Vol.7 No.4 2015
Research paper : Preparation of superconducting films by metal organic deposition (T. MANABE et al.)−241−Synthesiology - English edition Vol.7 No.4 (2015) breakers to ones with larger ratings. In contrast, when FCLs are introduced as shown in Fig. 1(b), the existing distribution lines and breakers can be used and the facilities can be laid out readily. Therefore, the realization of such FCLs is eagerly awaited. Here, FCL is a device that inhibits the overcurrent to flow into the circuit to protect the power network system (distribution and main lines) from fault current.Currently, passive (autonomous action) FCLs including the thin film resistive-type and rectifier-type, as well as active FCLs such as the semiconductor switch type and arc driven type are being developed. The thin film resistive-type FCL (Fig. 2) is a type of passive FCL that uses the phenomenon where the superconducting thin film changes instantly from superconducting to normal conducting states and large resistance is generated when the overcurrent flows through the superconducting thin film (this phenomenon is called the SN transition or quenching) to inhibit the fault current. Since there are no moving parts in this method, it is reliable compared to active FCLs. Since the series-parallel arrangement of superconducting film is capable of handling high voltage and large current, there is expectation for the application to high-volume interconnection of distributed power supply sites using the low-cost superconducting film.The functions required for the superconducting film for thin film resistive-type FCLs are as follows.(1) Large critical current (current can flow in the superconducting state)→ Critical current density (Jc: hereinafter, critical current per 1 cm2 of cross section at 77 K) must be high and the thin film be wide(2) When it shifts to a normal conducting state, it must have high resistance and produce high voltage→ Thin film must be long in the direction of the currentThe width and length of the superconducting film are related to the current and voltage, respectively, and the loss by the number of steps to obtain series-parallel arrangement and connection resistance increases as the number of sheets of superconducting film increases. Therefore, a superconducting film with high Jc and a large surface area is necessary. The developmental goals of the “R&D of the Core Technology for Superconducting AC Equipment” funded for the Technological Development for Diversification of Power Source were as follows:• High critical current density (Jc > 1,000,000 A/cm2)• A large surface area (10 cm × 30 cm)Here, the Jc value of the superconducting film is strongly dependent on the microstructure of the thin film, and it is necessary to have a single-crystal film where the YBCO particles are arranged three dimensionally to achieve high Jc. Therefore, it is necessary to manufacture a single-crystal superconducting thin film using the single-crystal with good lattice match (small difference of lattice constant) with YBCO as a substrate, and then epitaxially grow the YBCO on such a substrate. As it will be mentioned later, the sapphire (single-crystal alumina) substrate is highly regarded as the substrate for superconducting film for FCL from the perspective of thermal shock resistance and thermal conductivity. The largest size of commercially available sapphire was 10 cm × 30 cm. Since sapphire has poor lattice match with YBCO (about 10 % mismatch) and reacts with YBCO at high temperature, it is necessary to form an appropriate buffer layer between the two. Also, the superconducting film must be thick to increase the critical current, but the thermal expansion coefficient of YBCO (13 × 10−6/K) is about twice that of sapphire (5~7 × 10−6/K). When the film thickness of YBCO surpasses 300 nm (critical film thickness), micro-cracks may occur due to heat stress when cooling from the deposition temperature (700~800 °C), and therefore, the film thickness that can be obtained with sapphire is 300 nm or less.3 Comparison of the MOD method and conventional large-area deposition technology and the scenario to realize the goalAs it is clear from chapter 2, the establishment of synthesis technology for large-area superconducting films with high Jc is necessary for the development of thin film resistive-type FCLs. Meanwhile, the authors have been engaging in the research of a YBCO thin film preparation process using the MOD method immediately after the discovery of YBCO. In this chapter, we shall describe the R&D scenario to achieve the goal for the product requirement extracted in chapter 2, when preparing the large-area superconducting film by the MOD method for FCL application, after comparing the MOD and the conventional large-area deposition technologies.3.1 Comparison of the MOD method and other large-area deposition technologiesAs shown in Fig. 3, the MOD method and the conventional large-area deposition technologies for metal oxides can be compared as follows.1) Conventional technology(1)Gas phase method (vacuum evaporation, pulsed laser deposition (PLD), sputtering, and chemical vapor deposition): The component atoms (molecules) are dissociated in the gaseous phase and then deposited on a substrate. Dense and good quality epitaxial film can be manufactured.(2)Liquid phase method (slurry coating, sol-gel): The slurry, in which the powder of the target substance is dispersed in a solvent or a sol where a metal alkoxide is hydrolytically polycondensed, is coated onto a substrate, dried, and fired to manufacture a ceramic film.