【作者】 张贺新；

【导师】 赫晓东；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2008， 博士

【摘要】 SiO2气凝胶是一种由SiO2胶体粒子相互团簇聚集形成的纳米多孔网络结构材料,由于在常温下具有很低的密度和极低的热导率,因此成为最具有开发潜力的一种超级绝热材料。但是随着使用环境温度的升高,由于SiO2气凝胶在高温时对传递热辐射能量的红外电磁波具有穿透性,它的辐射热导率会随温度升高而迅速增加,从而限制了其在高温环境下的应用。本文针对SiO2气凝胶对高温环境下的红外光具有高透过率的问题,选用了TiO2和六钛酸钾晶须两种具有高红外反射和吸收性能的粉末材料作为遮光剂,采用溶胶-凝胶法和非超临界干燥技术制备了遮光剂掺杂的SiO2干凝胶(通常由非超临界干燥法制备的气凝胶材料被成为干凝胶),并通过X射线衍射(XRD)、扫描电镜(SEM)、高分辨透射电镜(HRTEM)、傅立叶红外光谱仪(FTIR)以及N2吸附-脱附等方法对掺杂后的SiO2干凝胶的组织性能和孔结构等进行了分析,采用了分子力学模拟方法对掺杂后遮光剂粒子与SiO2干凝胶粒子的界面能进行了模拟和计算,并通过将掺杂后的干凝胶填充入蜂窝隔热结构中,采用表面瞬态加热装置对掺杂遮光剂的SiO2干凝胶与纯SiO2干凝胶在高温环境下的隔热性能进行了评价。以正硅酸乙酯(TEOS)为硅源,无水乙醇为溶剂,盐酸(0.2mol/L乙醇溶液)和氨水(2mol/L乙醇溶液)分别作为酸碱两步法中的酸性和碱性催化剂,选用丙三醇(GLY)为化学干燥控制剂(DCCA),制备TiO2和六钛酸钾晶须掺杂含量均为1%~8%(质量比)的SiO2干凝胶材料。在制备过程中,TEOS: H2O:无水乙醇=1:4:8;酸性水解条件为:pH=2、水解时间为15min、水解温度为50℃;碱性聚合条件为:TEOS: NH3·H2O=1:0.036、凝胶时间<2min。TiO2粉末掺杂的SiO2干凝胶是一种具有纳米孔的非晶态网络结构材料,TiO2颗粒以物理形态包覆于SiO2网络骨架中,在1000℃以内,随掺杂含量的增加,TiO2-SiO2干凝胶的孔径分布逐渐变宽,掺杂量≤5%的TiO2-SiO2干凝胶均能够具有>1000m2/g较高的比表面积,平均孔径分布为30~50nm。TiO2颗粒的掺入能够有效的散射或吸收2~8μm波长区域内的红外电磁辐射,降低这一高温能量主要集中区域的红外透过率。六钛酸钾(K2Ti6O13)晶须掺杂SiO2干凝胶在小于870℃时同样是具有纳米孔网络结构的非晶材料,其中1%质量比掺杂的干凝胶具有最大的比表面积为843m2/g,相比于同质量比掺杂的TiO2-SiO2干凝胶来说比表面积有所降低,且平均孔径也有所减小,1%~8%不同晶须掺杂量的SiO2干凝胶的平均孔径均为20~30nm。六钛酸钾晶须的掺入能够大大降低SiO2干凝胶的结晶温度,使其在870℃就出现了多晶化现象。六钛酸钾晶须掺杂的SiO2干凝胶同样可以有效的降低SiO2干凝胶在2~8μm波段内的红外辐射透过率。采用分子模拟方法对TiO2和六钛酸钾晶须掺杂SiO2干凝胶的掺杂形态和界面能进行了模拟和计算,结果表明:TiO2与SiO2界面能仅为0.601J/m2,说明TiO2颗粒与SiO2干凝胶骨架间没有的化学反应结合,TiO2颗粒能够稳定地掺杂于SiO2网络结构中;六钛酸钾晶须与SiO2的界面能为3.32J/m2,也属于一种较弱的结合方式。但是与TiO2/SiO2界面不同的是,在模拟过程中,六钛酸钾晶须中有部分K+从其初始位置扩散到干凝胶结构内部,这种K+扩散所导致的界面变形重构,可能是使六钛酸钾晶须掺杂的SiO2干凝胶在较低温度下就出现内部分子重排,导致晶化的主要原因。利用有限元分析方法对遮光剂掺杂SiO2干凝胶填充的蜂窝隔热结构进行了瞬态热环境模拟,并采用表面瞬态加热装置对这一隔热结构进行真实热环境测试来验证模拟结果。结果表明:未填充干凝胶的蜂窝结构的传热方式主要是内部的空气对流和辐射传热,填充遮光剂掺杂的SiO2干凝胶能够有效地阻止这两种传热方式的产生,使热量主要集中在面积很小的蜂窝壁上,大大延迟了底部面板达到预计高温的时间。更多还原

