Date of Completion

12-17-2015

Embargo Period

12-15-2015

Keywords

SOFC Interconnect Spinel Crofer 22 APU

Major Advisor

Mark Aindow

Associate Advisor

Ramamurthy Ramprasad

Associate Advisor

George A. Rossetti, Jr.

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Modern solid oxide fuel cells (SOFCs) operate at sufficiently low temperatures to allow the use of alloy interconnects in the SOFC stacks. The alloys used are cheaper, more formable, and more conductive than the traditional ceramic materials used for interconnects. However, the long term SOFC performance is limited by oxidation of the alloy interconnect. To ameliorate the issues which result from oxidation, manganese cobaltite spinel coatings are typically applied.

The microstructural effects of the reduction step in the reactive consolidation of slurry processed Mn1.5Co1.5O4 (MCO) coated Crofer 22 APU were studied. The reduced coating contained particles of MnO with the NaCl structure and Co with the face centered cubic (FCC) structure. The interface exhibited a thin dense chromia layer with a thicker porous MnCr2O4 over-layer with needle-like protrusions into the reduced coating. The consequences of these observations for the complex microstructural variation during subsequent re-oxidation are discussed.

The effects of Cr, Ni, and Fe substitution into MCO spinels are of great interest due to the roles that the diffusion of these cations play in reaction layer development during high temperature exposure of MCO-coated alloys. Here a study is reported on a series of model Cr-, Ni-, and Fe-substituted MCO spinel ceramics. The cation site occupancies in these samples have been studied by X-ray spectrometry- based Atom Location by CHanneling Enhanced MIcro- analysis (ALCHEMI) experiments in the transmission electron microscope. These ALCHEMI data could provide a useful insight into the role of cation sub-lattice site preference in the formation of reaction layers in MCO-coated stainless steels and superalloys.

The use of pulsed laser deposition to produce high-quality manganese cobaltite spinel coatings on Crofer 22 APU substrates has been investigated. It is shown that deposition from ceramic MCO targets in a N2 environment results in smooth, dense polycrystalline coatings comprised of rock-salt (Mn,Co)O and FCC Co phases. Post-deposition annealing in laboratory air led to re-oxidation of the coatings to give spinel phases with a thin chromia layer at the interface with the substrate. There are local variations in the porosity and surface morphology of the coating, which appear to be correlated with the orientation of the underlying substrate grains. These observations indicate that control of grain structure in the substrate may be necessary to promote microstructural stability in such coatings.

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