Preparation and characterization of a heat storage material: Shape-stabilized KNO₃ using a modified diatomite-based porous ceramic as the skeleton

The applications of salts-based phase change materials (PCMs) are greatly restricted by their corrosion to containers and low thermal conductivity. To address these issues, a modified diatomite-based porous ceramic (MDPC) was prepared to shape-stabilize KNO₃ salt in this paper. Particularly, effects of graphite content on the pore structure of diatomite-based porous ceramic (DPC) and MDPC on the thermophysical properties of shape-stabilized KNO₃ were investigated. The results showed that 30 wt% graphite increased apparent porosity of DPC up to 67.21% and enlarged average pore diameter from 0.71 to 3.58 μm, while weakened its compressive strength from 19.12 to 5.51 MPa. The MDPC effectively prevented leakage of molten KNO₃ to avoid its corrosion problem, and possessed an excellent chemical compatibility with KNO₃ salt. Additionally, the MDPC hardly changed phase transition temperature of shape-stabilized KNO₃, while increased latent heat of composites from 51.70 to 60.21 J/g due to its higher apparent porosity. The MDPC was also shown to enhance thermal conductivity of shape-stabilized KNO₃ from 1.16 to 1.52 W/(m·K) at 25 °C, which was attributed to that the MDPC with a larger pore size was more easily infiltrated by molten salt, contributing to a lower apparent porosity of composites. Compared with pure KNO₃, thermal conductivity and thermal stability of shape-stabilized KNO₃ were seen to be significantly improved by 238% and 60 °C, respectively. This work therefore solved two key challenges for the application of KNO₃ salt, and proposed an effective way to improve thermophysical properties of such heat storage material.