Weigh a certain amount of sodium nitrate and silica nanoparticles, and stir them with a mechanical stirrer in a constant temperature salt bath at 400°C. The stirring speed is 750 r/min, and the stirring time is 15 min (Song et al., 2018b). Finally, a molten salt nanofluid with a mass fraction of SiO₂ of 1 wt% is obtained. The physical drawing and schematic diagram of mechanical mixing device are shown in Figure 1. The molten salt nanofluid with SiO₂ mass fraction of 0.1 wt% and TiO₂ mass fraction of 0.9 wt% was prepared by the same method.

The prepared NaNO₃/SiO₂ molten salt nanofluid was placed in a 400°C constant temperature salt bath, and different mass fractions (7, 10, 15, 20 wt%) of EG were added.

Stir with a stir bar until the liquid molten salt completely enters the pores of the EG, and the stirring time is about 60 min; take the mixture out, cool and grind in a desiccator, and then the powdered NaNO₃/SiO₂/EG composite material can be obtained.

The powder is pressed into a shape with a pressing machine, the molding pressure is 10 MPa, and the pressure holding time is 60 s (Yu et al., 2020), to form a cylindrical sample with a diameter of 12.5 mm and a height of 2.5 mm. Put the pressed sample into a muffle furnace for sintering. During sintering, the heating rate between 30–150°C was 2°C/min (the slow heating rate was to make a small amount of water in the original sample evaporate completely); the heating rate is 5 °C/min between 150 and 400°C, keep the temperature at 400°C for 90 min to make the composite material fully adsorb and mix (Ren et al., 2018; Yu et al., 2020). The sample was naturally cooled to room temperature, and a sintered NaNO₃/SiO₂/EG composite material was obtained. The process of preparing samples by mechanical dispersion method is shown in Figure 2.

In order to explore the optimal addition amount of EG, the prepared samples were characterized by morphology and mass analysis before and after sintering. In this study, the composite phase change materials with basically unchanged appearance before and after sintering and high adsorption degree of EG to molten salt need to be selected to determine the optimal addition amount of EG. The sintered appearance of the composite material with different mass fractions of EG is shown in Figure 3. The mass changes of composites before and after sintering are shown in Table 1.

Combining Figure 3 and Table 1, it is found that the composite material sample with 7 wt% EG has a serious leakage, and the mass loss rate is as high as 18.19%. The mass loss rate of the composite material with 10 wt% EG is 11.03%, there are many small pits on the surface of the sample, which is not enough smooth. The mass loss rate of the composites with 15 wt% EG and 20 wt% EG added is less than 5%, and the sample surface is smooth and flat. Therefore, this study determined that the optimal addition amount of EG was 15 wt%. Using the same method, NaNO₃/(SiO₂+TiO₂)/EG composite material was prepared. The addition ratios of SiO₂ nanoparticles, TiO₂ nanoparticles and EG were 0.1, 0.9, and 15 wt%, respectively. In order to explore the effect of adding nanoparticles and eg on the thermophysical properties of sodium nitrate, the comparison samples shown in Table 2 were prepared in this paper.

The microstructures and elemental analysis for the composite materials were characterized by scanning electron microscopy (SEM, ZEISS Gemini 300) and energy dispersive spectrometry (EDS). The sample is dried in a drying oven to prevent the effect of small amount of moisture inhaled in the sample on the characterization results. Due to the poor conductivity of the molten salt itself, it is necessary to spray gold on the dried samples to improve the conductivity of the samples.

The composites were characterized by X-ray diffractometer (XRD, D8-ADVANCE, BRUKER/AXS, Germany) to determine its chemical composition, crystal structure and other information of the sample. The diffractometer uses Cu target, the scanning range is 20^o-80^o.

The specific heat, latent heat and the phase change temperature of the sample were measured by a synchronous thermal analyzer (STA-449F3, NETZSCH). The sample was sealed in an aluminum crucible. In order to ensure uniform heat transfer between the crucible and the sample, the sample mass and heat ramp rate should be controlled at 5–15 mg and 10 K/min, respectively, with a nitrogen atmosphere with a nitrogen flow rate of 50 ml/min. The sample was kept at a constant temperature of 30°C for 10 min, and then heated from 30 to 400°C at a heating rate of 10 K/min, and kept at 400°C for 10 min. The extrapolated starting temperature on the differential scanning calorimetry (DSC) curve is selected as the melting temperature of the sample. This temperature can be determined by the intersection of the extension of the baseline and the tangent to the initial linear portion of the peak. The latent heat of phase change is calculated by integrating the area under the peak of the DSC curve. In the determination of specific heat capacity, sapphire is usually used as a standard material, and its data has been accurately determined. By comparing the measured values of the standard sample (sapphire) with known specific heat and the sample to be tested, the specific heat capacity of the sample to be tested can be obtained.

