HfB2 and SiB6 nanoparticles enhance heat-storage capacity, thermal conductivity, and thermal stability of 7NaNO3–53KNO3–40NaNO2 molten salt

Improving the heat-storage capacity, thermal conductivity, and high-temperature stability of nitrate–nitrite molten salts is essential for increasing the efficiency and operational reliability of concentrating solar power (CSP) systems. Accordingly, the effects of hafnium diboride (HfB 2 ) and silicon hexaboride (SiB 6 ) nanoparticles on the phase integrity, microstructural evolution, and thermophysical performance of 7NaNO 3 –53KNO 3 –40NaNO 2 molten salt were comparatively investigated. Nanocomposite formulations containing 0.5-2 wt% additive were synthesized and characterized by XRD, FT-IR, FE-SEM/EDX, DSC, TGA, and TPS measurements. The results confirmed that nanoparticle incorporation did not disrupt the fundamental nitrate-nitrite salt framework, and no evidence of newly formed phases or chemical bond generation was identified. Despite this structural and chemical preservation, marked improvements were obtained in heat-storage and heat-transfer characteristics. The melting enthalpy increased from 76.2 J/g for the reference salt to 89.4–90.6 J/g in the HfB 2 -containing compositions and to 92.6–95.6 J/g in the SiB 6 -containing compositions. The highest specific heat capacity enhancement in the HfB 2 series occurred at 1.5 wt%, reaching 55.17% in the solid phase and 66.65% in the liquid phase, whereas the SiB 6 series exhibited its optimum response at 1 wt% with corresponding increases of 107.78% and 117.79%. Thermal conductivity rose from 0.951 W/m.K to 1.689 W/m.K with 2 wt% HfB 2 and 1.529 W/m.K with 2 wt% SiB 6 . The degradation onset temperature also increased from 612 °C to 653 °C and 663 °C, respectively. These findings demonstrate that boride-based nanoadditives provide an effective route for improving the thermophysical performance and high-temperature durability of this molten salt, with HfB 2 delivering the strongest thermal conductivity enhancement and SiB 6 providing the most pronounced improvements in latent heat, specific heat capacity, and thermal stability.