540 / 2019-01-10 22:06:23

Preparation of silica nanoparticle /binary nitrate /expanded graphite as high-temperature thermal energy storage materials

binary nitrate; nanoparticles; composites; specific heat; thermal conductivity

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In order to study the effects of expanded graphite content on the specific heat and thermal conductivity of nano-SiO2/NaNO3-KNO3/EG composite heat storage materials, a series of nano-SiO2/NaNO3-KNO3/EG composites were prepared by aqueous solution method. Nano-SiO2/NaNO3-KNO3 was used as heat storage materials and expanded graphite (EG) with different mass fractions (5%, 10%, 15% and 20%) was used as matrix material. Then the specific heat and the thermal diffusivity of composite heat storage materials were measured, and the microstructural characteristics were analyzed by scanning electron microscopy (SEM). The results show that the specific heat and thermal conductivity of the nano-SiO2/NaNO3-KNO3/EG composites were significantly higher than that of pure nitrate. At the same time, the increase of EG mass fraction could reduce the leakage of composites. When the mass fraction of EG was 15% and 20%, the leakage of composites was very small, 2.78% and 1.47% respectively. Adding 15% of expanded graphite to the composite could significantly affect its average specific heat and thermal conductivity, with a measured value of 2.57 J/(g K) and 6.05 W/(m K), respectively, which were 1.69 times and 10.08 times higher than that of pure nitrate. This was because EG had good thermal conductivity and evenly distributed in the composite material, forming an excellent heat transfer network. Compared with NaNO3-KNO3/EG composite, the average specific heat capacity and thermal conductivity of the nano-SiO2/NaNO3-KNO3/EG composites increased by 44% and 60.9% respectively when the mass fraction of EG was 15%. This was owed to its high density network nanostructure with the large specific surface area and high surface energy after adding SiO2 nanoparticles, which could improve the specific heat capacity and thermal conductivity. When the mass fraction of EG was 5%, 10%, 15% and 20%, the latent heat of the composites was 95.56J/g, 86.26J/g, 81.61J/g and 58.81J/g, respectively. In composites, only phase change materials had latent calorific value in melting state, while EG was solid at high temperature and had no latent calorific value. The increase of EG would decreases the mass fraction of nitrate. Therefore, latent heat, specific heat and thermal conductivity should be considered comprehensively to determine the optimum mass fraction of expanded graphite. The preparation of composite heat storage materials by adding EG and nano-SiO2 into binary nitrate was a promising candidate material for improving heat transfer performance.