Characterization of a Laser-Based Heating System Coupled With In Operando Raman Spectroscopy for Studying Solar Thermochemical Redox Cycles

Abstract

A 200 W CO2 laser-based heating system coupled with in operando Raman spectroscopy has been developed. The system delivers highly concentrated radiation capable of driving thermochemical reactions and simulates heat fluxes expected by 3D solar concentrating systems. 10 mol% Gd-doped and pure ceria pellets were prepared and used to characterize the system because of their well-established thermodynamic and kinetic properties, as well as their strong Raman peak due to F2 g symmetrical mode at 460 cm−1. Reduction in an H2 atmosphere has been carried out to investigate the behavior of the full width at half maximum (FWHM) of the F2 g Raman peak resulting from changes in temperature and oxidation state. For both samples, an increase in temperature during heating in air (i.e., fully oxidized) resulted in a peak shift toward low wavenumber and an increase of FWHM. The FWHM versus temperature curves were then measured for controlled reduction extents ranging between sample averaged nonstoichiometries of δ = 0–0.209 as a function of temperature. At a fixed temperature, Gd-doped ceria exhibited an increase in FWHM with increasing reduction extent until δ = 0.056. At greater reduction extents, the FWHM decreased with increasing reduction extents. We attribute this to changes in the lattice parameter caused by the eventual formation of intermediate cubic Ce2O3 at the radiated surface. This study demonstrates the promise of utilizing Raman spectroscopy to probe thermochemical reactions in operando. Going forward, we expect that this will be an especially promising tool for characterizing emerging thermochemical materials with complex phase equilibria, especially for nonequilibrium processes.

In situ defect quantification and phase identification during flash sintering using Raman spectroscopy

“…Since current does not flow through the sample until the onset of the flash (which occurs at 670°C) the spectra collected before the flash during the CHR experiment are expected to follow trends that are known to occur when CeO 2 is heated. It is known that the maximum intensity of the F 2 g peak decreases with temperature while the full width at half maximum (FWHM) increases and the peak shifts to lower wavenumber 40 . As expected, these three trends are seen before current begins to flow in the CHR experiment, as shown in Figure 1(C,D).…”

“…It is known that the maximum intensity of the F 2g peak decreases with temperature while the full width at half maximum (FWHM) increases and the peak shifts to lower wavenumber. 40 As expected, these three trends are seen before current begins to flow in the CHR experiment, as shown in Figure 1(C,D). The F 2g peak integrated intensity is also known to decrease with temperature and has been explained as being the result of the decrease in sampling depth due to the increase in optical absorbance.…”

“…The F 2g peak integrated intensity is also known to decrease with temperature and has been explained as being the result of the decrease in sampling depth due to the increase in optical absorbance. 40 This trend can be seen in Figure 1(E). It has also been shown that the defect peak intensity increases relative to the F 2g peak, as seen in Figure 1(A).…”

A Laser-Based Heating System for Studying the Morphological Stability of Porous Ceria and Porous La0.6Sr0.4MnO3 Perovskite during Solar Thermochemical Redox Cycling

“…An annular pipe that isolates the delivered gas from the surroundings is connected to a 6-way ConFlat (CF) stainless steel cube sample chamber and is enclosed by a ZnSe window. More details of the laser-based heating system are described in a previous work [21].…”

“…An annular pipe that isolates the delivered gas from the surroundings is connected to a 6-way ConFlat (CF) stainless steel cube sample chamber and is enclosed by a ZnSe window. More details of the laser-based heating system are described in a previous work [21]. Porous ceria (Alfa Aesar, 5 μm, 99.9%) and LSM40 pellets (synthesized with a modified pechini method described prior [22,23]) were produced by mixing the oxide powders and graphite powder (Alfa Aesar, 2-15 μm, 99.9995%) with a volume ratio of 4:6.…”