Direct foaming driven synthesis and thermophysical characterization of silica-alumina foams: Applications for thermal insulation

Using earth-abundant materials such as quartz and corundum powders as precursors, we have developed 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. Specifically, 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 and structural characterization was performed via optical and electron microscopy, x-ray diffraction, and nuclear magnetic resonance, and the results from these studies are also discussed in the context of structural evolution of the foams as a function of porosity. While this work represents a straightforward and simple path towards obtaining silica-alumina foams with very low density, the developed method is directly applicable to obtaining a wider variety of multicomponent ceramic foams.