High-Performance Aerogels

From Liquids to Light Solids: High-Performance Aerogels with Different Drying Technologies Materials obtained by replacing the liquid inside the pores of a gel with air while largely preserving the typical structure of the pores and network are called aerogels. Aerogels are open colloidal or polymeric network structures composed of interconnected particles or fibers exhibiting very low density and high specific surface area properties [1]. The sol-gel process, the most commonly used method in aerogel production, begins with hydrolysis of colloidal particles dispersed in a liquid, continuing with condensation. The gel consists of a sponge-like, three-dimensional solid network with pores typically filled with a liquid. When gels are prepared by hydrolysis and condensation of metal and semi-metal alkoxides or other hydrolyzable metal compounds, the pore liquid consists essentially of water and/or alcohols. The resulting wet gels are named aquagels, hydrogels, or alcogels depending on the pore liquid. When the pore liquid is replaced with air without deforming the network structure or the volume of the gel body, aerogels are obtained; when the pore liquid is removed by freeze-drying, cryogels are obtained. Xerogel is obtained by conventional drying of wet gels at elevated temperature, accompanied by accompanying shrinkage and collapse of the initially uniform gel body (Figure 1). During drying, as liquid is drawn into the gel body, it causes capillary forces acting on the pore walls, and as the liquid in the pores evaporates, shrinkage occurs in the gel body. As a result, it causes collapse of the porous networks of hydrogels or alcogels. For this reason, the development of drying methods to prepare aerogels has become important. During drying of aerogels, undesired shrinkage may occur in the gel. A material is porous if it contains cavities, channels, or voids deeper than their width. Pores can be regular or irregular, but the common situation is an irregular pore structure. Pores are obtained through cross-linking of polymer chains, agglomeration of small particles, or selective removal of elements of a solid. The physical properties and reactivity of a porous solid are affected by the type, shape, and size of the pores [2]. The evaporation of liquid from a wet gel is very complex and different stages exist. In the initial stage, the gel shrinks by the volume occupied by the liquid. The liquid moves from inside the gel body toward its surface. With shrinkage, OH groups on the inner surface approach each other and can react with one another. For example, new siloxane bridges form in SiO2 gels. As drying progresses, the gel network gradually hardens. As a result of this, pore radii decrease and surface tension in the liquid increases. The second stage of the drying process begins at the point where surface tension can no longer deform the network. The gel body cannot shrink further and becomes very solid. At this stage, the tension in the gel becomes so large that it reaches a critical point where cracking is highly likely. There are two reasons for gel network collapse. First, slower shrinkage of the network inside the gel body creates a pressure difference that causes crack formation. Second, during drying, large pores empty faster than small ones; that is, if pores of different radii exist, the surface tension created by the liquid drops more rapidly in larger pores. For this reason, cracks form from unequal tension in the walls between pores of different sizes. There are certain key parameters affecting capillary forces, and by controlling these parameters, cracking can be prevented. For example, as pore size increases, capillary force decreases in inverse proportion to pore radius. The aging step makes the gel network structure more rigid. The addition of surfactants reduces surface energy and capillary forces. Taking these parameters into account, crack-free xerogels can be obtained; however, significant shrinkage cannot be prevented. Since more is needed for aerogel production, drying methods that prevent shrinkage while preserving the pore structure of wet gels have been developed.

Supercritical Drying

Supercritical drying is the process of replacing the solvent in the gel with a liquid at supercritical conditions. Liquids used in supercritical drying have unique properties that allow them to disperse without damaging the pore structure of the aerogel. Carbon dioxide, methanol, ethanol, and other liquids behave as supercritical fluids when they reach appropriate critical temperature and pressure values (Table 1). The most commonly used supercritical fluid for drying aerogels is carbon dioxide. Carbon dioxide is frequently preferred because it reaches critical temperature and pressure values more easily than other liquids (Figure 2). One of the main advantages of supercritical drying is that it prevents the effects of surface tension found in other drying methods. In conventional drying methods such as drying at ambient pressure, surface tension can cause collapse of the aerogel structure, resulting in loss of porosity and surface area. Supercritical drying eliminates this problem by using a liquid without surface tension. Supercritical drying also provides high-level control over drying speed. By adjusting the pressure and temperature of the supercritical fluid, it is possible to control the rate at which solvent is removed from the gel. This allows production of aerogels with specific pore size and structure that can be adapted to particular applications. Despite many advantages of supercritical drying, there are some disadvantages that should be considered. One of the disadvantages of supercritical drying is that it is a rather expensive process. The equipment required for supercritical drying is complex, and its installation and operation are costly. Supercritical drying can be a slow process, especially for large or thick samples. This is because the supercritical fluid must penetrate deep into the gel to remove all solvent. For this reason, drying time requires long periods depending on the volume of the gel, and this can create a risk of damaging the aerogel structure. If the pressure or temperature of the supercritical fluid is not carefully controlled, it can cause shrinkage or collapse of the aerogel. This condition can result in loss of porosity and surface area, which could negatively affect the performance of the aerogel [5].

