## 3 Results and Discussion
Unfortunately, because energy-dispersive X-ray spectroscopy analysis was not performed for the images in Figure 8, our hypothesis is not verified. However, it was visually conformed that the homogeneously dispersed particles existed in the nanofluids. Even though the images in Figure 8 were obtained in solid phase, the dispersion homogeneity of the graphite nanoparticles in liquid phase was well maintained under repeating thermal cycles in DSC. According to the DSC measurements and the SEM images, it may be concluded that the homogeneous dispersion of the nanoparticles favorably influences the specific heat enhancement of the nanofluids. Regarding the enhanced specific heat of molten salt nanofluids, the higher heat capacity of the nanoparticles, the interfacial thermal resistance at the solid-liquid interface, and the formation of the compressed liquid layer near the nanoparticles were proposed as mechanisms contributing to the increase of the specific heat [19; 23; 45]. Many researchers have presented SEM images of unique structures in nanofluids. Needle-shaped structures and network-like structures are often shown in research papers concerning the specific heat of molten salt nanofluids [23; 45]. It is reasonable to consider that the special structures are relevant to the specific heat increase. [PERSON] et al. experimentally observed ordered liquid aluminum atoms adjacent to the crystalline interface with sapphire [46]. Additionally, molecular dynamics simulations showed the formation of a dense (compressed) layer with a certain thickness at the interface between a solid nanoparticle and liquid molecules [30]. This compressed liquid layer in which the solvent molecules (molten salt molecules) acquire an ordered structure can be regarded as a quasi-crystalline structure. The compressed liquid layer of binary carbonate salt mixtures has different chemical compositions of the two salts from the overall (bulk) compositions of the base fluid [20; 28]. The compressed liquid layer has a higher concentration of potassium carbonate than the bulk salt mixture (base fluid). Because of the different chemical composition of the compressed liquid layer, this region has significantly different thermophysical properties than the bulk salt mixture (base fluid) [31]. Thus, the compressed liquid layers adjacent to the nanoparticles create special structures by joining each layer, and these structures enhance the specific heat of the nanofluids. Ultimately, the compressed liquid layers amplify the capacity of the nanofluids for thermal energy storage. Figure 8: SEM images of the nanofluid (1 wt % nanoparticles and 5 wt % GA): secondary electron images (**A**,**B**) and backscatter images (**C**,**D**). The red circles indicate the single compressed liquid layer. Figure 9 shows the nanoparticle-induced compressed liquid layer structures of the molten salt mixtures in solid phase. The network like structures in Figure 9 are presumed as the compressed liquid layer structures in solid phase. The dispersion effect can be explained by these special structures of the solvent material. If the nanoparticles are homogeneously dispersed in the base fluid, a significantly larger volume of the compressed liquid layers can be formed, and the compressed liquid layer structures should be increased. Thus, the individually dispersed nanoparticles shown in Figure 8 allow a large volume of the compressed liquid layer structures to be created in the nanofluids. Regarding the concentration effect of the nanoparticles, the specific heat of the nanofluids can be modified significantly via doping with a minute concentration of the nanoparticles in the salt mixtures, and the enhancement increases with the nanoparticle concentration. However, experimental analysis, such as energy dispersive spectroscopy, is necessary to reveal the identity of the structures in Figure 9.  
Conventional theoretical models are unable to explain and predict the specific heat enhancements achieved in this study.