In order to further ensure the reliability of the results, each sample in the experiment was measured at least three times, and the following equation was used to calculate the standard deviation of the measurement results. S = ∑ i = 1 n ( X i − X ¯ ) 2 n − 1 (1) where, S is the standard deviation of the sample; X _i is the measured value of the sample; X ¯ is the average of three measurements.

The heat storage density of composites can be calculated by Formula (2). Q m = ∫ T s T m m c p d T + m Δ H + ∫ T m T 1 m c p d T = m c p ( T m − T s ) + m Δ H + m c p ( T 1 − T m ) (2) Where, Q m means heat storage density (kJ/kg), m means mass (kg), c _p means specific heat (J/(gK)), T m means melting temperature (°C), T s means initial temperature (°C), T 1 represents the termination temperature (°C), Δ H represents the latent heat (J/g).

Thermal diffusivity of the composites was characterized by a laser thermal conductivity meter (LFA-45, NETZSCH). During the measurement, the laser pulse is emitted to the bottom of the sample through the laser beam, and after absorbing the light energy, the temperature of the bottom surface of the sample rises instantly. Suppose the heat conduction is one-dimensional, and the heat is transferred to the top surface. The temperature change of the center of the top surface of the sample is measured continuously by infrared sensor above the sample. The thermal diffusivity along the thickness of the sample can be obtained by increasing the surface temperature of the sample. The thermal diffusivity (α) can be calculated by Formula (3). α = d 2 t 1 / 2 (3) Where, α represents the thermal diffusivity (mm²/s), d represents the sample thickness (mm), and t _1/2 is the time for the maximum temperature to rise by 50% (s).

According to Formula (4), the thermal conductivity can be calculated. λ = α ⋅ ρ ⋅ c p (4) Where, λ represents the thermal conductivity (W/(mK)), ρ is the density (g/m³) (Calculated based on the mass of the weighed composite material and the measured volume), α is the thermal diffusivity (mm²/s), c _p is the specific heat (J/(gK)).

The phase change process of composites with temperature includes three stages: solid state, phase change process and liquid state. The specific heat of different samples before phase transition is shown in Figure 7, which increases with the increase of temperature. The specific heat of different samples before phase transition is shown in Figure 8, which remains basically unchanged with the increase of temperature. The standard deviation of specific heat capacity of all samples is less than 4%. Table 3 shows the average specific heat capacity of different samples in different temperature ranges according to the measurement curve.

It can be seen from Table 3 that adding nanoparticles and EG alone improves the specific heat of sodium nitrate, and the influence of adding two kinds of nanoparticles is better than adding one kind of nanoparticles. In addition, when adding nanoparticles and EG at the same time, the increase in specific heat is significantly greater than adding nanoparticles or EG alone.

The specific heat of molten salt can be increased by adding nanoparticles, because nanoparticles adhere to the surface of molten salt to form semi-solid nanolayer. This kind of semi-solid nanolayer has higher thermal performance than liquid, which helps to increase the specific heat of nanofluid (Shin and Banerjee, 2011; Lu and Huang, 2013). It can be seen from the above SEM image that after adding SiO₂ (SiO₂+TiO₂) nanoparticles to NaNO₃, a dense solid-like nanostructure is also formed. This structure has a large specific surface area and specific surface energy, thereby increasing the specific heat capacity of the composites. When Shin (Shin and Banerjee, 2014) studied pure molten salt nanofluid, it was also found that similar structures appeared on the surface. Luo et al. (Luo et al., 2022) proposed a 3D hierarchical ultralight SiC foam with 1 wt% SiO₂ added LiNO₃/NaCl inside to achieve high specific heat capacity. The average c _p is 4.86% higher than that of pure PCMs due to the high surface energy and interfacial thermal resistance induced by weak interaction between SiO₂ nano-particles and eutectics, as confirmed by molecular dynamics (MD) simulations.

The addition of EG can increase the specific heat of sodium nitrate. This is because the mechanical energy of NaNO₃ and EG is converted into the interface energy of the composite by the tablet press in the process of preparing the composite. The increase of the interface energy may promote the specific heat of the composites.

After adding nanoparticles and EG at the same time, the nanoparticles adhere to the surface of molten salt and EG, forming a dense network of nanostructures, and because the molten salt is adsorbed by porous structure of EG, it overcomes the phenomenon that the nanoparticles are not uniformly dispersed in the molten salt and agglomeration occurs in the process of long standing. Therefor the nanoparticles more uniformly dispersed in the composites. The synergy between nanoparticles and EG can greatly increase the specific heat capacity of the composites. In addition, on the basis of adding EG, the influence of adding (SiO₂+TiO₂) nanoparticles is better than adding only SiO₂ nanoparticles. This may be because molten salt and EG have different forces on SiO₂ and TiO₂ molecules. Therefore, when nanostructures are generated, the nanostructures generated by the former are more uniform and continuous in the composites than the latter, which in turn affects the specific heat of the composites.

