Preparation and thermal properties of exfoliated graphite/erythritol/mannitol eutectic composite as form-stable phase change material for thermal energy storage

The m-Erythritol (erythritol, ME)/D-mannitol (mannitol, DM) binary system was investigated by differential scanning calorimetry (DSC), and the phase diagram was then obtained. It was found that the eutectic composition was at the mole fraction of erythritol xME = 0.87. The latent heat of fusion (ΔmH) of the eutectic mixture was 322.8 J/g with a peak melting temperature (Tp, m) of 114.4 °C. A cyclic DSC experiment revealed that the ΔmH of the eutectic mixture slightly decreased, and the Tp, m remained stable over 50 DSC cycles. Exfoliated graphite (EG) was then applied to absorb the eutectic mixture to obtain the EG/eutectic composite form-stable phase change material (PCM) and enhance its thermal conductivity. Good form stability could be obtained when the loading of EG in the EG/eutectic composite reached 2.0 wt%. The thermal conductivity of the eutectic mixture was greatly enhanced by EG. When the loading of EG was 10.0 wt%, the thermal conductivity increased by 408% compared to that of the eutectic mixture. Additionally, severe supercooling of the eutectic mixture was also greatly alleviated by EG. The best result was obtained when 3.0 wt% EG was added, for which the degree of supercooling (ΔT) was 62.6 °C and the heat of crystallization (ΔcrH) was 185.7 J/g. X-ray diffraction (XRD) experiments revealed that the eutectic mixture was simply a physical mixture of erythritol and mannitol. Thermal gravimetry (TG) investigations showed that the eutectic mixture and the EG/eutectic composite form-stable PCM exhibited good thermal stability for its applications.