Apart from the colloidal chemistry routes discussed in Chapter 1 to Chapter 3, the sol-gel technique has been extensively used for the manufacturing of nanophased materials for the last three decades (Pathak, et al., 1997, Lessing, 1989, Anderton, et al., 1979). Among the physical and chemical methods devised for preparation of nanoscaled materials, synthesis from atomic or molecular precursors such as the sol-gel route can give better control of particle size and homogeneity in particle distribution.
The sol-gel processing, which is based on inorganic polymerization reactions, can loosely be defined as the preparation of inorganic oxides such as glasses and ceramics by wet chemical methods (Brinker, et al., 1988; Hench, et al., 1990). The goals of sol-gel processing in general are to control the composition homogeneity and the nanostructure during the earliest stages of productions. By controlling the chemical additives and processing, the products with nanostructured grains can be attained without vacuum conditions.
This technique exhibits a number of advantages, such as the potentially higher purity and homogeneity and the lower processing temperatures, over both conventional ceramic processing and traditional glass melting, especially in preparing multicomponent systems. Many unique processing characteristics of the sol-gel route provide unique opportunities to make pure and well-controlled composition inorganic composited oxides (Armelao, et al., 1995b; Zhang, et al., 1990; Kuhn, et al., 1995; Kawamuura, et al., 1996; Sánchez, et al., 1996; Choy, et al., 1995; Ding, et al., 1996; Ishikawa, et al., 1996; De, et al., 1996; Ma, et al., 1996; Wang, et al., 1996; Licciulli, et al., 1996; Fang, et al., 1996) and organic/inorganic hybrid materials (Huang, et al., 1987, 1989; Wen, et al., 1996; Huang, et al., 1989).
In fact the interest in the sol-gel processing of inorganic ceramic and glass materials began as early as the mid-1800s with Elelman (Elelman, 1846; 1847) and Graham's (Graham, 1864) studies on the silica gels. The hydrolysis of tetraethyl orthosilicate (TEOS), Si(OC2H5)4, under acidic conditions was found to lead to SiO2 in the form of a “glass-like” materials. Due to the lack of development in drying technology, however, no considerable progress had been achieved before 1950. The renewed interest in the sol-gel method was stimulated by some pioneering work of Iler and Stober (Iler, 1955; Stober, et al., 1968) in silica chemistry, and Roy's (Roy, et al., 1954; Roy, 1956, 1969; McCarthy, et al., 1971) in ceramic oxides till 1950s and 1960s. Iler commercially developed colloidal silica powders, Du Pont's colloidal Ludox spheres. Stober et al. extended Iler's findings to show that using ammonia as a catalyst for the TEOS hydrolysis reaction could control both the morphology and size of the powders, yielding the so-called Stober spherical silica powder. Roy and co-workers synthesized a large number of novel ceramic oxides by the sol-gel method, involving Al, Si, Ti, Zr oxide etc., which could not be made using traditional ceramic powder methods.
Extended from the classical silica sol-gel processing, the metal alkoxides (Armelao, et al., 1995b) gradually became the most common choice of precursors for the preparation of inorganic oxides. Metal alkoxides tend to be volatile, and most of them seem to be soluble in organic solvents (Mehrotra, 1988). By precursor selection and careful processing, a large number of nanoscaled oxides have been successively prepared (Varnier, et al., 1994; Merkle, et al., 1998; Armelao, et al., 1995a; Ravichandran, et al., 1996). However expensive starting materials of metal alkoxides for sol-gel applications became the obstacles to the commercialization of this technique. And as for the preparation of composite oxide, the required precursors, multialkoxides, are difficult to be synthesized in most cases. It is therefore a matter of technique interest to develop a preparation technique for special precursor.
A simple powder preparation via polymeric precursors made of alpha-hydroxycarboxylic acids, such as citric, lactic, glycolic acids etc., and ethylene glycol was first investigated by Pechini at 1967 (Pechini, 1967). Some high purity dielectric materials including Mg(Sr, Ca, Pb)TiO3, Ca(Sr, Ba, Pb)ZrO3, Mg(Ca, Sr, Ba, Pb)ZrO3 were produced with proper metal salts and careful processing. This invention not only allows the production of nanoscaled powders, thin films and fibers, and provides multicomponent oxides having a homogeneous composition (Cheng, et al., 1999; Lee, et al., 1998; Oh, et al., 1997), but also overcomes the drawbacks of metal alkoxide route mentioned above. It therefore stimulated the applications of the sol-gel technique to prepare various catalysts, high-Tc superconductors, magnetic materials, etc. In the meantime the control of the film morphology opens up interesting possibilities of the development of functional filters, catalysts and sensors.
Principles of the Synthesis Technique
The sol-gel processing can control the structure of a material on a nanometer scale from the earliest stages of processing. This technique to material synthesis is based on some organometallic precursors, and the gels may form by network growth from an array of discrete particles or by formation of an interconnected 3-D network by the simultaneous hydrolysis and polycondensation of organometallic precursors. The size of the sol particles and the cross-linking between the particles depend upon some variable factors such as pH, solution composition, and temperature etc. Thus by controlling the experimental conditions, one can obtain the nanostructured target materials in the form of powder or thin film. The detailed chemical processes and experimental parameters will be discussed in the next part of this chapter.
the silica process is a classic model of the sol-gel technique;
a little different process due to their different final products. The preparation of ceramic oxides from metal alkoxide precursors is complicated compared with that of conventional glass materials from silica process.
