Phase Equilibrium Of Bi-Sr-Ca-Cu-O High Temperature Superconducting System And Thermodynamic Stabilization Of Bi-2212 Thin Film

In this paper, we studied the phase equilibrium of the Bi-Sr-Ca-Cu-O system and the thermodynamic stabilization of Bi-2212 thin film.The decomposition of Bi₂Sr₂Ca₁Cu₂O_8+δ(Bi-2212) single crystal was studied in situ with high-temperature optical microscopy (HTOM). It was found that the decomposition of Bi-2212 led to the formation of a needle-like phase. Energy dispersive x-ray spectrometry (EDX) compositional analysis, combined with scanning electron microscopy (SEM) and X-ray diffraction (XRD), identified the needle-like phase as (Sr,Ca)1Cu1O2(1:1), which has an epitaxial relation: (0k0)1:1 // (001) Bi-2212. The lattice mismatching between 1:1 and Bi-2212 is about 4.34% or 4.73%. Thus, the present result is well interpreted by epitaxial stabilization theory.In order to make a further investigation of the phase evolution of Bi-2212, two kinds of Bi-2212 powders were used as samples for in situ observation by HTOM. It was clarified that during the same heating process to 885oC, the impure Bi-2212 powder specimen decomposed to 1:1 phase and (Sr,Ca)2Cu1O2(2:1) phase, respectively, while the pure Bi-2212 powder sample only decomposed to 1:1 phase. This difference is attributed to the existence of Bi-Sr-Ca-O phase in the impure Bi-2212 powder sample.In previous work, a novel superheating of 50K above the peritectic temperature (Tp) in the YBa2Cu3O7-x (YBCO) thin film deposited on a MgO substrate was reported by our lab. In order to confirm whether there is a universal superheating phenomenon in oxide thin films, we performed similar studies for the Bi-2212 thin film. Unlike the YBCO thin film, no obvious superheating phenomenon was detected during the melting of Bi-2212 thin film. This dissimilarity is caused most likely by the surface energy difference on the (001) plane between YBCO and Bi-2212.