Thermophysical and Photothermal Enhancement of Ternary Carbonate Molten Salt by SiC–TiN Composite Nanoparticles

Abstract The Li2CO3–Na2CO3–K2CO3 ternary carbonate molten salt is a promising storage medium for high-temperature concentrated solar power systems owing to its broad liquid-phase range and high sensible heat storage potential. However, its low thermal conductivity and weak solar absorption limit its performance under direct-absorption conditions. In this study, SiC–TiN composite nanoparticle-modified ternary carbonate molten salts were prepared, and their thermophysical, optical, photothermal heating, rheological, and cycling properties were investigated. The molten salt containing 1.5 wt % nanoparticles with a TiN:SiC mass ratio of 5:5 exhibited the best overall performance. Its specific heat capacity reached 2.38 J·g–1·K–1, representing a 43.3% increase relative to the base salt, while independent cross-validation based on thermal conductivity, density, and thermal diffusivity yielded a consistent value of approximately 2.37 J·g–1·K–1. The thermal conductivity increased to 1.35 W·m–1·K–1, and the calculated volumetric thermal energy storage density based on the density measured at 500 °C reached 2.05 ± 0.04 GJ·m–3, corresponding to an increase of approximately 49.2%. Meanwhile, the average viscosity increased by only 8.93%. The average solar absorptance reached 73.35%, and under xenon-lamp irradiation, the average heating rate and stabilized temperature increased from 9.15 to 16.38 °C·s–1 and from 650 to 737 °C, respectively. After 50 thermal cycles, the mass loss remained below 0.5% and the specific heat capacity attenuation was 5.51%. Static molten-state sampling at 600 °C further showed similar specific heat capacities in the upper, middle, and lower regions, indicating limited macroscopic stratification under the investigated conditions. These results demonstrate that SiC–TiN composite modification provides a favorable balance among thermal storage, heat transfer, solar absorption, photothermal heating response, and high-temperature stability.