Phase composition, microstructure and properties of refractory foams as insulation materials for high-temperature thermal energy storage

Abstract

High-alumina refractory foams (insulating firebricks) are suitable insulation materials for high-temperature thermal energy storage aggregates. In this contribution we present analytical approaches for estimating the effective properties (density, specific heat, thermal conductivity and elastic properties) of these materials on the basis of the phase composition (determined via X-ray diffraction), including estimates of the content, composition and properties of the glass phase. Based on the weight percentages and the phase densities (for the glass phase estimated via the empirical approach of Huggins and Sun), the volume fractions are calculated. Based on the volume fractions and the phase properties (for the glass phase estimated according to different empirical approaches), the effective density of the dense multiphase materials is calculated, while the effective thermal conductivity and elastic properties are estimated via the arithmetic means of the Wiener-Paul bounds and the Hashin-Shtrikman bounds as well as the volume-weighted geometric mean. The total porosity (calculated from the bulk density and the effective density) can be taken into account via the upper bounds (Wiener-Paul, Hashin-Shtrikman) and analytical model relations for convex pores (power-law, exponential). Experimentally measured values are compared to these predictions and the cross-property relations between relative Young’s modulus and thermal conductivity. Based on these findings, the temperature dependence of Young’s modulus is discussed.