## 3 Results and Discussion
As shown in Figure 6, no remarkable decrease was not found except one case (nanoparticle concentration of 0.1 wt % for the base fluid containing 62 mol % Li\({}_{2}\)CO\({}_{3}\)). Although a little decrease was observed as repeating thermal cycles (liquefaction and solidification), the degradations for most nanofluids were less than 10%. Thus, the nanofluid synthesis protocol employed in this study is appropriate for molten salt-based nanofluids. However, relatively large decreases were also observed  
Figure 3: Heat-flow curves of the pure salt eutectic and the nanofluids, obtained via DSC measurements. the previous section, the chemical compositions of the base fluid were identical for all of the samples in Group 2, and the concentrations of the nanoparticles were 0.025, 0.05, 0.1, and 1.0 wt %. Figure 7 shows the specific heat of the nanofluids in both the solid phase (averaged between 390 and 410 \({}^{\circ}\)C) and the liquid phase. The results for the solid phase are not readily convincing, because of the large increase even in the case of doping the salt mixture with small amounts of nanoparticles. However, in a previous study, a similar result was reported for carbonate salt mixture-graphite nanofluids [28]. A nanoparticle concentration of only 0.1 wt % led to a specific heat enhancement of more than 30% for a carbonate salt mixture with almost the same chemical composition of the two salts [20]. In addition, it appeared that the increase of the specific heat was limited even though the nanoparticle concentration increased. The specific heat increase was not significant for either the solid or liquid phases. Therefore, dispersing an abundant amount of the nanoparticles is meaningless, because surplus nanoparticles result in the agglomeration of the nanoparticles; thus, the addition of nanosized-particles is no longer effective. In the liquid phase, the effect of the nanoparticle concentration on the increase of the specific heat was observed, as shown in Figure 7. The specific heat of the nanofluids was measured to exceed 3.1 [J/g\(\cdot\)K] at a nanoparticle concentration of 1.0 wt %, which is almost double that of the pure carbonate eutectic. The dispersion of nanoparticles has been a fundamental issue since the early stages of nanofluid research. The two-step synthesis method for the molten salt nanofluids employed in this study utilizes the high water solubility of the salt components, but the evaporation process may cause unfavorable agglomeration of nanoparticles. In particular, this is a critical problem for carbon nanoparticles such as graphite and carbon nanotubes, which need a surfactant to be dispersed into water. To determine the effect of the nanoparticle dispersion on the specific heat enhancement of the carbonate salt mixture, the amount of the surfactant--GA--was increased from 1 to 5 wt %, and the specific heat was compared with that of the nanofluid at the same nanoparticle concentration with 1 wt % GA. As shown in Figure 7, the specific heat values of the nanofluids with 5 wt % GA were 1.91 [J/g\(\cdot\)K] in the solid phase and 3.36 [J/g\(\cdot\)K] in the liquid phase. Figure 8 shows SEM images for nanofluids produced with 5 wt % GA: secondary electron images and backscatter images. These SEM images differ significantly from those in other studies. Small particles exhibiting a spherical shape in the secondary electron images (Figures 8-A and 8-B) were homogeneously dispersed into the salt mixture (base fluid) without any agglomeration. The backscatter images (Figures 8-C and 8-D) show the dispersion homogeneity of the nanoparticles, which were black-colored. The diameters of the particles in Figure 8 appear larger than those of the graphite nanoparticles shown in Figure 1. It is thus hypothesized that the particles  
Figure 7: Specific heat of nanofluids in the solid and liquid phases with respect to the nanoparticle concentration. in Figure 8 are single structures of the nanoparticle-induced compressed liquid layer.