Confinement effect of silica mesopores on thermal behavior of phase change composites

SiO2-based phase change composites were prepared by sol-gel method using different phase change materials (PCMs), including stearic acid (SA), paraffin, and polyethylene glycol (PEG), and the effects of various mass fractions of PCMs were comprehensively investigated. The structures and thermal properties of these composites were characterized and analyzed by differential scanning calorimetry (DSC), X-ray diffraction (XRD), field emission scanning electron microscopy, and nitrogen adsorption-desorption measurements. The XRD and DSC results showed that the crystallinity and thermal enthalpy of paraffin in composite were the best among the three types of PCMs confined in SiO2. However, PEG in composites showed poor crystallinity and thermal enthalpy. After the extraction of the PCMs, the pore size distribution of SiO2 matrix and SEM images showed that SA/SiO2 composites possessed mesopores (20 nm) and macropores (50-150 nm), paraffin/SiO2 composites possessed mesopores and very large pores (2 mu m), and PEG/SiO2 composites contained only mesopores. Among the three types of composites, paraffin/SiO2 composites exhibited the least mesopore volume. Presence of more mesopores in the composite led to worse crystallinity and thermal enthalpy of PCMs in composites. The crystallinity and thermal enthalpy of PCMs in composites were affected by the pore size and volume. Mesopores confined the crystallization and thermal behavior.