Structural evolution of directionally freeze-cast iron foams during oxidation/reduction cycles
Structural evolution of directionally freeze-cast iron foams during oxidation/reduction cycles
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DOI：
10.1016/j.actamat.2018.09.054
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发表时间：
2019
期刊：
影响因子：
9.4
通讯作者：
S. Wilke;D. Dunand
中科院分区：
文献类型：
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作者：
S. Wilke;D. Dunand
铁基粉末和多孔颗粒的循环氧化/还原行为对于铁-空气电池、蒸汽铁和化学循环过程非常有意义，但长期循环受到烧结或粉碎引起的降解的限制。为了解决这些问题，我们使用定向冷冻铸造来制造多孔铁泡沫，该泡沫铁由平行的铁片层和足够宽度（分别为20-40和20-50μm）的开放通道组成，以适应氧化还原循环过程中铁/氧化铁体积的变化。通过水基冷冻铸造 Fe2O3 粉末，然后用 H2 还原并烧结，制备了三种不同初始通道孔隙率（48、61 和 65vol.%）的泡沫铁。使用光学显微镜、扫描电子显微镜和同步加速器 X 射线断层扫描，在 800°C（通过蒸汽和氢气）下，在 Fe3O4 和 Fe 之间进行 5 和 10 次氧化还原循环后，研究了这些泡沫铁的演变。随着铁片层越来越靠近，氧化还原循环会导致宏观泡沫收缩，从而减少（有时甚至消除）片层之间的通道宽度。单个铁片层内较小的微孔被部分保留，这与通过空位扩散和氧化物相聚集形成的新孔隙一致。此外，大多数样品的外表面上会形成致密的铁壳，这是由于氧化过程中片层接触和烧结造成的，随后在还原过程中形成了不渗透的铁层。提出了减少通道收缩和壳形成的策略，这是不希望的，因为它们限制了气相传输。
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 degradationviasintering 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 fabricatedviawater-based freeze casting of Fe2O3powders followed by reduction with H2and sintering. The evolution of these iron foams is examined after 5 and 10 redox cycles between Fe3O4and Fe at 800 °C (viasteam 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 formationviavacancy 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.