Optical and radiative characterisation of alumina–silica based ceramic materials for high-temperature solar thermal applications

Optical and radiative properties of alumina–silica based ceramic materials are determined in the spectral range of 0.2–2.5 µm. The effects of material thermal treatment on the material structure, composition as well as optical and radiative properties are investigated using the commercial CARBO HSP materials and in-house fabricated samples with sintering temperatures of 1400^∘C and 1500^∘C. The material structure and composition are characterised using scanning electron microscopy, X-ray micro computed tomography, powder X-ray diffraction and scanning transmission electron microscopy. Directional–hemispherical reflectance and transmittance of the samples are obtained using dispersive spectroscopy. A two-step inverse methodology consisting of an analytical solution based on the modified two-flux approximation and iterative Monte Carlo ray-tracing is developed and applied to infer the transport scattering albedo and the transport extinction coefficient, respectively. The structural and compositional investigation revealed that the presence of chemical reactions between Al₂O₃ and SiO₂ to form mullite during the thermal treatment in the preparation process. The investigated samples are strongly absorptive in the spectral range of 0.2–0.4 µm, in which the transport scattering albedo is close to 0. Scattering is the dominant attenuation mechanism in the spectral range of 1.5–2.5 µm, in which the transport scattering albedo is above 0.8. The absorptive index of the in-house prepared samples sintered at 1400^∘C is one to two orders of magnitude smaller than that of the commercial material in the spectral range of 0.2–2.5 µm. However, the absorptive index of the samples sintered at 1500^∘C has similar values and is 25% smaller than that of the commercial material in the spectral ranges of 0.4–0.7 µm and 1.5–2.5 µm, respectively. The sample absorption increases with thermal treatment temperature because of higher extents of sintering.