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Materials 2022, 15, 2946 8 of 20 under an argon atmosphere with a flow of 80 mL/min and at a heating rate of 10 (K/min) in a NETZSCH STA 409C/CD. A platinum–rhodium crucible (DSC/TG pan Pt-Rh) was used to hold the samples in these tests. The amount of each sample used was around 20 mg. All of the specific heat measurements were calibrated using a crystal of sapphire provided by NETZSCH. The measurements were extremely sensitive to experimental factors (e.g., crucible size, shape, mass, placement within the furnace, matching with the reference crucible, and sample characteristics such as distribution within the crucible and sample mass). In addition, the matching of each sample’s raw DSC signal to that of the sapphire standard was taken into account during the calibration and cp measurements of the samples. 4. Results and Discussion 4.1. Effects on Optical and Thermophysical Properties of Particulates 4.1.1. Particle Agglomeration Photographic images of the candidate particulate materials “as received” and after heating are shown in Table 2. In the beginning, three samples from each natural particulate material—Riyadh white sand, Riyadh red sand, and ilmenite—were heated to 800, 1000, and 1200 ◦C for 6 h and then cooled to ambient temperature. The results did not show any signs of agglomeration in all the white sand samples. In the case of the red sand, the sample heated to 800 ◦C did not show any signs of agglomeration, while a very weak adhesion between the particles was observed in the red sand after heating it to 1000 ◦C. A weak agglomeration began to form in the sample that was heated to 1200 ◦C. Generally, the agglomeration at 1200 ◦C was not significant, and it was easy to separate the agglomerated particles. The color of the sand was found to have become darker after heating. This was probably due to the removal of moisture and other impurities from the sand. The ilmenite particles began to show some signs of agglomeration at 800 ◦C, but when the ilmenite was heated to 1000 ◦C, large lumps were created. Lump formation increased at 1200 ◦C and was very hard to break. This agglomeration became harder at 1200 ◦C. Based on the aforementioned observations, the ilmenite was excluded from the next heating stage and replaced with the Carbobead CP sample for evaluation. The samples of white sand and red sand, which were heated to 1200 ◦C initially, were reheated to 1200 ◦C for 8 h and then cooled down to room temperature. Three samples of the Carbobead CP were tested at 1200 ◦C for different periods: one for 4 h, one for 32 h, and one joined to the white sand and red sand in the cyclic heating test. This cycle was repeated until reaching 500 h of heating to observe the aging effect on each material. The results also showed that the agglomeration did not appear in the white sand sample, while a slight agglomeration occurred in the Carbobead CP sample. However, the agglomerated masses were very easy to break. A hard agglomeration appeared in the red sand sample, and formed a large lump. 4.1.2. Particle Color Change Table 2 shows the gradual change in the appearance of the tested samples with temperature. The results showed a bit of change in the color of the white sand samples, which became more whitish. In the case of the red sand, the appearance of all the samples slightly changed and became darker after heating at 800, 1000 and 1200 ◦C for 6 h. After the aging process at 1200 ◦C for 500 h, the results showed that the white sand turned more whitish. In the case of the red sand, a significant degradation in its appearance occurred, in which the color changed entirely to white. The Carbobead CP exhibited a small color change compared to the original color, experiencing a slight darkening of its color to a tan or orange particle color. Changing the color of the particulate materials and agglomeration after cyclic heating was interpreted using EDX and XRD analyses. In general, the particulate samples appeared to be stable at temperatures ranging from 650 ◦C to 1000 ◦C. However, we cannot say that these particulate samples were not affected by heating, because they were heated for only 6 h and were not exposed to the cyclic heating at 800 and 1000 ◦C.PDF Image | Low-Cost Particulates Used as Energy Storage and Heat-Transfer Medium
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