*3.1. Components of Modified Salt*

Figure 2 shows the XRD spectra of samples S0, S4, and the product after centrifugation. There are characteristic peaks at 19.03◦, 23.52◦, 33.06◦, 33.84◦, 41.15◦, and 46.61◦ in sample S0, corresponding to the (110), (111), (200), (112), (221), and (113) crystal planes of KNO3. Two peaks at 29.41◦ and 38.99◦ represent the (104) and (113) crystal planes of NaNO3, respectively. In addition, the crystal plane (104) of LiNO3 could be found according to the characteristic peak at 32.21◦. It can be seen that the ternary nitrate was successfully prepared. After centrifugation, characteristic peaks can be seen at 36.89◦, 42.86◦, 62.22◦, 74.58◦, and 78.51◦, corresponding to (111), (200), (220), (311), and (222) crystal planes of MgO. Sample S4 possesses several characteristic peaks of MgO at 42.96◦ and 62.22◦. It can be seen that MgO is formed in situ in the ternary nitrate system. The characteristic peaks at 18.59◦, 32.84◦, 38.02◦, 50.85◦, 58.64◦, 68.87◦, and 72.03◦ mark to (001), (100), (101), (102), (110), (200), and (201) of Mg(OH)2 planes. However, no characteristic peaks of Mg(OH)2 were found in samples S0 and S4, indicating that the magnesium hydroxide precursor was completely decomposed when the nanoparticles were generated in situ. The reason why the characteristic peaks of Mg(OH)2 can be seen in the product after centrifugation might be because MgO combines with water during the centrifugation process to form a trace amount of Mg(OH)2.

**Figure 2.** The XRD patterns of samples S0, S4, and product after centrifugation.

Figure 3 shows the Raman spectra of samples S0, S4, and the product after centrifugation. In samples S0 and S4, the peak at 713 cm−<sup>1</sup> corresponds to the NO3 − in-plane bending vibration (710–740 cm−1), and the peak at the frequency of 1049 cm−<sup>1</sup> corresponds to the NO3 <sup>−</sup> symmetric stretching vibration (1020–1060 cm<sup>−</sup>1). In the Raman spectrum of sample S4 and the MgO nanoparticles obtained after centrifugation, peaks with frequencies of 1499 cm−<sup>1</sup> and 1936 cm−<sup>1</sup> can be observed. Combined with XRD analysis, it can be further known that MgO nanoparticles were successfully generated in situ in the ternary nitrate system [38].

**Figure 3.** The Raman spectra of samples S0, S4, and product after centrifugation.

## *3.2. The Structure of Modified Salt*

Figure 4 is SEM photos of samples S0, S2, S4, S6, and S7. Figure 4a is the SEM diagram of LiNO3–NaNO3–KNO3 ternary nitrate. The surface of the nitrate is relatively flat, and the material is uniform. The mass fractions of MgO nanoparticles in Figure 4b–d are 1 wt %, 2 wt %, and 3 wt %, respectively. It can be seen that the MgO nanoparticles are relatively evenly dispersed among the nitrates, which shows that the nanoparticles generated in situ have good dispersibility. Figure 4e,f are SEM of modified nitrate with 5 wt % MgO nanoparticles at different magnifications. When the content reached 5 wt %, the nanoparticles had obvious agglomeration. Due to the high surface energy of nanoparticles, they will agglomerate together and deposit in the nanofluid, resulting in poor system stability. In the modified salt, the size of the nanoparticles synthesized in situ is concentrated in the range of 50–200 nm.

The element distribution of the sample was determined by the area scanning method of the energy spectrum. Figure 5 is the element distribution of modified nitrate. Figure 5c is the distribution of the Mg element. Figure 5d shows the distribution of the N element, which represents the distribution of nitrate. It can be seen from the element distribution diagram that MgO nanoparticles are evenly dispersed among the ternary nitrates.

**Figure 4.** SEM micrographs of (**a**) sample S0, (**b**) sample S2, (**c**) sample S4, (**d**) sample S6, and (**e**,**f**) sample S7.

**Figure 5.** The elemental distribution of modified nitrate.
