Structural evolution of directionally freeze-cast iron foams during oxidation/reduction cycles

Stephen K. Wilke, David C. Dunand*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

15 Scopus citations

Abstract

Cyclical oxidation/reduction behavior of iron-based powders and porous pellets is of great interest for iron-air batteries, steam-iron, and chemical looping processes, but extended cycling is limited by degradation via sintering or pulverization. To address these problems, we use directional freeze casting to fabricate porous iron foams, consisting of colonies of parallel iron lamellae and open channels of sufficient width (20–40 and 20–50 μm, respectively) to accommodate iron/iron oxide volume changes during redox cycling. Iron foams of three different initial channel porosities (48, 61 and 65 vol.%) are fabricated via water-based freeze casting of Fe2O3 powders followed by reduction with H2 and sintering. The evolution of these iron foams is examined after 5 and 10 redox cycles between Fe3O4 and Fe at 800 °C (via steam and H2) using optical microscopy, scanning electron microscopy, and synchrotron X-ray tomography. Redox cycling causes a macroscopic foam shrinkage as the iron lamellae grow closer together, decreasing (and even sometimes eliminating) the channel width between lamellae. Smaller micropores within individual iron lamellae are partially preserved, consistent with new porosity formation via vacancy diffusion and clustering in the oxide phase. Additionally, a dense Fe shell forms on the exterior surface of most samples, caused by lamellae contacting and sintering during oxidation, followed by formation of an impermeable Fe layer during reduction. Strategies are proposed to reduce both channel constriction and shell formation, which are undesirable as they restrict gas phase transport.

Original languageEnglish (US)
Pages (from-to)90-102
Number of pages13
JournalActa Materialia
Volume162
DOIs
StatePublished - Jan 1 2019

Keywords

  • Freeze-casting
  • Iron-air battery
  • Metal foam
  • Steam iron process
  • X-ray computed tomography

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Ceramics and Composites
  • Polymers and Plastics
  • Metals and Alloys

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