Impregnation of porous mullite with Na2SO4 phase change material for thermal energy storage

• We prepare porous mullite/Na2SO4 composites by infiltration method.
• The porous mullite possesses unidirectionally aligned open pores.
• The composites possess improved heat storage properties.
• Increasing porosity and infiltration time will increase the infiltration ratio.
• Decreasing pore size and infiltration temperature will increase the infiltration ratio.

The porous mullite/Na2SO4 composites with both novel structure and improved heat storage properties were successfully prepared by infiltration of the molten Na2SO4 into the porous mullite matrix, which was fabricated by TBA-based freezing casting method. The effects of fabrication parameters on pore structure of the porous mullite matrix, the infiltration ratio of molten Na2SO4 and heat storage properties of the composites were investigated extensively. The results show that the infiltration ratio of the molten Na2SO4 decreases with decreasing the porosity and increasing the infiltration temperature and the average pore size of the porous mullite matrix. While it increases initially with increases of the infiltration time, and tends to remain constant thereafter. The optimal infiltration temperature and infiltration time are 950 °C and 1 h, respectively. The unidirectionally aligned open pores in the porous mullite matrix benefit the infiltration process, and the higher infiltration ratio of the molten Na2SO4 and the relatively larger specific heat capacity of the mullite powders both contribute to the higher heat storage density of the composites.