*3.5. Heat Transfer Characteristics*

Ternary nitrate has good heat transfer performance and can be used as a heat transfer fluid. Figure 8 shows the thermal diffusivity of samples S0, S2, S4, S6, and S7. Obviously, with the increase of MgO nanoparticles, the thermal diffusion coefficient increases significantly. When the mass fraction of MgO nanoparticles is 5%, the thermal diffusion coefficient is 0.425 mm2·s−1, which is 39.3% higher than that of the eutectic salt without MgO nanoparticles. It can be seen that the interface thermal resistance effect between

**Figure 8.** Thermal diffusivity of S0, S2, S4, S6, and S7.

To explore the heat transfer performance of the modified nitrate, the samples S0, S2, S4, and S6 were placed on a heating plate, and an infrared thermal imaging instrument was used to characterize the heat transfer performance of the material. Figure 9 shows the infrared thermal images of samples S0, S2, S4, and S6 heated on the heating plate for different times. The heat is transferred upward from the bottom end. The heat transfer rate from sample S0 to sample S6 shows an increasing trend, and the transfer rate of sample S6 is the fastest at the same time. Plot the temperature of samples S0, S2, S4, and S6 at the same geometric position with heating time, as shown in Figure 10. It can be seen that the heat transfer performance of the material increases with the increase of MgO nanoparticles. The doping of nanoparticles enables the material to transfer heat in the solid state quickly, accelerating the heat transfer process of the material, thus shortening the time required for the phase change of the material, and more efficiently storing thermal energy. At the same time, when the material acts as a nanofluid in a liquid state, the heat transfer performance is greatly improved, and the heat transfer process is accelerated. The results show that modified nitrate has high sensible heat and latent heat, a suitable phase transition temperature, and high thermal conductivity, which can improve the energy storage efficiency and can be used in thermal energy storage systems.

**Figure 9.** Infrared thermography images of samples S0, S2, S4, and S6 heated on the heating plate for different times: (**a**) 0 s, (**b**) 60 s, (**c**) 120 s, (**d**) 180 s, (**e**) 240 s, and (**f**) 300 s.

**Figure 10.** The temperature variation curve of the same geometric position of samples S0, S2, S4, and S6 with heating time.
