Facilitated synthesis and thermal performances of novel SiO2 coating Na2HPO4⋅7H2O microcapsule as phase change material for thermal energy storage

Abstract

Novel microencapsulated phase change material (MEPCM) with SiO2 coating Na2HPO4⋅7H2O were synthesized by one-pot method. A possible formation mechanism of Na2HPO4⋅7H2O core and SiO2 shell was preliminarily analyzed. The morphology, composition, crystalline phase and thermal performances of the synthesized MEPCM was characterized by scanning electron microscopies (SEM), Transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetry analysis (TG) and thermal constants analyzer. Preliminary analysis indicated that the Na2HPO4⋅7H2O nanocrystals as the core was obtained by anti-solvent process with partially dehydration (loss of five crystal water molecules) from Na2HPO4⋅12H2O, and the SiO2 as the shell was formed by sol-gel method with hydrolysis and condensation of tetraethyl orthosilicate. Analysis results of FT-IR and XRD illuminated that only physical interaction occurred between the SiO2 shell and the Na2HPO4⋅7H2O core in the MEPCM. The results of SEM and DSC showed that the synthesized MEPCM displayed a flat cylinder structure and the particle size of about 8 μm, and exhibited a phase change temperature of 50.12 °C, latent heat of 159.8 kJ/kg and encapsulation ratio of 82.40%. Moreover, the formation of the phase change microcapsules could reduce the supercooling of Na2HPO4⋅7H2O to some degree and the MEPCM displayed a suitable thermal conductivity (0.3838 W/m⋅K) and an excellent thermal stability. The thermal properties of the MEPCM make it suitable for solar energy storage.