Their processing steps involved in making the sol-gel-derived materials and the critical factors to produce nanostructured materials are discussed in detail below.
Silica Sol-Gel Processing
Step 1. Hydrolysis and polycondensation
The hydrolysis and polycondensation reactions initiate at numerous sites within the TMOS + H2O solution as mixing occurs. When sufficient interconnected bonds are formed in a region, they respond cooperatively as colloidal particles or a sol. The final size of the spherical silica particles is a function of the initial concentration of water, the type and concentration of catalyst, the type of silicon alkoxide (methyl, ethyl, pentyl, esters, etc.) and alcohol (methyl, ethyl, butyl, pentyl) mixture used, and reaction temperature (Hench, et al., 1990).
Since it is the relative rates of hydrolysis (Eq. 4.1) and condensation (Eq. 4.2) that determine the structure of the gel, it is essential to understand the kinetics of the hydrolysis and condensation reactions and the ratio of the rate constants (k H/k C). Studies (Orcel, et al., 1988) show that the shape and size of polymeric structural units are determined by the relative values of the rate constants for hydrolysis and polycondensation reactions (k H and k C), respectively. To reduce the particle size, k H should be larger than k C. Fast hydrolysis and slow condensation favor formation of linear polymers; on the other hand, slow hydrolysis and fast condensation lead to larger, bulkier, and more ramified polymers(Brinker, et al., 1982).
The dominant factor in controlling the hydrolysis rate is the electrolyte concentration (Orcel, 1987). The k H increases linearly with the concentration of H+ or H3O+ in acidic media and with the concentration of OH− in basic medium (Aellon, et al., 1950). As a result, the distribution of polysilicate species is much broader for basic conditions of hydrolysis and condensation, characteristic of branched polymers with a high cross-linking degree, whereas for acidic conditions there is a low cross-linking degree due to steric crowding (Hench, et al., 1990). Therefore base catalysis produces colloidal particles giving rise to meso- or macroporous xerogels composed of dense, non-porous particles; acid catalysis gives weakly branched polymeric sols and gels leading after drying to highly microporous xerogels with a very fine texture.
The nature of the solvent and the temperature dependence of the reaction have a “secondary” effect on k H. The k H varies in the different solvents as follows: acetonitrile>methanol>dimethylformamide>dioxane>formamide, and k H for acetonitrile is about 20 times larger than k H for formamide (Orecel, 1987; Artaki, et al., 1985). Additionally 10-fold increase in k H occurs when the temperature varies from 20° to 45.5° (Hench, et al., 1990). The nature of the alkoxy groups on the silicon atom also influences the rate constant. As a general rule, the longer and the bulkier the alkoxide group, the slower the rate constant (Schmidt, et al., 1981).
In fact, the condensation reaction and the hydrolysis reaction are not separated in time but take place simultaneously. It has been well established that the presence of H3O+ in the solution increases the rate of the hydrolysis reaction, whereas OH-ions increase the condensation reaction (Prassas, et al., 1984). As for the solvent effect, formamide decreases the hydrolysis rate but slightly increases the condensation rate. This can be attributed to the ability of HCONH2 to form hydrogen bonds and to its high dielectric constant (=100) (Orcel, et al., 1986). However, there are little data on the rate constant of the condensation reaction so far.
Step 2. Gelation
As the sol particles grow and collide, condensation occurs and macroparticles form. The sol becomes a gel when it can support a stress elastically. This is typically defined as the gelation point or gelation time, tg. The change is gradual as more and more particles become interconnected. All subsequent stages of processing depend on the initial structure of the wet gel formed in the reaction bath during gelation.
Polymerization reactions are usually thermally activated, for example, a molar solution of TEOS in methanol gels in 49 h at 4°C, and in about 0.3 h at 70°C (Mackenzie, 1986). In view of the dependence of t g on solution pH, the gelation by a destabilization of a silica sol can be either acid or base catalyzed (Sakka, et al., 1980; Debsikdar, 1986). The gelation has an S shape with the maximum around the isoelectric point of silica (pH∼2) and a minimum near pH 5–6 (Iler, 1955).
The amount of water for hydrolysis has a dramatic influence on gelation time (Colby, et al., 1986). For a R ratio (mole of water/mole of silicon alkoxide) of 2, t g is about 7 h (gelation process at 70°C) and decreases to 10 min for R = 8, generally an increase of the amount of hydrolysis water decreases the gelation time for low water contents, whereas it increases the gelation time for the higher water contents owing to the dilution effect.
The anion and solvent also play a role in the kinetics of gelation (Mackenzie, 1986). But it is difficult to separate the effect of the alkoxy group from the effect of the solvent since gelation kinetics depends on the quantity of the solvent concentration. However, the trend is the longer and the larger the solvent molecule, the longer the gelation time (Mackenzie, 1986).
However the activation barrier to aggregation increases linearly with the size of two equal particles. Thus, the rate of aggregation would decrease exponentially with their size. Smaller particles will aggregate with larger ones at a much higher rate. Thus two distributions of particles are predicted, small newly formed particles and large aggregating particles (Himmel, et al., 1987; Dwivedi, 1986).
