一、设备概述
SiO2气凝胶是由胶体粒子或高聚物分子相互聚结构成的纳米多孔网络固态非晶材料,其多孔率可达到80-99.8%，比表面积可高达800-1000m2/g以上。气凝胶具有很低的密度,当前制备出的气凝胶的密度可以达到0.003g/cm3，其密度仅为空气的三倍,被称为“固体烟”。目前SiO2气凝胶的制备通常包含溶胶-凝胶和干燥两个过程,而干燥方法通常采用超临界干燥。在溶胶-凝胶过程中有机硅醇盐通过水解与缩聚反应形成具有微细网络孔洞结构的凝胶,利用超临界干燥过程则可以在保持原有结构的前提下去除凝胶内的大量液体而制得气凝胶。但超临界干燥过程需要高压设备且条件控制非常苛刻,因而气凝胶的制备成本昂贵,危险性大,限制了块状气凝胶的大规模推广应用。
SiO2气凝胶是一种新型的结构可控的孔状材料,具有多种独特的性质,例如低的折射率、低的弹性模量、低声阻抗、低热导率、强吸附性、典型的分形结构等,可被制作成声阻抗耦合材料、过滤材料、高温隔热材料等多种高性能材料,在切仑可夫探测器、催化剂及催化剂载体、宽带减反射、可充电电池、防眩光涂层、低介电常数绝缘层、超高速集成电路基片、高激光损伤阈值增透薄膜、绝热涂层等众多领域,都具有广阔的应用前景。
溶胶-凝胶过程中溶胶粒子先聚集形成一个个团簇，这些团簇不断扩大且相互连接形成网络状的大团簇，当扩展到整个容器即得到凝胶。凝胶形成后并不等于溶胶-凝胶过程结束，还要经过一系列后处理(包括:老化、防开裂、干燥等)才能得到性能独特的气凝胶。
凝胶形成后，溶液中的溶胶粒子和小凝胶团簇继续聚集粘连，从而扩展到整个凝胶络，该过程即为老化。这些老化过程是使凝胶网络变粗、变滑、总体比表面积下降，网络的孔径分布、组成网络的胶体颗粒半径的分布变窄。
在凝胶的后处理过程中将不可避免的引起凝胶表面的开裂，而导致凝胶开裂的应力主要源于毛细管压力，这种由填充于凝胶骨架孔隙中的液体的表面张力所引起的毛细管压力，使凝胶收紧重排、体积收缩。可以采用以下几种措施减少干燥过程中的开裂程度:
(1)减小溶剂的表面张力
在反应过程中经水解和缩聚形成的醇凝胶，其网络孔洞中充满的溶剂主要是水和醇，由于水的表面张力很大,因此在干燥过程中毛细管的附加压力很大，这是造成气凝胶制备过程中开裂破碎的直接原因。如果通过溶剂替换，用表面张力小的溶剂将水和醇替换出来，这些表面张力小的溶剂蒸发干燥时，附加压力将大大减小，从而降低干燥过程中的开裂。
(2)改善凝胶中孔洞的均匀性
由于有机金属化合物直接水解和缩聚得到的凝胶网络结构一般不可能形成得非常均匀，这就造成凝胶内部的孔道有粗有细，这样在同一块凝胶内部应力的不均衡往往造成凝胶在干燥过程中的开裂或粉碎。因此在溶胶-凝胶过程中添加控制干燥的化学添加剂，它能促使醇凝胶的网络孔道均匀，产生比较均匀的凝胶孔结构，从而可以减少干燥过程中凝胶破裂的可能性，缩短干燥周期。
(3)凝胶的表面修饰
如果对醇凝胶的表面进行修饰改善，调节和控制凝胶表面羟基的数量和表面电性，使凝胶骨架表面具有一定的憎水性，从而使骨架和溶剂之间的接触角增大，这样就能大大减小毛细管附加压力，进而减少干燥过程中的开裂程度。
二、主要技术参数
干燥釜：2L/30MPa，300℃
分离器1L/20MPa，85℃
总功率：5kW
I. Overview of equipment
SiO2 _ 2 aerogel is a nano-porous network solid-state amorphous material which is composed of colloidal particles or polymer molecules. Its porosity can reach 80-99.8% and its specific surface area can be as high as 800-1000m2/g or more. Aerogels have very low density, and the density of aerogels prepared at present can reach 0.003g/cm3, which is only three times that of air, so it is called "solid smoke". At present, the preparation of SiO2 _ 2 aerogels usually includes sol-gel and drying, and the drying method usually adopts supercritical drying. In the sol-gel process, silicone alkoxide forms gel with fine network pore structure through hydrolysis and polycondensation, and aerogel can be prepared by removing a large amount of liquid in the gel under the premise of maintaining the original structure by supercritical drying. However, the supercritical drying process needs high-pressure equipment and the conditions are very strict, so the preparation of aerogels is expensive and dangerous, which limits the large-scale popularization and application of bulk aerogels.
