Elastic properties, biaxial strength and fracture toughness of nickel-based anode materials for solid oxide fuel cells
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abstract
NiO-YSZ composites are widely used as precursors for Ni-YSZ anode materials for solid oxide fuel cells (SOFCs) with YSZ electrolytes. Prior to operation, the anode material is reduced in hydrogen to obtain a cermet comprised of metallic Ni and YSZ. The reduction process is accompanied by changes in the microstructure and mechanical properties of the Ni-based anodes, which needs to be understood and optimized to maximize the reliability of SOFCs. In this study the elastic properties, biaxial strength and fracture toughness of Ni-based anode materials were determined at room temperature as a function of initial porosity before and after full reduction in hydrogen. The elastic properties were determined by impulse excitation, while biaxial strength and fracture toughness were determined using the ring-on-ring and double torsion test methods, respectively. It was found that the magnitude of Young's and shear moduli decreases significantly with porosity and that the magnitude of the elastic moduli decreases almost by 50% due to H 2-induced reduction. The characteristic strength of the distributions of biaxial strengths of Ni-based anode materials decreased with increasing porosity, and it was always found to be lower (by as much as 50%) than that of the initial NiO-YSZ material. The decrease in the magnitude of the biaxial strength after reduction was attributed mainly to the increase in porosity of the material. Conversely, the fracture toughness of fully reduced Ni-based anodes was found to be significantly higher than that of unreduced anodes, which results from the ductile behavior of nickel.
