Faculty Publications
Permanent URI for this communityhttps://idr.nitk.ac.in/handle/123456789/18736
Publications by NITK Faculty
Browse
Search Results
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 Thermal expansion and microstructure evolution of atmospheric plasma sprayed NiCrAlY bond coat using in-situ high temperature X-ray diffraction(Elsevier B.V., 2023) Abhijith Vijay, V.; Santhy, K.; Govindarajan, G.; Rajasekaran, B.The paper focuses on in-situ high-temperature X-ray diffraction (HT-XRD) study on atmospheric plasma sprayed NiCrAlY coating. The sample was in-situ heated from 25 °C to 1150 °C in a controlled atmosphere (3 × 10−4 bar), and the corresponding X-ray diffraction patterns for different temperatures were recorded. The effect of temperature on crystallite size, lattice strain, and coefficient of linear thermal expansion was studied. Major phases identified are γ-Ni, γ’-Ni3Al, β-NiAl, and α-Cr. The formation of stable α-Al2O3 and spinel was found above 1000 °C. The transformation of β to γ’ and γ phase was observed as a function of temperature. The equilibrium phases and the thermal expansion of disordered Face Centered Cubic (FCC) and Body Centered Cubic (BCC) phases were predicted and supported by Thermo-Calc prediction for the stable temperature range. Results showed that the non-equilibrium microstructure produced by thermal spray process did not alter the thermal expansion behaviour. In-situ treatment resulted in microstructure and elemental homogenization. The thermal expansion and mechanism of phase evolution were discussed. © 2022 Elsevier B.V.Item 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 In Situ High-Temperature X-ray Diffraction Study on Atmospheric Plasma and Detonation Sprayed Ni-5 wt.%Al Coatings(Springer, 2023) Purushotham, N.; Santhy, K.; Suresh Babu, P.; Govindarajan, G.; Rajasekaran, R.In situ high-temperature x-ray diffraction (HT-XRD) was used in the present study to assess the coefficient of thermal expansion and recrystallization of Ni-5 wt.%Al coatings. Atmospheric plasma spray (APS) and detonation spray (DSC) techniques were used to deposit Ni-5 wt.%Al coatings on IN718 substrates. The coatings were examined using HT-XRD at ambient conditions (25 °C) up to high temperatures (1150 °C) under a vacuum pressure of around 10−4 mbar. Coefficients of thermal expansion (CTE), crystallite size (D) and lattice strain (ε) were determined by the Scherer and Williamson-Hall (W-H) method with a uniform strain model (UDM) using x-ray peak profile analysis (XPPA). The microstructure of the Ni-5 wt.%Al coatings was analyzed by field emission scanning electron microscopy (FESEM). No phase changes were observed in either coating, as the Ni-5 wt.%Al coatings consisted mainly of γ-Ni crystals with a face-centered cube (FCC) phase in both coating techniques. Lattice parameters as a function of temperature were used to calculate linear thermal expansion coefficients. The linear thermal expansion of Ni-5 wt.%Al coatings deposited by both thermal spray methods was discussed on the basis of process-induced microstructures. © 2023, ASM International.