Effects of bridging fibers on the evolution of lamellar architecture during H2/H2O redox cycling of Fe-foams

Abstract

Fe/Fe3O4 redox cycling via cyclic H2/H2O exposure at 800 °C is studied in lamellar Fe foams with 15 vol% fibers, created by freeze-casting. Fibers were integrated in the foams to mitigate densification during cycling by mechanically supporting neighboring lamellae, thus preventing buckling and sintering at contact points. Three fiber types are examined: short (0.1 mm) and long (1–2 mm) stainless-steel fibers, and long zirconia fibers. Long fibers bridge lamellae and have a marked effect on the architecture by increasing the initial interlamellar porosity (from < 60 to > 85%), with a corresponding decrease in foam shrinkage during initial reduction and sintering (from > 80 to < 55% volumetric loss). Though performance improves as compared to fiber-free foams, fiber effectiveness against damage decreases with cycling: after 10 redox cycles, porosity falls from 85 to 50% for foams with long fibers. One novel degradation mechanism is identified: fiber engulfment. This mechanism occurs over successive redox cycles, as material from the lamellae cyclically engulfs (as Fe3O4) and withdraws (as Fe) from the fibers, with a net transport from lamellae to fibers after each cycle. This cyclic coarsening mechanism alters foam architecture from bridged-lamellar (with evenly distributed porosity) to mixed lamellar/fibrous (with unevenly distributed porosity).