Shape-Stabilized NaNO3 Phase Change Materials Supported by CO2-Mineralized Basic Oxygen Furnace Slag for Photothermal Heat Storage
NaNO3 shape-stabilized composite phase change materials (PCMs) supported by CO2‑mineralized basic oxygen furnace (BOF) slag were developed for photothermal heat storage. The effects of slag mineralization degree and NaNO3 loading on structure, phase transition behavior, thermal stability, thermal conductivity, and cycling stability were systematically investigated. CO2 mineralization converted active Ca/Mg-bearing phases in BOF slag into carbonate phases, increasing the weight gain from 9.37 to 17.45% as mineralization time increased from 30 to 120 min. Among the mineralized samples, CSS-60 provided the most favorable balance between mineralization degree and accessible porosity. Using CSS-60 as the supporting skeleton, the composite PCMs showed good shape stability. The melting enthalpies of OPT-N40, OPT-N50, and OPT-N60 were 67.43, 83.22, and 97.38 J g-1, respectively. Compared with pure NaNO3, the composites exhibited improved thermal stability and higher thermal conductivity. After 300 thermal cycles, all samples maintained good thermal cycling stability, with shifts in onset and peak temperatures below 0.5 °C and enthalpy decay below 5%. OPT-N50 showed the best overall performance. These results indicate that CO2-mineralized BOF slag is a promising low-cost and low-carbon supporting matrix for NaNO3-based medium-temperature photothermal heat storage.