Evolution of Directionally Freeze-Cast Fe2O3 and Fe2O3+NiO Green Bodies during Reduction and Sintering to Create Lamellar Fe and Fe-20Ni Foams
Evolution of Directionally Freeze-Cast Fe2O3 and Fe2O3+NiO Green Bodies during Reduction and Sintering to Create Lamellar Fe and Fe-20Ni Foams
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DOI：
10.1016/j.jallcom.2021.161707
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发表时间：
2022
影响因子：
6.2
通讯作者：
S. Wilke;Jacob B. Mack;C. Kenel;D. Dunand
中科院分区：
文献类型：
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作者：
S. Wilke;Jacob B. Mack;C. Kenel;D. Dunand
粉末悬浮液的定向冷冻铸造（FC），然后冷冻干燥和烧结是一种通用且可扩展的加工路线，用于制造具有高度细长孔的金属泡沫。由于金属粉末极易氧化，因此本文研究了前体氧化物粉末的使用，并在烧结前附加了将氧化物氢气还原为金属的步骤。然而，由于氧化物还原导致的大体积收缩导致泡沫变形，使得难以优化FC参数以获得特定的泡沫结构。我们使用准原位 X 射线显微断层扫描技术来分析定向冷冻铸造、层状 Fe2O3 和 Fe2O3+NiO 生坯在 725 °C 下分别被 H2 还原为 Fe 和 Fe-20Ni (at%) 并在 900 °C 下烧结时的三维结构演化。这些温度和气体条件导致连续的还原和烧结步骤，可以单独分析。通过图像分析量化泡沫孔隙率、孔宽度、薄片厚度和宏观收缩率。氧化物生坯结构符合典型的 FC 关系：孔隙率随着 FC 悬浮液中粉末含量的减少而增加，片层间距周期或 FC 波长随着冷冻速度的增加而减少。在 H2 还原时，泡沫铁中的片层由于空间不均匀还原率产生的不匹配应力而弯曲，导致各向异性变形。 Fe-20Ni 泡沫中不存在屈曲，因为 Ni/NiO 的还原动力学更快，导致空间上的还原更加均匀。总体积收缩率的 73-86% 是由还原造成的，而其余的收缩则由烧结引起，这对于所有泡沫来说几乎是各向同性的。观察到的 FC 参数、生坯和金属泡沫结构之间的关系可以帮助指导特定技术应用的金属泡沫的设计和优化。
Directional freeze-casting (FC) of powder suspensions followed by freeze-drying and sintering is a versatile and scalable processing route for creating metallic foams with highly elongated pores. Because of the high propensity for oxidation of metal powders, the use of precursor oxide powders is studied here with an additional step of H2-reduction of oxides to metal before sintering. However, the large volume shrinkage due to oxide reduction causes foam deformations, making it difficult to optimize the FC parameters to obtain a particular foam structure. We use quasiin situX-ray microtomography to analyze the three-dimensional structural evolution of directionally freeze-cast, lamellar Fe2O3and Fe2O3+NiO green bodies as they are reduced by H2at 725 °C to Fe and Fe-20Ni (at%), respectively, and sintered at 900 °C. These temperature and gas conditions result in sequential reduction and sintering steps that can be individually analyzed. Foam porosity, pore width, lamellae thickness, and macroscopic shrinkage are quantified by image analysis. Oxide green body structures match typical FC relationships: porosity increases with decreasing powder content in the FC suspension, and the lamellae spacing period, or FC wavelength, decreases with increasing freezing velocity. Upon H2-reduction, lamellae in Fe foams buckle due to mismatch stresses from spatially-inhomogeneous reduction rates, leading to anisotropic deformation. Buckling is absent in Fe-20Ni foams due to the faster reduction kinetics of Ni/NiO that lead to more spatially uniform reduction. Reduction is responsible for 73–86% of the total volumetric shrinkage, with sintering causing the remaining shrinkage, which is nearly isotropic for all foams. The observed relationships between FC parameters, green body and metal foam structure can help guide the design and optimization of metal foams for specific technological applications.