Large-scale Fabrication and In-operando Crystallization Study of Expanded Graphite/Zinc Nitrate Hexahydrate-Potassium Nitrate Eutectic Composites with Robust Thermal Properties.

High enthalpy of fusion (ΔH_Fusion) and suitable melting point (M.P.) makes zinc nitrate hexahydrate (ZNH) and its eutectics popular as phase change materials (PCM) for low temperature thermal energy storage (TES). Primary challenges in salt hydrates are their low thermal conductivity (k) and stability after thermal cycling. Past studies have attempted to enhance k by adding loose carbonaceous fillers like expanded graphite (EG). However, infiltrating salt hydrates within binder incorporated, hydrophilic-treated, robust EG matrices that are reliable up to ~1000 thermal cycles is yet unexplored. For practical application of PCM composites like electronics, solar plants, green buildings, etc. reliable thermal performance throughout its operational lifetime is essential. Nevertheless, the reliability of thermal conductivity in EG/salt hydrate composites over relevant numbers (~10³) of melt/freeze cycles has barely been studied. Here we propose, a methodology for large-scale fabrication of PDMS-bonded, ZNH/KNO₃ composites, tailoring enhanced k and ΔH_Fusion, with reduced supercooling (ΔT_Sup), M.P. and endothermic range. The binder imparts strong connections between the EG to improve thermal conductivity and stability of TES properties up to 1,000 melt/freeze cycles. The thermal conductivity increased with EG content, although its initial magnitude and long-term retention were observed to be tradeoffs. In-operando crystallization studies to further understand the variation of thermal conductivity, revealed the influence of carboxymethyl cellulose (CMC) thickener in attenuating k fading over prolonged melt/freeze cycling. The thickener prevented the formation of large PCM crystals that could potentially impair thermal connections between EG. We believe the outcome of this study introduces a new dataset to the scientific community regarding robust ZNH eutectic composites for low temperature TES applications.