The Investigation On The Relationship Between The In-plane Orientation Of The YBCO Thin Film And Its Thermal Stability

In this paper, we studied the melting behaviours and thermal stability of c-axis YBCO films with different in-plane orientation, in various atmospheres.First, the melting behaviours of two kinds of c-axis oriented YBa2Cu3Ox (YBCO) films with 0o and 45o in-plane orientations were observed in-situ by using high temperature optical microscopy. Before the experiment, the in-plane orientation was detected by pole figures and the atomic configuration in the vicinity of the interface of the samples was evaluated by electron diffraction and high-resolution transmission electron microscopy (HRTEM). It is found that the melting mode and thermal stability of YBCO thin films with different in-plane orientations differ completely from each other: the 0o oriented film is in the metastable state during the melting process, and shows obvious superheating phenomenon; on the other hand, the 45o oriented film is in the unstable state during the melting process, and there is no superheating phenomenon exist in this film. Such an experimental result is greatly influenced by the geometrical lattice matching between YBCO thin films and the MgO substrate, as well as the terminal plane of the YBCO thin films.In order to make a further investigation on the relationship between the thermal stability of YBCO thin films and the oxygen partial pressure in the experimental environment, the melting behaviours of the 0o and 45o oriented YBCO films were observed in-situ under oxygen and nitrogen atmosphere, respectively. The experimental results show that, under the oxygen atmosphere, both the 0o and 45o oriented YBCO films have obvious superheating phenomenon, and the thermal stability of the 45o oriented film is even better than that of the 0o oriented film. The traditional geometrical lattice matching theory can not explain this abnormal phenomenon. On the basis of thermodynamic theory and the melting experiment on the simulative interface, we finally found that the thermal stability of the terminal layer of the 45o oriented YBCO thin film will become better with the increase of the oxygen content in the experimental environment. This would be the reason that cause the great improvement of the thermal stability of 45 o oriented YBCO thin film under oxygen atmosphere. While under the nitrogen atmosphere, the melting points of both films decreased obviously comparing with the corresponding temperatures in the air and oxygen atmosphere. Besides, the coarsen experiment of the c-axis YBCO thin films, which have 8 symmetry in-plane orientation, also shows that the 0o oriented YBCO grains have better thermal stability under the air environment, but under the oxygen atmosphere, the 45o oriented grains are more stable. This result is consist with the conclusion we made about the melting experiments.