Metallic Composites Phase-Change Materials for High-Temperature Thermal Energy Storage

Inorganic materials and organic salts are usually used as phase change materials (PCMs) for thermal energy storage. Some of these materials have high latent heat of fusion; however one major drawback of these materials is the low thermal conductivity, which limits the rate of charging and discharging process. In this paper, we studied metallic alloys (eutectic alloys or alloys with a narrow melting temperature range) as phase-change materials, which have both high thermal conductivity and high latent heat of fusion. A formula was presented from entropy change to predict the latent heat of fusion of metallic alloys. We found that the latent heat of fusion of alloys can be expressed from three different contributions: the latent heat from each element, the sensible heat, and the mixing entropy. From the theory we also showed that latent heat of fusion could be greatly increased by maximizing the entropy of mixing, which can be realized by introduce more elements in the alloys, i.e., form ternary alloys by adding elements to binary alloys. This idea is demonstrated by the synthesis and measurement of the binary alloy 87.8Al-12.2Si (at%) and ternary alloy 45Al-40Si-15Fe (at%). The metallic alloy is synthesized by hot pressing method. The latent heat of fusion of 45Al-40Si-15Fe (at%) is about 865 kJ/kg with melting temperature from 830 °C to 890 °C from the differential scanning calorimetry (DSC) measurement, comparing with 554.9 kJ/kg and 578.3 °C for the binary alloy 87.8Al-12.2Si (at%). From the binary to the ternary alloy, the contribution to the latent heat from mixing entropy increases by 17%.