Publication Date

8-22-2013

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

Supervisory Committee Chair

Darryl P. Butt, Ph.D.

Supervisory Committee Member

Alex Punnoose, Ph.D.

Supervisory Committee Member

Rick Ubic, Ph.D.

Abstract

Efficient and reliable materials for gas separation, syngas production, and hybrid nuclear power plants must be capable of reliably operating at a high-temperature range of 700-1000°C and under exposure to highly oxidizing and reducing conditions. Candidate materials for these applications include alkaline metal doped lanthanum ferrite.

In the first study, the impact of A site substitution by different alkaline metals on lanthanum ferrite (LMF, M=Ca, Sr, and Ba) was investigated. The study focused on thermal expansion near the Néel transition temperature and a magneto-elastic contribution to thermal expansion was identified for each sample. Iron oxidation, Fe3+ to Fe4+, was identified as a preferred charge-compensation mechanism for Ca substitution while a mix of iron oxidation and oxygen-vacancy formation was identified for Sr and Ba substituted samples.

The second study focused only on calcium substituted lanthanum ferrite but with a comparison between stoichiometric and sub-stoichiometric quantities of iron on the B site. The samples were heat treated in oxidizing (air), mildly reducing (Ar), and very reducing (5% H2-N2) atmospheres to compare the impact of iron sub-stoichiometry and PO2 on the Néel transition and orthorhombic-to-rhombohedral transition temperatures. Treatment in reducing conditions caused the Néel transition temperature to increase for all samples. The orthorhombic-to-rhombohedral transition temperature was determined to decrease for samples treated in Ar and to occur gradually over a broad temperature range when treated in 5% H2-N2. Iron deficiency during preparation was determined to cause a decrease in calcium actual content and a general increase in both phase transition temperatures in all samples. Iron vacancy formation was also determined to be unlikely due to the high energy of the defect and the samples compensated for iron sub-stoichiometry by rejecting calcium on the A site in favor of lanthanum.

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