Enhanced thermophysical properties of NaNO3-KNO3 Eutectic Salt incorporated with liquid gallium for medium temperature thermal energy storage

The intrinsically low thermal conductivity of nitrate-based phase change materials remains a major limitation for efficient heat storage. Conventional solid additives can improve heat transfer slightly, but they often suffer from limited interfacial contact, particle agglomeration, and a trade-off between thermal conductivity enhancement and latent heat retention. In this work, liquid gallium (Ga) was introduced into a NaNO 3 -KNO 3 eutectic salt matrix as a thermally conductive and interfacially adaptive enhancer. Compared with conventional solid fillers, liquid Ga is expected to provide better interfacial contact with the molten salt matrix and to form localized heat-transfer pathways owing to its high thermal conductivity and fluidity. The addition of Ga slightly decreased the melting temperature and reduced the latent heat mainly because of the dilution effect of the non-phase-change component. Meanwhile, the specific heat capacity of the composites increased by 68-85% compared with pristine nitrate salt, and the thermal conductivity increased by 19.6% at 5 wt% Ga loading. However, the high-temperature stability margin decreased with increasing Ga content. After 100 melting–solidification cycles, the mass of CPCM 3 remained nearly unchanged, whereas its latent heat decreased from 128.16 to 112.43 J/g, corresponding to a retention of 87.73% and indicating measurable cycling-induced degradation. A two-dimensional transient melting simulation further showed that CPCM 3 developed a larger liquid region and melted faster than the pristine PCM under identical single-wall heating conditions. These results indicate that liquid Ga can serve as a promising thermal enhancer for nitrate-based phase change materials.