低密度SiO₂及其复合气凝胶的制备与特性研究

【作者】 任洪波；

【导师】 张林；

【作者基本信息】 中国工程物理研究院 ， 凝聚态物理， 2010， 博士

【摘要】 二氧化硅(SiO2)及其复合气凝胶以其高孔隙率、高比表面积和低密度等特性广泛应用于惯性约束聚变(ICF)研究中。ICF靶要求气凝胶材料具有好的加工成型性能、高的环境稳定性及结构与密度均匀等特性。但是低密度气凝胶目前在制备过程中,普遍存在着溶胶-凝胶过程难于控制、材料强度低、易塌陷、难于成型以及微观结构不均匀等技术难题。本论文在较为系统地研究溶胶-凝胶动力学过程和低密度SiO2及其复合气凝胶的制备工艺的基础上,成功制备出密度1.8mg/cm3-200mg/cm3的SiO2气凝胶及其复合气凝胶块体材料,并已在ICF实验研究中得到成功地应用。通过对TEOS-乙醇体系凝胶化过程的动力学特性分析发现,凝胶化过程主要受反应物浓度即气凝胶目标密度的控制,随着反应物浓度的降低,体系的凝胶化时间呈指数增加,当气凝胶目标密度低于10mg/cm3时,体系无法凝胶。在以正硅酸乙酯(TEOS)为硅前驱体的乙醇-水为溶剂体系时,无法获得密度低于3mg/cm3的SiO2气凝胶材料。通过合成新型硅前驱体,并优化溶剂体系,即以多聚笼形烷氧基硅氧烷与八甲氧基六面体倍半硅氧烷(OMOPOSS)为前驱体,以N’N-二甲基甲酰胺(DMF)、乙腈和二甲基亚砜(DMSO)为溶剂体系,成功地制备出密度低于3mg/cm3的SiO2气凝胶材料。吸附特性及微观结构分析表明,不同前驱体制备的气凝胶具有类似的规律,即随着密度的降低,样品的比表面积呈现先增加后下降然后趋于稳定的趋势。气凝胶的孔径大部分小于50nm,主要分布在10-20nm间,具有典型的中孔特征。为提高气凝胶的生产效率和成品率,经一系列探索,开发出了一套低密度气凝胶快速制备工艺,使制备SiO2气凝胶的时间周期缩短至传统工艺的1/10。在低密度SiO2气凝胶加工成型性能研究方面,着重探讨了后处理法、原位共凝胶法等工艺参数对成品气凝胶微观结构、形貌及性能等的影响。采用凝胶后处理法和原位共凝胶法实现了气凝胶的表面疏水改性和机械力学性能改性。在后处理工艺中采用六甲基二硅胺烷为疏水改性试剂,对凝胶进行了表面处理,使产品气凝胶的抗潮解性能大为提高；而在原位共凝胶改性中,以甲基三乙氧基硅烷和TEOS作为混合前驱体,经一系列实验,成功制备出具有较高机械力学性能和环境稳定性的SiO2气凝胶材料。动态热机械性能分析结果表明,经过改性后的气凝胶材料由原来的弹脆性材料逐渐转变为弹塑性材料。上述两种改性技术途径所制备的气凝胶均具有较强的疏水特性,与水的接触角超过130°，最高可达151°，在湿度大于80%,温度为50℃的条件下,气凝胶受潮气的影响甚微,环境稳定寿命可达60天以上,其环境储存稳定性和加工特性已基本达到ICF实验用靶的需求。在SiO2掺杂复合气凝胶方面,着重开展了金属粒子掺杂、金属氧化物及金属硫化物掺杂复合气凝胶的制备工艺、结构与性能等研究。基于种子原位还原技术,探讨了Au在SiO2溶胶颗粒表面原位还原形成掺金SiO2凝胶的机理及掺杂气凝胶的性能,首次成功制备出纳米Au掺杂的SiO2气凝胶,Au掺杂重量百分比可达2.7%-4.2%；通过纳米粒子粘合法及二次凝胶化技术获得了掺杂量可达80 wt%以上Cu掺杂复合气凝胶；采用广义两步法和共水解法成功制备出复合均匀的GeO2/SiO2、Ta2O5/SiO2复合气凝胶,产品GeO2/SiO2气凝胶的典型密度为3-30mg/cm3,Ta2O5/SiO2密度为30-150mg/cm3;以常压单源化学气相沉积法开展了SiO2气凝胶表面硫化镉(CdS)薄膜沉积技术研究,成功地在气凝胶内外表面分别制备出CdS涂层,内外表面的CdS涂层具有同样的镉硫原子比率(约1：1)。更多还原

