**1. Introduction**

The exploitation of solar energy is essential to sustainable development. To solve the problem of intermittent solar energy in solar thermal utilization, it is necessary to use the thermal energy storage (TES) system to store and release heat when solar radiation is weak or absent [1,2]. As an excellent heat storage carrier, molten salt heat storage material has the advantages of sizeable latent heat, high energy storage density, low subcooling, good thermal stability and low cost, which is widely used as a heat storage medium for solar heat. Currently, solar salt is widely used in TES technology. The components of solar salt are NaNO3 and KNO3. The melting point of solar salt is 220 ◦C. Due to its high melting point, the pipeline needs to be heated to a higher temperature to prevent the pipeline from freezing, resulting in additional energy input and power generation costs. The higher melting point of molten salt limits the application of molten salt in the field of heat storage. Therefore, the ideal heat transfer fluid is supposed to have a low melting point, reducing the risk of freezing and heating energy consumption of the pipeline. Studies have shown that mixing several molten salts in a certain proportion forms a eutectic salt that can reduce the melting point while ensuring the thermal stability of the molten salt. The main melting salts are nitrates, carbonates, and sulfates [3,4]. In recent years, the development of low-melting, high-stability multi-element molten salt systems has become a research

**Citation:** Tong, Z.; Li, L.; Li, Y.; Wang, Q.; Cheng, X. The Effect of In Situ Synthesis of MgO Nanoparticles on the Thermal Properties of Ternary Nitrate. *Materials* **2021**, *14*, 5737. https://doi.org/10.3390/ma14195737

Academic Editor: Andrea P. Reverberi

Received: 19 August 2021 Accepted: 29 September 2021 Published: 1 October 2021

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hotspot in molten salt modification. Wu et al. [5] formulate 19 kinds of binary mixed molten salts in different proportions, the main component of which is KNO3-Ca(NO3)2·4H2O. The results showed that the thermodynamic properties of these molten salts performed well. Ren et al. [6] further explored the Ca(NO3)2–NaNO3 binary salt and modified it with expanded graphite, which effectively improved the thermophysical properties of the molten salt. In recent years, more and more ternary and quaternary molten salts have been developed [7–13]. The main research systems are LiNO3–NaNO3–KNO3, NaNO3– NaNO2–KNO3, Ca(NO3)2–NaNO3–KNO3, and LiNO3–NaNO3–KNO3–Ca(NO3)2. These multi-molten salts have lower melting points and higher stability. The use of relevant phase diagrams to create ternary or higher salt mixtures can obtain low melting point molten salts. The ideal freezing temperature for Hitec and Hitec XL is 120–140 ◦C, and they can withstand temperatures exceeding 500 ◦C. The LiNO3–NaNO3–KNO3 ternary mixture is considered as a promising heat transfer and storage medium, with a low melting point (120 ◦C) and high thermal stability (550 ◦C). Multi-element eutectic molten salt has a wide operating temperature range (low melting point and high decomposition point), which is very suitable as a heat transfer fluid and heat storage carrier in the TES system of a concentrating solar power plant to store solar energy.

Nanomaterials have special physical and chemical properties due to their unique structure, so they have important applications in heat storage [14–18]. For example, nano-SiC and nano-MgO have not only higher specific heat capacity but also better heat transfer efficiency, and they are very good heat storage materials. Therefore, the research of nanomaterials is of great significance to the development of heat storage materials. Researchers tried to add nanoparticles to molten salt to increase the specific heat capacity of molten salt. Among the research of using nanoparticles to modify molten salt, the most common materials are SiO2 and Al2O3 nanoparticles, most of which have been observed to have an increase in the specific heat and thermal conductivity [19–21]. Dudda et al. [22] and Seo et al. [23] explored the effect of nanoparticle size on the specific heat capacity of the nanoparticle/molten salt eutectic mixture. It was observed that the salt compounds around the nanoparticles formed a large number of nano-sized structures, which may be the main reason for the increase in specific heat. From the view of structure, one reason for the increased specific heat is the thermal resistance of the interface between the nanoparticles and the molten salt. Another reason is that a semi-solid layer is formed between the nanoparticles and the molten salt. From the perspective of energy, the high surface energy of nanoparticles can also store part of the thermal energy. Hu et al. [24] performed molecular dynamics simulations on Al2O3 nanoparticles doped in solar salt and explored the reason for the specific heat enhancement from the view of energy. The result shows that the change of Coulomb energy is the reason for the change of specific heat capacity.

