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

Stephen K. Wilke; Jacob B. Mack; Christoph Kenel; David C. Dunand

doi:10.1016/j.jallcom.2021.161707

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

Stephen K. Wilke^*, Jacob B. Mack, Christoph Kenel, David C. Dunand

^*Corresponding author for this work

Materials Science and Engineering

Research output: Contribution to journal › Article › peer-review

8 Scopus citations

Abstract

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 quasi in situ X-ray microtomography to analyze the three-dimensional structural evolution of directionally freeze-cast, lamellar Fe₂O₃ and Fe₂O₃+NiO green bodies as they are reduced by H₂ at 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.

Original language	English (US)
Article number	161707
Journal	Journal of Alloys and Compounds
Volume	889
DOIs	https://doi.org/10.1016/j.jallcom.2021.161707
State	Published - Jan 5 2022

Funding

This work was supported by the National Science Foundation ( NSF CMMI-1562941 ). Experiments and characterization made use of the Materials Characterization and Imaging Facility, the NUANCE Center (supported by SHyNE under NSF ECCS-1542205, MRSEC under NSF DMR-1720139, the International Institute for Nanotechnology, the Keck Foundation, and the State of Illinois), and the Center for Advanced Molecular Imaging (NCI CCSG P30 CA060553) at Northwestern University (NU). Tomography experiments were performed at Sector 2-BM of the Advanced Photon Source (APS), operated by Argonne National Laboratory (DOE DE-AC02-06CH11357). The authors gratefully acknowledge Mr. Pavel Shevchenko (APS) and Ms. Kristen Scotti (NU) for their help in collecting tomography data and for insightful discussions. This work was supported by the National Science Foundation (NSF CMMI-1562941). Experiments and characterization made use of the Materials Characterization and Imaging Facility, the NUANCE Center (supported by SHyNE under NSF ECCS-1542205, MRSEC under NSF DMR-1720139, the International Institute for Nanotechnology, the Keck Foundation, and the State of Illinois), and the Center for Advanced Molecular Imaging (NCI CCSG P30 CA060553) at Northwestern University (NU). Tomography experiments were performed at Sector 2-BM of the Advanced Photon Source (APS), operated by Argonne National Laboratory (DOE DE-AC02-06CH11357). The authors gratefully acknowledge Mr. Pavel Shevchenko (APS) and Ms. Kristen Scotti (NU) for their help in collecting tomography data and for insightful discussions.

Keywords

Freeze-casting
Metal foams
Reduction
Sintering
X-ray tomography

ASJC Scopus subject areas

Mechanics of Materials
Mechanical Engineering
Metals and Alloys
Materials Chemistry

Access to Document

10.1016/j.jallcom.2021.161707

Cite this

@article{f549842fa03b45119ec8fdcfe203b714,

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

abstract = "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 quasi in situ X-ray microtomography to analyze the three-dimensional structural evolution of directionally freeze-cast, lamellar Fe2O3 and Fe2O3+NiO green bodies as they are reduced by H2 at 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.",

keywords = "Freeze-casting, Metal foams, Reduction, Sintering, X-ray tomography",

author = "Wilke, {Stephen K.} and Mack, {Jacob B.} and Christoph Kenel and Dunand, {David C.}",

note = "Publisher Copyright: {\textcopyright} 2021 Elsevier B.V.",

year = "2022",

month = jan,

day = "5",

doi = "10.1016/j.jallcom.2021.161707",

language = "English (US)",

volume = "889",

journal = "Journal of Alloys and Compounds",

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T1 - Evolution of directionally freeze-cast Fe2O3 and Fe2O3+NiO green bodies during reduction and sintering to create lamellar Fe and Fe-20Ni foams

AU - Wilke, Stephen K.

AU - Mack, Jacob B.

AU - Kenel, Christoph

AU - Dunand, David C.

PY - 2022/1/5

Y1 - 2022/1/5

N2 - 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 quasi in situ X-ray microtomography to analyze the three-dimensional structural evolution of directionally freeze-cast, lamellar Fe2O3 and Fe2O3+NiO green bodies as they are reduced by H2 at 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.

AB - 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 quasi in situ X-ray microtomography to analyze the three-dimensional structural evolution of directionally freeze-cast, lamellar Fe2O3 and Fe2O3+NiO green bodies as they are reduced by H2 at 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.

KW - Freeze-casting

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KW - Reduction

KW - Sintering

KW - X-ray tomography

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