Step 3. Drying
addition of surfactants;
hypercritical evaporation, which avoids the solid-liquid interface;
obtaining monodisperse pore sizes by controlling the rates of hydrolysis and condensation.
It has been generally accepted that there are three stages of drying (Dwivedi, 1986; Brinker, et al., 1989; Kawaguchi, et al., 1986). During the first stage of drying the decrease in volume of the gel is equal to the volume of liquid lost by evaporation. The greatest changes in volume, weight, density, and structure occur during stage 1 of drying. Stage 1 ends when shrinkage ceases. Stage 2 begins when the “critical point” is reached. The critical point occurs when the strength of the network has increased, due to the greater packing density of the solid phase, sufficient to resist further shrinkage. The third stage of drying is reached when the pores have substantially emptied and surface films along the pores can not be sustained. During this stage, there are no further dimensional changes but just a slow progressive loss of weight until equilibrium is reached (Hench, et al., 1990). It was found (Dwivedi, 1986) that 87% of the initial liquid present in the gel was removed in stage 1, 10% in stage 2, and only 3% in stage 3. And the cracking almost always occurred at some point in stage 2, where the water-air interface moves inward and was strongly dependent on the thickness of the gel. At dried thickness less than 80 μm the gel shrinkage was primarily perpendicular to the surface and cracks occurred. At 40 μm thickness no cracks occurred. At thickness >80 μm shrinkage was both radial and perpendicular and cracking was prevalent. The thickness sensitivity of cracking may be caused by the differences in the rate of diffusion of water in the transition between stages 2 and 3 (Dwivedi, 1986).
During the whole process of drying, the nanostructure of the particles can be maintained, but with pores in the gel.
As for optical applications, the gel is usually identified to prepare monoliths. Heating the porous gel at high temperature causes densification to occur. The pores are eliminated, and the density ultimately becomes equivalent to fused quartz or fused silica. The densification temperature, usually between 1000°C and 1700°C, depends considerably on the dimensions of the pore network, the connectivity of the pores, and the surface area, etc.
The surface silanol groups of silica gel have capacity to covalently bind to silica coupling agents, the silica surface can be modified with these organic groups by direct reaction, thereby the silica gel behaves as an important matrix for some applications (Gushiken, et al., 1985; Leal, et al., 1975; Vranken, et al., 1995; Nassar, et al., 1999). A common route to modify the sol-gel-derived oxide networks is the use of organosilanes containing Si—C bonds which are stable towards hydrolysis-condensation reactions (Dire, et al., 1998). When the silicon alkoxide precursor is linked by only one Si—C bond, the hydrolysis of the Si(OR)3 group leads to a cross-linked solid in which most of the organic groups are located on the surface of monodisperse particles (Nassar, et al., 1999). The preparation of functionalized silica by sol-gel process has been shown to be convenient and efficient, and the modified silica can have better mechanical properties and chemical durability.
Metal Alkoxide Method
The reason for the success of alkoxides as precursors of the sol-gel process is their facility to undergo hydrolysis because the hydrolysis is the main step in the transformation of alkoxides to oxides. All metals are capable of forming an alkoxide in which an alkyl group is bonded to the metal by means of oxygen atom. The number of alkyls bonded depends on the valence of the metal. Metal alkoxides have the general formula M(OR)x in which M is a metal with valence x and O is an oxygen that is used to bond the alkyl group, R, to the metal at each valence site. The choice of alkyl group may be made according to availability, but one must be careful because the reaction rates vary throughout the process depending on the alkyl group used.
Step 1. Synthesis of metal alkoxides
Reactions of metals with alcohols:
direct method: Li, Na, K, Cs, Sr, Ba,
with catalyst (I2, HgCl2, HgI2): Be, Mg, Al, Tl, Sc, Y;
Reactions of halides with alcohols:
direct method: B, Si, P,
ammonia method: Si, Ge, Ti, Zr, Hf, Nd, Ta, Fe, Sb, V, Ce, U, Th, Pu,
sodium alkoxide method: Ga, In, Si, Ge, Sn, Fe, As, Sb, Bi, Ti, Th, U, Se, Te, W, La, Pr, Nd, Sm, Y, Yb, Er, Gd, Ho, Ni, Cr;
Reactions of metal hydroxides and oxides with alcohols;
Alcohol interchange reactions;
Reactions of dialkylamides with alcohols.
In addition to monometal alkoxides, some heterobimetal alkoxide with the same molar ratio of two elements as target composited oxides can also be synthesized as precursors. The typical one is [MgAl2(O n BU)8] n with n ≥ 2 for the preparation of nanoscaled MgAl2O4 spinel powder (Varnier, et al., 1994).
——the percentage of ionic character of the M—O bond due to electronegativity differences between oxygen and the M element;
——the electronic effect of the alkyl (or aryl) R group on the oxygen atom; this can modify the intrinsic polarity of the M—O bond through donation or substitution;
——the formation of oligomers due to the expansion of the metal coordination sphere by means of intermolecular dative bonds with donor atoms of neighboring alkoxide groups.
