Synthesis and Characterization of Thermally Insulating Silica-Alumina Foams

Abstract

Aluminosilicate derived engineering materials such as geopolymers, zeolites, Gorilla Glass, and amorphous silica-alumina systems have found immense uses as catalysts, filters, drug delivery, thermal barriers, refractory coatings, and mechanical protection systems. Partly inspired by previous investigations carried out on zeolites, and geopolymers, the focus of this dissertation is on developing a new class of silica-alumina foams (SAF) for thermal insulation applications. Specifically, using earth-abundant materials such as quartz and corundum powders as precursors, a simple, but effective low energy direct foaming based method for synthesis of thermally insulating porous foams with thermal conductivity as low as 0.08 W/mK has been established. The adopted synthesis procedure provides a straightforward avenue for obtaining closed-cell silica-alumina foams, with densities ranging from 1.7 to 0.22 g/cc. The ability to tune the densities is achieved by controlled addition of blowing agents (H2O2) and surfactants (stearic acid) respectively. The thermal conductivity of these foams decreased monotonically with increasing porosity (or decreasing densities), and the density-thermal conductivity relationship is governed by a power law. In addition, morphological, microstructural, structural, and compositional characterization was performed via a combination of optical and scanning electron microscopy (SEM), x-ray diffraction, X-ray microcomputed tomography (μCT), and nuclear magnetic resonance (NMR) methods. In particular, μCT, optical microscopy, and SEM clearly indicated the closed cell form of the foams; in addition, the ratio of average pore-size to average strut thickness decreased with increasing mass density of the foams. An analysis of SEM images demonstrated that the compositional and structural features of both the strut walls and the interior walls of the pores were very similar, primarily consisting of inorganic silica and alumina particles. SEM data also indicated that the SAF foams demonstrate a hierarchical pore structure. Further insights obtained from EDX maps, XRD data, and NMR spectra, clearly indicated the presence of a crystalline ‘film’ phase (NaAlSiO4) in close proximity with silica particles. It is hypothesized 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 particles. While this work represents a straightforward and simple path towards obtaining silica-alumina foams with very low densities, the developed method is directly applicable to obtaining a wider variety of multicomponent ceramic foams.