Thermal storage performance of NaNO₃/cordierite-mullite ceramic (CMC) composites deriving from metallurgical slag

Although inorganic salts are considered outstanding thermal storage materials, drawbacks such as low thermal conductivity, corrosion issues, and leakage severely hinder their practical industrial application. Porous ceramics are suitable to serve as the supporting skeleton for inorganic salt due to good corrosion resistance, high thermal conductivity and high porosity. In this study, secondary aluminum dross (SAD) and ferronickel slag (FNS) were used as raw materials to prepare cordierite-mullite porous ceramic (CMC). The NaNO3/CMC composites were synthesized using spontaneous melting-infiltrating method, and their thermal storage performance were studied in detail. The results indicated that CMC had a favorable pore structure with high porosity and micro-sized pores, as well as high compressive strength exceeding 30 MPa. Excellent chemical compatibility between CMC and NaNO3 was demonstrated by XRD analysis. The latent heat of the NaNO3/CMC5, NaNO3/CMC15 and NaNO3/CMC30 composites, as determined by DSC, were 76.4 J/g, 76.7 J/g and 73.6 J/g, respectively, accounting for 43.5%,43.6% and 41.9% of pure NaNO3. The supercooling of NaNO3 was significantly decreased after infiltration into CMC supporting skeleton. A 100-cycle thermal cycles experiment was conducted to verify the long-term thermal reliability of the NaNO3/CMC composites, including latent heat and heat transfer property. The melting latent heat of the NaNO3/CMC5, NaNO3/CMC15 and NaNO3/CMC30 decreased by nearly 5.6%, 3.7% and 2.9%, respectively, after 100 thermal cycling tests, indicating the NaNO3/CMC composites had good thermal reliability. The thermal conductivity of all the NaNO3/CMC composites exceeded 1.8 W m(-1) K-1 at 350 degrees C. Additionally, the compressive strength of all the composites showed only minimal decrease after thermal cycling test, remaining higher than 100 MPa. The NaNO3/CMC composites offer promising prospects in the medium temperature thermal storage utilization field, such as industrial waste heat recovery, owing to low production cost, high thermal conductivity and high mechanical strength.