Aerogels and Xerogels
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- Bu səhifədəki naviqasiya:
- II. Sol-Gel Processing
- A. Inorganic Gels
- B. Chemical Differences between Silica and Other Oxides Sol-Gel Chemistry
- C. Supercritical Drying
- D. Insulati ng Propertie
- E. Chemistry of Hydrophobicity
- F . Applications
- References Livage, J.; Henry, M.; Sanchez, C. Sol-Gel Chemistry of Transition Metal Oxides. Prog. Solid State Chem. 1988
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Aerogels
Aerogels designate dried gels with a very high relative pore volume. These are versatile materials that are synthesized in a first step by low-temperature traditional sol-gel chemistry. However, while in the final step most wet gels are often dried by evaporation to produce so-called xerogels, aerogels are dried by other techniques, essentially supercritical drying. As a result, the dry samples keep the very unusual porous texture which they had in the wet stage. In general these dry solids have very low apparent densities, large specific surface areas, and in most cases they exhibit amorphous structures when examined by X-ray diffraction (XRD) methods. In addition, they are metastable from the point of view of their thermodynamic properties. Consequently, they often undertake a structural evolution by chemical transformation, when aged in a liquid medium and/or heat treated. As aerogels combine the properties of being highly divided solids with their metastable character, they can develop very attractive physical and chemical properties not achievable by other means of low temperature soft chemical synthesis. In other words, they form a new class of solids showing sophisticated potentialities for a range of applications. The particularly interesting properties of aerogels arise from the extraordinary flexibility of the sol-gel processing, coupled with original drying techniques. The wet chemistry is not basically different for making xerogels and aerogels. This common basis has been extensively detailed in recent books1 . Compared to traditional xerogels, the originality of aerogels comes from the technique used to evacuate the liquid. Conventionally, the term "aerogels" has been used to designate gels dried under supercritical conditions. More recently, materials dried by other techniques such as freeze-drying, produce materials initially known as cryogels, now also termed aerogels. However, such an extension can be argued. It can only be justified if the pores actually occupy a very high percentage of a sample volume, e.g., above 90%, although no official definition exists. All materials that can be synthesized as wet gels by the sol-gel process can then be dried by the supercritical method to obtain aerogels. New properties based upon the very open texture of these materials make them outstanding for some applications. To cite a few examples, aerogels can indeed contribute to high added value technical solutions for domestic applications such as in buildings double window glazing as transparent thermal super-insulators or for more restricted applications demanding a high degree of technology in space exploration and electronics. In any case, a striking difference appears between two classes of compounds: on one hand those wet gel monoliths which can be dried to aerogel monoliths without cracking, on the other hand those wet gels which can only be obtained as fluffy aerogel powders after supercritical drying. The former categories comprise mostly silica or the silicates where silica is a major component, organic polymeric aerogels, and carbon aerogels derived from these organic aerogels. For other oxides, monolithicity with a mechanical resistance sufficient for easy handling has not really been achieved. For instance, alumina-actually boehmite AlO(OH)-monolithic gels have been made by Yoldas.2 But these are xerogels dried by evaporation, which therefore undertake a significant contraction during drying and reach a dry specific pore volume (~60%) much lower than silica aerogel monoliths. In what follows, we therefore first summarize very briefly the chemistry of sol-gel processing, simply to stress out the main chemical differences between silica, carbon, and organic polymers on one hand, the other oxides on the other. II. Sol-Gel Processing Sol-Gel processing designates a type of solid materials synthesis procedure, performed in a liquid and at low temperature (typically T < 100 Regarding inorganic gels, that is to say mostly oxides, independent solid colloidal particles (i.e., nanoparticles with a size below a micrometer) are often formed, in a first step of the process. Each colloidal particle has a more or less densely cross-linked internal structure, as illustrated in Figure 1a. It is usually easy to maintain such particles in a dispersed state in the solvent, in which case a colloidal suspension also termed a sol is obtained. In a second step, these colloidal particles can be made to link with each other, while they are still in the solvent, so as to build a three-dimensional open grid, termed a gel (Figure 1b). Hydrophilic colloidal solutions are formed of colloidal particle that remain separated because of electrical charges on their surface and in the surrounding solution. These charges create repelling forces which prohibit coagulation of the colloidal particle. The point at which the hydrogel formation, called gelation, occurs depends on the micelle concentration, temperature, ionic strength of the solution and especially the pH. The overall process produces a reticulate, metastable polymer with an open structure in which the primary number units are held together by either chemical bonds, hydrogen bonds, dipole forces or van der Waals interactions. This framework includes almost all the solvent. The gelation time ranges from a few minutes to a 100 h and the density of the hydrogel increases with increasing concentration of the original salts in the solution and with the gelation rate. The homogeneity of the hydrogel depends on the solubility of the reagents in the solvent used, the sequence of addition of reactants, the temperature and the pH. The transformation of a sol to a gel constitutes the gelation process, and the gels which are obtained are then termed colloidal gels. On the other hand, it is also quite possible to directly form some gels from rather linear polymer formed from a precursor solution, without the intermediate occurrence of individual particles. When this occurs, the gels are termed polymeric gels. 
