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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 FeWO4to inhibit sintering for Fe3O4in 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 FeWO4reduction. This influence is mitigated by the formation of nanocrystalline W‐rich phases due to the chemical vapor transport (CVT) mechanism during the reduction of FeWO4to boost the reaction kinetics during redox cycling.
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Tungsten Strongly Inhibits Sintering of Porous Iron During High‐Temperature Redox Cycling
Abstract Freeze‐cast Fe‐25 W (at%) lamellar foams show excellent resistance to degradation at 800 °C during steam‐hydrogen redox cycling between the metallic and oxide states, with fast reaction kinetics maintained up to at least 100 redox cycles with full Fe utilization. This very high stability stems from the sintering inhibition of W combined with the freeze‐cast architecture and the chemical vapor transport (CVT) mechanism of reduction. These three factors create a hierarchical porosity in the foam, consisting of i) macroscopic elongated channels, ii) micro‐scale sintering inhibition pores, and iii) submicron CVT pores. Microstructural characterization via SEM and EDS is combined with in situ XRD to fully explore the phase evolution and microstructural impact of W on Fe during redox cycling. Comparison with tapped Fe‐25 W (at%) powder beds reveals that the freeze‐cast channels and lamellae are not critical to the performance of the material.
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- Award ID(s):
- 2015641
- PAR ID:
- 10504351
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Small
- Volume:
- 20
- Issue:
- 37
- ISSN:
- 1613-6810
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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