Mariko Hayashi

Growth Mechanism During Firing Process of Single-Coated Thick YBCO Films by TFA-MOD

Single-coating technology for metal organic deposition using trifluoroacetate has various advantages: no inhomogeneous interface in the case of multilayers, no random orientation from the interface, and no deterioration of material through two or more heat treatments. However, single-coated thick film inevitably has many voids. In order to minimize voids, both a calcining process and a firing process are required. This paper focuses on the firing process. During the firing process, balance of <i>V</i>_d and <i>V</i>_g is important. <i>V</i>_d and <i>V</i>_g are the drop rate of quasi-liquid and the growth rate of YBa₂Cu₃O_{7 -}<i>x</i>, respectively. We found that the volume of voids can be reduced in the condition of V_d > V_g, where gas flow is low, temperature high, and humidity low. We prepared gel films whose thickness was about 50 μm and calcined them to realize thick calcined films. The films are fired to realize 5.2-μm-thick superconductor on LaAlO₃ single crystal. Transmission electron microscopy observation revealed oriented layers. The film is still 20% void by volume. However, the film must be 40% void by volume without optimal firing process based on a quasi-liquid network model. The quasi-liquid network model is effective for reducing voids. Optimization of the calcining process will spur further development.

Required Crack-Preventing Chemicals and Chemical Background for Single-Coated Technology to YBCO Thick Films by TFA-MOD

Single-coating technology for metal-organic deposition using trifluoroacetate (TFA-MOD) is indispensable, because single-coated film has no intermediate layer that causes fatal deterioration in superconducting properties. This technology requires a crack-preventing chemical (CPC). Our group reported several CPCs at ASC2008. With the CPCs, we obtained single-coated thick films with thickness of over 1 micrometer. Some CPCs cause crack formation and increase carbon and fluorine residues in the resulting superconducting films. The objective of this study is to discover CPCs that cause no crack formation. Our study concludes that CPCs for TFA-MOD must have the following attributes: no branch structure, no ether bonds, no chemical double bonds in the carbon chain, strong acidity (pH -1 to 1), -(CF2)- chain, and hydrogen terminations. Among more than 1 000 000 organic chemicals, there are only two series of chemical groups that work effectively as CPCs in TFA-MOD : H-(CF2)n-COOH and HOCO-(CF2)n-COOH.