Preparation And Characterization Of Shape-stabilized Phase Change Materials For Thermal Energy Storage In Asphalt Pavement

Though asphalt pavement is widely used, it faces a series of temperature diseases at high temperature, such as rutting, transposition, upheaval, bleeding and so on, which affect the pavement performance. Furthermore, asphalt pavement release volatile organic compounds and vast amounts of heat under high temperature, not only pollute the surrounding environment, but also increase the high-temperature action time at low ground levels and then exacerbate the urban heat island effect. In this paper, supported by National Natural Science Foundation project and based on phase transition theories and experiments, appropriate organic solid-liquid phase change materials and inorganic porous materials are selected as thermal energy storage materials and supporting materials, respectively. Then, sol-gel method is operated to develop shape-stabilized phase change composites (SS-PCMs), which can be replaced fine aggregates of asphalt pavement to cool pavement temperature. In these composites, it is extremely difficult to exfoliate each other between organic PCMs and inorganic porous materials, they own excellent integrity and good thermal properties.Firstly, according to construction temperatures, service temperatures and working conditions of asphalt pavement engineering, the structures and properties of certain organic PCMs, such as neopentyl glycol, fatty acids, fatty alcohols, paraffin wax, polyethylene glycol (PEG) and so on, were determined, and PEG as suitable PCMs was selected, because of its applicable phase transition temperatures, high enthalpies and thermal stabilities, stable crystallization characteristics and chemical structures and so on. On the other hand, based on pore structure, aperture size, strength, absorption characteristics, encapsulation effects and other features, macroporous carriers (porous ceramics, allochroic silica gel, molecular sieves, lightweight shale ceramsite, porous volcanic rock, etc.), mesoporous carriers (silica gels (SiO2), etc.) and microporous carriers were studied. Owing to appropriate aperture, large pore volume, good adsorption characteristics and excellent encapsulation effects, silica gels as carriers were selected to absorb and encapsulate PEG.Secondly, In order to discuss the interactions between silica sol and PEG, and analyze adsorption and filling mechanism of silica gel in gelation process, the physical and chemical characteristics of silica sol and PEG were dissected from molecular levels, respectively. Then, according to the conditions of gelatinization, sol gel methods were divided into coagulant-assisted sol gel method and temperature-assisted sol gel methoddue to two aspects: countra-ion coagulation and thermodynamic instability. The gelatinization was carried out by adding coagulant and adjusting temperature, respectively. The corresponding coagulation accelerators were potassium chloride solution, calcium chloride solution and aluminum chloride solution, the temperature-assisted conditions were the temperature from room temperature to 70?, either of which would accelerate the gelation process. SS-PCMs, called SS-PCMs 1 and SS-PCMs 2, were prepared by two sol gel methods, respectively.Then, the thermal properties of two SS-PCMs were characterized by DSC, Phase change enthalpies of SS-PCMs 1 prepared by CaCl2-assisted one were 32.6-91.5J/g; SS-PCMs 2 prepared by temperature-assisted one were 63.4-128.4J/g. Comparing these date, it is notable that little Ca2+ significantly affects the phase transition behaviors of SS-PCMs 1. By studying the influence of Ca2+ and SiO2 on the properties of SS-PCMs, it could be reached that Ca2+ and O atom of PEG formed coordination bonds and the coordination number was about 4. Therefore, PEG from crystalline state transformed into amorphous state, lost its original crystalline structure, and crystallization characteristics were greatly reduced. SiO2 as an impurity inhibited the perfect PEG crystallization. On the one hand, there existed hydrogen bonding interaction between silicon hydroxyl from the interface of SiO2 and alcohol hydroxyl of PEG chains, beyond that, the surface tension and capillary forces also were existed between SiO2 and PEG These forces interacted to restrict the free movement of PEG chains during the crystallization process, so PEG crystallization was limited. On the other hand, the steric effect and drag effect of SiO2 mesoporous structure hindered the crystal arrangement and orientation of PEG chains, which led to the decline of regularities of crystalline regions and the increase of lattice defects. Therefore, the actual enthalpies of SS-PCMs were lower than theoretical enthalpies. Various characterization techniques, such as DSC, XRD, FTIR, TG and POM, were employed to investigate some properties of SS-PCMs 2. As PEG molecular weights and effective contents increased, the change rule of enthalpies and phase transition temperatures were analyzed; the internal and surface distribution of PEG on SiO2 skeleton structure, PEG crystal morphology, action mechanisms between components, phase transformation behaviors of SS-PCMs 2 and so on were investigated. Then, using unsaturated polyester resins (UPR) with high consolidation strength and graphite, copper powder, aluminium powder to enhance the mechanical properties and thermal conductivities of SS-PCMs 2, respectively. When UPR just wrapped SS-PCMs 2 and formed the skeletal structure, the mechanical properties of SS-PCMs 2 were optimal. Graphite was the best to improve thermal conductivities of SS-PCMs 2, thermal conductivities of SS-PCMs with appropriate graphite reached up to 0.588 W·m-1?-1, which were raised 110.75%.Finally, the mixtures of SS-PCMs 2 and asphalt were prepared by physical method, and their compatibilities between all the components, thermal properties, crystallization properties and thermal stabilities were detected by same characterization techniques. The thermoregulation tests of heat storage asphalt mixtures prepared by SS-PCMs 2 particles as fine aggregates was measured by water bath at 70?, and then the thermoregulation effects were analyzed. When SS-PCMs 2 contents were 3.5%, the temperature of heat storage asphalt mixture was about 4.0? lower than common asphalt mixture.