Thermo-mechanical/structural properties and oxygen permeation behavior of mixed ionic electronic conductors lanthanum-strontium-cobalt oxide.

摘要

Mixed ionic-electronic conducting ceramics (MIEC) with perovskite structure are good candidates for gas sensors, gas separation, and/or SOFC electrodes. Due to high operating temperatures in these systems, physical compatibility and chemical stability of MIEC, as well as effective oxygen electro-catalysis and higher ionic transport properties, are the essential criteria for the successful application of MIECs. This provides thermodynamic, kinetic, and mechanical properties of La1-xSr xCoO3-delta (LSC) and/or other MIEC ceramics, and insight into how these properties can be used to aid material design and development in high temperature electrochemical applications.; The equilibrium thermal and chemical expansivities, beta T and betaC, in LSC were systematically measured by vacuum-controlled dilatometry as functions of oxygen partial pressure, temperature, and oxygen nonstoichiometry. The chemical expansion resulted from a 2% change in oxygen content were found equivalent to the thermal expansion attained from a temperature change of about 100°C. Normalized chemical expansion was discovered to follow a consistent nonlinear trend with oxygen content, which may result from the cooperative relaxation of lattice strain with increasing defect concentration. A slowly-relaxing secondary expansion effect (and/or chemical expansion hysteresis) was also discovered in equilibrium chemical expansions and has been found to relax in long time scales and be PO2 dependent. The studies of ex-situ and in-situ XRD of LSC revealed that LSC undergoes phase transition from high to low symmetry and/or phase separation to form non-perovskite materials. The cation rearrangement associated with the phase instability mainly contributes to the discovered expansion hysteresis.; Analysis of transient chemical expansion relaxation profiles can provide an understanding of change in oxygen permeation with temperature and oxygen partial pressures. However, macro- and micro-structures of MIEC (associated with phase instability as well as thermal and chemical stresses) change with thermal history, which makes it challenging to study oxygen surface exchange and bulk diffusion behaviors accurately in this family of MIEC via chemical expansion relaxation techniques or by other techniques.