The phase change temperature and latent heat have an important influence on the heat storage performance of the composite phase change material. Figure 9 shows the DSC curves of different samples. The thermophysical parameters obtained according to the curve analysis are listed in Table 4. It can be seen from the DSC curve that there is a solid-solid phase change peak of sodium nitrate at about 275°C (Tamme et al., 2010), and another sharp peak represents the solid-liquid phase change. It can be seen from the table 4 that the phase change temperature of sodium nitrate is 306.9°C, the phase change temperature of the composites remained basically unchanged after adding nanoparticles, and after adding EG, the phase change temperature dropped by about 5°C. Wang (Wang et al., 2012) pointed out that the weak interaction between EG and binary eutectic nitrate, such as capillary force and surface tension, will cause a decrease in the phase change temperature.

According to reference (Wang et al., 2012), the theoretical latent heat of composites can be derived from Formula (5). Δ H theory = ( 1 − φ ) Δ H PCM (5) Where, Δ H theory is the theoretical latent heat of the composites; φ is the mass fraction of EG and nanoparticles; Δ H P C M is the latent heat of sodium nitrate. It can be seen from Table 4 that the addition of nanoparticles has little effect on the latent heat of the composites, but when 15 wt% EG is added, the latent heat of the composites decreases greatly. This is because the addition of EG reduces the proportion of phase change materials in the composites, and EG keeps solid without phase change, so it has no contribution to the latent heat. On the other hand, the measured value of the latent heat of the composites is significantly lower than the theoretical value. The reason is that during the sintering process of the sample, some phase change material leakage is unavoidable. At the same time, Zhao (Zhao et al., 2016) and Feng (Feng et al., 2011) pointed out that the crystal arrangement and orientation of nitrate molecular chain would be limited by space effect and resistance due to the existence of insertion mechanism, which will lead to a decrease in the regularity of the crystal line region and an increase in lattice defects. As a result, compared with the theoretical value, the latent heat of phase change of the composites will be reduced.

The decomposition temperature of industrial-grade NaNO₃ is 380°C, so the operating temperature should not exceed 380°C. Assuming that the working temperature range of the composite material is 100–380°C, the heat storage density of the composites can be calculated by Formula (2). As shown in Table 5, the specific heat, fusion heat, melting point and other physical parameters of the composites studied in this paper and two commonly used low-melting mixed nitrates are listed. Using Formula (2), the thermal storage density of NaNO₃/(SiO₂+TiO₂)/EG composites was 679.2 kJ/kg. In the temperature range of 100–380°C, the heat storage density of the commonly used low-melting Hitec salt is 465.4 kJ/kg, and the heat storage density of the mixed quaternary nitrate developed by this research group is 401 kJ/kg. It can be seen that although the addition of EG reduces the heat of fusion of sodium nitrate, because the specific heat capacity of the composite heat storage material increases, when the sensible heat and latent heat are used at the same time, the composite material still has a high heat storage density, and the heat storage density is much larger than two commonly used low melting point mixed nitrates.

Thermal conductivity is an important index to characterize the heat transfer rate of materials. The thermal diffusivity of different samples is measured in the temperature range of 200–400°C. Figure 10 shows the thermal diffusivity of different samples vary with temperature. Figure 11 shows the thermal conductivity of different samples vary with temperature. In the range of 200–400°C, the thermal diffusivity and thermal conductivity both first increases and then decreases with the increase in temperature, and the turning point is near the melting point. The measured thermal diffusivity and calculated thermal conductivity are listed in Tables 6, 7. The results show that the addition of nanoparticles has a small increase in the thermal conductivity of NaNO₃, while the addition of EG can effectively increase the thermal conductivity of the composites sharply. At the same time, under the synergistic effect of nanoparticles and EG, the influence of improving the thermal conductivity is the best.

The addition of EG can improve the thermal conductivity of the composites, because it is evenly distributed in the composites, it forms an excellent heat transfer network. Heat can be quickly transferred to the inside of the nitrate along the mesh wall of EG, at the same time, EG itself has good thermal conductivity, and its own temperature quickly rises to a temperature close to the heating boundary. Therefore, in the subsequent heating process, nitrate is heated by multiple heating surfaces at the same time, the rate of heat transfer can increase exponentially. The schematic diagram of the heat transfer process is shown in Figure 12. On the other hand, after adding nanoparticles, nanostructures similar to solids will be formed, and these nanostructures will form channels that facilitate heat transfer and increase the overall thermal conductivity of the composites.