Tungsten's Role in Enhancing Sintering Resistance of Fe‐W Hierarchical Foams during Redox Cycling

Abstract

Directional freeze‐cast Fe‐W lamellar foams with 10–33 at.% W show distinct microstructural evolutions during steam/hydrogen redox cycling between oxidized and reduced states at 800 ⁰C, depending on W concentration. The Fe‐18 W and Fe‐25 W foams exhibit a sufficient volume fraction of W‐rich phases – λ‐Fe2W to inhibit sintering for α‐Fe in the reduced state and FeWO4 to inhibit sintering for Fe3O4 in the oxidized state – thus forming ligaments comprising two phases (Fe/λ‐Fe2W and Fe3O4/FeWO4, respectively). In contrast, a Fe‐10 W foam with a lower volume fraction of W‐containing phases (λ‐Fe2W and FeWO4) shows lamellae densification as well as core‐shell structure formation, due to Fe outward diffusion during oxidation. While higher W concentration enhances the stability of lamellar structure in Fe‐W foams, degradation still occurs, via buckling of lamellae and swelling of foams after extensive cycling. In situ XRD characterization shows that W addition has a minor effect on the oxidation process but slows reduction due to the sluggish kinetics of FeWO4 reduction. This influence is mitigated by the formation of nanocrystalline W‐rich phases due to the chemical vapor transport (CVT) mechanism during the reduction of FeWO4 to boost the reaction kinetics during redox cycling.