Ternary Al-Cu-Si Alloy-Based Microencapsulated Phase Change Material for Middle-High Temperature Applications

To achieve a decarbonized society, the utilization of renewable energy and industrial waste heat is essential. High-temperature heat storage and regulation technologies are becoming increasingly important. Latent heat storage (LHS) using alloy-based phase change materials (PCMs) is promising due to their high thermal conductivity and heat storage density. Recent advancements include the development of microencapsulated PCMs (MEPCMs) with Al alloy cores and ceramic shells that address leakage and corrosivity issues. In our previous work, we reported MEPCMs with various melting temperatures using various metals and alloys as cores, spanning a wide temperature range. Among these, Al-Cu-Si MEPCMs with a melting temperature of approximately 520 °C, show potential for large-scale heat storage in power-to-heat-to-power systems. MEPCMs can also be utilized as fine particles with thermal controllability at specific temperatures, exploiting the ability of PCMs to absorb or release heat at their melting temperature. Particularly, the Sabatier reaction, ammonia decomposition, and next-generation solid oxide fuel cells —key technologies in hydrogen utilization—require heat storage and thermal regulation at around 500 °C. Al-Cu-Si alloy MEPCMs thus hold promise for these applications. However, significant supercooling has been reported in alloy-based MEPCMs for most compositions, including Al-Cu-Si systems. Supercooling is a critical issue in LHS, as it reduces exergy and impairs thermal regulation ability. Therefore, it is essential to investigate and mitigate the supercooling behavior of MEPCMs for LHS applications. In this study, Al-Cu-Si alloy MEPCMs were fabricated, and their structures and melting/solidification behaviors were examined.<br/>Al-26.5%Cu-5.4%Si (mass%) alloy powder (D &lt; 38 µm) was used as the raw material. Initially, 20 g of the powder was added to 300 mL of boiling water containing 16.7 g L⁻¹ Al(OH)₃, and chemically treated for 3 h to form a precursor sample with boehmite on the alloy surface. This precursor was heated at 10 °C min⁻¹ to 1100 °C and maintained for 3 h in an O₂ atmosphere using TG for the thermal oxidation treatment. Sample morphologies were observed using XRD, SEM, and EDS. The melting and solidification properties of the MEPCM were analyzed using DSC, with heating and cooling rates of ±5 °C min⁻¹, and the sample heated to either 700 °C or 800 °C before cooling.<br/>SEM-EDS observations of the sample cross-section revealed an Al oxide layer approximately 1-2 µm thick on the surface of the Al-Cu-Si alloy particles. The XRD pattern of the MEPCM showed alloy phases of Al, Al₂Cu, Si, as well as oxide phases of α-Al₂O₃ and CuAl₂O₄, confirming the formation of microcapsule structure with an Al-Cu-Si alloy core and an Al oxide shell. The DSC curves show an endothermic peak at 524 °C during heating, corresponding to the melting of the alloy core, with a latent heat of 235 J g⁻¹. The cooling DSC curve from 700 °C showed an exothermic peak of solidification at 483 °C. Cooling from 800 °C revealed two distinct exothermic peaks at 481 °C and 409 °C. The degree of supercooling defined as the temperature difference between the melting and solidification peaks, was 41 °C during cooling from 700 °C and 43 °C or 115 °C during cooling from 800 °C. This variation in supercooling suggests that higher temperatures may lead to the loss of potential heterogeneous nucleation sites, such as clusters. Furthermore, the differences in exothermic solidification behavior observed in DSC imply variations in the microstructural formation process of the alloy.<br/>Understanding the melting and solidification temperatures is crucial for determining the operating conditions for heat storage and thermal control applications using MEPCM. Future studies shall focus on elucidating the relationship between these solidification processes and the resulting differences in alloy microstructure to enable better control of supercooling within MEPCM.