A series of fatty acid binary eutectic mixture/expanded graphite (FABEM/EG) composite phase change materials (CPCMs) were prepared by absorbing a liquid FABEM into the EG. The thermal properties, thermostability, and thermoreliability were determined by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and thermal cycling tests, respectively. Fourier transform infrared spectrometry (FT-IR) and scanning electron microscopy (SEM) were used to analyze the chemical structure and microstructure, respectively. Thermal conductivity measurements and heat storage/release experiments were carried out to study the heat transfer performance. The DSC test results show that the phase transition temperatures and latent heat of these CPCMs are in the range of 17.8–55.2 °C and 134.9–176.2 J/g, respectively. FT-IR analysis indicates there is only a simple physical adsorption between EG and the PCM and no chemical reaction occurs. SEM results show that the fatty acid binary eutectic mixtures are well adsorbed in the pores of EG, and EG can provide a certain mechanical strength and prevent phase change materials from leaking. The thermal conductivity measurement and heat storage/release experiment show that the addition of EG greatly improves the thermal conductivities of the CPCMs. TGA test results and thermal cycle tests show that the CPCMs have excellent thermal stability and long-term cycling thermal reliability.

A series of fatty acid binary eutectic mixtures were prepared by using capric acid, lauric acid, myristic acid, palmitic acid, and stearic acid (CA, LA, MA, PA, and SA) as raw materials. The phase diagrams of these fatty acid binary eutectic mixtures were drawn using the Schrader equation. The thermal properties and structure were determined using differential scanning calorimetry (DSC). The thermal stability was assessed using thermogravimetric analysis (TGA) and thermal cycling tests. DSC analysis results showed that the phase change temperature of these fatty acid binary eutectic mixtures is between 17.7 °C and 57.1 °C, and the phase change latent heat is between 145.2 J/g and 193.0 J/g. The results of TGA and thermal cycle tests showed that these fatty acid binary eutectic mixtures have good thermal stability and long-term cycle thermal reliability. These results indicated that these binary eutectic mixtures of fatty acids were suitable as thermal energy storage materials for low-temperature systems.