Abstract

Abstract Phase change materials (PCM), allow the storage of a large amount of thermal energy during the change from one phase to another (usually solid-liquid) at a specific temperature, presenting a high heat of fusion. Once a certain amount of thermal energy has been accumulated, it can be later released in a simple and predictable way, while phase change occurs. PCMs can be classified as organic, inorganic and eutectic. Among them, inorganic PCMs stand out for presenting high latent heat, relatively high thermal conductivity, being non-toxic, non-flammable and of relatively low cost compared to organic PCM. In addition, the inorganic salts generated as waste from the nonmetallic mining industry can be considered as a very economical source of inorganic PCM. At the same time, the search for applications to these wastes would add extra value to the mining processes. However, these materials (mostly inorganic salts) present some problems when applied to thermal storage systems, such as subcooling and phase segregation. One of the most successful methods used to address these disadvantages is the encapsulation of PCMs, providing a large area of heat transfer, decreasing subcooling and controlling changes in the volume of storage materials when the phase transition occurs.. The encapsulation methods, depending on the material design, can be classified as core-shell encapsulation (EPCM) and stabilized (SS-PCM). SS-PCMs are composed of PCM with a support material (MS) that stabilizes the PCM during the phase transition, by surface forces and capillarity; retaining the shape of the solid structure and avoiding the segregation of the liquid. In addition, they can increase the overall thermal conductivity and give the material good compatibility with other materials in thermal storage systems. It is essential to maintain or improve the main properties of the PCM once encapsulated. The main properties analyzed are the melting and solidification temperatures (Tf and Ts (0C)), conductivity (k (Wm-1K-1)), latent heat (ΔHf and ΔHs (Jg-1)) and the cyclic stability (number of heating / cooling cycles) and thermal. In addition, it is necessary to reduce or eliminate sub-cooling and phase segregation. Among the most common methods used to develop inorganicS S-PCMs are melt infiltration, cold compression and vacuum impregnation, but the sol-gel method applied directly for this purpose has not been used according to the revision made in this work. The meticulous control of the microstructure and the molecular level with the ability to shape the material in the form of powder, bulk and monolith presented by the sol-gel process make it a very attractive approach to obtain inorganic SS-PCM. The optimization of the synthesis parameters (monomers, solvents, temperature and monomer / crosslinker ratio, among others) are necessary to improve the performance of the materials obtained by these techniques. Therefore, the main objective of this work is to develop a method of obtaining stabilized phase change materials of type SS-PCMs using sol-gel techniques based on polymerization of SiO2, using inorganic salts and disposable wastes in processes of the non-metallic mining industry. In addition, the influence of PCM content and pH, as well as monomers and solvent on the thermal properties of SS-PCMs was studied. Tetraethyl orthosilicate (TEOS), trimethoxy [3- (methylamino) propyl] silane (PTMOS), (3-glycidyloxypropyl) trimethoxysilane (GPTMOS) and trimethoxy (2-phenylethyl) silane (PhTMOS) were used as silane monomers for the synthesis; while ethanol, acetonitrile and cyclohexane were used as solvents for the sol-gel process. The samples were also characterized by infrared spectroscopy (IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry and differential scanning calorimetry (TG-DSC). The formation of the SiO2 polymers and the different SS-PCMs were confirmed by IR and XRD. Several inorganic salts were used as PCM: Na2SO4.10H2O, MgCl2.6H2O, MnCl2, LiCl, LiNO3, Li2CO3, CH3COOLi.2H2O. In addition, the developed method was used for the encapsulation of bischofite and carnallite, wastes from the non-metallic mining industry of northern Chile. Using SEM, the tendency to the formation of a mixed compound of PCM and SiO2 particles was observed. In general, these compounds have a morphology that depends not only on the monomers used, but also on the PCM content. The PTMOS and GPTMOS monomers were not recommended for the synthesis of SS-PCM, since the materials obtained with these monomers showed a very low thermal storage efficiency. The highest values of latent heat (230.4 Jg-1) were obtained for LiNO3 SS-PCM (80%), synthesized with ethanol and using only TEOS as monomer. This new compound had a melting point at 255.0 0C and a RL value of only 2.8 0C. The thermal properties of the SSPCMs of LiNO3 and LiCl stood out for achieving reductions in the subcooling and the latent storage range RL for all materials with respect to the pure salts.