Faculty Publications

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Author: search.filters.author.Rajasekaran, B.Subject: search.filters.subject.X ray diffraction

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    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. © 2023
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    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.
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    Effect of microwave treatment on structural characteristics and energy bandgap of electrochemically synthesized hydrated tungsten oxide quantum dots
    (Elsevier Ltd, 2024) Salot, M.; Santhy, K.; Pramanick, A.K.; Rajasekaran, B.; Awasthi, G.; Singh, S.G.; Chaudhury, S.K.
    Quantum Dots (QDs) of hydrated tungsten oxide were synthesized via electrolysis using sintered tungsten carbide-6 wt% cobalt (WC–6Co) scrap as anode, Ti plate as cathode, and sulfuric acid as electrolyte at room temperature. The as-synthesized powder was characterized using X-ray diffraction (XRD), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, Electron paramagnetic resonance spectroscopy (EPR), and Ultraviolet–visible spectroscopy (UV–Vis). The XRD analysis confirmed the formation of orthorhombic hydrated tungsten oxide (WO3.H2O) QDs via electrochemical oxidation of WC. As-synthesized WO3.H2O QDs were thermally-treated using microwave radiation and conventional furnace at 150 °C for 8 min and 45 min, respectively. Thermal treatment of as-synthesized QDs produced partially dehydrated powder consisting of both orthorhombic WO3.H2O and cubic WO3.H0.5 crystal structures. The TEM analysis showed that the average particle size of QDs was 7.60 nm. Further, an increase in lattice strain was observed on microwave treatment owing to the non-equilibrium phase transformation (i.e., rapid heating) from orthorhombic to cubic crystal structure resulting in the generation of oxygen vacancies. The increase in oxygen vacancy concentrations in QDs on microwave heating was confirmed by XPS, FTIR, EPR, and Raman spectroscopy. The energy bandgaps of as-synthesized and thermally-treated QDs were in the range of 2.4307–2.4979 eV. The relatively low energy bandgap of QDs is attributed to the change in crystal structure and increase in the oxygen vacancy concentration. An improved CO gas sensing characteristics of microwave-treated QDs was noted. © 2024 Elsevier Ltd and Techna Group S.r.l.
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    Effect of Build Orientation on Anisotropy in Tensile Behavior of Laser Powder Bed Fusion Fabricated SS316L
    (Springer, 2024) Thanumoorthy, R.S.; Chaurasia, J.K.; Anil Kumar, V.A.; Pradeep, P.I.; Balan, A.A.S.; Rajasekaran, B.; Sahu, A.; Bontha, S.
    In the present study, Stainless steel 316L (SS316L) cylindrical specimens were fabricated at two different build orientations and two different laser powers using Laser powder bed fusion process (LPBF). Microstructural characterization such as XRD, SEM, EBSD analysis and tensile testing were carried out on fabricated specimens in stress relieved condition to understand the anisotropic behavior of LPBF printed specimens. Horizontally oriented specimens showed higher tensile strength when compared to vertically oriented specimens for both laser powers. XRD and EBSD phase maps did not reveal the presence of any secondary phases. However, build orientation and laser power affected the crystallite size of the samples. Bimodal grain structure comprising coarse columnar grains and fine equiaxed grains were observed from the micrographs. With variation in build orientation, there was a significant change in the average grain size of the specimens. High dislocation density was observed in horizontally oriented samples built at low laser power because of dislocation annihilation that can occur at high temperatures. However, EBSD analysis revealed random weak crystallographic texture which does not vary significantly with laser power or build orientation. Variation in grain size, grain morphology, sub-grain features and dislocation density are the reasons for the anisotropic tensile behavior observed in LPBF printed SS316L coupons in stress relieved condition. © ASM International 2023.
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    On the merit of solute segregation and low angle grain boundary for thermal stability and thermal expansion of cold-sprayed CuCrZr
    (Elsevier Ltd, 2025) Abhijith Vijay; Sreerag, M.P.; Varalakshmi, S.; Santhy, K.; Singh, R.; Kondás, J.; Makineni, S.K.; Rajasekaran, B.
    The precipitation hardenable CuCrZr alloy is a potential alternative to copper for inner liners in rocket thrust engines. Cold spray manufacturing has been seen as a promising processing route to manufacture bulk additive structure of CuCrZr. This work reveals that the cold-sprayed as-deposited Cu-Cr-Zr alloy, in its inherent non-equilibrium state. It is highly stable up to 950 °C and exhibits lower thermal expansion than the equilibrium Cu-Cr-Zr alloy, deduced using HT-XRD and Thermo-Calc. Atomic-scale compositional and diffraction analysis using Atom Probe Tomography (APT) and Electron Backscatter Diffraction (EBSD) support the Zener pinning effect of Cr segregation near the grain boundaries, along with a large fraction of low-angle grain boundaries (LAGBs), that contribute to the high thermal stability and controlled thermal expansion of the deposit. Cold spray deposition naturally yields microstructural features that are conducive to high thermal stability and controlled thermal expansion, features which are comparable to the self-organized microstructures observed in segregation engineering (SE). © 2025 Elsevier B.V.