Ceramics oxides with mixed ionic and electronic conductivity have received a lot of attention due to their wide range of applications in solid oxide fuel cells, interconnects, gas sensors, and ion transport membranes. However, owing to harsh operating conditions, the choice of proper materials and engineering their properties are still challenging. Perovskite and fluorite structures are two promising structures for ceramic membrane applications. The objective of this research is to explore the stability of lanthanum chromite-based perovskite ((La,Sr)(Cr,Fe)O3-δ) as single phases and dual-phase composites with fluorite phases under fabrication and operating conditions of Oxygen Transport Membranes (OTM). The current research has been categorized into two sections: structural and chemical stability of perovskite phases and dual-phase perovskite/fluorite composites. Also, investigation on both categories has been conducted with two separate approaches: experimental examinations and computational Thermodynamic. In the computational part, independent methods have been considered for the single-phase perovskite and dual-phase perovskite/fluorite composites. In the experimental section, the bulk chemical stability of the dual-phase samples has been examined under controlled oxygen partial pressure p(O2) atmospheres at 1400ᵒC for 10 hours with slow and fast cooling rates. Besides, the phase stability of the perovskite structures as a single-phase has been also examined under OTM fabrication conditions. The results present new phenomena in the chemical stabilities of the materials. They include formations of liquid phases, Sr-segregation, and perovskite phase separations. The correlations between compositions/ temperature/ p(O2) and secondary phases have been investigated to improve the chemical stability and extend the lifetime of the materials. The findings in this thesis enhance the knowledge about the chemical stabilities of OTMs and help to develop more reliable materials for ceramic-based OTMs.

Ceramics oxides with mixed ionic and electronic conductivity have received a lot of attention due to their wide range of applications in solid oxide fuel cells, interconnects, gas sensors, and ion transport membranes. However, owing to harsh operating conditions, the choice of proper materials and engineering their properties are still challenging. Perovskite and fluorite structures are two promising structures for ceramic membrane applications. The objective of this research is to explore the stability of lanthanum chromite-based perovskite ((La,Sr)(Cr,Fe)O3-δ) as single phases and dual-phase composites with fluorite phases under fabrication and operating conditions of Oxygen Transport Membranes (OTM). The current research has been categorized into two sections: structural and chemical stability of perovskite phases and dual-phase perovskite/fluorite composites. Also, investigation on both categories has been conducted with two separate approaches: experimental examinations and computational Thermodynamic. In the computational part, independent methods have been considered for the single-phase perovskite and dual-phase perovskite/fluorite composites. In the experimental section, the bulk chemical stability of the dual-phase samples has been examined under controlled oxygen partial pressure p(O2) atmospheres at 1400ᵒC for 10 hours with slow and fast cooling rates. Besides, the phase stability of the perovskite structures as a single-phase has been also examined under OTM fabrication conditions. The results present new phenomena in the chemical stabilities of the materials. They include formations of liquid phases, Sr-segregation, and perovskite phase separations. The correlations between compositions/ temperature/ p(O2) and secondary phases have been investigated to improve the chemical stability and extend the lifetime of the materials. The findings in this thesis enhance the knowledge about the chemical stabilities of OTMs and help to develop more reliable materials for ceramic-based OTMs.