A series of progress in silica aerogels made by our group

As a typical representative of the large family of aerogels, silica aerogel has far surpassed other types of aerogels, such as carbon aerogels, graphene aerogels, polymer aerogels, etc., and has taken the lead in realizing industrial production. It has extremely wide application value in high-tech fields such as aerospace, national defense, and civil fields such as construction and industrial pipeline insulation. However, silica aerogels still face problems such as long production cycle, high production cost, weak mechanical strength, and lack of functionalization. In order to solve the above problems, our team through precursor design, sol-gel chemical process control, and gelation parameter adjustment methods to optimize and coordinate, and achieved a series of important progress:


First, an atmospheric drying method for functional silica composite aerogel preparation was developed by add functionalized nanoparticles into the organopolysiloxane precursor and then subjected to gelation, aging, solvent replacement, and hydrophobic modification process. (RSC Adv. 2014, 4, 51146). In order to further shorten the preparation time and reduce the amount of organic solvent, we mix the solvent with low surface tension with the precursor before gel. By this simple adjustment, a large amount of organic solvent is saved (>1000%) due to the solvent replacement process is omitted, and shorten the preparation time of silica aerogel to less than 2 hours (Micropor. Mesopor. Mater. 2015, 218, 192), which has become the fastest record for preparing silica aerogel till now. However, the above techniques still need to consume a certain amount of organic reagents in the hydrophobization modification process. Adding the hydrophobizing agent and the low surface tension solvent to the precursor before the gel is expected to further simplify the hydrophobizing step. Therefore, our research group selected a suitable hydrophobizing agent, mixed the low surface tension solvent and the hydrophobizing agent with the precursor, and then gelled, which can be directly dried to prepare high-performance silica aerogels, and successfully realized the aerogel Super-hydrophobic modification of the surface of living bodies. (J. Mater. Chem. A 2016, 4, 11408). Figure 1 shows the simplified methods of the above work, and Figure 2 shows the superhydrophobic modification of various interfaces by silica aerogel.

Figure 1. Schematic diagram of atmospheric drying preparation method of silica aerogel and continuous optimization method


 

Figure 2. Hydrophobic silica aerogel

   

Secondly, the extremely high crosslinking density and the fragile interactions between silicon oxide primary particles lead to poor mechanical properties of silica aerogels, and extremely fragile and difficult to secondary processing. The traditional reinforcement method is to increase the interaction between the primary particles through the aging process or the introduction of organic crosslinking agents and polymers. Our group used molecular structure design to reduce the molecular-level crosslink density, that is, the introduction of organic carbon long chains between trifunctional siloxanes, and successfully prepared a series of highly elastic, autocatalytic silica aerogels (RSC Adv. 2015), 5, 91407). These aerogels can be compressed by more than 90%, and the absorption of dichloromethane can reach more than 2200%; Further, use equal molar isocyanate propyltriethoxysilane (ICPTES) and 3- Aminopropyltriethoxysilane (APTES) as silicon source, the completely symmetric structure of bridged silane was prepared, and then through processes such as hydrolysis and polycondensation, a silica aerogel with a Young's modulus of 34MPa is finally obtained. The surface of the aerogel quickly shrinks and collapses when it encounters water. Based on this, the water sculpture of aerogel is proposed for the first time, and various characters and graphics are carved on the surface of the aerogel with water as a "carving knife". Related work was published in New J. Chem. 2017, 41, 1953.


Finally, silica aerogel microspheres have important applications in the fields of energy devices, sensing, medicine, and chromatographic separation. Most of the traditional silica aerogel microspheres are prepared by using surfactants to form an oil-in-water or water-in-oil structure. However, the size of the aerogel microspheres prepared by this method is uncontrollable and irregular, and it is easily affected by many factors such as stirring speed, temperature, and shape of the reactor. In addition, the removal of surfactants is also extremely difficult and requires a large amount of reagents, which increases costs and puts great pressure on the environment. For this reason, through long-term exploration, our team have found that the CS silicone oil can form a regular shape and controlled particle size silica gel microparticles by introduction of ammonia gas in n-heptane solution. After atmospheric drying, the obtained aerogel microspheres possess bulk density of 62-230 mg/cm3, a specific surface area of ​​800-960 m2/g, a statistical average particle size of 0.8-1.5μm. Through comparative experiments, we proposed a completely different microsphere formation mechanism from emulsion polymerization. After the ammonia gas diffuses in the sol, the sol forms primary particles. Due to the presence of n-heptane, the fusion between the gel particles is inhibited, and the growth of the primary particles dominates to form aerogel microspheres. Related work has been published online in Journal of Colloid & Interface Science 2018, 515, 1-9, and served as the cover highlight of Volume 515.


Figure 3. Schematic of in-situ formation mechanism of silica aerogel