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Item Thermal expansion of Crofer 22 APU steel used for SOFC interconnect using in-situ high temperature X-ray diffraction(Elsevier Ltd, 2023) Manjunath, N.; Santhy, K.; Rajasekaran, B.Crofer 22 APU is ferritic stainless steel extensively used as metallic interconnect material in Solid Oxide Fuel Cell (SOFC) applications. The interconnects are exposed to both oxidizing and reducing atmospheres at high temperatures. As SOFCs are operated above 700 °C, understanding the thermal expansion behavior of the interconnect material with other components (anode, cathode, electrolyte) of the fuel cells is essential. Metallic interconnects should have a matchable thermal expansion to other ceramic materials such as anode, cathode, and solid electrolyte used in SOFCs. The present study evaluates the thermal expansion of Crofer 22 APU steel from 25 to 950 °C in a controlled atmosphere (10-4 mbar pressure) using in-situ high-temperature X-ray diffraction (XRD). The XRD patterns were analyzed using the ‘High Score Plus Software’ attached to the system, and the phases were identified using the standard Crystallographic Open Database (COD). The coefficient of thermal expansion (CTE) was determined based on the change in lattice parameter/peak shift to a lower 2θ value as a function of temperature. The normal XRD data showed no oxide formation on the Crofer steel after heating until 950 °C in in-situ high-temperature conditions. The peak shift to the lower 2θ degree observed in the XRD data was due to the relaxation of residual stress upon heating. The isothermal section and phase fraction of Crofer 22 APU alloys are analyzed with the help of thermo-calc with the iron database of TCFE7. The Fe-rich bcc phase was found to be stable up to high temperatures. The major phases are the Fe-rich bcc, Cr-rich BCC, and sigma phase in the solid state. The minor phases are FCC, M3P, TiC, Laves, and Ti4C2S2. The calculated lattice parameter of the Fe-rich BCC phase matches with the experimentally calculated data using XRD. The thermal expansion of Crofer 22 APU was found to be 11.9181 × 10-6 /°C at 950 °C. The in-situ high-temperature XRD technique has been an effective methodology for determining the thermal expansion behavior of the as-received Crofer steel. © 2023Item The effect of strain induced phase transformation on the thermal expansion compatibility of plasma sprayed spinel coating on SOFC metallic interconnect – A study using in-situ high temperature X-ray diffraction(Elsevier Ltd, 2023) N, M.; Santhy, K.; Rajasekaran, R.A new and novel approach has been adopted in this study to evaluate thermal mismatch induced by thermal expansion in substrate-coating contact pairs using in-situ high-temperature X-ray diffraction (HT-XRD). Atmospheric plasma sprayed (APS) Mn1.0Co1.9Fe0.1O4 (MCF) coating on Crofer 22 APU steel interconnect was investigated. In-situ HT-XRD was performed individually for substrate and coating from 25 °C to 900 °C. Diffraction data were recorded for different temperatures to obtain lattice parameters and strain as a function of temperature. The coefficient of thermal expansion (CTE) of MCF coating was slightly higher than steel substrate and showed no significant thermal expansion mismatch till 700 °C. The increasing lattice strain measured by Scherrer and Williamson-Hall methods indicates strain-induced phase transformation of MCF coating with temperature, supporting the phase transformation-induced self-healing phenomenon of MCF coating. The merit of in-situ HT-XRD as a tool for optimizing operating temperature and measuring thermal mismatch of solid oxide fuel cell (SOFC) stacks has been discussed. © 2023 Hydrogen Energy Publications LLCItem Understanding early-stage oxidation mechanism of Crofer 22 APU solid oxide fuel cell steel interconnect using glow discharge optical emission spectroscopy and grazing incidence X-ray diffraction(Elsevier B.V., 2023) Naik, M.; Santhy, K.; Rajasekaran, R.Crofer 22 APU is used as metallic interconnects in stacking solid oxide fuel cells (SOFCs) operated at elevated temperatures (above 700 °C) owing to their excellent oxidation resistance. Understanding the protective layer formation in the initial oxidation stage would be useful for optimizing and designing protective coatings for extended life. Initial stage oxidation of Crofer 22 APU steel using surface analytical tools such as glow discharge optical emission spectroscopy (GD-OES), grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, and atomic force microscopy (AFM) are studied in the paper. An oxidation test on as-received Crofer 22 APU steel was carried out in a controlled atmosphere (0.01 Pa) in an in-situ high-temperature X-ray diffraction (XRD) stage at 950 °C. Normal XRD showed no indication of oxidation, while GIXRD revealed the formation of two-layer oxides: Top layer spinel MnCr2O4 and fine-grained inner layer Cr2O3, which was confirmed and quantified by GD-OES depth profiling. The Cr2O3 formed initially led to the formation of MnCr2O4 spinel during the initial stage. The rapid diffusion of Mn through the fine-grained Cr2O3 layer results in an increased growth rate of MnCr2O4 spinel on the top of the fine-grained Cr2O3 layer. © 2023 Elsevier B.V.