SiO2 _ 2 aerogel is a new type of porous material with controllable structure, which has many unique properties, such as low refractive index, low elastic modulus, low acoustic impedance, low thermal conductivity, strong adsorption, typical fractal structure, etc. It can be made into a variety of high-performance materials, such as acoustic impedance coupling materials, filter materials, high-temperature thermal insulation materials, and can be used in Cherenkov detectors, catalysts and catalyst carriers, broadband antireflection, rechargeable batteries, anti-glare coatings, and low-temperature insulation materials.
In the process of sol-gel, sol particles first gather to form clusters, and these clusters expand and connect with each other to form network-like clusters, and when they extend to the whole container, the gel is obtained. Gel formation does not mean the end of the sol-gel process, and aerogels with unique properties can only be obtained after a series of post-treatments (including aging, cracking prevention, drying, etc.).
After the gel is formed, the sol particles and small gel clusters in the solution continue to aggregate and adhere, thus extending to the whole gel network, which is the process of aging. These aging processes make the gel network coarsened, slippery, the overall specific surface area decreased, and the pore size distribution of the network and the radius distribution of colloidal particles forming the network narrowed.
In the post-treatment process of gel, it will inevitably lead to the cracking of gel surface, and the stress leading to gel cracking mainly comes from capillary pressure, which is caused by the surface tension of liquid filled in the pores of gel skeleton, making the gel tighten, rearrange and shrink in volume. The following measures can be taken to reduce the degree of cracking during drying:
(1) reducing the surface tension of the solvent
In the process of reaction, the alcohol gel formed by hydrolysis and polycondensation is mainly filled with water and alcohol. Because of the great surface tension of water, the additional pressure of capillary is great in the drying process, which is the direct cause of cracking and breaking in the preparation of aerogel. If water and alcohol are replaced by solvents with small surface tension, the additional pressure will be greatly reduced when these solvents with small surface tension are evaporated and dried, thus reducing the cracking during drying.
(2) improving the uniformity of holes in the gel.
Because the gel network structure obtained by direct hydrolysis and polycondensation of organometallic compounds is generally impossible to form very uniform, the pores in the gel are coarse and fine, so the uneven stress in the same gel often leads to cracking or crushing of the gel during drying. Therefore, adding chemical additives to control drying in the sol-gel process can make the network pores of alcohol gel uniform and produce a relatively uniform gel pore structure, thus reducing the possibility of gel breakage during drying and shortening the drying cycle.
(3) Surface modification of gel
If the surface of alcohol gel is modified and improved, the number of hydroxyl groups on the gel surface and the surface electrical properties are adjusted and controlled, so that the surface of the gel skeleton has certain hydrophobicity, and the contact angle between the skeleton and the solvent is increased, so that the additional pressure of the capillary can be greatly reduced, and the cracking degree in the drying process can be reduced.
Second, the main technical parameters
Drying kettle: 2L/30MPa, 300℃
Separator 1L/20MPa, 85℃
Total power: 5kW
南通立凯机电工程有限公司专业设计、制造、销售超临界流体设备和石油科研仪器。产品主要包括：
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