Silica aerogels are one of the most widely used aerogels at present. It is composed of silica particles and has a multi-dimensional network structure with a porosity of about 90%. Silica aerogels are widely used in thermal insulation, catalytic carrier, adsorption cleaning, biomedicine and other fields.
There are many methods to prepare silica aerogel, and the technology of preparing silica aerogel varies greatly according to the different silicon source materials, gel process, hydrophobic agent and desiccant used.
The preparation method of silica aerogel with tetraethoxysilane as silicon source is mainly introduced here.
Tetraethoxysilane is the most commonly used precursor for silica preparation. Aerogel products prepared with this precursor system have high purity and good quality and are widely used.
The production process of silicon oxide aerogel can be roughly divided into several steps, such as gel, aging, drying and modification, or cross-use.
1. The gel
Its reaction equation is as follows:
Gel aging is a process of dissolution and recondensation of inhomogeneous gel particles. In the sol-gel process of silica aerogel, there are few connecting bonds between silica secondary particles, and only a few silicon-oxygen bonds hold the secondary particles together. After the dissolution and recondensation of the gel particles, the connections between the secondary particles can be increased, and larger agglomerated particles can be obtained to strengthen the aerogel skeleton. Gel aging is generally done by soaking the gel in an alcohol/water mixture of the original sol.
In the preparation process of silica aerogel, the solvents in the pores of silica aerogel should be removed, and the pore structure should be protected from destruction by capillary force to maintain the integrity of the pore structure.
Supercritical drying technology is one of the most effective methods to prevent gel rupture in the drying process. This method makes the solvent reach its critical point in the drying process through the control of pressure and temperature and completes the supercritical transition from the liquid phase to the gas phase. Because the solvent has no obvious surface tension in the drying process, the volume shrinkage or cracking caused by the surface tension of the solvent during the drying process can be avoided or reduced so that the aerogel with the original shape and structure can be prepared.
Atmospheric drying is to select a solvent with low surface tension to infiltrate silica aerogel and make the surface of aerogel appear hydrophobic through surface modification. During the drying process, the low capillary force will be generated when the solvent volatiles and the network pores of aerogel will not be destroyed, and the shrinkage effect will be minimized, and finally, the drying effect can be achieved.
Compared with supercritical drying, atmospheric drying equipment is simple and cheap, as long as the technology is mature, it can carry out continuous and large-scale production. However, when drying at atmospheric pressure, the movement of fluid in the pores will produce a capillary force of the liquid, resulting in the contraction and collapse of the aerogel structure.
4. The modified
Aerogel material itself has the shortcoming of low strength and high brittleness, in order to overcome this shortcoming, it is necessary to modify the aerogel material. At present, the most commonly used method of aerogel material modification is doping, that is, adding dopants or reinforcing/toughening materials to prepare composite aerogel materials.
There are two ways to prepare composite aerogel materials: one is to add doping materials before the gel process; The other is to prepare aerogel particles or powders, then add doping materials and binder, and make a secondary molding complex by molding or injection molding. The commonly used doping materials are glass fiber, mullite fiber, rock wool, aluminum silicate fiber and so on. The selection of doping materials mainly depends on the application purpose of aerogel composites.
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