Molecular association of alkoxides is an important feature, which influences many of the physical and chemical properties of these compounds and may affect the processes which lead to the sol-gel transformation. The degree of oligomerization or polymerization can directly induce differences in the processing and in the resulting final structures of multicomponent systems, for instance, affecting homogeneity at the molecular level; an indirect influence arises from changes of other parameters important for the process such as reaction kinetics and physical properties (solubility, volatility, viscosity).
The degree of association depends on the nature of the central metal atom and the alkoxy group, in some cases, it also depends on the nature of solvent and the solute concentration. In general, the dimension of molecular complexity increases with the size of the central atom and decreases with the increase in branching and bulkiness of the alkoxy group because of the steric hindrance effects (Guglielmi, et al., 1988).
If the molecular association between alkoxides can not occur, it will be difficult to prepare a multicomponent homogeneous system since different alkoxides of different elements show a wide range of reactivities toward water. This can be overcomed by the sequential hydrolysis technique in which different reactive precursors are added to the system following an appropriate sequence (Levene, et al., 1972). Another possibility is to control hydrolysis rates of highly reactive alkoxides by using chelating organic ligands such as glycols, organic acids or β-diketones. In general, the chelated complexes are more stable than non-chelated ones, so the formation of the metal complex with a multidentated ligand will decrease the hydrolysis rate.
The high volatility of some alkoxides is another important property because it allows the possibility to easily achieve pure precursors by distillation. However, it must be considered during the sol-gel synthesis to avoid unwanted changes of composition in multicomponent solutions. The volatility of alkoxides is related to their degree of molecular association. In the case of strong metal-alkoxide-metal bridges, the greater the degree of oligomerization, the lower is the volatility.
Viscosity is also affected by the chain length and branching of alkyl groups and by the degree of molecular association. It seems obvious that highly polymerized products should be more viscous. The viscosity depends strongly on the concentration, too. In those applications where viscosity becomes a particularly critical parameter, for example in thin films or fiber production, a specific characterization of the solution is necessary.
Step 2. Mixing
Most sol-gel processes are preferably accomplished in a common solvent (Van Vlack, 1985). Alcohol is the substance usually chosen as the common solvent although the addition of extra alcohol is not necessary in some systems. The choice of alcohol should be made by availability, toxicity, solubility of the possible salts used in the selected alcohol and properties such as boiling point. Some possible selections are methanol, isopropyl alcohol and 2-methoxyethanol.
Although hydrolyzing ingredients may be added during sol-gel synthesis, a more controllable and time saving method from an experimental viewpoint is to make a large amount of “stock solution”. This can be done by mixing different optimized precursors together before or after addition of the common solvent. Distillation of the unwanted products may be needed with each separate precursor or after all of the precursors have been mixed. The final solution should be stirred long enough to ensure homogeneity.
Similar to the silica sol-gel processing discussed above, hydrolysis and polycondensation reactions of metal alkoxides lead to the formation of metal oxides. The fundamental chemical process involved in this processing is influenced by several parameters which allow the control of homogeneity (or heterogeneity) and the nano- and microstructure of the derived materials.
As in silica sol-gel processing, basic conditions tend to produce both partially and totally hydrolyzed monomers which will in turn create more densely, highly branched, crosslinked polymers. A densely crosslinked structure will result in a larger pore size upon gelation. In contrast, acidic conditions tend to produce partially hydrolyzed monomers which condense into a more linear, lightly crosslinked network (Brinker, et al., 1988). Correspondingly, the particulate granular morphology can be accomplished via hydrolytic polycondensation of alkoxides under acidic conditions, whereas the honeycomb morphologies of the material are usually formed under basic conditions with ammonia.
Water may be added along with an acid or base to further tailor these pores controlling reactions. Because water is usually a by-product of a condensation reaction, an addition of excess water can inhibit condensation (Le Chatelier's principle) and therefore shift the reaction toward further hydrolysis. The effect of water/alkoxide ratio during the hydrolytic condensation of metal alkoxides on the behavior of the resultant oxides is considerable. The higher the content of the hydrolysis water, the finer the texture of matrix (Yoldas, 1986a; 1986b; 1982). Under certain hydrolysis condition, localized condensations of the particles can be prevented, resulting in clear solutions. Generally when the water/alkoxide ratio exceeds a critical concentration, localized condensation occurs. The solution turns milky, reflecting that it has become a two-phase suspension. One of the reasons for the condensation of particulates in these systems is the near-complete removal of OR groups from the molecular structure. The presence of a small critical concentration of OR groups in the molecular structure is required for these molecules to be soluble in alcohols (Yoldas, 1979).
Various factors in addition to the water/alkoxide ratio also affect the kinetics of network forming reactions. Such factors include the type of alkyl groups in the alkoxide, the host medium, molecular separation of species, catalysts, and the temperature. The alkoxides with large alkyl groups hydrolyze slowly and also diffuse slowly. Since polymerization requires partial hydrolysis and diffusion, larger alkoxides tend to produce smaller polymeric condensations; as a consequence the oxide component of their molecules is smaller. Therefore the use of the larger alkyl groups of alkoxides will be helpful for achieving nanostructured products (Yoldas, 1986a; 1986b).
