A series of forma-stable n-alkanes/silica composite phase change materials (PCMs) was developed through sol–gel synthesis using sodium silicate as a silica precursor. Fourier transform infrared spectra confirm the chemical composition of the synthesized microcapsules, and wide-angle X-ray scattering patterns indicate good crystallinity for the n-alkanes inside silica microcapsules. Scanning electric micrographs demonstrated that these forma-stable n-alkanes/silica composite PCMs synthesized at pH 2~3 presents an irregular-shaped morphology and a core-shell microstructure. Because the pH value of reaction solution determines the silica condensation rate and, thus, influences the balance between the self-assembly and polycondensation of silica precursors on the surface of n-alkanes droplets, the forma-stable n-alkanes/silica composite PCMs could achieve a high productivity and good heat-storage performance only at pH 2~3. The form-stable n-alkanes/silica composite PCMs also exhibits achieves a high encapsulation rate and high encapsulation efficiency in this synthetic condition. The encapsulation of n-alkanes into silica can impart a high thermal conductivity and a good anti-osmosis property to the form-stable composite PCMs and can also improve the thermal stability of the composite PCMs by preventing inside n-alkanes from thermally evaporating. Owing to the easy availability and low cost of sodium silicate, this synthetic technique indicates a high feasibility in industrial manufacture for the forma-stable silica-based composite PCMs.
