Sol-gel technology

Sol-gel technology describes those processes where a mixture of precursors undergoes chemical reactions forming a colloidal solution, which end up with a solid network (Bergna & Roberts, 2006).

Sol-gel technology has proved to be a versatile and valuable method for production and processing of materials. Metallic, organic, inorganic and hybrid materials are examples of the precursors that can be used for this process. The end products can range from highly advanced materials to materials of general daily use. The importance of the sol–gel process arises from two main causes: 1) production of highly pure materials; 2) creation of novel valuable materials (Sakka, 2002). Fig. 2 shows a general

10 Sol-gel technology

sketch of the main most commonly used steps in the sol-gel processes. Typical sol-gel preparation starts with mixing the precursors, f. i., metal oxides, with a hydrolysis agent, and a solvent. The precursors undergo a series of hydrolysis and polycondensation reactions, which can be catalyzed using an acidic, a basic catalysts or a combination of both (two-steps). A sol colloidal solution is eventually formed, which can be considered as a dispersion of polymers or fine particles (~1-1000 nm) in a solvent. Further reactions result in connecting these fine particles. Eventually, the sol converts to a wet gel containing the solvent. Evaporation of the solvent from the wet gel results in a dry gel “xerogel” (Kaufman & Avnir, 1986), heating this dried gel to several hundred degrees results in dense material in form of films, fibers, particles or monoliths (Kumar et al., 2008; Lu et al., 2007;

Mukherjee et al., 2006).

Fig. 2: General steps involved in the processing of materials using the sol-gel technology and some possible final products structure.

For a broad number of applications, the gel porous network is the key feature for their use.

Hence, it is important to remove the solvent, residues and the unreacted chemicals from the

Precursor

Furnace Condensation Polymerization

Colloidal solution

Xerogel film

Gel

Uniform particles

Xerogel Aerogel

Dense film

Dense ceramic Ceramic fibers

Cryogel

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network in a way that preserved the internal textural properties of the gel. During solvent evaporation from the gel network the curvature of vapor-liquid interface changes. The curvature of the meniscus decreases with time (Fig. 3). As a result capillary forces take place. The pressure difference between the liquid and vapor phase can be given by Laplace‟s equation:

Fig. 3: Change in liquid-vapor meniscus radius as a function of drying time at the pore surface.

Where σ is the liquid/vapor interfacial surface tension, R is the meniscus radius and θ is the contact angle at which the liquid/vapor interface meets the solid surface. Accordingly the gel structure is subject to compression stresses. Because of the high capillary pressure induced upon solvent evaporation and the fragility of the gel structure, cracks and shrinkages are obtained. Hence, a reduction of the textural properties of the dry gel will be observed.

However, it is possible to reduce the capillary pressure induced during drying by using a solvent which has a low surface tension value (equ. 1.1). Table 1 shows the interfacial surface tension of some liquids that might be used as a solvent for the gel. By means of solvent exchange it is possible to reduce the capillary forces; using solvent with lower surface tension. However, since the capillary

R, t₁ R, t₂;t₂>t₁

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force depends also on the radius of the capillary, pore radius, which is in the nano-scale, the capillary forces induced by the lowest possible interfacial surface tension will be large enough to destroy the textural structure of the gel.

Table 1: The surface tension of some fluids (Rideal, 2007).

Fig. 4 shows the capillary forces induced by different solvents as a function of the pore radius. It can be seen that the capillary forces are reduced by using solvents with lower surface tension, however, at small pore size, the capillary forces can be as large as several thousands of bars (Weissmüller et al., 2010). Furthermore, a gradient of capillary forces is induced due to the pores size distribution, resulting in inhomogeneous distribution of the forces acting on the fragile porous gel, which leads definitely to the destruction of the gel network.

Accordingly, the gel structure can be preserved only if the capillary forces emerge during drying process are avoided. This can be achieved only if the interfacial surface tension between the phases ceased. Freeze drying and supercritical extraction of the solvent from the gel are among the most intensively investigated processes to produce intact dried gel structures. Freeze drying consists of lowering the temperature of the solvent below the crystallization temperature. The solvent is then removed as a vapor by reducing the pressure (sublimation). The product of this process is usually

Solvent σ [mN/m] T [°C]

Water 72.80 20

Acetone 25.20 20

Acetonitril 29.10 20

methanol 22.61 20

n-Hexane 18.43 20

Carbon dioxide 1.16 20

Nitrogen 6.6 -183

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called a cryogel (Jungbauer & Hahn, 2004; Kumar et al., 2003; Mukai et al., 2004; Plieva et al., 2008;

Plieva et al., 2004; Rey & May, 2004). However, many obstacles are associated with freeze drying, among them are: the slow rate of sublimation; solvent exchange maybe required; increase of the solvent volume upon crystallization, this induces stresses directed from the crust toward inside, resulting in shrinkages and breakage of the crust layers as small particles. This phenomenon explains the fact that most of freeze drying products are powders (production of monoliths is extremely difficult).

Fig. 4: Capillary pressure of different solvent at different pore sizes (assumption: θ is 0).

The other possibility to maintain the textural structure of the gel upon removal of the solvent is the supercritical drying (extraction). The resulting product of this process called aerogel.