【Abstract】 Aerogel is a sort of nano porous material with many fancy characterizes, such as high porosity, high specific surface area, low density, and so on. It can be used as the catalyst and carrier, super thermal insulation material, particle trapping material, and adsorption material and target material of the inertial confinement fusion (ICF). Hence, aerogel has a promising application prospect in the fields of military, industry and civilian. In ICF experiment, aerogel with the property of low density and porosity is usually used to eliminate or decrease the fluid mechanic instability in the transportation process, homogenize the X-ray radiation, or used as an adsorption medium of liquid deuterium and tritium. Further more, aerogel, as an ICF target, should possess good processability, high environment stability and uniformity in structure and density.At present, SiO2 and its complex aerogel is one of the most used aerogels in ICF experiments. However, a lot of crucial technical problems appear widely in preparation of the low density aerogels, such as uncontrollability in sol-gel process, low mechanical strength, easy collapse in structure, poor forming and nonuniformity of microstructure. Hence, base on the investigation of relevant theories, farther improvement in preparation technology of aerogel, enhancement in material properties and reduction in production cost have been of intriguing interest in the field of aerogel study. This dissertation presents a detailed investigation in kinetics of sol-gel process and preparing process of low density SiO2 and its complex aerogels, and the block SiO2 aerogels with the densities range from 1.8 to 200mg/cm3 have been prepared successfully.In investigation of low density of SiO2 aerogels, this dissertation discussed the correlative mechanisms, sol formula and production procedure, and the low density SiO2 aerogels with good properties have been obtained. Based on the analysis results of dynamic characteristics in tetraethoxysilane (TEOS)-ethanol system, we found that gelation time increases in index as the density decreases, when the density of the target aerogel is lower than 10 mg/cm3, the system can not be gelatinized. Therefore we synthesized two kinds of novel precursors (poly ethoxyl silane, PES and octamethoxy polyhedral oligosilsesquioxane, OMOPOSS) and optimized the solvent system. Through the solvent substitution by dimethylformamide (DMF), acetonitrile (AN) and dimethyl sulfoxide (DMSO), the samples with the density lower than 3mg/cm3 was successfully achieved. In addition, in order to improve the production efficiency and yield of aerogel, a rapid preparation process based on the result of a mass of experiments was developed, and the preparing period of SiO2 aerogels was shortened to one tenth compared with that of the traditional method. The results of adsorption property and microstructure analysis indicated that aerogels prepared with different precursors have the same rule:the specific surface area of the sample presents the trend of first increase then decrease with the decrease of aerogel density, after that it tends to be steady. The hole-size of aerogel is smaller than 50 nm, and the main distribution arrangement is from 10nm to 20nm, which means the aerogel possesses the typical characteristic of mesopore.As for the research of the processability of low density SiO2 aerogel, we have investigated the effects of the microstructure, morphology and property of SiO2 aerogel by the process parameters of post-treatment method in mother liquor, sol-gel process and hydrophobical modification. We have employed post-treatment method in mother liquor and modification of in-situ toughening process to improve the hydrophobical property and the mechanical strength of SiO2 aerogel. In post-treatment process of mother liquor, we chose hexamethyldisilazane as a hydrophobic modification reagent to deal with the surface of aerogel, which lead a great improvement in anti-deliquescent property of aerogel. In the in-situ hydrophobic and toughening processes, the methyl triethoxysilane and TEOS are used as complex precursors to prepare SiO2 aerogel with excellent mechanical properties and environment stability. The analysis results of dynamic thermo-mechanical show that the SiO2 aerogel becomes elastic-plastic material from elastic-brittle material after modification process. Aerogels prepared by the two modification methods mentioned above exhibit excellent hydrophobicity. The contact angles are larger than 130°, and the largest angle is 151°. When the humidity is over 80% and the temperature is 50℃, the characteristics of aerogel would not be affected by humidity, and the environment stable period is longer than 60 days. As a conclusion, the stability and processability can meet the requirements to prepare ICF target.With respect to investigation of the doped SiO2 aerogels, this dissertation focused on the studies of the fabrication process, structure and properties of composite aerogels doped with metal particles, metallic oxides and metallic sulfide. We have investigated the principles and properties of Au-doped SiO2 aerogel which was prepared by in-situ reduction of Au on the surface of SiO2 sol by seeding method, and the Au nano particle doped SiO2 aerogel with the Au content range from 2.7% to 4.2% were firstly synthesized. Based on the same method, we prepared the Cu-doped SiO2 aerogel with the density from 90 to 110 mg/cm3, and the hole and framework sizes are all less than 100 nm. The highest doping quantity of Cu is more than 80 wt%, and the specific surface area of the corresponding complex aerogel ranges from 150 to 200m2/g. We have also investigated the preparation process of the GeO2/SiO2, Ta2O5/SiO2 composite aerogel with crosslinking co-polymerized framework by the generalized two-step and cohydrolysis method. The typical density of GeO2/SiO2 and Ta2O5/SiO2 aerogels is 3-30 mg/cm3 and 30-150mg/cm3 respectively. CdS film was deposited on the inner and external surface of SiO2 aerogel by the single-source CVD method under the atmospheric-pressure. When the flow of Ar is 1-3 L/min, CdS film is only deposited on the external surface. As the flow of Ar improves to 5L/min, there is no CdS film deposited. If the flow of Ar is controlled at 4 L/min, CdS film is deposited on the inner and external surface. The EDS results show that the thin film deposited on the inner and external surface of the SiO2 aerogel has the similar atom ratio (about 1:1) of sulfur and cadmium. The XRD results reveal that the CdS thin film deposited on the inner and external surface of SiO2 aerogel possesses the hexagonal crystal structure.更多还原