Temperature-Dependent Diffuse Reflectance Measurements of Ceramic Powders in the Near- and Mid-Infrared Spectra

TL;DR

This paper presents novel temperature-dependent diffuse reflectance measurements of ceramic powders, including ACCUCAST, alumina, and silica, in the infrared spectrum, revealing their thermal properties and changes relevant for solar energy applications.

Contribution

It introduces a new technique for measuring diffuse reflectance above the background material's limiting temperature, providing first-time data for ACCUCAST at high temperatures.

Findings

ACCUCAST, alumina, and silica exhibit high emittance at room temperature.

Reflectance decreases with temperature, notably for ACCUCAST at 1000 C.

Thermal cycling causes visible color change and increased reflectance in ACCUCAST.

Abstract

This study focuses on experimentally measuring temperature-dependent diffuse reflectance in the near- and mid-infrared spectra for ceramic particles with applications as heat-transfer and thermal-storage media in concentrated solar power (CSP) plants. Specifically, a commercially available sintered bauxite ceramic powder, ACCUCAST ID80, and its primary chemical constituents, alumina (Al2O3) and silica (SiO2), are measured using a Fourier transform infrared spectrometer (FTIR) coupled with a specialized diffuse reflectance accessory and a heated stage. Room-temperature diffuse reflectance measurements show increased absorption in tests with greater mass fractions of the ceramic samples. There is a strong correlation in the measured reflectance spectra of ACCUCAST with alumina and silica in the spectral range 2000-500 cm-1. For the first time, temperature-dependent diffuse reflectance measurements are reported for ACCUCAST, including a novel technique for accessing reflectance values above the limiting temperature of the background material KBr. All three materials exhibit a calculated emittance of ~0.9 at room temperature. However, this value drops to 0.68 at 1000 C for ACCUCAST and ~0.43 for alumina and silica. Thermal cycling in air from 25 C to 1000 C resulted in a visible color change from dark grey to light orange for ACCUCAST and a subsequent 5X greater increase in reflectance at 4000 cm-1 as compared to ACCUCAST thermally cycled at 1000 C in vacuum. Alumina and silica spectra proved to be largely unaffected by thermal cycling under atmospheric and evacuated conditions.