The fluorine-free metal organic decomposition (FF-MOD) method is considered to be most suitable for mass production of REBCO thin film layer with low cost among various methods such as Pulsed-Laser-Deposition (PLD), Metal-Organic-Chemical-Vapor-Deposition (MOCVD), trifluoroacetate (TFA)-MOD and Reactive Co-Evaporation by Deposition & Reaction (RCE-DR), because of fast crystal growth, flat and clean surface, relatively low cost production system and less material loss. However, development of FF-MOD processed REBCO tapes was not very active due to low Jc under magnetic field originated in very high crystallinity of REBCO layer with less defects acting as pinning centers. Recent successful studies on the introduction of fine nonsuperconducting precipitates in GdBCO layer, increase in the thickness of the layer by multiple sintering process and coating of raw solutions on long metallic tapes [1] suggested that high future potential of FF-MOD method. A problem remains in starting raw materials used to prepare solutions of FF-MOD. Acetates or acetylacetonates (acac) of constituent metals are used in most of studies, however, they are not inexpensive and some of them are hydrates, resulting in difficulty in precise control of metal composition. In the present study, we have attempted to develop a new method to prepare FF-MOD solutions with low cost, high productivity, high homogeneity, high cation concentration and well-controlled cation compositions.

The discovery of easy dissolution of YBCO fine powder in propionic acid at room temperature was a breakthrough of this study. In addition, we found the various REBCO powder and sintered bulks can be dissolved in propionic acid with high concentration by increasing temperatures up to the boiling point of the acid 414 K. After evaporating excess solution, propionate crystals containing RE, Ba and Cu were obtained. By using new FF-MOD solutions, which were prepared by dissolving these propionate crystals in a mixed solvent of methanol, butanol and water, high J_cREBCO films on SrTiO₃ single crystals and IBAD substrates were successfully synthesized with high reproducibility under wider sintering conditions expanding lower temperature down to 993 K. Details of the superconducting properties of REBCO films and preparation process of the new FF-MOD solutions will be shown and future prospects of the FF-MOD processed REBCO films will be discussed.

[1] T. Yoshihara, G. Honda, T. Nagaishi, S. Kobayashi, K. Kanie, T. Okada, and S. Awaji, IEEE Trans. Appl. Supercond., 33, 6600205 (2023)