Publication Date

5-2014

Date of Final Oral Examination (Defense)

5-7-2014

Type of Culminating Activity

Dissertation

Degree Title

Doctor of Philosophy in Materials Science and Engineering

Department

Materials Science and Engineering

Supervisory Committee Chair

Darryl Butt, Ph.D.

Supervisory Committee Member

William B. Knowlton, Ph.D.

Supervisory Committee Member

Dmitri Tenne, Ph.D.

Supervisory Committee Member

Charles Lewinsohn, Ph.D.

Abstract

Increasing world energy demands are still heavily dependent on fossil fuels. To meet these demands, crude oil is increasingly being extracted from remote locations where natural gas pipelines do not exist. It is not profitable for oil and gas companies to transport natural gas by ship or truck from these remote locations so it is combusted on site in the form of gas flares. Critics oppose this practice due to the environmental impact from pollution and carbon emissions created during the flaring process. Oil and gas companies are eager to find profitable ways to eliminate natural gas lines in order to reduce environmental impacts and avoid costly lawsuits and political opposition. Building new pipeline infrastructures have been pursued, however construction is slow and costly. A cost effective alternative to gas transportation is the onsite conversion of natural gas to transportable liquid products such as ammonia or methanol. The first intermediate step in this process is the conversion of natural gas to synthesis gas, or “syngas” for short. In this process natural gas (CH4) is oxidized at high temperature to form a gas mixture composed principally of hydrogen, carbon monoxide, and carbon dioxide.

Syngas reactors may use oxygen ion conducting membranes which transport oxygen from one side to the other when a chemical potential gradient is applied at high temperatures, typically in the range 750-900°C. The chemical potential is created when one side of the syngas membrane is exposed to air and the other side is exposed to a reducing, methane atmosphere. Materials selected for syngas membranes need to be chemically stable in reducing environments at high temperatures, have good chemical selectivity with high flux, and have mechanical properties which allow for the fabrication of thin membranes operated under pressure.

Perovskite materials are a class of complex oxides with a common ABO3 crystal structure. Of these, lanthanum ferrites (LaFeO3) have been shown to display superior chemical stability at high temperatures under reducing atmospheres making them a good choice for syngas membranes. In perovskites, oxygen conduction occurs via the hopping of oxygen to available oxygen vacancy sites. Lanthanum ferrite does not intrinsically have a high concentration of oxygen vacancies and therefore is not a good ion conductor. Oxygen flux can be increased by doping LaFeO3 with a divalent alkaline metal cation (M2+) on the trivalent lanthanum site (La3+) site creating a charge imbalance resulting in the formation of additional oxygen vacancies. Of the available alkaline cations, calcium and strontium are the most promising substituents for syngas membrane applications.

This study focuses primarily on calcium substituted lanthanum ferrites. Despite the great interest in these materials, the phase stability of the lanthanum calcium ferrite (LCF) materials system is not very well understood in the scientific literature. In this study, we have synthesized LCF materials using solid state reactions and characterized the phase stability under oxidizing and reducing atmospheres.

This investigation began by accessing the phase stability of LCF materials in air and argon atmospheres. The calcium solubility in the lanthanum ferrite perovskite structure was identified for samples sintered at 1250°C using room temperature x-ray diffraction (XRD), scanning electron microscopy, and energy dispersive spectroscopy. We identified structural transitions using thermal analysis techniques and characterized the resulting crystal structure and changes in thermal expansion with high temperature XRD. We used thermal analysis techniques to identify the phase transition temperature of the previously reported order-disorder transformation. The behavior of two phase materials composed of the perovskite and Grenier structures treated above the order-disorder transition temperature was studied and show that they form single grains with a nominal composition between the two initial phases.

Thermal expansion and magnetic behaviors of divalent, alkaline-doped lanthanum ferrites (La0.9M0.1FeO3, M=Ca, Sr, Ba) were assessed using a combination of dilatometry, magnetometry, time of flight neutron diffraction, and high temperature XRD. Néel temperatures were determined through vibrating sample magnetometry and correlated well with changes in thermal expansion behavior observed during both dilatometry and x-ray diffraction. Differences in changes of the observed Néel temperatures due to divalent substitution were shown to be related to a combination of charge compensating mechanisms and differences in the Fe-O-Fe bond angle associated with the superexchange interaction.

The stability of lanthanum calcium ferrites in reducing atmospheres were investigated through measured mass changes associated with decomposition of the material. The partial pressure of oxygen (PO2) was controlled by varying H2/H2O ratios via pre-mixed gases and by bubbling hydrogen through water baths at controlled temperatures. Three regions of mass loss were identified, two of which were discovered to be associated discrete decompositions. Calcium substituted samples are shown to decrease the thermal stability of the compound; but rather than incrementally increasing the required for decomposition, substituted samples partially decompose at a single , the extent of which was dependent on the amount of Ca substitution and the isothermal temperature. All samples were found to fully decompose at the same oxygen partial pressure as pure lanthanum ferrite.

The oxidation and resulting atomic structure of the LaCa2Fe3O8 compound, also known as the Grenier phase, was investigated before and after the order-disorder transformation (ODT). The Grenier compound was synthesized in air using traditional solid state reactions. Thermal analysis characterization shows that the material undergoes the order-disorder transformation in both oxygen and argon atmospheres with dynamic, temperature dependent, oxidation upon cooling. Results from scanning transmission electron microscope (STEM) images suggest that the Grenier phase has preferential segregation of Ca and La on the two crystallographic A-sites before the ODT, but a random distribution afterwards. Furthermore, STEM images suggest the possibility that oxygen excess may exist in La rich regions in microdomains rather than at microdomain boundaries as previously suggested.

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