【Abstract】 SiO2 aerogels belong to a kinds of non-crystal solid material which consist of SiO2 colloid particles molecule and form continuous random and porous network structure filled with gaseous dispersive medium. Because of their nano-size particles and porous characteristic, aerogels have many potential applications especially as super-insulation materials in modern aerospace and industrial fields.However, at ambient and higher temperatures, aerogel has poor thermal insulating property because it is highly transparent in the 2-8μm wavelength regions and the infrared electromagnetic wave can transmit in the SiO2 aerogels,therefore the radiative thermal conductivity will increase with the temperature increasing, which limits its actual application at high temperature.Due to its high transmittance in the infrared region at elevated temperature, this paper select TiO2 and K2Ti6O13 whisker as main opacifiers, considering it is an efficient opacifer due to its high reflection index and thermal stability. The pure and composite aerogels were fabricated by sol-gel method and subsequent non- supercritical drying technique (usually it maybe called as xerogels prepared with this method).The microstructure and physicochemical properties of the prepared aerogels were investigated by the menas of XRD, SEM, TEM, HRTEM, as well as Nitrogen gas adsorption, BET and FTIR method. Moreover, the MS method was adopted to simulate and analyze the interfacial and bonding properties between the SiO2 xerogels matrix and doped TiO2 particle or K2Ti6O13 whisker. Also the thermal insulation properties of the pure and doped-opacifer aerogels were evaluated using oxy-acetylene heating equipment.In the course of sol-gel, two-step acid-base catalyzed silica gels were prepared by using of tetraethoxysiliane (TEOS) as precursor, absolute alcohol as solvent, 0.2mol/L hydrochloric acid (diluting with absolute ethanol) and ammonia as acid and base catalyzer respectively. The results show that the most optimal molar ratio of TEOS, H2O, alcohol is 1: 4: 8. The hydrolyzation and condensation temperature is 50℃, and the hydrolysis time is 15min under acid solution. The most optimal molar ratio of TEOS and NH3·H2O is 1:0.036 under alkaline condition and the gel time is less than 2 min.Silica aerogels doped nano-sized TiO2 particle belong to amorphous and nano-porous network materials. TiO2 particles are physically embedded by silica aerogel and most TiO2 particles were adhered to silica aerogels.The average diameter of the pores obtained by BET analysis was about 30-50 nm. The BET surface area was in excess of 1000 m2/g for the doped TiO2 is less than 5wt%, while the specific surface area was slightly decreased to 785 m2/g for the doped TiO2 reaches 5wt%. The results showed that the SiO2 aerogel doped TiO2 can effectively scatter and adsorb the infrared electromagnetic wave between 2-8μm wavelengths, which results in the reducing infrared transmittance.Silica aerogels doped K2Ti6O13 whisker also have a characteristic of amorphous and nano-porous structure. The maximum BET surface area was 843 m2/g for the doped K2Ti6O13 whisker is 1wt%, and the specific surface area was slightly decreased compared with the doped-TiO2 aerogels. The average diameter of the pores for the doped content between 1wt% and 8wt% obtained by BET analysis was about 20-30 nm. The doped K2Ti6O13 whisker can greatly reduce the crystallization temperature, and thus the SiO2 crystalline was found at 870℃. The results showed that the SiO2 aerogel doped K2Ti6O13 whisker can also scatter and adsorb the infrared electromagnetic wave effectively between 2-8μm wavelengths, which results in the reducing infrared transmittance.The MS method was adopted to simulate and analyze the microstructural feature and interfacial energy between the SiO2 xerogels matrix and doped TiO2 particle or K2Ti6O13 whisker. The results revealed that the interfacial energy between TiO2/SiO2 is only 0.601J/m2, showing that no obvious chemical reaction on interface when the TiO2 is added into SiO2 aerogel. However, the interfacial energy between TiO2/ K2Ti6O13 whisker increased to 3.32 J/m2, although it is also attributed to weak interfacial bonding. It is noticeable that the some K+ can diffuse into the SiO2 aerogel and therefore displace its original position, which will result in interfacial reconstitution. This phenomenon will explain the reason that the SiO2 crystalline occur at relatively low temperature.The transient temperature distribution of honeycomb structure filled with and without doped TiO2 or K2Ti6O13 whisker aerogel was simulated by finite element modeling (FEM) analysis. Moreover, the modeling results were compared with the experimental equipment by oxy-acetylene heating system. The experimental and simulated results showed that the heat transfer style is mainly air convection and radiation transfer. However, the heat transfer of the doped-opacifer TiO2 or K2Ti6O13 can effectively inhibit the occurrence of the above two transferring style. The heat was mainly transferred by the small-area honeycomb wall and greatly delays the time to reach the back surface.更多还原