Evolution of directionally freeze-cast Fe2O3 and Fe2O3+NiO green bodies during reduction and sintering to create lamellar Fe and Fe-20Ni foams

Directional freeze-casting (FC) of powder suspensions followed by freeze-drying and sintering is a versatile scalable processing route for creating metallic foams with highly elongated pores. Because the high propensity oxidation metal powders, use precursor oxide powders studied here an additional step H2-reduction oxides to before sintering. However, large volume shrinkage due reduction causes foam deformations, making it difficult optimize FC parameters obtain particular structure. We quasi in situ X-ray microtomography analyze three-dimensional structural evolution directionally freeze-cast, lamellar Fe2O3 Fe2O3+NiO green bodies as they are reduced H2 at 725 °C Fe Fe-20Ni (at%), respectively, sintered 900 °C. These temperature gas conditions result sequential steps that can be individually analyzed. Foam porosity, pore width, lamellae thickness, macroscopic quantified image analysis. Oxide body structures match typical relationships: porosity increases decreasing content suspension, spacing period, or wavelength, decreases increasing freezing velocity. Upon H2-reduction, buckle mismatch stresses from spatially-inhomogeneous rates, leading anisotropic deformation. Buckling absent faster kinetics Ni/NiO lead more spatially uniform reduction. Reduction responsible 73–86% total volumetric shrinkage, causing remaining which nearly isotropic all foams. The observed relationships between parameters, structure help guide design optimization specific technological applications.