Due to the possibility of obtaining high-surface-area solids with homogeneous compositions and controlled porosity, sol–gel methods have been widely employed for the synthesis of micro and mesoporous materials. There are three strategies that enable control of pore size :
pyrolysis of the organic constituents that serve as micropore templates.
A. Inorganic Gels The first inorganic gels accidentally synthesized by chemists were silica gels made by Ebelmen in 1846.28 But natural gels of an organic nature are common in the living world. As an example, the eye vitrea is a natural gel. Overall, gels are materials at the border between organic, inorganic, and biological chemistry. Aerogels, which are only at the beginning of their development, are certainly among the most amazing of these new materials because they can retain, in the dry state, the very open type of network which they had in the wet state. The first precursors used in sol-gel p Overall, a model termed the partial charge model was proposed by Livage et al.,1 as a development of the electronic transfer concept developed by Sanderson.29 This model makes it possible to establish rather satisfying numerical predictions on the complexes formed in solution. It relies on the building of transition states by nucleophilic or electrophilic attacks, followed by redistribution of the electronic cloud (i.e., modification of the polarity of each bond) among all the atoms in a given transition state. As a result, the partial charge as well as the electronegativity of each atom in this transition state are modified. When a group of near neighbor atoms in the transition state reaches an added partial charge which is an integer (e.g., -1, 0, or +1, etc.), this group of atoms becomes electronically "self sufficient", and it separates as a leaving group. A leaving group can be an anion, a cation, or a neutral molecule such as H2O when the added partial charge (2 For silica gels, an important inorganic precursor was sodium metasilicate Na2SiO3, also termed water-glass, which reacts with an acid such as HCl according to reactions similar to eq 3:
As indicated by this reaction, a salt is produced, which must be eliminated by tedious dialysis. This synthesis method was first used by Kistler to produce the first aerogels.30 As water-glass is cheap, an industrial process based on this precursor was developed for some time by BASF.31 The main inconvenience of these precursors is that, most frequently, they require water as the solvent, followed by dialysis to eliminate the foreign ions. This does not give much flexibility in adjusting the hydrolysis and condensation reaction rates at the desired level, hence, to tailor the texture of a gel. The use of metallic salts as sol-gel precursors has recently seen a renewed interest with the use of organic solutions, in which an organic, slow "proton scavenger" is dissolved.32 The metallic salt must be in its hydrated form, e.g., Cr(NO3)3·9H2O. The solvent can be ethanol, and the slow proton scavenger can be propylene oxide or an epoxide (e.g., 1,2-epoxybutane, 1,2 epoxypentane, 2,3-epoxy(propyl)benzene, trimethylene oxide, glycidol, epichlorohydrin, epibromohydrin). The hydrated cation [M(H2O)N]z+ must be acidic, so that a slow deprotonation reaction occurs with the proton scavenger. The protonated scavenger undertakes an irreversible ring-opening reaction with the metal salt anion, while the solvated cation transforms to aquo-hydroxo species [M(OH)x(H2O)N-x](z-x)+ which undertake condensation reactions later on. Nice wet gel monoliths can in this way be obtained with Cr, Fe, Al, Zr, and other cations.32 The second generation of precursors, which are now largely used, are alkoxides M(OR)n. These chemicals are compounds formed by combination of a metal M with an alkoxide group OR, where R designates an alkyl group. They are characterized by the existence of M-O polar covalent bonds in their molecules. In practice, they are often available as more or less small polymerized complexes, often in solution in their parent alcohol. Their transformation is carried out in an organic solvent and water becomes a reactant added in controlled proportion. The sol-gel reactions in which they participate are hydrolysis reactions (eq 4), which lead to the replacement of OR ligands by OH ones, followed by condensation reactions similar to those occurring with metal salts. B. Chemical Differences between Silica and Other Oxides Sol-Gel Chemistry The mechanisms of hydrolysis reactions depend on nucleophilic chemical attacks on the metal M,1 which depends on the partial positive electronic charge Silicon precursors, mostly alkoxides Si(OR)4, constitute an important exception which explains that they have been much more extensively studied than precursors of other metals M. In these alkoxides, R often is an alkyl CH3 group, so that the precursor is termed tetramethoxysilane, or TMOS, or an ethyl C2H5, in which case the precursor is termed tetraethoxysilane, or TEOS. This exception comes from the fact that Si atoms carry a reduced partial positive charge, e.g., A general flowchart for a complete sol-gel process is shown in the Figure. After gelation, the wet gel can optionally be aged in its mother