Preparation And Heat Transfer Characteristics Of SiO₂Aerogel Composites

It has well been acknowledged that the super-insulation material has super-low thermalconductivity （e.g. lower than that of the still air under the intended conditions）. Silica aerogelsand their composites are currently studied as a type of typical nanoporous super-insulationmaterial. Silica aerogels are a type of non-crystal solid material which has continuous randomand porous network structure filled with still gas. It is considered to be the solid-state materialwith the lowest density and the highest insulating performance. The thermal conductivity ofsilica aerogels are low to0.013W/（m·K） at room temperature, while their porosity andspecific surface area are up to99%and1000m2/g, respectively. However, owing to the highporosity and porous network structure of the silica aerogels, the aerogels have low strengthand poor flexibility, which limites their application as bulk insulation materials. Moreover, athigh temperatures, silica aerogels have poor thermal insulating property since they are highlytransparent in the3-8μm wavelength regions.In this study, a series of SiO₂aerogels composites were successfully fabricated from thetetraethoxysiliane （TEOS） precursor reinforced by carbon nanotubes, glass fibers and washedcotton via sol-gel process and drying at ambient pressure. The Fourier Transfrom InfraredSpectroscopy was used to characterize the refractive properties of the silica aerogelcomposites. The scanning electron microscopy and the automatic surface area and porousphysical adsorption analyzer were used to characterize the morphology and microstructure ofthe silica aerogel composites. The thermal constant instrument and electronic universal testingmachine were used to measure the thermal conductivity and compressive and flexural strengthof the silica aerogel composites. In this study a heat transfer model of the alignedfiber-reinforced aerogel composites was also established to explore the heat transfermechanism of the silica aerogel composites. The calculated thermal conductivities at threedirections were compared to and validated with the experimental results. Based on thenumerical model, the effects of fiber layers, fiber spacing, volume fraction on the thermalconducitivity of the silica aerogel composites were accordingly evaluated.The results show that the aerogel composites have super-low thermal conductivity of0.0206W/（m·K） as the content of carbon nanotubes is0.2%. The addition of the carbonnanotubes significantly improved the compressive strength of the silica aerogel from0.081MPa （10%deformation）、0.77MPa （50%deformation） to0.22MPa （10%deformation）and1.26MPa （50%deformation）. Owing to the shading performance of the CNTs to infered,the thermal conductivity of aerogel composits was low to0.0246W/（m·K） after400℃heat treatment. The addition of the glass fiber into the silica aerogels greatly resists the destructionof silica aerogels.Based on the established heat transfer model in this thesis, the effective thermalconductivities of the aligned glass fiber-reinforced aerogel composites at the heat flowparallel to the fiber orientation, and horizontal and vertical fiber dimensional orientation were0.0541W/（m·K）,0.0372W/（m·K）,0.0221W/（m·K）, respectively. The relative errors of thecalculated thermal conductivities compared to the experimental results were3.6%,3.0%,10.0%at three directions, respectively. It could be concluded that the heat transfer modelcould be used to determine the effective thermal conductivity and explore the heat transfermechanism of the fiber-reinforced aerogel composits.