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- Kim, Jun Woo, et al.
(författare)
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3D EVALUATION OF THERMAL STRESSES ON CERAMIC-METAL COMPOSITES USED AT HIGH TEMPERATURE OPERATION
- 2017
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Ingår i: 5TH IIR INTERNATIONAL CONFERENCE ON THERMOPHYSICAL PROPERTIES AND TRANSFER PROCESSES OF REFRIGERANTS (TPTPR). - : INT INST REFRIGERATION. - 9782362150203 ; , s. 771-775
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Konferensbidrag (refereegranskat)abstract
- Ceramic-metal composites (CMC) have been used for various high temperature applications including combustion engines, steam and gas turbines, industrial heaters and ceramic fuel cells. Reliable incorporation of the CMC at elevated temperatures, however, is very difficult in practice for the following reasons. First, meting and sublimation points of those solids are different causing undesired diffusion and mixing of elements across the material boundaries degrading functions of the materials. Secondly, maintaining temperature and pressure regimes for desired phases of the component materials is challenging during operation in many of practical cases. Lastly, thermal expansion rates of those two materials are significantly different frequently causing mechanical stresses and fractures. There have been numerous efforts to evaluate and design the CMC materials to minimize the thermo-mechanical stresses. Among various techniques, the focused ion beam-scanning electron microscope (FIB-SEM) tomography has been proved as a state-of-art technique to obtain 3D compositional and structural information of the CMC materials. In this study, we have evaluated thermal stresses applied on nickel-zirconia CMCs by using the FIB-SEM 3D tomography and finite element analysis.
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| 2. |
- Kim, Jun Woo, et al.
(författare)
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Three-dimensional thermal stress analysis of the re-oxidized Ni-YSZ anode functional layer in solid oxide fuel cells
- 2018
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Ingår i: Journal of Alloys and Compounds. - : ELSEVIER SCIENCE SA. - 0925-8388 .- 1873-4669. ; 752, s. 148-154
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Tidskriftsartikel (refereegranskat)abstract
- Nickel-yttria-stabilized zirconia (Ni-YSZ) cermet is widely used as an anode material in solid oxide fuel cells (SOFCs); however, Ni re-oxidation causes critical problems due to volume expansion, which causes high thermal stress. We fabricated a Ni-YSZ anode functional layer (AFL), which is an essential component in high-performance SOFCs, and re-oxidized it to investigate the related three-dimensional (3D) microstructural and thermo-mechanical effects. A 3D model of the re-oxidized AFL was generated using focused ion beam-scanning electron microscope (FIB-SEM) tomography. Re-oxidation of the Ni phase caused significant volumetric expansion, which was confirmed via image analysis and calculation of the volume fraction, connectivity, and two-phase boundary density. Finite element analysis (FEA) with simulated heating to 500-900 degrees C confirmed that the thermal stress in re-oxidized Ni-YSZ is concentrated at the boundaries between YSZ and re-oxidized NiO (nickel oxide). NiO is subjected to more stress than YSZ. Stress exceeding the fracture stress of 8 mol% YSZ appears primarily at 800 degrees C or higher. The stress is also more severe near the electrolyte-anode boundary than in the Ni-YSZ cermet and the YSZ regions. This may be responsible for the electrolyte membrane delamination and fracture that are observed during high-temperature operation. (C) 2018 Elsevier B.V. All rights reserved.
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| 3. |
- Woo Kim, Jun, et al.
(författare)
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Compositional optimization of gadolinia-doped ceria treatment for enhanced oxygen reduction kinetics in low-temperature solid oxide fuel cells
- 2017
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Ingår i: Thin Solid Films. - : Elsevier. - 0040-6090 .- 1879-2731. ; 624, s. 95-100
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Tidskriftsartikel (refereegranskat)abstract
- We report the optimization of the dopant concentration of gadolinia-doped ceria (GDC) to enhance the oxygen reduction reaction (ORR) kinetics for low-temperature solid oxide fuel cells (LTSOFCs). Crystalline GDC layers with uniform nano-granular structures were prepared by aerosol-assisted chemical vapor deposition (AACVD) on the cathode side of a GDC electrolyte. The current-voltage (I-V) characteristics and power performances of the AACVD GDC-treated cells were measured and their cathodic reactions were analyzed by electrochemical impedance spectroscopy (EIS) to investigate the effect of GDC treatments with various dopant compositions. The cell treated with Gd0.14Ce0.86O2-delta was found to show an optimized power performance with the best ORR kinetics, due to its nano-granular structure and high concentration of oxygen vacancies. (C) 2017 Published by Elsevier B.V.
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