The addition of nanoparticles can also improve the heat transfer performance of molten salt to a certain extent. Gupta et al. [25] added different types of nanoparticles (TiO2, ZnO, Fe2O3, and SiO2) to the phase change material (PCM) of Mg(NO3)2·6H2O and formed the PCM–metal oxide nanocomposite material through the melting and mixing technology. The PCM–metal oxide nanocomposite with a 0.5 wt % nanoparticle addition increased the thermal conductivity by 147.5% (TiO2), 62.5% (ZnO), 55% (Fe2O3), and 45% (SiO2), respectively. Ho et al. [26] discussed the effect of nanoparticle concentration on the convective heat transfer performance of molten nano-HITEC fluid laminar flow in microtubes. The heat transfer performance of HITEC fluid with Al2O3 nanoparticle concentration as high as 0.25 wt % has been improved. The study of Yu et al. [27] observed that SiO2 and TiO2 nanoparticles can improve the thermal conductivity of molten salt. Under normal circumstances, the thermal conductivity of molten salt is about 0.2–2.0 W/(m·K), and the specific heat is about 1.35 J/(g· ◦C). The specific heat of the molten salt added with these two kinds of nanoparticles increased by 28.1%, and the thermal conductivity increased by 53.7%. Studies have shown that there are heat transfer channels in high-density nanostructures, which can contribute to the enhancement of thermal conductivity. D. Shin et al. [28]

found that in traditional nanofluids, nanoparticles can form fractal fluid nanostructures to enhance thermal conductivity.

Poor particle dispersion can reduce the specific heat capacity of the molten salt [29]. Therefore, to achieve the particular heat enhancement of nanomaterials, the preparation method must be carefully controlled. There are many methods of using nanoparticles to modify molten salts, such as the high-temperature melting method, aqueous solution method, combustion method, and in situ synthesis method. The high-temperature melting method is to directly melt and stir molten salt and nanoparticles at high temperature to form a uniform eutectic system. The aqueous solution method is to dissolve the molten salt in water, then add nanomaterials to form a stable suspension, and finally, by heating, precipitation to obtain the eutectic salt. The combustion method is to mix the precursor, molten salt, and fuel together, then ignite the fuel and generate a lot of heat through violent combustion, so that the molten salt forms a eutectic system. The in situ synthesis method is to mix the precursor and molten salt, and then, the precursor reacts in the molten salt at a certain temperature to generate nanoparticles. Li et al. [20] and Zhang et al. [30] used SiO2 and Al2O3 nanoparticles as additives and added the nanoparticles to the molten salt by the high-temperature melting method, and they successfully prepared the modified salt. Xiong et al. [21] used the aqueous solution method to prepare the SiO2/molten salt nanofluid successfully. Lasfargues et al. [31,32] used copper sulfate pentahydrate and titanium sulfate as precursors to synthesize CuO and TiO2 nanoparticles in situ in solar salt. The specific heat of solar salt was observed to increase. In our previous research, we successfully synthesized MgO nanoparticles in situ in solar salt, which significantly increased the specific heat capacity of solar salt [33].

At present, there are relatively few studies on the performance improvement of multielement molten salt by nanoparticles [19]. In the research of nanoparticle modification of molten salt, MgO nanoparticles are an excellent modified particle. MgO has several types of bulk intrinsic defects, including oxygen and magnesium vacancies, interstitials, their agglomerates, etc. [34,35]. This has aroused the interest of many researchers. In this work, we prepared a ternary eutectic nitrate and applied the in situ generation method to generate MgO nanoparticles in molten salt. By testing the specific heat capacity, latent heat of phase change, and thermal conductivity of the prepared nitrate-based composite materials, the influence of MgO nanoparticles on the heat transfer and heat storage performance of ternary nitrate was studied.
