Enhancing phase change thermal properties of unitary and binary nitrates by carbon/ceramic-based nanoparticles via interfacial effect

Molten salt phase change materials (MSPCMs) provide strong support for the long-term and stable operation of new energy systems. This study focuses on LiNO3 (high latent heat, low melting temperature), NaNO3 (an important component in solar salts), and binary eutectic nitrate (LiNaNO3). The aim is to explore eutectic nitrate that matches lower working temperature requirements by eutectic reaction. The other aim is to enhance the specific heat and thermal conductivity of nitrates by adding nanoparticles (NPs) with different microstructures. The NPs include multi-walled carbon nanotubes (MWCNT), graphene nanoplatelets (GNP), AlN, BN, and SiC. The distribution patterns of nitrate molecules near NPs were investigated under the influence of interaction strength through molecular dynamics. The enhancement of thermal transport is decided by phonon vibrations, interface thermal resistance, and heat pathway. NPs can significantly weaken the melting enthalpy and limit the enhancement of thermal conductivity by disrupting the eutectic microstructure of LiNaNO3, which is different from the mechanism of NPs' action on unitary nitrates. MWCNTs exhibit the most significant enhancement in thermal conductivity and specific heat of nitrates, which is due to the diffusion of nitrate molecules in the MWCNT. The in-plane phonon vibrations of GNP contribute slightly to the thermal transport in the vertical plane direction, resulting in limited enhancement of thermal transport. Therefore, we propose principles for selecting NPs doped into eutectic salts, which should ensure that the interaction strength between NPs and both unitary nitrates is similar or that their microscopic structure can maintain the integrity of the eutectic structure.