Mechanical Characterization of Coating-Interconnect Interfaces and Anode-Electrolyte Interfaces for Solid Oxide Fuel Cells

A planar solid oxide fuel cell (SOFC) consists of multiple layers of dissimilar materials with distinct physical, mechanical, and thermal properties. High operating temperatures and mechanical loadings during service can significantly weaken the interfaces of different components in an SOFC. The strength and integrity of various interfaces, for example, coating-interconnect interfaces and electrode-electrolyte interfaces play an important role in increased power density of an SOFC.In the first part of the present investigation, the interfaces between oxide coatings and interconnects are characterized. The repeating anode-electrolyte-cathode units in a planar SOFC stack are physically separated by electrically conductive interconnects. With the reduction of operating temperature to 800oC, it is possible to replace lanthanum based ceramics with less expensive, more readily available chromium alloyed iron metals as interconnects. However, when incorporating chromium-alloyed interconnects, steps must be taken to inhibit chromium poisoning of cathodes. To prevent the chromium poisoning, a dense manganese cobalt spinel oxide (MCO) coating is applied on the cathode side surface of interconnect prior to its installation in the fuel cell. But highly ceramic brittle nature of MCO coatings makes them susceptible to damage under mechanical loads and thermal stresses developed during cooling down the fuel cell from operating temperature to room temperature. A room temperature four-point bend experiment is designed to assess the quality of coatings and coating adhesion. Resulting tensile cracking patterns on the coatings on the convex side of the bend specimen are used to quantify the interfacial shear strength from a shear lag model. In addition, the onset strain of coating spallation is incorporated in an energy based fracture mechanics model to obtain the interfacial fracture energy. Images from scanning electron microscopy (SEM) of the tested coating surfaces are processed to analyze the interface failure mechanisms, the crack spacing, and the spalled areas at higher strains. The analysis obtained from the present investigation is able show distinct differences between coatings processed with different parameters. In addition, based on the results obtained from the bend experiments, coating lifetime is predicted. Lifetime prediction of coatings will greatly assist in optimizing the coating process parameters and assessing the reliability of coated interconnects.In the second part of this dissertation, anode-electrolyte interfaces at which the important electro-chemical hydrocarbon fuel reactions take place are investigated. Frequent anticipated and unanticipated shut down and startup of fuel cells can cause delamination and failure of the anode-electrolyte interfaces. Room temperature four-point bend experiments are performed to obtain the interfacial fracture energy of the anode-electrolyte interfaces. The notched bend test specimens consist of NiO-YSZ anode and ScSZ electrolyte bi-layers are sandwiched between two steel stiffeners. A stable crack is forced to propagate along the interfaces and is monitored with a long distance camera lens. The constant load at which the stable crack propagates is recorded and utilized to obtain the critical strain energy release rate of the interfaces. The cracked surfaces are studied with SEM and energy dispersive spectroscopy (EDS).

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor dc:publisher

The Ohio State University

Year dc:date

2012

Author dc:creator

• Akanda, Sajedur R.

Contributors dc:contributor

• Walter, Mark

• Mechanical Engineering

Statement dc:rights

• unrestricted
• This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.

Language dc:language

English

Repository record dc:identifier

http://rave.ohiolink.edu/etdc/view?acc_num=osu1356969023

OAI identifier oai:identifier

oai:etd.ohiolink.edu:osu1356969023