liquor, or in another solvent, and washed. Then it must be dried. C. Supercritical Drying The traditional drying technique, at the origin of the label "aerogel", is supercritical drying. This type of drying prevents the formation a liquid-vapor meniscus which recedes during the emptying of the pores in the wet gels. As this meniscus is due to the surface tension of the liquid, a capillary pressure gradient is built in the pore walls, which is able to collapse most part of the pore volume. To avoid this event, it is necessary to annihilate the liquid surface tension and one convenient way to do so is to transform this liquid in a supercritical fluid with a null surface tension. Supercritical evacuation of the liquid-phase impregnating the porous volume of the wet gel permits the recovery of a kind of "dry solid image" of the wet material. This drying procedure releases the porous texture quite intact by avoiding the pore collapse phenomenon. D. Insulati  ng Propertie
The above figure shows typical order of magnitude of the thermal conductivity of silica aerogel compared to other insulating materials, for 10 mm thick slabs E. Chemistry of Hydrophobicity A second quality is required in order to be used in a sustainable manner, i.e, silica aerogels must exhibit good hydrophobic properties with time on stream. One pertinent method to reach this goal is to co-gel some silicon precursor containing at least one nonpolar chemical group, such as a CH3-Si bond, with the normal silicon precursor. Hydrophobic aerogels were synthesized from methyltrimethoxysilane (MTMS) and TMOS by hydrolysis in basic conditions followed by supercritical drying in methanol. In these high-temperature drying conditions, the aerogel surface was dehydrated, and these authors found that aerogels made from more 20% MTMS would float on water and hence were hydrophobic. However, for a higher proportion of MTMS, the condensation degree of silica was reduced. As the hydrolysis and condensation rates of MTMS are much lower than those of TMOS, a quasi two-step gelation process was achieved. Indeed, as this was well shown by Hüsing et al., this particular gelation occurs in two-steps: first, the TMOS reacts and then the MTMS is grafted through the silanol surface groups of the gel giving surface Si-0-Si-CH3 functions.107 The methyl groups in MTMS could also be replaced by other types of functionalities with similar hydrophobization and gel structure results.107 The hydrophobization of silica aerogels with MTMS can be summarized by the reaction schem Thermal Insulation, Acoustic Insulation, Catalyst Support, Optical applications, Nuclear Waste Storage, Filler for paints or others, low dielectric constant materials, Batteries. References
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C). As previously mentioned, the physics and chemistry involved in sol-gel synthesis has been detailed in many review papers as well as in books.3,4 The solids which were addressed were then exclusively inorganic: mostly oxides or hydroxides, formed by chemical transformation of chemical solutes termed precursors. The solid is formed as the result of a polymerization process which involves the establishment of M-OH-M or M-O-M bridges between the metallic atoms M of the precursor molecules. Such transformations are the equivalent of the polymerization process well-known to occur in organic chemistry and which consists of the establishment of direct bonds between the carbon atoms of organic precursors. Actually, the traditional division between inorganic and organic chemistry has been such that, 
rocessing were metallic salts MXn in which a metal M is linked to some number n of anions X. In solution in aqueous solvents, these precursors are present as ionic species in which the metal atoms exist as solvated cations M[H2O]N z+. T
he reactions to form sol particles and gels comprise hydrolysis reactions which replace H2O groups by OH ones with loss of protons and, condensation reactions leading to the construction of M-OH-M "ol" bridges or M-O-M "oxo" bridges with elimination of water molecules. Several concurrent mechanisms have been proposed for these reactions. For instance, an H2O ligand can first be replaced by an OH- one by direct substitution. A proton exchange mechanism through transition hydrogen bonds according to eq 1 proposed by Livage et al.,1 similar to that occurring in the ionization of water itself, is also possible. Next, various condensation reactions can lead to the formation of ol bridges or oxo bridges, as in the reaction mechanism in eq 2.1
H + 
0.32 in Si(OEt)4. Hence, the global gelation kinetics of Si(OR)4 alkoxides can be very slow. The hydrolysis and condensation reactions of Si alkoxides therefore need to be catalyzed, either with bases which carry strong negative charges (e.g., OH-, but also strong Lewis bases such as F- ions) or with acids (e.g., H+) which carry a strong positive charge and are able to attack the O(
enser colloidal silica particles and colloidal gels. The ability to control these kinetics has important consequences regarding the adaptation of silica chemistry for a given type of aerogel application, as this is discussed in the related sections.
e.
. Applications