Preparation of YBCO by TFA-MOD

At present one of the most promising and widely investigated chemical solution deposition processes for the preparation of YBCO layers within the YBCO coated conductor development is the TriFluoroAcetate (TFA) process. The major advantage of this fluorine-based process is the suppression of the formation of the highly stable critical current degrading intermediate BaCaO₃ through the formation of thermodynamically more stable BaF₂. During the precursor solution synthesis, trifluoroacetates are formed from metal (metal: Y, Ba, Cu) acetates dissolved in de-ionized water and trifluoroacetic acid. The resulting aqueous solution is refined to yield a glassy blue gel and dissolved in methanol to form the final coating solution. During pyrolysis, a film microstructure of CuO nanocrystallites embedded in an amorphous oxyfluoride Y-Ba-O-F matrix is formed, which is subsequently converted into the highly textured YBCO phase during firing at temperatures between 725°C and 850°C. An following oxygen anneal below 500°C is required to dope the superconductor yielding high T_c. Reproducible high critical current carrying YBCO layers can be obtained by this process.

At the IFW uniform high critical current densities up to 4--5 MA/cm² were achieved on 200 nm thick TFA-YBCO layers deposited on single crystalline SrTiO₃. The typical microstructure of a TFA-YBCO layer and the measured E(J)-curve are shown in Figure 2 and 3. More recent work is focused on the incorporation of artificial pinning centres like BaHfO₃ in the YBCO matrix during the TFA process.

Fig. 1

Fig. 2	Fig. 3

Fig. 1:  Schematically drawing of steps during the TFA-process from the as-deposited precursor layer to the crystallized YBCO layer. A humid atmosphere is necessary during the heat treatment to form HF.

Fig. 2:  SEM picture of a typical TFA-YBCO layer on SrTiO₃.

Fig. 3: E-J-curve of a 200 nm thick TFA-YBCO on STO showing a critical current density J_c of 4.05 MA/cm² (electrical field criteria: 1 µV/cm).

selected publications

Engel, S., Thersleff, T., Hühne, R., Schultz, L., Holzapfel, B.
Enhanced flux pinning in YBCO layers by the formation of nanosized BaHfO₃ precipitates using the chemical deposition method
Appl. Phys. Lett. 90 (2007) 102505

Knoth, K., Engel, S., Apetrii, C., Falter, M., Schlobach, B., Hühne, R., Oswald, S., Schultz, L., Holzapfel, B.
Chemical solution deposition of YBa₂Cu₃O_7-x coated conductors
Curr. Opin. Solid State Mater. Sci. 10 (2006) 205-216

Falter, M., Demmler, K., Häßler, W., Schlobach, B., Holzapfel, B., Schultz, L.
Chemical solution deposition (CSD) of YBa₂Cu₃O_7-x films and oxide buffer layers by dip coating
IEEE Trans. Appl. Supercond. 13 (2003) 2751-2754

Falter, M., Häßler, W., Schlobach, B., Holzapfel, B.
Chemical solution deposition of YBa₂Cu₃O_7-x films by dip coating
Physica C 372-376 (2002) 46-49