Date of Completion

7-20-2015

Embargo Period

7-19-2017

Keywords

Perovskite/Fluorite, Lanthanum chromite, Doping effect, Processing, Stability and Oxygen transport membrane

Major Advisor

Prof. Prabhakar Singh

Associate Advisor

Dr. Jonathan Lane

Associate Advisor

Prof. Steven Suib

Associate Advisor

Prof. Ramamurthy Ramprasad

Associate Advisor

Prof. Ugur Pasaogullari

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Judicious selection of mixed ionic–electronic conducting (MIEC) perovskite oxide as oxygen transport membrane (OTM) offers the potential to enhance overall process economics and systems performance for a wide variety of industrial applications ranging from clean and efficient energy conversion (oxy-combustion) to selective gas separation (high purity oxygen production) and value added chemicals (syngas and liquid fuel) production with near-zero greenhouse gas emissions.

Lanthanum chromite (perovskite) and aliovalent doped zirconia (fluorite) based composites have been considered as promising material of choice for use as electrochemically active components in oxygen transport membrane. Inter–cationic diffusion and formation of secondary compounds due to the interaction between the perovskite and fluorite, however, modifies the thermo-physical and electrochemical properties of such systems leading to the performance and structural degradation. The properties of the lanthanum chromite, required for optimum oxygen transport can be tuned and stabilized through selection of dopant's type and level.

In this study, phase transformation, stability and thermal-electrical properties of un-doped lanthanum chromite (LaCrO3) have been evaluated. Furthermore, the role of various A (Sr) and B-site (Mn, Fe, Ni and Ti) dopants on the crystal structure, densification, electrical conductivity, thermal expansion, electrochemical performance and thermochemical stability of lanthanum chromite is investigated to enlighten ‘composition-structure-property-stability’ correlations and achieve OTM materials requirement. The effect of oxygen partial pressure and Cr: M (B-site dopants; e.g. Fe) ratio on the processing (sintering behavior), thermal-electrical properties and stability (surface, bulk and interfacial) of La1-xSrxCr1-yMyO3-δ (with and without fluorite phase) are studied in detail. Perovskite – fluorite interactions in the composite results in the secondary compound formation (e.g. SrZrO3) during exposure to reducing atmospheres. Mechanisms for the formation of the secondary compounds are established.

This study demonstrates the stability of lanthanum chromite based materials increases with increase in Cr:M ratio. In contrast, densification and performance decreases with the increase in the ratio. Optimization of the required properties is accomplished by introducing variation in the doping type and level of A-site (alkaline earth metals; Sr) and B-site (transition metals; Mn, Fe, Ni and Ti) dopants. In summary, this thesis presents the research work performed on the fundamental understanding and development of the perovskite/fluorite based materials for oxygen transport membrane system. The materials stability and performance are also demonstrated utilizing real-world OTM fabrication and operating conditions.

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