Non-supercritical drying technologies include atmospheric drying, subcritical drying, and freeze drying. Subcritical drying is similar to supercritical drying, except that the temperature and pressure are below the critical point, and there is less research on its mechanism. Freeze-drying relies on low temperature to convert the liquid-gas interface to the solid-gas interface. The solvent is removed by sublimation, which can also avoid the adverse effects of surface tension.
The basic principle of the atmospheric drying process is to first replace the pore solution in the wet gel with one or more solvents with low surface tension and to hydrophobize the gel surface through modification to prevent excessive shrinkage deformation and structure during the drying process. damage. Studies have shown that the method of network enhancement and surface modification can reduce or eliminate the degree of aerogel fragmentation. The properties of silica aerogel obtained by reasonable atmospheric drying are basically the same as those obtained by supercritical drying process.
The key to the atmospheric drying process is the effective treatment of the wet gel before drying. Generally, it can be carried out by the following measures:
(1) Increase the skeleton strength of the gel network and replace it with a solvent with a small surface tension to reduce the destruction of capillary forces between the pores when the gel dries;
(2) Increase the pore size of the gel and make it uniform in size. During the sol-to-gel process, improve the coagulation by adding chemical additives to control drying, such as phthalamide, dimethylformamide, and glycerol. Uniformity of pores in the glue to reduce internal stress differences during drying;
(3) The surface modification of silica particles can effectively prevent the hydroxyl groups on the surface of the framework particles from shrinking when the gel is dried;
(4) The cross-linking and strengthening of the framework of the organic polymer is adopted to enhance the strength of the framework structure.