## 3 Results and Discussion
### Microstructural Observations  
Figure 3 displays the microstructural images of the sintered samples Sample S3 and Sample S8 and the associated EDS maps. Figure 3a,b show a cross section of Sample S3 at two different magnifications (100\(\times\) and 500\(\times\)) while Figure 3d,e show the same for the Sample S8. SEM images at a magnification of 100\(\times\) give overall views of the surface morphology of different samples, and using these we can find that the microstructure shown in Figure 3d is more porous than the microstructure shown in Figure 3a. Pores are marked using circles in the SEM images: circles in orange are of smaller size; circles in yellow are of larger size. It can be seen in Figure 3d that Sample S8 has a large number of small pores and also some large pores with a diameter larger than 100 \(\upmu\)m. Conversely, Sample S3 has fewer observable small pores but some larger pores and cracks, as seen in Figure 3a. SEM images at a magnification of 500\(\times\) show more detailed information of the microstructure; SiC particles are observable in Figure 3e with sizes of \(\sim\)40 \(\upmu\)m to \(\sim\)80 \(\upmu\)m, which could be identified by the EDS image shown in Figure 3f. In addition, when comparing Figure 3b,e, Sample S8 can be seen to display a looser structure, with more small pores observed (\(\sim\)10 \(\upmu\)m). Figure 3c,f are the EDS mappings corresponding to Figure 3b,e, which give the element distributions on the sample surface. One can see a uniform distribution of the K\({}_{2}\)CO\({}_{3}\)-Na\({}_{2}\)CO\({}_{3}\) salt, MgO, and SiC, as reflected by the Na, K, Mg, O, and C elements. To compare the porosity of Sample S3 and S8 quantitatively, a mercury intrusion porosimeter was used to measure the porosity and pore size distribution of these samples. In Figure 4a, one can see that Sample S8 has a higher porosity (30.42%) with an average pore diameter of 66.0 nm. Meanwhile, Sample S3 has a lower porosity (22.44%) with an average pore diameter of 62.8 nm. As a result, Sample S8 has a lower bulk density than Sample S3. In Figure 4b, it can be found that Sample S8 has a large number of pores with a diameter of 16 \(\upmu\)m, which is consistent with the observations shown in Figure 3d,e. By contrast, Sample S3 has a smaller quantity of pores with a diameter of 16 \(\upmu\)m but some large pores and cracks. [PERSON] et al. [27] have indicated that the microstructure and the porosity of composite PCMs are related to the wettability between the liquid molten salt and solid particle surfaces. They concluded that a good wettability could densify the final composite, whereas a poor wettability would lead to a loose structure [26]. They also found that a eutectic molten salt (Na\({}_{2}\)CO\({}_{3}\)-Li\({}_{2}\)CO\({}_{3}\)) has a good wettability with a skeleton material (MgO) and hence a dense structure [27]. In this work, Sample S3, which does not consist of SiC, shows a denser or less porous structure than Sample S8. This was observed via SEM images and was also measured using the mercury posimeter. Therefore, our work is consistent with the findings of [PERSON] et al. [27]. The loose structure due to the addition of SiC is very likely due to the molten salt not wetting silicon carbide well. However, since no report has been made on the wetting behavior of the carbonate salts on SiC surfaces, the validation of this explanation needs further high temperature wetting experiments. Figure 2: The thermal conductivity of composite PCMs with different SiC contents using a mass ratio of salt to MgO of 60:40. Figure 4: Pore size, porosity, and density of Sample S3 and Sample S8. Figure 3: SEM images for Sample S3: (**a**) 100\(\times\); (**b**) 500\(\times\); (**c**) EDS mapping, 500\(\times\); SEM images for Sample S8: (**d**) 100\(\times\); (**e**) 500\(\times\); (**f**) EDS mapping, 500\(\times\). Figure 5 shows the SEM images and EDS maps of Sample S8 after 100 thermal cycles. It can be seen that the components of the composite remain uniformly distributed; see Figure 5c.