Single-coated thick superconducting films for metal–organic deposition using trifluoroacetates

Abstract

Metal-organic deposition using trifluoroacetates (TFA-MOD) is known to yield uniform superconducting wires by a liquid growth mode. However, it has been difficult to prepare thick films because of drying stress during the calcining process. To avoid the drying stress, conventional crack-preventing chemicals such as H(CH₂)₈COOH are applied in conventional metal–organic deposition. However, large amounts of hydrogen atoms react with fluorine atoms during calcining process in TFA-MOD, and the consequent increased harmful carbon residue decreases superconductivity of the resulting films. To avoid the chemical reaction, new crack-preventing chemicals such as H(CF₂)₈COOH were applied to prepare single-coated thick films. A low ratio of hydrogen atoms decreases the chemical reaction and generates hydrogen fluorine gas, consequently suppressing the carbon residue. Above the calcining temperature, the crack-preventing chemical is decomposed into low-boiling-point chemicals such as CF₂CF₂ or CF₃CF₃. Consequently, single-coated thick film having low carbon residue and sufficient superconducting current per width was realized. For a long time, the authors have studied other possible candidate crack-preventing chemicals. Newly introduced fluorine ion measurement of decomposed materials during the calcining process revealed the nature of the crack-preventing chemicals. Based on the accumulated results, we have concluded that among over one million chemicals there are only two groups suitable for preparing single-coated thick superconducting films by TFA-MOD. One group is hydrogenated perfluoro-carboxylic acids such as H(CF₂)₈COOH and the other group is perfluoro di-carboxylic acids. With H(CF₂)₈COOH, using a single-coating process we were able to achieve a 560 nm-thick YBa₂Cu₃O_6.93 film having J_c of 4.70 MA/cm² (77 K,0T). Compared with a standard 150 nm-thick YBa₂Cu₃O_6.93 film having J_c of 7.70 MA/cm² (77 K,0T), the critical current per width is improved to about 227 %.