Azelaic Acid/Expanded Graphite Composites with High Latent Heat Storage Capacity and Thermal Conductivity at Medium Temperature.

A novel azelaic acid/expanded graphite (AA/EG) phase change composite (PCC) was fabricated as a shape-stabilized phase change material (PCM) for latent heat storage at medium temperatures. The composite exhibited a low supercooling degree and high heat storage capacity. Despite the impregnation of a high quantity of AA (85 wt %) in the porous network of EG, there was no leakage of liquid AA. This was attributed to the capillary forces and surface tension forces. The pure AA exhibited a melting temperature of 108.0 °C, with an intrinsically low supercooling degree of 5.8 °C. The melting temperature of AA in the PCC decreased slightly to 105.8 °C, and there was a significant decrease in the supercooling degree to 1.0 °C. The AA/EG PCC exhibited a high latent heat storage capacity of 162.5 J/g, and there was a significant gap between the decomposition temperature and the phase change temperature range. Therefore, the composite exhibited high thermal stability during operations. The results of an accelerated thermal cycling test (200 cycles) indicated the high cycling durability and chemical stability of the PCC. The thermal conductivity of AA increased by 15.7 times after impregnation in EG, as compared to that of the pure AA, and thus, thermal kinetics of the PCC was improved. The results of a heat storage/release test with 15 g of the PCM revealed that the melting and solidification of the AA/EG PCC were 5.0-fold and 7.4-fold faster, respectively, than those of the pure AA. This was attributed to the high thermal conductivity of the PCC.