The host medium of the condensation process also affects the diffusion rates. The hydrolytic condensation of alkoxides carried out in lower alcohol yields material with a higher equivalent oxide content, reflecting a greater degree of polymerization as well as the ester exchange of heavier alkyls with lighter alkyls. The catalysts dramatically influence the resultant morphologies. The hydrolysis temperature has a bearing on the morphology and the particle size. The cold-water-hydrolyzed materials appear coarse granular, but the hot-water-hydrolyzed ones have a fine and fluffy structure (Yoldas, 1986a; 1986b).
Step 3. Drying
The drying procedure is routinely performed as described in silica sol-gel processing. In some cases, the hypercritical drying is chosen to eliminate cracking due to stresses.
After drying, a porous and homogeneous aerogel can be obtained. Once the gel has been dried, a sintering step may be needed to collapse the pore structure and solidify the gel. If further crystallization is desired, the gel can then be calcinated.
Step 4. Thermal decomposition
In this step, the complex decompositions of organic precursor take place, and the organic substances added for the preparation of gels are almost completely removed, leading to amorphous powders. DTA-TGA experiments may be performed to study the decompositions of organic precursors, from which one can determine a minimal calcination temperature. This minimal temperature should be higher than the decomposition temperature of the organic substances.
Step 5. Sintering
During calcination, pore reduction occurs due to a process of particle bonding by thermal energy. The driving force behind sintering is a reduction in surface area and therefore is a reduction in surface energy of the system (Van Vlack, 1985). So the increase in particle size is always accompanied with the increase in calcination temperature although calcination at higher temperature for longer time can produce better crystallized samples.
Oxygen-rich atmospheres are usually used in calcination not just to ensure a clean burnout of organics, but also to enhance the formation of target crystal phase at lower temperature. However, the powders calcinated in oxygen-rich atmospheres show dramatic crystallite coarsening and necking on the surface of particles. This abnormal grain growth is probably due to the surface diffusion or evaporation-condensation, which can be enhanced by the more oxidizing atmosphere. On the other hand, the additional oxygen in the calcination atmosphere is of benefit only near the end of the organics-burnout stage, but not at the oxide formation stage at high temperature. Hence the oxygen can not be brought into the calcination furnace before the end of the organics burnout to prevent the undesirable ignition of the ceramic precursors. Thus the benefits of using an oxygen-air mixture during calcination should be optimized to a maximal degree.
During this period, the amorphous particles resulted from thermal decomposition process can form well crystallized products. If gels are heat-treated by successively elevating temperature at a certain heating rate, thermal decomposition of gels and crystallization of oxides take place in sequence.
Pechini processing involves the dissolution of metal salts in a mixture of a weak hydroxycarboxylic acid such as citric acid, and a polyhydroxy alcohol such as ethylene glycol. When the solution is heated at a certain temperature usually lower than 100°C, a viscous organic precursor develops, which can then be converted to the oxide by postheating. The CA-EG (citric acid-ethylene glycol) system, which involves the most features of this processing, will be discussed below.
Step 1. Formation of gel
Mixtures of reagent-grade CA and EG in various molar ratios are magnetically stirred in beakers on hot plates. A minimum amount of water (about 0.5 ml water per gram of citric acid) is blended into each sample to help dissolve CA. Three drops of nitric acid for every 100 ml of mixture is added to each mixture to catalyze the esterification between CA and EG. These well-mixed samples are then aged in a drying oven at 100°C for three days.
The reaction of polymerization is usually enhanced by making one of the reactants excess in amount, but excessive EG has been historically preferred because of its low cost and superior solubility. Actually, the esterification reaction takes place in all compositions of EG-CA mixtures, and it ceases when one of the reactants is consumed in that mixture.
A soft and porous resin intermediate is always the initial requirement for making a single-phase, fine-grain, and nonagglomerated powder. Proper control of the processing parameters is necessary. These factors include mass ratio of the organic substances to metal salts, water content in the mixture, pulverization before calcination, and the calcination conditions.
Organic substances in the ceramic precursor decompose and burn off during both charring and calcination steps. Those organic residues in charred resin intermediates can provide additional combustion heat for powder calcination. However, this uncontrollable burning results in partially sintered agglomerates. Also for economic considerations, it is preferred to cut such organic content to a minimum. Experiments for the optimization of the ratio of organic substances to metal salts are therefore needed. In general, the precursors with excess polymeric gel are first charred to a solid piece of black resin, thus fierce ignition occurs when calcined, as a result of which, hard agglomerates always form. On the other hand, the use of insufficient organic substances produces the gel in which the metal ions are dispersed insufficiently, so these resin intermediates, after calcination, form coarse powders which even show second phases in some instances.
At any temperature, the viscosities of CA-EG mixtures increase with the increase of CA content and reach a maximum at 50 mole% CA. For the CA content changing from 30 mole% to 40 mole%, the resultant viscosity can suddenly increase even two orders of magnitude. This indicates that a gelling process starts rapidly at this composition. Citric acid can not be completely dissolved in the sample containing 80 mole% CA (Lone, et al., 1992).
