| Thermal storage materials (TSM) as the key of thermal energy storage technology have been widely applied to solar-thermal area, such as concentrated solar power, building energy efficiency and solar hot water systems, which are belong to the industries of energy conservation and emission reduction. However, high cost of TSM restrictes development of these terminal industries, and affectes solar-thermal wide application. Hence, low cost manufacturing technologies for high performance TSM are urgently needed. But, the relationship between micro structure and property of TSM is hard to establish due to lack of basic research, and hence, it is unable to master the key technology of TSM’s property control. Minerals have the superiority, such as unique structure, abundant morphology and good thermal stability; also minerals possess cost advantages on account of simplified and low cost. Therefore, exploring thermal storage characteristics of natural mineral, the high performances mineral-based composite TSM will be low cost obtain by combining minerals and the TSM. The microstructure-property relationship will be established and thermal storage property control will be realized.In this thesis, mineral-based composite TSM were prepared by means of the structure and morphology of natural minerals, and the thermal storage properties were enhanced by modification and combination technique. The microstructures of samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), atomic force microscope (AFM), petrography analysis, Fourier transformation infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric (TG) and nitrogen gas adsorption-desorption isotherms; the properties of samples were evaluated by laser flash technique, thermal constant analyzer, low-temperature DSC and thermal storage and release test set-up.Novel sensible TSM were first synthesized via procedures of mixing, pressure forming, natural drying and sintering, by using natural clay as the binder, hematite as the aggregate and kaolin tailings as a thermal conductivity "modulator". The energy density of sensible TSM reached2.18×106J·m-3·K-1. The thermal conductivity of the green compact were1.11～1.64W·m-1·K-1after thermally treated at200～1000℃. Kaolin tailings could act as a "modulator" for adjusting the thermal conductivity from1.42to1.92W·m-1·K-1. The change of phase and morphology of clay during the thermally treated were characterized, indicating that the clay showed a predominant effect on the thermal conductivity of TSM. The microstructure-property relationship was established and affecting mechanism of mineral structure on thermal storage property was revealed.Three different forms of kaolin (platelet, PK; layered, LK; and rod, RK) were used to stabilize paraffin and prepared the PK/paraffin, LK/paraffin and RK/paraffin composites. The latent heats of melting and freezing are determined as107.2and105.8J·g-1for the PK/paraffin,94.8and93.0J·g-1for LK/paraffin, and84.1and82.7J·g-1for the RK/paraffin, respectively. The crystallinity (Fc) of the paraffin in the PK/paraffin, LK/paraffin and RK/paraffin are96.1%,98.4%and87.9%, respectively. The effects of morphology on thermal storage properties were investigated. The results clearly demonstrated that the less pore volume with the pore size smaller than5nm, the higher crystallinity of paraffin in the composites. The higher crystallinity, the less phonon scattering in the interface zone and then the larger phonon mean free path of paraffin in the composite. The relationship microstructure of kaolins and thermal storage property was established and affecting mechanism of mineral morphology on thermal storage property was ascertained.Pristine vermiculite was first thermally expanded to produce expanded vermiculite (EVM), and paraffin was impregnated into selected EVM by vacuum impregnation to synthesize novel EMT/paraffin composites. The crystal structure, chemical structure and morphology of vermiculite after sintered at different temperature were compared. It was proved that EVM (900℃) existed in the form of phlogopite structure, which has a good thermal conductivity. Thermal properties and microstructure of EVM before and after composite were investigated, verifying that paraffin was successfully loaded into the EVM and no chemical reaction between EVM and paraffin before and after thermal cycling. Thermal cycle experiment showed that EMT/paraffin composites have good thermal stability and chemical compatibility. The paraffin maximum loadage of EVM was67%. The EVM/paraffin composite had higher latent heats, the latent heats of melting and freezing of EVM/paraffin composite were135.5,137.6J·g-1, respectively. The composite had a good thermal conductivity of0.545W·m-1·K-1, which is121%higher than that of pure paraffin (0.246W·m-1·K-1). Mechanism of thermal storage characteristics enhanced by mineral modification was investigated via structural modification and carbon nanofibers (CNF) modification, respectively. And mechanism of thermal storage property enhanced by mineral modification was explored.Bentonite was used as support to stabilize the SA and prepared SA/B. Then, the graphite (G) was introduced to enhance the thermal conductivity of the composite (SA/GB). To synergetic increase the Thermal capacities and conductivities, the graphite and bentonite mixture was subtly implemented by microwave-acid treatment and SA/GBm with the maximum loadage of45.5wt.%SA was obtained. It is of interest that the thermal storage capacity of the SA/GBm is84.6J·g-1, which is62%larger than that of SA/GB and75%than that of SA/B. The thermal conductivity of the SA/GBm is0.77W/m K, which is31%higher than that of SA/B and196%than that of pure SA. Thermal durability analysis suggested that the initial decomposition temperature of SA/GBm is240℃, which is higher than that of SA/B and pure SA (208℃). The pore structures of the supports before and after microwave-acid treatment were comparative studied, indicating that the pore sizes of bentonite were expanded by microwave-acid treatment and the supports possessed lager hold space and higher crystallinity of SA. And then, the reason of improved thermal storage capacity of composites was uncovered. Furthermore, interfaces between SA and support surfaces were depicted, and synergistic effects of thermal storage properties enhanced by mineral combination were illuminated. Based on the academic thought of mineral resources with fine processing, thermal storage characteristics of natural mineral were explored and unique advantages of minerals used in the TSM were developed. Basic research of mineral-based composite TSM were carried out in this thesis. The microstructure-property relationship was established and thermal storage property control was realized. All of these will provide new idea for preparing of high performance TSM and show new direction for mineral-based functional materials. |
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