High-temperature optical and radiative properties of alumina–silica-based ceramic materials for solar thermal applications

Abstract

Optical and radiative properties of alumina–silica-based ceramic materials are determined in the spectral range of 3–10 μm as a function of temperature in the range of 150−650°C. The investigated materials are commercial CARBO HSP samples and in-house fabricated samples with sintering temperatures of 1400°C and 1500°C. Spectral normal emittance of the samples is measured in the spectral range of 3–20 μm using Fourier transform infrared spectroscopy. A two-step inverse methodology consisting of analytical and numerical solutions to the radiative transfer equation is employed. The analytical solution is based on the modified two-flux approximation and the transport approximation, and is used to determine the transport scattering albedo. The iterative Monte Carlo ray-tracing method is used to obtain the transport extinction coefficient. The spectral normal emittance increases non-monotonically with the thermal treatment temperature of the samples due to the sintering extent increasing with the temperature during the preparation process. The spectral normal emittance of all investigated samples also increases non-monotonically with the increasing actual sample temperature. The maximum difference in total normal emittance at studied temperatures is 7.9%, 22% and 5.5% for the commercial sample and the in-house prepared samples sintered at 1400°C and 1500°C, respectively. With the sample temperature varying in the range of 150–650°C, the spectral normal emittance in the spectral range of 4–20 μm maximally varies by 23.2%, 44.3% and 23.4% for the commercial sample, and in-house prepared samples sintered at 1400°C and 1500°C, respectively. The transport scattering albedo decreases with increasing temperature for all samples in the considered spectral range. For all samples, the absorptive index increases non-monotonically with the increasing sample temperature. The most significant increase in the absorptive index, as the sample temperature increases, is found in the spectral range of 3–5 μm for the commercial material, and it increases by up to 31.6%, 116%, 170%, 186% and 194% when the temperature increases from 150°C to 250°C, 350°C, 450°C, 550°C and 650°C, respectively.