Effect of Si content on the thermal response and stability of Al-40Sn-XSi-Mg composite PCMs

Metallic Phase Change Materials (m-PCMs) are gaining interest for medium-temperature thermal energy storage due to their high thermal conductivity, mechanical robustness, and chemical stability. Prior studies showed that adding 40mass%Sn to Al and commercial Al–Si alloys yields effective composite m-PCMs, where Sn provides high latent heat and the Al/Al–Si matrix ensures dimensional stability during cycling. Building on this concept, this work examines the thermal response and stability of Al-40Sn, Al-40Sn-3Si-Mg and Al-40Sn-4Si-Mg alloys to clarify how the Si content affects the overall thermal behavior. Induction-melted ingots were characterized through microstructural analysis, Differential Scanning Calorimetry, and Laser Flash Analysis in the as-produced state and after thermal cycling across the activation temperature. Measured latent heat, specific heat capacity, and thermal diffusivity were compared with equilibrium thermodynamic simulations to evaluate the influence of Si on thermal response and cycling stability. Results show that varying Si from 0 to 4mass% does not significantly affect the melting temperature and the latent heat associated with Sn melting, while it modifies the temperature spread of the phase transition through secondary Mg2Sn–Sn eutectic reaction. Si additions strongly enhance microstructural stability during cycling by improving Sn confinement within the Al–Si matrix and limiting porosity evolution. Consequently, the phase-change thermal response remains highly reproducible, and thermal diffusivity is markedly more stable than in the Al–Sn system. These findings clarify how composition, microstructure, and porosity jointly govern the thermal performance of Al-Sn-Si-Mg m-PCMs, providing guidelines for efficient and durable thermal energy storage materials.