Preparation And Application Of Nanoencapsulated Phase Change Materials

In last decades, especially after the energy crisis in 1970s, thermal energy storage (TES) through phase change materials (PCMs) became an important field of scientific research. PCMs can storage/release large amounts of thermal energy during phase change process, with high energy storage density at near constant temperature. A majority of practical valuable PCMs perform thermal storage through solid-liquid phase transition, with low costs, high phase change enthalpies, and minor volume change during the phase change process. However, if the solid-liquid type PCMs are used without encapsulation, leakage and pollution to surrounding environment will occur. In addtion, most organic PCMs possess low thermal conductivity and slow thermal response, and are potential sources of fire danger due to their flammability.Micro/nano encapsulated phase change materials can provide solutions on the above mentioned limits for solid-liquid PCMs. After encapsulation with polymeric or inorganic shells, leakage of PCMs can be prevented, efficiency of thermal energy storage/release can be improved, and the volume change during phase change process can be controlled. The size of microencapsulated phase change materials (McroPCMs) ranges from 1 to 1000μm, while that of nanoencapsulated phase change materials (NanoPCMs) generally ranges from several tens of nanometers to 1μm. Micro/nanoencapsulated phase change materials have found applications in many fields, such as energy efficient buildings, solar energy storage, and refrigeration systems.Traditionally, various organic polymers were adopted as shell materials of micro/nano encapsulated phase change materials. Organic shell materials have excellent structural flexibility and can withstand repeatedly volume change of PCMs during the phase change process. However, there are some limits for organic shell materials, including emission of poisonous gases and low thermal conductivity. In recent years, MicroPCMs with inorganic shell materials have attracted much attentions, because inorganic materials usually have high thermal conductivity, excellent thermal and chemical stability, and nonflammability, and do not release poisonous gases. However, inorganic shells are somewhat brittle, their mechanical strength are not quite ideal, and mesoporous structures are often formed. Furthermore, few works have been reported on NanoPCMs with inorganic shell materials. Hybrid materials, which can combine the advantages of both organic polymer materials and inorganic materials, possess superior performances and have great potential to serve as the shell materials of encapsulated PCMs. However, relevant studies on this topic are very few, and efficent methods for preparation of encapsulated PCMs with hybrid shell materials are highly desired. Based on encapsulated PCMs, a number of thermoregulated composite materials have been developed. However, not enough attention have been paid to the compatibility between encapsulated PCMs and matrices, and results in deteriorated microstructure and mechanical properties of thermoregulated composite materials. Therefore, following works have been performed on preparation and application of NanoPCMs in this thesis.1. Nanoencapsulation of n-octadecane phase change material with silica shell was performed through interfacial hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS) in miniemulsion. The chemical composition and crystallinity of the synthesized n-octadecane@SiO2 nanocapsules were characterized by Fourier transform infrared (FT-IR) spectroscopy and X-ray diffraction (XRD) analysis. differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) results demonstrated that the as-prepared nanocapsules have high heat storage capability and good thermal stability. The melting enthalpy and encapsulation ratio of the nanocapsules were as high as 109.5 J·g-1 and 51.5%, respectively. Most importantly, n-octadecane@SiO2 nanocapsules with different morphologies and sizes (169-563 nm) have been conveniently obtained via tuning water-to-ethanol ratio in continuous phase of the miniemulsion. With decreasing size of the n-octadecane@SiO2 nanocapsules, the phase change temperatures move to lower values due to Gibbs-Thomson effect. Moreover, the as-prepared nanocapsules possess high thermal conductivity, and can maintain their phase transition properties perfectly after 500 melting-solidifying thermal cycles, making them ideal candidates as thermal energy storage materials.2. NanoPCMs with polystyrene-silica (PS-SiO2) hybrid shells were prepared in miniemulsion, through combined free radical polymerization of vinyl monomers and sol-gel reaction of silane precursors. FT-IR spectroscopy and XRD analysis were used to verify the chemical composition and crystallinity of the as prepared NanoPCMs. SEM and TEM results demonstrated that these NanoPCMs possessed regular bowl-like morphology and well-defined core-shell structure. With increasing core-shell ratio, shell thickness of the NanoPCMs gets lower, and the shape of the NanoPCMs turns more flat. Phase change properties of the NanoPCMs were investigated by DSC and temperature-dependent XRD methods. Effects of core/shell ratio and usage of y-methacryloxypropyltrimethoxysilane (MPS) on crystallizing behavior of the NanoPCMs were studied. TGA results indicated that, compared with pristine n-octadecane, onset thermal decomposition temperature of these NanoPCMs were improved obviously. After 1000 melting/solidifying thermal cycles, the chemical structure, morphology, and phase change enthalpies of these NanoPCMs were maintained perfectly, and super cooling behaviors were mitigated largely, which is favorable for thermal energy storage/release under constant temperature. This work provides a facile and efficient approach for preparation of NanoPCMs with organic-inorganic hybrid shells, which possess superior thermal stability and reliability, and might be applied in fields of thermal energy storage, energy efficient buildings, and smart textiles.3. Rigid polyurethane (RPU) foams containing nanoencapsulated phase change materials (NanoPCMs) with silica shell were fabricated, in order to improve their thermal energy storage capacity and suppress adverse effect of the fillers on the morphology and mechanical property of the thermoregulated composite materials. The chemical composition of the NanoPCMs/RPU composite foams was verified by FT-IR spectroscopy. SEM analysis indicated that the NanoPCMs are homogeneously dispersed in polyurethane matrices. Thermal energy storage capacities of the composite foams were characterized by DSC methods. The melting and solidifying enthalpies are 17.93 and 18.75 J/g, respectively, for the foam containing 17.40 wt% of NanoPCMs. Thermal stability of the foams was evaluated by TGA and dynamic mechanical thermal analysis (DMTA) methods. The results of uniaxial compression tests indicated that the specific compressive strength and modulus of the composite foams decrease gently with increasing NanoPCMs content. At 17.40 wt% of NanoPCMs content, the specific compressive strength and modulus retain 57% and 66% of the original values of pure RPU foam, respectively. In addition, the compressive property-density relationship of the NanoPCMs/RPU composite foams is in good agreement with the classical Gibson-Ashby’s power law.