Investigation of heat capacity and viscosity enhancements of binary carbonate salt mixture with SiO₂ nanoparticles

A binary carbonate salt mixture (Li₂CO₃-K₂CO₃ at 62:38 molar ratio) was doped with SiO₂ spherical nanoparticles at 1% concentration by weight. A differential calorimeter and a rheometer were used to study the thermal and rheological characteristics of the binary carbonate salt mixture & its nanofluids mixed with SiO₂ nanoparticles. The results showed that the viscosity was enhanced by 34.0–94.4% and the heat capacity was enhanced by 19%. Furthermore, the nanofluids showed significant non-Newtonian behavior (shear-thinning). Nanofluids are known to show non-Newtonian behavior when nanoparticles have a high aspect ratio (e.g., rod-like structure, nanotube, etc.) at high concentrations. Literature study showed salt mixture can be micro-segregated near a nanoparticle. The segregated salts can crystallize on the nanoparticle surface and grow further to form dendritic nanostructures. To verify the nanostructural change, we added a minute concentration of hydroxide (0.03 wt.%) into a nanofluid to disrupt the nanostructural change and measured their properties as reported in the literature. The result showed that the heat capacity enhancement decreased from 19% to 9%. Also, the viscosity decreased from 34.0% to 8.4% (at the highest shear rate) and 94.4% to 62.8% (at the lowest shear rate), respectively. Moreover, the theoretical viscosity model predicted well at the highest shear rate (250/s), where the effect of the nanostructural change is minimal, while it failed to predict the viscosity enhancement at a low shear rate, where the nanostructural change is expected to dominate the flow. Our experimental results support that the dendritic salt nanostructures are primarily responsible for increasing both heat capacity and the shear-thinning behavior of molten salt nanofluids.