A multiscale microstructural characterization of SiO2-Al2O3 foams

Abstract

Recent advances in engineering of porous ceramics have enabled the ability to fabricate silica-alumina foams (SAF) with controllable porosity. In an earlier investigation (doi.org/10.1016/j.ceramint.2020.01.042), we examined the thermophysical properties, as well as the efficacy of Na-activated SAF foams for thermal insulation applications. As an important follow-up, in this work, we focus on microstructural characterization of SAF foams and provide fundamental insights into the chemical and structural features that underlie SAF foams. In this regard, we carry out X-ray micro computed tomography (μCT), in conjunction with scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), and nuclear magnetic resonance (NMR) spectroscopy. μCT images clearly show the closed cell nature of the foams; in addition, the ratio of average pore-size to average strut thickness decrease with increasing mass density of the foams. An analysis of SEM images demonstrates that the compositional and structural features of both the strut walls and the interior walls of the pores are very similar, primarily consisting of inorganic silica and alumina particles. SEM data also indicate that the SAF foams demonstrate a hierarchical pore structure. Further insights obtained from EDX maps, XRD data, and NMR spectra, clearly indicate the presence of a crystalline ‘film’ phase (NaAlSiO4) in close proximity with the inorganic particles. We hypothesize that the observed NaAlSiO4 phase is formed due to the high pH conditions pertaining to the synthesis protocols and serves as a binder phase between constituent silica particles.