Synthesis of a novel three-dimensional Na₂SO₄@SiO₂@Al₂O₃-SiO₂ phase change material doped aerogel composite with high thermal resistance and latent heat

A novel Na₂SO₄@SiO₂@Al₂O₃-SiO₂ phase change material doped aerogel composite (PDAC) using the sol-gel technique is prepared. This new kind of composite material is composed of a core-shell structured Na₂SO₄@SiO₂ phase change material (PCM) with high melting temperature and Al₂O₃-SiO₂ aerogel with high thermal resistance and low thermal conductivity. The effects of PCM amounts and heat treatment temperatures on the physicochemical properties of Na₂SO₄@SiO₂@Al₂O₃-SiO₂ PDCA are investigated. X-ray diffraction (XRD) shows that when the mass percentage of Na₂SO₄@SiO₂ PCM powders increases to 36.7%, the peaks corresponding to Na₂SO₄ can be well observed. The boehmite phase within Na₂SO₄@SiO₂@Al₂O₃-SiO₂ PDAC can be transformed to γ-Al₂O₃ via structural adjustment after heat treatment at 600 °C. The pore size distribution doesn't shift after 600 °C heat treatment, and the most probable pore diameter is at around 30 nm. Transmission Electron Microscopy (TEM) shows that the Na₂SO₄@SiO₂ PCM powders with diameters at around 500 nm can be effectively trapped and are homogeneously distributed within the aerogel matrix even after further heat treatment at 1000 °C, therefore, liquid leakage can be inhibited during melting process. The peak latent heat of Na₂SO₄@SiO₂@Al₂O₃-SiO₂ PDCA can be as high as 153.84 J/g with the PCM mass percentage at 36.7%. The Brunauer-Emmett-Teller (BET) specific surface area increases from 419.3 m²/g to 630.6 m²/g due to structural adjustment, and then decreases to 277.7 m²/g caused by volume shrinkage. Thermal simulation results show that the total melting time can be 254.8 s, and development of the temperature distribution keeps well with that of the liquid fraction. The velocity distribution of the liquid PCM keeps at 0 due to the constraint of aerogel matrix. This Na₂SO₄@SiO₂@Al₂O₃-SiO₂ PDAC can be a promising high temperature PCM in thermal energy storage applications.