of thermal decomposition of YBCO and CuO, and areas in which the YBCO films readily become a-axis oriented (orientation that the superconducting current does not easily flow) with low Jc in the low-temperature side or areas of high oxygen-partial pressure, just as in the gas phase method.[23][24] Using this property, the c-axis oriented film is deposited in the c-axis oriented area by the gas phase method.[25] On the other hand, in the MOD method, since the prefired film that was once deposited underwent final heat treatment, the crystal growth of the a-axis grains is likely to occur locally as the substrate surface area increased as it passed through the a-axis oriented area in the heat process in the ordinary electrical furnace with a small heating rate. It was thought that the inclusion of the a-axis orientation occurred due to this phenomenon.Therefore we introduced an infrared image furnace that enabled rapid heating. As a result of investigation on the heating rate and uniform heating conditions, the c-axis oriented film was obtained by rapid heating, i.e., by quickly passing the low-temperature zone where the a-axis orientation tended to occur and the a-axis oriented growth was inhibited. The YBCO film with a thickness of 700 nm manufactured on the lattice-matched LaAlO3 (LAO, mismatch about 2 %) with a diameter of 5 cm was extremely dense and smooth, and Jc measured by the inductive method was extremely high (>2 MA/cm2).[26][27] Even with rapid heating, since YBCO and LAO are lattice matched and the thermal expansion coefficients are close (YBCO: 13 × 10−6/K; LAO: 12.6 × 10−6/K[9]), no cracks occurred. Thus,it was possible to obtain a YBCO thick film with high Jc on a lattice-matched LAO substrate. However, the maximum size that can be manufactured for a LAO substrate is about 5 cm in diameter, and a larger surface area cannot be obtained. Also, the thermal shock resistance and heat conductivity are low, and the substrate tends to be damaged due to heat stress when it is cooled in liquid nitrogen in the quenching process, and therefore it is considered unsuitable for FCL application.5.2 Formation of a buffer film on a sapphire (lattice-mismatched) substrateAs a substrate material for superconducting films for FCL application, sapphire (single-crystal alumina) is optimal since heat conductivity and thermal shock resistance are high and large-surface area substrate is available. However, sapphire chemically reacts with YBCO, has a different crystal structure, and has large lattice mismatch (about 10 %), and these make the direct epitaxial growth of YBCO difficult. Therefore, similar to the gas phase methods,[10]-[13] CeO2 (lattice mismatch of about 1 %) was used as the buffer layer to mitigate the lattice mismatch as well as to inhibit chemical reaction.When the CeO2 buffer layer was formed by a vacuum vapor deposition method by changing the deposition conditions (temperature, deposition rate, oxygen pressure, and plasma gasification conditions) on the sapphire substrate, the orientation of the CeO2 could be arranged in desirable directions (100) by plasma gasification of oxygen by a radiofrequency (RF) antenna and by increasing the substrate temperature. Then, the CeO2 buffer layer with a smooth surface at nanometer level could be obtained.[28][29]Concurrent to the buffer layer deposition, we attempted tuning with the YBCO deposition by the MOD method on the buffer layer. Although the heat treatment condition was about the same as on the lattice-matched substrate, when CeO2 was used for the buffer layer, the production of BaCeO3 by reaction with YBCO became an issue. When BaCeO3 is produced, the amount of Ba in the film decreases, and not only does the metal composition ratio depart from 1:2:3 but also the crystallization property of YBCO decreases and the superconductivity degrades significantly. When we investigated the heat-treatment condition of the YBCO film when the CeO2 buffer layer was used, it was found that BaCeO3 was likely to be produced in high temperature or low oxygen partial pressure side, as shown in Fig. 6. It was also found during the optimization of the YBCO deposition condition on the CeO2 buffer layer, that although CeO2 had small lattice mismatch with YBCO, it had a fluorite-type crystal structure that was different from YBCO, and the YBCO crystal growth rate became relatively small on CeO2. Therefore, no rapid heating using the infrared image furnace was required as in the lattice-matched substrate, and only heating with a tubular furnace was necessary. As a result of tuning the buffer layer deposition method and the heat treatment conditions, we succeeded in depositing YBCO with high Jc at maximum heat-treatment temperature of about 750 °C, with a CeO2 buffer layer of 40 nm (achievement of Goal II-2).[30][31]5.3 Achievement of large-area superconducting/buffer/sapphire multilayersNext, we attempted to deposit the buffer layer on a large-area sapphire substrate and to form the superconducting multilayer on this layer. Here, the key issue was the uniformity of thickness of both layers deposited and of temperature and atmosphere of heat treatment.For the buffer layer deposition, two vapor deposition sources were installed to improve uniformity, the decrease of substrate temperature was prevented by devising a heater and shield, and oxygen was plasma activated by a RF antenna. By increasing the power of RF and maintaining the substrate

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