Multiscale modeling of the high-temperature radiative properties of ceria ceramics under thermochemical redox conditions

Abstract

A multiscale modeling approach is proposed to determine the normal spectral emissivity of a 1.92 mm thick and 98% dense cerium oxide ceramic containing a small amount of micron size pores. The model is based on measurements of the electrical conductivity of the ceramic for the thermal operating regime set by the actual solar thermochemical production of H${}_2$, i.e. from 900 to 1500 °C at a oxygen partial pressure of 10⁻⁵ atm. From a radiative point of view, the ceramic is defined as the volumetric distribution of spherical pores in a set of grains forming a radiatively homogeneous and continuous matrix. Room temperature normal hemispherical reflectance and transmittance measurements give analytically the scattering coefficient of this absorbing and scattering medium, considered independent from the temperature, using the modified two-flux approximation. The absorption coefficient is given at a fixed temperature and atmosphere by a semi-quantum Drude-Lorentz model whose parameters are derived from electrical conductivity measurements. The combination of both coefficients allow to determine critical optical thickness of the sample depending on the temperature, and to model emissivity. The multiscale model predicts the increase of an emissivity plateau between 2500 and 20000 cm⁻¹ with the temperature, going from 0.70 at 900 °C to 0.84 at 1500 °C, higher than the value of 0.20 measured at room temperature.