*3.3. Specific Heat Capacity*

Figure 6a,b are the specific heat capacity curves at 50–80 ◦C (solid phase) and 150–200 ◦C (liquid phase), respectively. The overall sensible heat capacity can be judged by selecting the specific heat capacities at 60 ◦C (solid phase) and 170 ◦C (liquid phase). Table 2 shows the specific heat capacities of ternary nitrate and modified salt in the solid phase and liquid phase. For the same sample, the specific heat in the liquid state is improved compared with that in the solid state. The reason for this phenomenon is that in the molten state, the ions in the molten salt perform randomly free movement, which can carry more energy. The specific heat in the solid state increases first and then decreases with the growth of nanoparticles. When the content of nanoparticles is 2%, the solid specific heat is the largest, which is 1.479 J/(g· ◦C). Compared with LiNO3–NaNO3–KNO3 ternary nitrate, it has increased by 51.54%. With the increase of nanoparticles, the specific heat in the liquid state shows an increasing trend at the first stage and then a decreasing tendency at the following stage. When the content of nanoparticles is 2%, the specific heat of the material at the liquid state reaches the peak, which is 1.878 J/(g· ◦C): an increase by 44.50%.

**Figure 6.** Specific heat capacity of ternary nitrate and modified nitrate: (**a**) solid phase, (**b**) liquid phase.


**Table 2.** Specific heat capacity of ternary nitrate and modified nitrate.

For MgO nanoparticles to increase the specific heat capacity of nitric acid ternary salt, the first possible reason is that MgO nanoparticles have higher specific surface energy. The second reason is the interface thermal resistance between MgO nanoparticles and nitrate, which can store and release additional energy. Another reason is that the MgO nanoparticles and the surrounding molten salt form a semi-solid layer. Hu et al. [24] explained the specific heat enhancement from the perspective of Coulomb energy. Molecular dynamics simulation is used to analyze the influence of nanoparticles on the energy composition of each atom type. The results show that the change in the Coulomb energy of each atom contributes the most to the enhanced specific heat capacity.
