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Materials 2022, 15, 2946 Error! Reference source not found. shows the variations in spectral absorptance of the Carbobead CP over a wavelength range of 350–2500 nm and under heat treatments at 1200 °C for different periods. The results show that the spectral absorptivity of the parti- cles decreased as the wavelength increased, and does not reach a steady value within the 500 h test limit. The Carbobead CP that was heated for 4 h at 1200 °C still almost simulated the solar absorptivity of the “as received” particulate due to the short time of heating to which the Carbobead CP was exposed. The results showed an increase in the values of spectral solar absorptance with an increase in the heating exposure time. However, the exposed to cyclic heating for 500 h, structural changes took place. The results also show that change in optical properties with exposure time was considerably slower between 4 h of Materials 2022, 15, x FOR PEER REVtIEhWe peaks of the samples “as received” and the samples subjected to heat treatmen1t4teosft2s3 heating operation and the samples exposed to cyclic heating for 32 h and 500 h. The change for 6 h were assigned to α-SiO2 as the major phase and aluminum silicate (Al2O3·SiO2) as in solar weighted absorptance for all three samples is shown in Error! Reference source the minor phases; while the β-SiO2 beside α-SiO2 was the major phase in the aged sample, not found.. The solar weighted absorptance dropped only 4% from the “as received” and keatite (SiO2) and aluminum silicate (Al2O3·SiO2) were the minor phases. value after being exposed to cyclic heating at 1200 °C for 500 h. Element White Sand (weight %) Element Fresh 800 ◦C 800 °C 1000 ◦C 1000 °C 1200 ◦C 1200 °C 1200 ◦C—500 h 1200 °C—500 h O 64.94 66.83 67.40 63.52 57.59 Al 1.16 1.00 1.17 1.26 0.99 O 64.94 66.83 67.40 63.52 57.59 Al 1.16 1.00 1.17 1.26 0.99 Fresh 12 of 20 changes did not take place as a result of heating for 6 h. However, when the sample was 4.2. Elemental Analysis In order to determine the physical mechanism responsible for the observed change in color and in the optical properties of the particles that were subjected to cyclic heating, the candidate particulate materials were examined using EDX and XRD. The elemental composition of the white sand is tabulated in Error! Reference source not found., while the XRD results are presented in Error! Reference source not found.. The results show the major contents of the samples were silicon (Si) and oxygen (O2), with a small percentage of other elements such as aluminum. The EDX results show a decrease in the O2 weight percentage and an increase in the silicon weight percentage with temper- ature. The higher percentages of Si and O2 refer to the greater existence of silica SiO2 in the white sand sample and a very small percentage of Al2O3. The XRD analysis showed that all the peaks for all the samples represented SiO2. The changes in the intensity of the peaks were due to the exposure of the samples to high temperatures. It was clear that the peaks matched for the samples “as received” and the samples subjected to heat-treatment tests for 6 h, which indicated that the structural changes did not take place as a result of heating for 6 h. However, when the sample was exposed to cyclic heating for 500 h, structural changes took place. The results also show that the peaks of the samples “as received” and the samples subjected to heat treatment tests for 6 h were assigned to α-SiO2 as the major phase and aluminum silicate (Al2O3·SiO2) as the minor phases; while the β-SiO2 beside α- Figure 5. Spectral absorptance of the white sand samples (“as received” and after aging test) over SiO2 was the major phase in the aged sample, and keatite (SiO2) and aluminum silicate Figure 5. Spectral absorptance of the white sand samples (“as received” and after aging test) over the the wavelength. (Al2O3·SiO2) were the minor phases. wavelength. Table 4. Elemental composition of white sand. Table 4. Elemental composition of white sand. White Sand (Weight %) Si 33.90 32.17 31.41 35.22 41.42 Si 33.90 32.17 31.41 35.22 41.42 Total 100.00 100.00 99.99 100.00 99.99 Total 100.00 100.00 99.99 100.00 99.99 FFigiguurere66. .XRDdaattaafforrwhiitteessandiintthe“asreceiiveed””ccondiittioionn,,ffoollolowiningghheeaatitninggininaairiraattdififfeerreenntt temperatures, and after cyclic heating at 1200◦°C for 500 h. temperatures, and after cyclic heating at 1200 C for 500 h. The EDX results for the red sand (“as received” and after each heating test) are sum- marized in Error! Reference source not found.. The EDX results show that the red sand (as received) contained more elements than the white sand, indicating the presence of more impurities. Since the red sand had a minor component of ferrous oxide, it may havePDF Image | Low-Cost Particulates Used as Energy Storage and Heat-Transfer Medium
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