This title appears in the Scientific Report :
2008
Please use the identifier:
http://hdl.handle.net/2128/3615 in citations.
Ferritic Steel Interconnectors and Their Interactions with Ni Base Anodes in Solid Oxide Fuel Cells (SOFC)
Ferritic Steel Interconnectors and Their Interactions with Ni Base Anodes in Solid Oxide Fuel Cells (SOFC)
In recent years high Cr ferritic steels such as Crofer 22 APU became the most widespread construction materials for solid oxide fuel cell (SOFC) interconnects mainly due to low cost and the ease of fabrication compared to ceramic materials. It was shown that optimum properties with respect to oxide...
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Personal Name(s): | Froitzheim, Jan (Corresponding author) |
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Contributing Institute: |
Werkstoffstruktur und Eigenschaften; IEF-2 |
Imprint: |
Jülich
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
2008
|
Physical Description: |
169 S. |
Dissertation Note: |
Aachen, RWTH, Diss., 2008 |
ISBN: |
978-3-89336-540-1 |
Document Type: |
Book Dissertation / PhD Thesis |
Research Program: |
Rationelle Energieumwandlung |
Series Title: |
Schriften des Forschungszentrums Jülich : Energie & Umwelt / Energy & Environment
16 |
Subject (ZB): | |
Link: |
OpenAccess |
Publikationsportal JuSER |
In recent years high Cr ferritic steels such as Crofer 22 APU became the most widespread construction materials for solid oxide fuel cell (SOFC) interconnects mainly due to low cost and the ease of fabrication compared to ceramic materials. It was shown that optimum properties with respect to oxide scale growth and adherence could only be obtained by very low, carefully controlled concentrations of minor alloying additions such as Al and Si. This required sophisticated alloy manufacturing methods, including vacuum induction melting, resulting in higher manufacturing costs. The high alloy purity also has as result a low creep strength of the material which indirectly resulted in increased oxidation rates of thin components. The aim of the present project was the development of a ferritic steel with improved creep strength and less restrictions with respect to alloy purity (especially Al or Si levels) than in the case of Crofer 22 APU For this purpose modifications of Crofer 22 APU with various amounts of W, Nb, Al and Si were investigated with respect to SOFC relevant properties. The alloying additions W and Nb were chosen because they are potentially suitable to increase creep strength by solid solution strengthening (W) and precipitation strengthening (Nb). The investigations included oxidation tests in air and anode atmosphere at 800 and 900°C for up to 10000h, creep tests, investigation of the electrical resistance of the surface oxide scale as well as measurements of the coefficient of thermal expansion (CTE). While the addition of W did not have a major effect on the oxidation behaviour, the addition of Nb resulted in significantly increased oxidation rates. Further it could be shown that the Nb containing precipitates of the Laves phase type had a high affinity for Si which results in a reduced amount of Si in the alloy matrix. Thus, in Nb containing alloys Si impurities can be tolerated which causes lower alloy manufacturing costs. At the same time Si additions suppress the adverse effect of Nb on the oxidation behaviour by stabilising the Laves phase which results in an oxidation rate which, for 1-2mm thick specimens, is almost identical to that of Crofer 22 APU. Due to the higher creep strength the new alloys exhibited only a slight thickness dependence of the oxidation rates with the consequence, that for thin components of a few tenths of a mm the oxidation rates for the new alloy were smaller than for Crofer 22 APU. Thus, if the amounts of W, Nb and Si were carefully adjusted the new material had, compared to Crofer 22 APU, similar CTE, similar ductility and lower oxidation rates for thin components. Apart from the interconnect material itself, issues related to compatibility of the interconnect with other cell components are a major problem in the SOFC. While compatibility related issues on the cathode side are widely known and have been extensively studied, diffusion related problems on the anode side have been hardly addressed until now. In the second part of the project a number of metallic coatings were investigated to check their potential suitability for inhibiting interdiffusion processes between ferritic steel and Ni base anode or wire mesh. However, none of the tested coatings could fulfil the requirements. In contrast, CeO$_{2}$ turned out to be an efficient diffusion barrier, and its electronic conductivity appeared to be sufficient for SOFC purposes. Measurements of ceria coatings on Crofer 22 APU substrates showed that a considerable part of the area specific resistance is related to the thermally grown oxide scale on the steel surface and to interfacial resistances. The latter can be reduced substantially if an intermediate Cu coating is applied between the steel substrate and CeO$_{2}$ barrier layer. |