超临界CO2干燥技术可以有效地消除纳米材料孔隙喉道内液体聚集的毛管压力效应, 能制得具有较好分散性的超细孔隙结构气凝胶。随着研究工作的深入, 应用超临界CO2流体干燥技术制备气凝胶已经具备很好的发展前景。超临界CO2 萃取干燥法是超临界萃取技术和超临界流体干燥技术的结合先进技术。
一、设备概述
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：5L/30MPa，300℃
分离器2L/20MPa，85℃
制冷系统：5100kcal/h，风冷，进口核心部件
储罐：4L/16MPa
高压输送泵：50L/40MPa
总功率：15kW
Supercritical CO2 drying technology can effectively eliminate capillary pressure effect in the pore throat of nanomaterials, and can produce ultrafine pore structure aerogels with good dispersibility. With the development of research work, the preparation of aerogels by supercritical CO2 fluid drying technology has good prospects. Supercritical CO2 extraction drying is an advanced technology combining supercritical extraction technology and supercritical fluid drying technology.
1、 Equipment Overview
SiO2 aerogel is a new type of porous material with controllable structure. It has many unique properties, such as low refractive index, low elastic modulus, low acoustic impedance, low thermal conductivity, strong adsorption and typical fractal structure. It can be made into many high-performance materials such as acoustic impedance coupling material, filter material, high temperature insulation material and so on. Catalyst and catalyst support, broadband antireflection, rechargeable battery, anti glare coating, low dielectric constant insulating layer, ultra-high speed integrated circuit substrate, high laser damage threshold antireflection film, thermal insulation coating and many other fields have broad application prospects.
During sol-gel process, the sol particles first aggregate into clusters, and these clusters expand and connect to form large clusters of networks. Gel formation is not equal to the end of the sol gel process, but also after a series of postprocessing (including aging, cracking prevention, drying, etc.), the unique aerogels can be obtained.
After the gel is formed, the sol particles and small gel clusters in the solution continue to adhere and expand to the whole gel network, which is called aging. These aging processes make the gel network coarser, slippery, and the overall specific surface area decreases. The pore size distribution of the network and the distribution of colloidal particle radius of the network are narrowed.
The postprocessing of gel will inevitably cause cracking on the gel surface, and the stress that causes the gel cracking is mainly from capillary pressure. The capillary pressure caused by the surface tension of the liquid filled in the gel skeleton pores will make the gel tighten up rearrangement and volume contraction. The following measures can be taken to reduce the degree of cracking during drying:
(1) Reduce the surface tension of the solvent
In the process of hydrolysis and condensation, the pores in the gels are mainly water and alcohol. Due to the large surface tension of the water, the additional pressure of the capillary is very large during drying, which is the direct reason for the cracking and fragmentation of the aerogels. If water and alcohol are replaced by solvents with low surface tension through solvent replacement, the additional pressure will be greatly reduced during evaporation and drying of these solvents with low surface tension, so as to reduce the cracking during drying.
(2) improving the uniformity of the pores in the gel.
Due to the direct hydrolysis and condensation of organometallic compounds, the gel network structure can not be formed very uniformly, which results in coarse and fine pores in the gel, so that the stress imbalance in the same gel often causes the cracking or crushing of the gel in drying process. Therefore, adding controlled dry chemical additives in sol-gel process can promote the uniform network pore size of the alcohol gel and produce a more uniform gel pore structure, thereby reducing the possibility of gel breaking in drying process and shortening the drying cycle.
(3) surface modification of gelatin
If the surface of the alcohol gel is modified, the number and surface electrical conductivity of the gel surface can be adjusted and controlled, so that the hydrophobic surface of the gel skeleton will have a certain degree of hydrophobicity, so that the contact angle between the skeleton and the solvent will increase. This will greatly reduce the additional pressure of the capillary and reduce the degree of cracking in the drying process.
2、 Main technical parameters
Drying kettle: 5L / 30MPa, 300 ℃
Separator 2L / 20MPa, 85 ℃
Refrigeration system: 5100kcal / h, air cooling, imported core components
Storage tank: 4L / 16MPa
High pressure transfer pump: 50L / 40MPa
Total power: 15kw
