Aerogel Supercritical Preparation Technology Introduction
2018-03-14 17:29:34 From: Tradematt-Aerogel
The aerogel preparation generally obtains the desired nanoholes and corresponding gel skeletons from the precursors through a sol-gel process, and then undergoes aging to enhance the framework structure. After drying, the liquid in the wet gel is replaced with gas to obtain aerogel.
Supercritical fluid drying (abbreviated as SCFD, also known as SD or CFD) technology is a new type of drying method developed using the special properties of supercritical fluids. It is a new chemical technology developed in recent years. The point where the interface between the liquid and gas phases disappears is called the supercritical point. Under supercritical conditions, the fluid is a supercritical fluid.
The aerogel preparation process is:
(1) The alcohol gel is placed in a supercritical fluid-dried dry pressure vessel and its temperature is lowered by a temperature controller;
(2) Open the pressure reducing valve of the CO2 cylinder and introduce CO2 from the upper part of the high pressure vessel. As the CO2 gas continues to flow into the CO2, a two-phase balance of the liquid and gas phases is reached. The lower layer is liquid CO2. The ethanol solvent in the gel can Gradually replaced by liquid CO2;
(3) At a certain rate, the liquid CO2 begins to expand gradually. The pressure first reaches the critical pressure and continues to increase. By releasing a small amount of CO2, keeping the pressure constant, it finally reaches the critical temperature selected in advance, that is, reaching the critical state;
(4) Maintaining the critical state for a certain period of time so that the liquid in the pores of the gel can be converted into a critical liquid, and then maintaining the critical temperature constant, the CO2 fluid of the drying medium can be slowly released through the exhaust valve and reach Normal pressure
(5) The system changes along the critical isotherm during the release of CO2, and the critical fluid will not be reversed to liquid, so the gel dispersed phase can be driven off without the liquid surface tension. When the temperature drops to room temperature, the system Get aerogel.
The supercritical fluid drying unit operation was first proposed by S. S. Keller, whose sole purpose was to prepare an aerogel. Supercritical fluid drying can generally be divided into three types:
(1) Drying of High-Temperature Supercritical Organic Solvents Hydrogels are not suitable for direct supercritical drying because of the high critical temperature of water, large critical pressure, and the problem of hydrogel dissolution in the supercritical state. The preparation of hydrogels using inorganic salts requires the alcohol to first displace the water of the hydrogel to obtain an alcohol gel, which is subjected to supercritical drying.
(2) Low-temperature supercritical CO2 drying The critical temperature of CO2 is close to room temperature, and it is non-toxic, non-flammable and explosive. If supercritical drying is performed using CO2 instead of organic solvent as the drying medium, it is low-temperature supercritical CO2 drying. Under operating conditions, CO2 is almost chemically inert to the backbone of the gel and belongs to a purely physical process. However, before the low-temperature supercritical CO2 is dried, there is a relatively time-consuming solvent replacement process, ie, the liquid solvent in the pores of the gel needs to be first After replacement with CO2, supercritical carbon dioxide drying was performed.
(3) Low-temperature supercritical CO2 extraction drying The liquid used in the solvent replacement process of low-temperature supercritical CO2 drying is changed to a supercritical CO2 fluid, which is the low-temperature supercritical CO2 extraction process, compared with low-temperature supercritical CO2 drying, low temperature supercritical CO2 extraction drying can further shorten the drying time and greatly reduce operating costs. However, the operating temperature and pressure of the system must be guaranteed to be above the critical point of the binary mixture during the operation. This is because the solute and CO2 are completely miscible above the critical curve and there is no surface tension.