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。
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