Drying is a critical process in fabricating low-density oxides from sol-gel alcogels as it ensures the integrity and improved performance of the resulting materials. However, drying at ambient conditions is accompanied by several challenges such as shrinkage which are attributed to capillary pressure. This makes it challenging to fabricate low-density materials. Typically, the capillary pressure is significantly influenced by the liquid’s surface tension, which exhibits a nonlinear relationship with temperature. The surface tension also approaches zero at critical temperature. Due to this, supercritical drying has been effectively used to address these challenges. However, supercritical drying necessitate many solvent exchange steps to eliminate the residual water, which is time-consuming. Additionally, supercritical drying requires expensive apparatus. Therefore, developing effective methods for producing silica materials at ambient conditions is a popular topic that has drawn significant research attention.
Over the years, numerous methods have been developed to minimize or prevent shrinkage and cracking during drying processes. They include applying drying control chemical additives such as propylene carbonate to achieve a lowpressure gradient and a complete solvent exchange. Consequently, other methods such as hydrophobization of the gel matrix and the addition of precursor alkoxides also reduce cracking and shrinkage. These approaches result in hydrophobic microporous and mesoporous silica materials with low bulk density as the problems associated with the drying of alcogels are minimized. Unlike liquids in bulk volume, the physical properties of those confined in porous media can be easily altered due to the possible pressure enhancement in the confined phase, allowing processes to occur that normally require high pressures in bulk phases. Moreover, previous research reveals that changes in the bulk pressure can result in the amplified in-pore pressures and also the properties of liquids in nanopores can significantly differ from the properties in bulk volume.
Researchers from the University of Tartu in Estonia: Associate Professor Martin Timusk, reasearchers Triin Kangur and Martin Jarvekülg, in collaboration with Professor Janis Locs and Associate Professor Andris Sutka from Riga Technical University, proposed a method for producing low-density silica from sol-gel alcogels at ambient conditions. This approach is different from previously reported supercritical drying and is based on the combustion of sol-gel derived alcogels. Specifically, the authors experimentally demonstrated the effectiveness of combining the nanoporous nature of the sol-gel alcogels and temperature-dependent surface tension of alcohols to prepare low-density silica materials in a simple and more feasible manner without drying, hydrophobization or using chemical additives. Their work is currently published in the research journal, Microporous and Mesoporous Materials.
Results showed that the combustion of sol-gel derived alcogel created appropriate temperatures and pressures in gel pores that significantly reduced the liquid’s surface tension to obtain granular silica with remarkably lower density than that of xerogels obtained at room temperature. The approach also allowed for obtaining such properties despite the high annealing temperatures associated with combustion. Furthermore, low-density SiO2 granules with uniform sizes, average pore size, high porosity and specific surface area were obtained.
In summary, the authors demonstrated the combustibility of sol-gel derived silica alcogels. Confining alcohol on the silica alcogel pores allowed combustion to obtain the desired conditions, reduced surface tension, and produce low-density silica with a density significantly lower than that of xerogels, approaching the density range of aerogels produced by using supercritical drying. In a statement to Advances in Engineering, first author, Associate Professor Martin Timusk stated that the importance of the study resides in a conceptually new pathway for processing inorganic alcogels for obtaining low-density porous materials, with an insight to bring together the internal nanoporous structure of the starting material and externally imposed combustion to reach desired drying conditions otherwise impossible at ambient pressure.
Timusk, M., Kangur, T., Locs, J., Šutka, A., & Järvekülg, M. (2021). Aerogel-like silica powders by combustion of sol-gel derived alcogels. Microporous and Mesoporous Materials, 315, 110895.