Study of viscosity and heat capacity characteristics of molten salt nanofluids for thermal energy storage

In this study, we synthesized molten salt nanofluids by dispersing spherical SiO₂ nanoparticles at a minute concentration (1 wt%) into a binary mixture of NaNO₃-KNO₃. The results showed that the heat capacity was enhanced by 15% and the viscosity was enhanced by 41–429%. Moreover, the nanofluids have shown significant non-Newtonian behavior (shear thinning). Nanofluids are known to show non-Newtonian behavior when particles have high aspect ratio (e.g., nanotube, rod-like structure) at high concentrations; however, only spherical nanoparticles were dispersed in molten salt at an extremely low concentration (1 wt%). The observed enhancement in heat capacity and the shear thinning behavior could result from the formation of dendritic salt nanostructures. Hence, we added hydroxide at an extremely low concentration (0.03 wt%) to disrupt the formation of such dendritic nanostructures to confirm their effects on the heat capacity and shear thinning behavior. The result showed that the heat capacity enhancement diminished from 15% to 3%. Moreover, the viscosity enhancement decreased from 429% to 148% at low shear rate (10/s) and from 41% to 10% at high shear rate (240/s). Furthermore, the enhanced viscosity of 10% at the highest shear rate (240/s), where the effect of the dendritic nanostructures is minimal, agreed well with a theoretical model developed for the viscosity of a simple liquid doped with nanoparticles. It supports that the dendritic salt nanostructures are primarily responsible for the enhanced heat capacity and the shear-thinning behavior of molten salt nanofluids. Material characterization using an electron microscope confirmed the presence of the dendritic salt nanostructures.