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

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‐Fe 2 W to inhibit sintering for α‐Fe in the reduced state and FeWO 4 to inhibit sintering for Fe 3 O 4 in the oxidized state – thus forming ligaments comprising two phases (Fe/λ‐Fe 2 W and Fe 3 O 4 /FeWO 4 , respectively). In contrast, a Fe‐10 W foam with a lower volume fraction of W‐containing phases (λ‐Fe 2 W and FeWO 4 ) 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 FeWO 4 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 FeWO 4 to boost the reaction kinetics during redox cycling.