Crystal growth of YBCO coated conductors by TFA–MOD method

Abstract

The crystal growth mechanism of TFA (trifluoroacetates)–MOD (metal organic deposition) derived YBa 2Cu 3O y has been investigated to understand the process for higher production rates of the conversion process. YBCO films were prepared by TFA–MOD on CeO 2/Gd 2Zr 2O 7/Hastelloy C276 substrates. The growth rates of YBCO derived from Y:Ba:Cu = 1:2:3 and 1:1.5:3 starting solutions were investigated by XRD and TEM analyses. YBCO growth proceeds in two steps of the epitaxial one from the substrate and solid state reaction. The overall growth rate estimated from the residual amounts of BaF 2 with time measured by XRD is proportional to a square root of P(H 2O). The trend was independent of the composition of starting solutions, however, the growth rate obtained from the 1:1.5:3 starting solutions was high as twice as that of 1:2:3, which could not be explained by the composition of BaF 2 included in the precursor films. On the other hand, the growth rate measured from the thickness of the YBCO quenched film at the same process time showed no difference between the samples of 1:2:3 and 1:1.5:3. The epitaxial growth rate of 1:1.5:3 was also the same as the overall growth rate of that, which means there was no solid state reaction to form YBCO after the epitaxial growth. The YBCO growth mechanism was found to be as follows; YBCO crystals nucleate at the surface of the substrate and epitaxially grow into the precursor by layer-by-layer by a manner with trapping unreacted particles. The amounts of YBCO and the unreacted particles trapped in the YBCO film are independent of the composition of the starting solution in this step. Unreacted particles react with each other to form YBCO and pores by solid state reaction as long as there is BaF 2 left in the film. The Ba-poor starting solution gives little BaF 2 left in the film and so the solid state reaction is completed within a short time, resulting in the fast overall growth rate.