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.

O uso do cálculo termodinâmico e consequentemente base de dados desenvolvidas para esse propósito é hoje uma das ferramentas fundamentais para o aprimoramento e desenvolvimento de superligas a base de níquel. A proposta deste trabalho foi avaliar a confiabilidade de uma base de dados comercial, específica para superligas a base de níquel, utilizada em softwares de cálculos termodinâmicos computacionais. Comparou-se os dados experimentais da superliga Inconel 713LC com informações obtidas através do calculo termodinâmico para a composição química específica da superliga estudada. As amostras da superliga comercial Inconel 713LC foram retiradas de barras da liga fundida a vácuo (VIM). Os tratamentos térmicos foram realizados sob atmosfera de argônio em cápsulas de quartzo nas temperaturas de 600?C, 900?C e 1250 ?C nos tempos de 10h, 24h, 50h, 100h, 250h, 500h, 1000h e 2000 horas, de acordo com a temperatura de tratamento utilizada, visando acompanhar o desenvolvimento microestrutural até alcançar a condição de equilíbrio termodinâmico. Previamente, a amostra no estado bruto de fusão e as amostras após os tratamentos de equilíbrio foram caracterizadas microestruturalmente via microscopia eletrônica de varredura (MEV) e a composição das fases medidas por microanálise de energia dispersiva (EDS). As temperaturas das transformações de fases foram medidas por analise térmica diferencial (DTA). Nas amostras também foram realizadas medidas de microdureza para acompanhar indiretamente a evolução microestrutural da fase gama prime. Para os cálculos termodinâmicos foram utilizados o software Thermo-Calc® e a base de dados TTNi8 da empresa Thermotech Ltd®. / The computational thermodynamic simulation and the databases designed for this purpose are today one the most important tools to develop new nickel based superalloys. The aim of this work was to check the reliability of a commercial database, designed specifically for Ni-based superalloys. The experimental phase diagram data of Inconel 713LC, such as, liquidus and solidus temperatures as well as chemical phase compositions were compared with thermodynamic information extracted from computational simulations. Samples of a commercial superalloy Inconel 713LC were extracted from an ingot produced by Vacuum Induction Melting (VIM) and heat treated under argon in quartz capsules at 600?C, 900 ?C and 1250?C for 10h, 24h, 50h, 100h, 250h, 500h, 1000h and 2000h, according to the treatment temperature used, aiming at reaching the thermodynamic equilibrium conditions. The as-cast and all heat-treated samples were characterized by Scanning Electron Microscopy (SEM) and the chemical compositions of the phases were measured via Energy Dispersive X-ray Spectroscopy (EDS). Temperatures of phase transformations were determined by Differential Thermal Analysis (DTA). Microhardness measurements were performed to indirectly follow the microstructural evolution of gamma prime. The Thermo-Calc® software and the TTNi8 database from Thermotech Ltd® were used for thermodynamic calculations.

O uso do cálculo termodinâmico e consequentemente base de dados desenvolvidas para esse propósito é hoje uma das ferramentas fundamentais para o aprimoramento e desenvolvimento de superligas a base de níquel. A proposta deste trabalho foi avaliar a confiabilidade de uma base de dados comercial, específica para superligas a base de níquel, utilizada em softwares de cálculos termodinâmicos computacionais. Comparou-se os dados experimentais da superliga Inconel 713LC com informações obtidas através do calculo termodinâmico para a composição química específica da superliga estudada. As amostras da superliga comercial Inconel 713LC foram retiradas de barras da liga fundida a vácuo (VIM). Os tratamentos térmicos foram realizados sob atmosfera de argônio em cápsulas de quartzo nas temperaturas de 600?C, 900?C e 1250 ?C nos tempos de 10h, 24h, 50h, 100h, 250h, 500h, 1000h e 2000 horas, de acordo com a temperatura de tratamento utilizada, visando acompanhar o desenvolvimento microestrutural até alcançar a condição de equilíbrio termodinâmico. Previamente, a amostra no estado bruto de fusão e as amostras após os tratamentos de equilíbrio foram caracterizadas microestruturalmente via microscopia eletrônica de varredura (MEV) e a composição das fases medidas por microanálise de energia dispersiva (EDS). As temperaturas das transformações de fases foram medidas por analise térmica diferencial (DTA). Nas amostras também foram realizadas medidas de microdureza para acompanhar indiretamente a evolução microestrutural da fase gama prime. Para os cálculos termodinâmicos foram utilizados o software Thermo-Calc® e a base de dados TTNi8 da empresa Thermotech Ltd®. / The computational thermodynamic simulation and the databases designed for this purpose are today one the most important tools to develop new nickel based superalloys. The aim of this work was to check the reliability of a commercial database, designed specifically for Ni-based superalloys. The experimental phase diagram data of Inconel 713LC, such as, liquidus and solidus temperatures as well as chemical phase compositions were compared with thermodynamic information extracted from computational simulations. Samples of a commercial superalloy Inconel 713LC were extracted from an ingot produced by Vacuum Induction Melting (VIM) and heat treated under argon in quartz capsules at 600?C, 900 ?C and 1250?C for 10h, 24h, 50h, 100h, 250h, 500h, 1000h and 2000h, according to the treatment temperature used, aiming at reaching the thermodynamic equilibrium conditions. The as-cast and all heat-treated samples were characterized by Scanning Electron Microscopy (SEM) and the chemical compositions of the phases were measured via Energy Dispersive X-ray Spectroscopy (EDS). Temperatures of phase transformations were determined by Differential Thermal Analysis (DTA). Microhardness measurements were performed to indirectly follow the microstructural evolution of gamma prime. The Thermo-Calc® software and the TTNi8 database from Thermotech Ltd® were used for thermodynamic calculations.