Preparation and Thermal Energy Storage Characteristics of Coal Gasification Slag and Fly Ash Based Composite Phase Change Materials

Utilizing coal gasification slag (CGS) and fly ash (FA) as matrix materials and Al-Si alloy as the phase change medium, composite phase change materials (PCMs) for medium-to-high temperature thermal storage were prepared by powder sintering. The effects of matrix ratio, phase change material content, and forming pressure on material properties were systematically investigated. The composites were characterized using a universal testing machine, ash fusion tester, thermal conductivity analyzer, DSC, and metallographic microscope. Results show that the optimal composite—composed of 20 wt% CGS, 30 wt% FA, 40 wt% Al-Si alloy, and 10 wt% silica sol, formed under 8 t pressure and sintered at 800 °C—exhibits the best overall performance. It achieves a compressive strength of 9.26 MPa and a thermal conductivity of 0.7848 W·m−1·K−1, which is 343% higher than that of pure FA. Microstructural analysis confirms that the molten Al-Si phase effectively fills matrix pores, enhancing densification. The composite demonstrates excellent high-temperature stability with no softening or melting below 1500 °C. After 15 thermal cycles, the mass loss is only 0.238% with preserved structural integrity, indicating good cyclic reliability. This study validates a feasible path for high-value solid waste utilization and low-cost thermal energy storage material development.