Abstract
One of the candidates for metallic interconnects of solid oxide fuel cells is a ferritic stainless steel, Crofer22 APU. Ferritic stainless steel Crofer22 APU samples with different roughness were prepared by grinding with SiC grinding papers of various grits, and a polished Crofer22 APU sample was also prepared. The prepared samples were then thermally cycled. The variations of their oxidation behavior with surface roughness and the number of thermal cycles were investigated. After 120 thermal cycles (a total 3000 h of exposure at 800 °C), the polished Crofer22 APU had a relatively flat, continuous Cr2O3 layer (thickness, ~ 1 μm), while the Crofer22 APU ground with 80 grit showed an undulating, continuous Cr2O3 layer (thickness, ~ 2 μm). For the samples that were thermally cycled 4 times (at 800 °C for 100 h total), the area specific resistance (ASR) increased as grit number increased for all measured temperatures (600 – 850 °C). Generally, for the samples that were thermally cycled 20 times, the ASR decreased slowly as the grit number increased. For the samples thermally cycled 40 times, the ASR decreased in general at all the measuring temperatures as the number of grit increased, indicating that the polished Crofer22 APU is better than those with rougher surfaces for the application of Crofer22 APU to an interconnect of SOFC.
Highlights
Materials for the interconnect of a solid oxide fuel cell (SOFC) [1 – 4] are required to have several excellent properties from the viewpoints of electronic conductivity and ionic conductivity, chemical stability in both fuel and air and with respect to other cell parts, thermal expansion match to other cell parts, strength and toughness, thermal conductivity, and ability to make gas-tight seals with other cell parts, and to be of low cost.Metallic interconnects [5 – 10] can be used since the SOFC operating temperature reduced down from 900 – 1000 °C to 600 – 850 °C recently [11]
When samples are ground with an emery paper with a larger grit number, the ridges formed on the surface of samples are lower than those formed on the surface of samples ground with an emery paper with a smaller grit number
The area specific resistance (ASR) is considered to decrease as the grit number increases. This indicates that the polished Crofer22 APU is better than those with rougher surfaces for the application of Crofer22 APU to an interconnect of SOFC
Summary
Materials for the interconnect of a solid oxide fuel cell (SOFC) [1 – 4] are required to have several excellent properties from the viewpoints of electronic conductivity and ionic conductivity, chemical stability in both fuel and air and with respect to other cell parts, thermal expansion match to other cell parts, strength and toughness, thermal conductivity, and ability to make gas-tight seals with other cell parts, and to be of low cost. Iron was rich in the spinel top layer of the scale on the air side of Crofer APU (~ 23 % Cr) after an isothermal heating at 800 °C, and the anomalous oxidation was accelerated by increasing both temperature and thermal cycling. They reported that, for the ferritic stainless steels with relatively low Cr content, the presence of hydrogen led to an enrichment of iron in the scale probably by accelerating the iron transport in the scale. The variations of their oxidation behavior with surface roughness and the number of thermal cycles were investigated
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