Proper amount of hot water in the precursory mixture can modify viscosity of the starting solution and aid in homogeneous mixing. However, excess water in a starting solution has two drawbacks. First, when the aqueous mixture is rapidly heated to the charring temperature within a preheated oven, strenuous evaporation of water will bring out some quantities of polymeric substances out of the boiling solution. Second, since the steaming and polyesterification take place simultaneously in the precursory solution, the energetic water vapor will keep breaking the newly formed, soft polymeric network, and thus a less porous resin will arise, which is unexpected for the producing of fine powders.
Steps 2 and 3. Thermal decomposition and calcination
The resultant gels of Pechini-type processing are very similar to those of alkoxide method in status, therefore the performance of the last two steps, the thermal decomposition and calcination, is also the same as the description in that foregoing part.
In some cases, however, the intermediate resin with a large volume needs to be broken, for both the most porous resin and those less optimal ones in which some aggregates might exist, to proper size for the following calcination process. It can be understood that material heating efficiency is lower when calcining a big porous foam inside a tube than when calcining an evenly spread powder bed of the same mass. As a result, the temperatures required for calcining pulverized resins are lower than those for unbroken ones, which is beneficial for making fine powders. In addition, organic substances in a ground resin are more easily burned out than in an unground resin. However, charred resins usually contain large amount of organic mass. These soft organics can work as a binder to cause dense reaggregates of resins during grinding, thus coarsened crystallines and hard particles of oxides are usually produced in their calcination step. Therefore a proper pulverization should also be optimized for the preparation of nanostructured materials if this step is necessary.
Sol-gel Thin Film Processing
Numerous unique properties originate from their microstructure composed of primary particles and pores in the nanometer scale. A growing number of magnetic, electronic and optical applications require the nanoscaled materials as thin layers on plannar substrates. A solution containing the desired oxide precursor can be applied to a substrate by spinning or dipping. Dip coating can be used in a batch process or a continuous process when the substrate is a long, flexible sheet (Seriven, 1988). The thickness of the film may be changed by controlling both the viscosity of the sol-gel and the removal rate of the substrate. Due to the nature of the dip coating process, objects of many different shapes may be used, but all external surfaces will be coated. Spin coating is more of a batch process in which one must use a rigid flat disk, plate, or a slightly curved bowl or lens (Seriven, 1988). Spin coating involves a deposition stage, followed by spin-up and spin-off stages. Spinning has recently been widely used to apply sol-gel thin films to semiconducting substrates (Dey, et al., 1988; Vest, et al., 1986).
The basic physical properties of the coating solution are viscosity, surface tension, and time-to-gel. In order for a sol-gel solution to be used in the preparation of a thin film, the viscosity must be kept within a reasonable range so that the solution can be applied onto the substrate. This implies that the solution must be hydrolyzed past the percolation threshold. The time-to-gel is especially important when it comes to coatings because film formation, drying, and creation of pores must be rapid. Optimum film formulations correspond to those solutions that lose tackiness quickly. In the TEOS system, this corresponds to stoichiometric water for complete hydrolysis. Generally a low surface tension is desirable for coating a low energy substrate. The surface tension can be adjusted by changing the solvent. To do so, low water content solutions are preferred (Klein, 1993; Melpolder, et al., 1988).
Once the thin films have been deposited, they must be dried to remove residual organics. When drying films a large volume change usually accompanies the removal of solvents. However, the shrinkage of the film produces tensile stress in the film and a compressive stress in the substrate (Scherer, 1986; 1987). If the thickness of the film is much smaller than the thickness of the substrate, the stress in the substrate may be negligible and not cause any deformation of the system. When the gel adheres strongly to the substrate, there may be practically no strain in the film-substrate plane and all of the contraction will occur along the plane normal to the film-substrate surface. In general, cracking during drying is not a problem in organometallic films of less than 0.5 micron (Hrubesh, et al., 1995). Once a thin film has been dried, annealing process is needed to obtain desired crystalline structure.
Examples of the Synthesis Process
Sol-gel approach not only allows the preparation of known materials in a new way (often combined with physical and technological advantages), but also materials with novel compositions and properties. In this part, we will describe several applications of sol-gel technique in the preparation of nanostructured materials.
Organic/Inorganic Hybrid Network Materials Based on Silica sol-gel Approach—PTMO TMOS nanocomposites (Huang, et al., 1987)
The organic/inorganic hybrid materials with pure and well-controlled composition are normally nanocomposites and have the potential for providing unique combinations of properties, which cannot be achieved by other materials (Huang, et al., 1987; 1989; Wen, et al., 1996). For the past decade, organic/inorganic nanocomposites prepared by the sol-gel process have attracted a great deal of attention, especially in the fields of ceramics, polymer chemistry, organic and inorganic chemistry, and physics.
The materials of this kind can be made through the incorporation of low molecular weight and oligomeric/polymeric organic molecules with appropriate inorganic moieties at temperature under which the organics can survive. The key for the development of hybrid materials is a suitable molecular precursor of the general type (RO)3Si-X-A, where (RO)3Si behaves as the hydrolysable moiety forming the inorganic structure during sol-gel processing, A is a (functional) organic group, and X is a chemically inert spacer permanently linking E and A.
