Study The Effects Of The Fabrication Conditions On The Thermal Conductivities Of The SiO₂ Aerogel

SiO₂ aerogels has been acknowledged as the lightest solid nano materials in the world , Owing to its special nanoporous network,SiO₂ aerogel has lots of specific functions including the super -low thermal conductivity,As the thermal insulation materials SiO₂ aerogels are attacting more and more attentions. So as to obtain the super-thermal insulating SiO₂ aerogels the effects of the fabrication conditions and the microstructures are needed to be further studied.In this thesis the SiO₂ aerogels were fabricated through the drying at the ambient pressure.Then the thermal conductivities of the as-fabricated SiO₂ aerogels were measured by using the Hot Disk instrument.The effects of the fabrication conditions (e.g. the ratios of the ethanol,deionized water,tetraethoxysilane and acid,alkali,the modification concentrations,the modification time,and ageing time,and the gelation temperature)on the thermal conductivities of the SiO₂ aerogels were accordingly studied. And the effects of microstructures of the SiO₂ aerogels the thermal conductivities of the SiO₂ aerogels were profoundly studied.And the effects of the fabrication condictionson the thermal conductivities of the SiO₂ aerogels were profoundly studied. A theorectical model of the thermal conductivity of the SiO₂ aerogels was also used to study the the mechanism of the combined radiative and conductive heat transfer within the SiO₂ aerogel.The results showed that the fabrication conditions (e.g. the ratios of the ethanol,deionized water,tetraethoxysilane and acid, the modification concentrations, the modification time and ageing time,and the gelation temperature)had great effects on the thermal conductivity of the SiO₂ aerogels.The thermal conductivityof the SiO₂ aerogels firstly decreased and then increased as the amount of the ethanol,deionized water ,tetraethoxysilane was increased.In the acid conditions,the thermal conductivity of the SiO₂ aerogel firstly inceased and then decreased as the pH value was increased.The lowest thermal conductivity of 28.11 mW/(m?K) could be obtained at pH=3. In the alkali conditions,he thermal conductivity of the SiO₂ aerogels firstly decreaed and then inceased as the pH value was increased.The lowest thermal conductivity of 29.87 mW/(m?K) could be obtained at pH=8.5. The thermal conductivity of the SiO₂ aerogels decreased as the the modification concentrations,the modification time,and ageing time was increased.The results also showed that the acid,alkali, modification time in the fabrication condictions had little effects on the raiation thermal conductivity of the SiO₂ aerogels.The effects of the microstructures of the SiO₂ aerogels on the thermal conductivity were also studied in this thesis. The results showed that the microstructures of the SiO₂ aerogels were the key factors to affect the thermal conductivity of the SiO₂ aerogels.The thermal conductivity of the SiO₂ aerogels increased as the average pore diameter of the SiO₂ aerogels was increased and specific ratio of area to volume was decreased.A theorectical model of the thermal conductivity of the SiO₂ aerogel showed that the radiative heat flux was much lower than the conductive heat flux at the ambient temperature.However,the radiative heat transfer will be domiant in the heat transfer wthin the SiO₂ aerogel at high temperature(e.g.higher than 500K).The computational results indicated tha the thermal conductivity of the solid network in the SiO₂ aerogels increased as the density of the SiO₂ aerogels was increased.As a comparison, the thermal conductivity of the gas in the SiO₂ aerogels decreased as the density of the SiO₂ aerogels was increased.Therefore,the optimum density of the SiO₂ aerogels which had the lowest thermal conductivity could be obtained.