Transient liquid phase-assisted fluorine-free chemical fabrication of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7–<i>δ</i></sub> and complete [Ba-Cu-O]<sub>L</sub> liquid phase films: Phase transfo

The epitaxial orientation of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7–<i>δ</i> </sub>grown via the oxygen partial pressure jump pathway in transient liquid-phase assisted chemical solution deposition (TLAG-CSD) depends on the barium-to-copper ratio in the precursor phase. To explore the mechanism behind this phenomenon, in this work we investigate the effects of different oxygen partial pressures and barium-to-copper ratio components on the barium-copper-oxygen liquid phase ([Ba-Cu-O]<sub>L</sub>) and the intermediate phase transition in the medium-high temperature heat treatment process. The research shows that the formation of the liquid phase exhibits a point-to-surface characteristic; the temperature and morphological differences in the liquid phase are mainly determined by the composition, with oxygen partial pressure only playing a supporting role. Y∶Ba∶Cu = 0∶3∶7 (0-3-7) components all appear before Y∶Ba∶Cu = 0∶2∶3 (0-2-3) components in the liquid phase, with a temperature difference of 20 ℃ (high oxygen partial pressure) or 40 ℃ (low oxygen partial pressure). Experimental results indicate that there are differences in the intermediate phase properties between these two components. Under high oxygen partial pressure, the intermediate phase BaCuO<sub>2</sub> exhibits a single characteristic peak in the 0-3-7 component, with large and dispersed grains; the 0-2-3 component has multiple characteristic peaks, with small and dense grains. The surface area of the liquid phase region in the 0-3-7 component is smaller than that in the 0-2-3 component, resulting in different supersaturation levels of Y<sup>3+</sup> in the liquid phases of the two components and causing orientation differences in YBCO. Finally, the basic model for the formation of fluorine-free liquid phase is summarized, and the complete [Ba-Cu-O]<sub>L</sub> film can be generated from the 0-2-3 component at high oxygen partial pressure and 750 ℃.