Oligomeric poly(dimethyl siloxane) (PDMS) with silanol terminal groups was initially chosen as the species for the incorporation. In order to overcome the poor mechanical property of PDMS containing composites, hydroxyl terminated PTMO [poly(tetramethylene oxide)] with better mechanical properties can be a substitution. However, a difference in the reactivity of the end groups of PTMO and that of tetraethyl orthosilicate (TEOS) can result in poor incorporation of the oligomeric component. This can be altered often by endcapping the oligomeric or polymeric components with triethoxysilane groups. An instance of organic/inorganic nanocomposites preparation by the use of sol-gel technique will be described.
Eight ml of isopropanol and 2 ml of THF were first added to a 100 ml flask. Then 10 ml of TMOS and an appropriate amount of PTMO were added and mixed thoroughly until the solution appeared to be homogeneous. Then, a mixture of HCl and deionized water was added to the base solution under rapid agitation. The reaction usually proceeded rapidly at ambient temperature, and the system quickly turned into a viscous liquid. After approximately 30 seconds, the clear liquid was cast into Teflon coated Petri dishes and covered with parafilm. After 1–2 days, the parafilm was removed and the gel was dried under ambient conditions for another week prior to testing.
Although the exact stepwise reaction mechanisms are still unknown, the overall sol-gel reaction of the present TMOS-PTMO system can be illustrated by the simplified scheme shown below.
The resulting hybrid networks can vary from soft and flexible to brittle and hard materials and still maintain optical transparency. In addition, some other organic groups can also be incorporated into inorganic networks to achieve: modification of mechanical, optical or electrical properties, electrochemical, chemical or biochemical reactivities.
Composited Oxide Powders—MgAl2O4 spinel from a heterometallic alkoxide (Varnier, et al., 1994)
Double alkoxides are very attractive precursor for sol-gel processes (Caulton, et al., 1990) since they can directly provide the desired combination of the two metal ions in a single compound. As an instance of the applications of double alkoxides, MgAl2O4 spinel powder will be discussed.
Mg-Al spinel is a refractory oxide with good mechanical and optical properties. It is generally prepared by reactive sintering of the two component oxides or thermal decomposition of a mixture of alum (aluminum ammonium sulfate) and magnesium ammonium sulfate at temperatures higher than 1000°C (Baker, et al., 1967; Yu, et al., 1988; Bratton, 1969). MgAl2O4 spinel powder with nanostructure has been prepared by the hydrolysis-condensation reaction of a heterobimetallic aluminum magnesium n-butoxide modified by polyethylene glycol (PEG200) (Varnier, et al., 1994).
To a solution of the precursor in EtOH (87 g/l to 350 g/l), OHCH2(CH2OCH2) n CH2–OH (PEG200, n = 3.1) was added in a proportion of M=1 to 8 (M=[PEG200]/[alkoxide]), and the resultant clear solution was stirred at room temperature for 15 h.
A water/ethanol solution (5 wt% of water) was progressively added to the limpid precursor solution. Rigid and transparent gels were obtained after 6 min to 480 h with a degree of hydrolysis, h′ (h′=[H2O]/[MgAl2(O n Bu)8]) which varied from 4.5 to 7 in the temperature range of −20°C to 75°C. The optimum conditions, defining the maximum concentration of inorganic material and a minimum concentration of organic constituents, are: [MgAl2(O n Bu)8[=350 g/l; M=1; T=−20°C; h′=4.5. Gelation occurred in about 4h with a spinel concentration equivalent to 37 g/l.
Nanocrystalline Thin Films of Composited Oxides—Co and RE(rare earth)-doped Co ferrite nanocrystalline films (Cheng, et al., 1998; Yan, et al., 1998; Cheng, et al., 1999)
Cobalt spinel ferrite thin films are of great interest for magneto-optical (MO) recording because of their large Faraday rotation in the spectral range of 400–500 nm and 700–800 nm as well as their good chemical stabilities (Ahrenkiel, et al., 1975; Martens, et al., 1985; Suzuki, et al., 1988). And nanostructured MO films can lower media noise in recording applications. Therefore the sol-gel route has been used to fabricate Co and RE-doped Co ferrite nanocrystalline films with an attempt to search for new MO materials having improved recording performance.
Fe(NO3)3 · 9H2O, Co(NO3)2 · 6H2O, and RE(NO3)3 were used as raw materials. These nitrates were mixed in the required metal atomic ratio and dissolved in aqueous solution of citric acid. After the chelating reaction of 1–2 h, ethanol and polyethylene glycol (PEG20000) were added. The filtered solution was spincoated on the pretreated silicon substrates at the speed of about 4000 r/min for 8 s and the obtained films were dried in 100U–110°C for 20 min to remove the mixed solvent. They then were preheated in 400°C also for 20 min to exclude the organic substances. The operation circle of spincoating, drying and preheating was repeated to reach the required thickness of the films. Finally the sol-gel-derived precursor films were calcinated to form well-crystallized spinel films.
The coating solution is highly critical for preparing high quality films. A small volume ratio of water to ethanol is applied to decrease surface tension of coating solution. Large content of citric acid can make the resultant films loose, and probably lead to cracking of the films. For its long-chain molecular structure, PEG20000 as a surfactant can enclose the colloidal particles resulting from the reaction between metal ions and citric acid, and thereby avoid the aggregation of those colloids. Therefore the proper amount of PEG can modify the homogeneity of the coating solution. Additionally the content of PEG influences the rheological behavior of the coating solution severely. Based on the above considerations, the volume ratio of the mixed solvents water/ethanol, the concentration of citric acid and that of PEG were optimized as 1:8, 0.2 mol/l and 40–80 mg/ml, respectively, in the coating solution.
