Influence of monomers and solvents in the direct sol-gel synthesis of LiNO3 shape stabilized phase change materials

Sol-gel techniques based on silicon dioxide SiO2 have been proposed for encapsulation of organic phase change materials (PCMs), avoiding phase segregation, and enhancing its thermal properties. Otherwise, the encapsulation of inorganic PCMs by direct sol-gel method has barely been applied for this purpose. Moreover, the evaluation of synthesis parameters (monomers, solvents, temperature and monomer/crosslinker ratio, among other) has never been done in order to improve the performance of the inorganic shape stabilized phase change materials (SS-PCMs) obtained by this technique. In this work, tetraethyl orthosilicate, trimethoxy [3-(methylamino)propyl] silane, (3-glycidyloxypropyl) trimethoxysilane and trimethoxy (2-phenylethyl) silane were employed as monomers for synthesis; and ethanol, acetonitrile and cyclohexane were used as solvent, for the evaluation of the sol-gel process. LiNO3 was selected as PCM, estimating the salt content of 50 and 70 wt% in the final material. Samples were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD) and differential scanning calorimetry (DSC). The achievement of LiNO3 SS-PCM was confirmed by XRD; while a mixed composite of the lithium salt and SiO2 particles was observed by SEM, with a morphology depending not only in the employed monomers but in the PCM content. High values of latent heat (206.2 J g(-1)) were achieved during the SS-PCM synthesis with ethanol, 70 wt% of PCM content and employing only TEOS as monomers; and a reduction in latent range R-L, respect to the pure LiNO3 salt, was observed. The thermal performance of the materials demonstrated its potential as SS-PCM for medium-temperature thermal energy storage applications.