  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.