XRD determinations showed that Co ferrite film with the pure target structure, spinel phase, can be formed at 575°C and wellcrystallized above 630°C, which is much lower than the required temperature in the conventional ceramic method (Lee, et al., 1995). The relatively lower process temperature is due to the initial and uniform mixing of Co2+ and Fe3+ ions in the coating solution.
The RE ions tend to decrease the room-temperature magnetization as well as Curie point of products, and may have a certain contribution to coercive force except for nonmagnetic Lu3+ ion. For the optical structure in which MO layers are substrated on silica, some Co-RE ferrite films exhibit interesting enhancement of MO effect.
Glass/Non-Oxide Nanocomposites by sol-gel Technique—LaF3 microcrystals in sol-gel silica
Sol-gel silica is also of scientific and technological importance for the preparation of glass/nanoparticle composites with electrical or optical functions (Tanahashi, et al., 1996; Zayat, et al., 1997). Some silica/non-oxide composites such as calchogenides and halides, in spite of difficulty in promoting preferred formation of non-oxide particles in oxide matrix, have been successfully prepared by modified silica sol-gel method (Zayat, et al., 1997; Blanco, et al., 1994; Rywak, et al., 1996; Nogami, et al., 1990; Yeatman, et al., 1994; Minti, et al., 1991; Righini, et al., 1996). An approach named pore doping technique (Nogami, et al., 1990; Yeatman, et al., 1994; Minti, et al., 1991; Righini, et al., 1996) has been used to the preparation of semiconductor nanoparticles such as CdS. The porous silica is prepared by starting from TEOS, and semiconductors can be introduced into pores. The average size of the pores is controlled by the pH of the starting solution. If a metal (e.g., Cd) salt precursor had been previously added to the starting solution, one can obtain CdS nanocrystals by annealing the samples in an H2S atmosphere (Yeatman, et al., 1994). SiO2/MgF2 composites were prepared by a“two-step” sol-gel process: (1) preparation of MgF2 sol from methanolic H2O2 and Mg(OCH3)2 treated in HF and (2) the mixing of the resultant MgF2 sol and a silicon alkoxide sol.
More recently, Fujihara have developed sol-gel techniques to produce LaF3 microcrystals in sol-gel silica (Fijihara, et al., 1999) by a usual “one-step” drying/heating process. Gelation occurred after mixing the tetramethylorthosilicate(TMOS) solutions and the La/TFA (TFA, trifluoroacetic acid) solutions with nitric acid as catalyst, and the gels were then dried at temperature from 60°C to 150°C within seven days. The presence of the La/TFA in silica gels did not cause the fragmentation. The dried gels were cut into pieces and heat treated. XRD analysis showed the peaks due to LaF3 start to be observed when heating at 300°C and only the LaF3 peaks appear at temperatures between 300°C and 800°C without any other crystalline phases.
La3+ ions may participate in the polycondensation reaction and are incorporated into the gel structure (Bansal, 1990), as a result of which La3+ ion will not be segregated as LaF3 after the heat treatment. The addition of TFA can be effective to prevent the ions from being accommodated in the networks. In the La/TFA solution, the La3+-CF3COO− coordination forms and is sustained in the mixed solution with the TMOS. Because it can work as screening of the electronic charge, this coordination is still sustained in the polycondensation stage. Thus the Si—O—Si network formed during gelation is not affected by the LaF3 crystallization, which did not occur until heating stage. The lanthanide salts of TFA decompose to form LnF3 around 300°C (Rillings, et al., 1974), which is in accordance with the above XRD analysis. LaF3 grains by the normal sol-gel grew with heating temperatures; however. the particle size of LaF3 microcrystals in silica is about 10–30 nm even after heat treatment at 800°C, and the LaF3 crystals are uniformly distributed without phase separation.
Current Status of the Technique, Limitations and Prospects
All three branches of sol-gel processing have several disadvantages: high cost of raw materials, low yield and density of the products, the residual carbon, the health hazards of some organic compounds and the possible cation segregation in the thermal decomposition (alkoxide route and Pechini processing). However the mild conditions of synthesis (“soft chemistry”) with respect to other techniques have allowed it to be successfully used in producing glasses, multicomponent oxides, and amorphous and nanostructured materials so far. This technique has also provided the possibility to prepare some composite nano-materials such as organic/inorganic hybrid materials and metal nanocrystals in amorphous matrix. The processed materials almost cover all fields of functional materials such as optics, magnetism, electronics, high Tc superconductor, catalyst, energy, sensor, etc.
The sol-gel route has become a very powerful and reliable way for novel materials. It may not be exaggerating if regarding the sol-gel technique as one key of scientists, especially of chemists, to open the door to the creation of powerful new materials for innovative applications. This technique also provides chemists and physicists the opportunity to cooperate in the research field of novel materials. Hence by this disciplinary approach, much scientific and technologic progress is expected in the future. Surly speaking, in spite of several problems to overcome, it is still expected that the sol-gel technique will become a routine technique to make novel materials in the coming century.