## 2 Nanofluid Synthesis and Specific Heat Measurements
### Synthesis of Nanofluids  
Figure 2 shows the procedures for synthesizing the molten salt-based graphite nanofluids [20; 28]. First, graphite nanoparticles were dispersed into distilled water with the GA (1 wt %). Then, the aqueous graphite suspension (aqueous nanofluid) was sonicated in an ultrasonic bath (Branson 2510) for 2 h. After the sonication process, the water-salt solution was added to the graphite nanofluid. The mixture (graphite suspension and water-salt solution) was subjected to the sonication process again for 3 h. Finally, the water was removed via evaporation on a hot plate at 100 \({}^{\circ}\)C using a large petri dish 10 cm in diameter. After the water was completely evaporated, the dry power of the molten salt nanofluids was scraped from the entire whole petri dish, and lastly, a grinding and mixing process was performed to obtain a uniform chemical composition of the base fluid. Pure salt mixtures were prepared though the same procedures but were subjected to sonication only once, for 2 h.  
Figure 1: TEM image of a graphite nanoparticle.  
Two sample groups were prepared in this study: Group 1 and Group 2. With the nanofluids in Group 1, the effect of the base fluid (chemical composition of the two carbon salts) on the increase of the specific heat was investigated. That is, in the Group 1 samples, the chemical composition of the base fluid was varied, and the nanoparticle concentration was fixed. In contrast, the nanofluids of Group 2 were used to explore the effect of the nanoparticle concentration on the specific heat. Therefore, the chemical composition of the base fluid (mode fraction of the two salts) was fixed (75 mol % of Li\({}_{2}\)CO\({}_{3}\) and 25 mol % of K\({}_{2}\)CO\({}_{3}\)), and the concentration of the graphite nanoparticles was varied from 0.025 to 1.0 wt %. As reported in a previous study [31], because the specific heat of the binary carbonate salt mixtures is strongly influenced by the chemical composition of the base salt, the mole fraction of the two salt components (lithium carbon and potassium carbonate) was experimentally measured using inductively coupled plasma-mass spectrometry (ICP-MS) for the sample after DSC tests.  
The mole fractions of the base fluid for the nanofluids in Group 1, which were measured via ICP-MS, are presented in Table 1. The mole fraction of lithium carbonate varied from 40% to 90%, and the mole fraction of potassium carbonate varied from 10% to 60%. The initial concentrations of graphite nanoparticles were 0.1 and 1.0 wt %, and GA of 1 wt % was included in each nanofluid. The mole fraction of lithium carbonate and potassium carbonate for the Group 2 nanofluids was 75:25 in molar ratio, where the maximum specific heat was measured for the binary carbonate salt mixtures in a previous study [31]. The mole fractions of the base fluid obtained via ICP-MS for the Group 2 nanofluids were not exactly the same for all of the samples. However, the deviations in the mole fraction of the two salts were between 0.8 and 1.9 mol %, which is acceptable for analyzing measurements of the specific heat. Finally, the amount of GA was increased to 5 wt % to improve the dispersion homogeneity of the nanoparticles. The specific heat of the nanofluids produced with GA of 5 wt % was compared with that of other nanofluids with GA of 1 wt % for the same base fluid and nanoparticle concentration.