Freeze-Drying

Another method by which phase boundaries between liquid and gas phases can be eliminated during drying is freeze-drying. The liquid in the pores is frozen and sublimed under vacuum. To stabilize the gel network, extended aging times, replacement of the solvent with another solvent with a lower expansion coefficient and higher sublimation pressure, or addition of salts to obtain low freezing temperatures are required. The disadvantage is that crystallization of the solvent in the pores can damage the gel network structure. In such a case, cryogels are obtained only in powder form [2]. The freeze-drying method consists of three steps; freezing, primary drying, and secondary drying (Figure 3). The freezing step is the most critical step in the entire freeze-drying process. To ensure that all drying is accomplished by sublimation, voids with concentrated liquid must be avoided. Freezing temperatures are typically between -50 and -85°C. The freezing method used during freeze-drying affects the structure of the ice formed, the water vapor during primary drying, and the quality of the final dried product. It also has an effect on ice crystal morphology and size distribution. Excessive crystal size can irreversibly affect cellular membranes and consequently the tissue of the final product. The freezing rate controls the size of ice crystals and thus their porosity, which can affect the drying time of the dry layer. In general, the slower the freezing rate, the larger the size of ice particles and the lower their number and specific surface. The primary drying step is based on solid solvent sublimation. At this stage, approximately 95% of the water present in the material sublimes and while the pressure is reduced to very low levels, the temperature on the shelves is raised to facilitate solvent sublimation. It is the longest stage of the freeze-drying process. The purpose of the secondary drying stage is to remove unfrozen water molecules since ice was removed during the primary drying stage. At this stage, the temperature of the shelves is raised to higher values than required in primary drying both to facilitate removal of residual solvent and to reduce vacuum pressure [7].

Drying at Atmospheric Pressure

Aerogels are largely dried using supercritical drying methods; however, since this process requires high temperature and pressure to approach the critical point, it has certain limitations in terms of cost effectiveness, process continuity, and safety. To overcome these difficulties, drying methods at ambient pressure have been developed [9]. Atmospheric pressure drying technique for aerogel preparation is a current area with restrictive aspects but much research has been conducted. When drying aerogels under ambient conditions, the network must be strengthened to prevent collapse, because if collapse occurs, irreversible shrinkage takes place. One of the alternative solutions is to change the contact angle between the pore liquid and pore walls by modifying the inner surface and solvent exchange to minimize capillary forces. The ability to dry aerogels at atmospheric pressure makes industrial application economically feasible.   References [1] I. Smirnova and P. Gurikov, "Aerogel production: Current status, research directions, and future opportunities," J. Supercrit. Fluids, vol. 134, pp. 228–233, 2018. [2] N. Hüsing and U. Schubert, "Aerogels - Airy Materials: Chemistry, Structure, and Properties," Angew. Chemie - Int. Ed., vol. 37, pp. 22–45, 1998. [3] C. A. García-González, M. C. Camino-Rey, M. Alnaief, C. Zetzl, and I. Smirnova, "Supercritical drying of aerogels using CO2: Effect of extraction time on the end material textural properties," J. Supercrit. Fluids, vol. 66, pp. 297–306, 2012. [4] M. Perrut and E. FRANÇAIS, " Aerogel Drying," Sepatek-Hp.Com, vol. 4, pp. 129–134, 2004. [5] S. Shafi, T. Rasheed, R. Naz, S. Majeed, and M. Bilal, "Supercritical CO2 drying of pure silica aerogels: effect of drying time on textural properties of nanoporous silica aerogels," Journal of Sol-Gel Science and Technology, vol. 98, no. 3. pp. 478–486, 2021. [6] T. Błaszczyński, A. Ślosarczyk, and M. Morawski, "Synthesis of Silica Aerogel by Supercritical Drying Method," Procedia Eng., vol. 57, pp. 200–206, 2013. [7] C. Simón-Herrero, S. Caminero-Huertas, A. Romero, J. L. Valverde, and L. Sánchez-Silva, "Effects of freeze-drying conditions on aerogel properties," J. Mater. Sci., vol. 51, no. 19, pp. 8977–8985, 2016. [8] R. Ganesamoorthy, V. K. Vadivel, R. Kumar, O. S. Kushwaha, and H. Mamane, "Aerogels for water treatment: A review," J. Clean. Prod., vol. 329, p. 129713, Dec. 2021. [9] J. L. Gurav, I.-K. Jung, H.-H. Park, E. S. Kang, and D. Y. Nadargi, "Silica Aerogel: Synthesis and Applications," J. Nanomater., vol. 2010, pp. 1–11, 2010     Emine Yapıcı - Research and Development Senior Engineer Ömer Lütfü Özgül Kimyevi Maddeler İthalat İhracat ve Ticaret A.Ş.   Berat Keçeci - Research and Development Senior Engineer Ömer Lütfü Özgül Kimyevi Maddeler İthalat İhracat ve Ticaret A.Ş.   Prof. Dr. Sevil Yücel - Research and Development Advisor Department of Bioengineering - Faculty of Chemistry and Metallurgy Yıldız Teknik Üniversitesi