Robocasting of Hierarchical Porous Al₂O₃ Structures: A Computational and Experimental Methodology for Porosity Estimation and its Effect

Porous alumina is utilized across diverse fields like catalyst supports, filtration membranes, and biomedical implants due to its advantageous properties such as high surface area, thermal stability, and biocompatibility. Optimizing and characterizing the porosity percentage is crucial for enhancing functionality, mechanical strength, and fluid permeability in these applications. This study aimed to develop alumina structures with hierarchical porosity and establish a reliable methodology for accurately determining their porosity. The research employed various techniques: Robocasting for fabricating structures using alumina pastes with different porogen concentrations, analysis of porogen effects on paste rheology, SEM combined with image analysis for geometric assessments, and the use of Representative Volume Elements to explore porosity's impact on elastic properties. Additionally, Micro-computed Tomography (μ-CT) scans evaluated structure and porosity, nanoindentation measured bulk mechanical properties, compression tests determined effective properties, and Finite Element Analysis simulated compression behavior. Results from μ-CT scans indicated consistent macro-porosity across all samples due to uniform infill patterns during 3D printing. Samples with two different porogen concentrations exhibited internal porosity within struts, measured at approximately 9.32 and 23.61%, respectively via μ-CT and 13.29 and 27.44%, respectively via SEM image analysis. The proposed numerical-experimental approach estimated similar values of 14.20 and 28.53%, respectively, without the need for physical sectioning or potential uncertainties from superficial region analysis and thresholding. This comprehensive methodology provided valuable insights into the characteristics and mechanical behavior of hierarchical porous structures, offering effective predictive tools for internal micro-porosity assessment.