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6 Advanced Ceramic��������������������������������������������������� المرحلة الرا

Advanced Ceramic��������������������������������������������������� المرحلة الرابعة/فرع السيراميك ومواد البناء�

3.1.1 SOL–GEL PREPARATION TECHNIQUES

Having considered some of the basic physics and chemistry of the sol–gel method, we come now to some of the practical issues involved in the preparation of gels. For the production of simple oxides (e.g., SiO2), the preparation techniques are fairly straightforward. Further considerations must be taken into account for the production of complex oxides; for we must ensure that the desired chemical composition and uniformity of mixing are achieved during the sol–gel processing. Gel compositions with one type of metal cation (such as silica or alumina gel) yield simple oxides on pyrolysis and are referred to as single-component gels.

�� Multicomponent gels have compositions with more than one type of metal cation and yield complex oxides on pyrolysis. A wide range of ceramic and glass compositions have been prepared by sol–gel processing.

3.1.1.1 Particulate Gels

Single-Component Gels

For single-component gels, colloidal particles are dispersed in water and peptized with acid or base to produce a sol. Two main methods can be employed to achieve gelation: (1) removal of water from the sol by evaporation to reduce its volume or (2) changing the pH to slightly reduce the stability of the sol.

Earlier in this chapter we discussed the sol–gel processing of aqueous silicates and mentioned the preparation of SiO2 gels from fine particles made by flame oxidation. Aluminum alkoxides such as aluminum secbutoxide and aluminum isopropoxide are readily hydrolyzed by water to form hydroxides. Which hydroxide is formed depends on the conditions used in the hydrolysis. The initial hydrolysis reaction of aluminum alkoxides can be written

������������������������������������������������������������������������������������������ (3.1)

The reaction proceeds rapidly with further hydrolysis and condensation:

������������������������������������������������������������������������������������������� (3.2)

�� Assuming the formation of polymers that are not too highly cross-linked, the incorporation of n aluminum ions into the chain is given by the formula AlnOn_1(OH)(n+2)-x(OR)x. As the reaction proceeds, the number of OR groups (i.e., x) relative to n should decrease to a value that depends on the hydrolysis temperature and the concentration of OR groups in the solvent.� Hydrolysis by cold water (20�C) results in the formation of a monohydroxide that is predominantly amorphous. The structure contains a relatively high concentration of OR groups. It is believed that the presence of the OR groups is directly related to the structural disorder in the amorphous phase since their removal (e.g., by aging in the solvent) inevitably leads to conversion of the amorphous hydroxide to a crystalline hydroxide, boehmite [AlO(OH)], or bayerite [Al(OH)3]. Aging at room temperature leads to the formation of bayerite by a process involving the solution of the amorphous hydroxide and subsequent precipitation as the crystalline phase. Aging of the amorphous hydroxide above 80�C leads to rapid conversion to boehmite. Since the conversion of the amorphous hydroxide to boehmite or bayerite is accompanied by the liberation of OR groups, the rate of conversion is inhibited by the presence of alcohol in the solvent during the aging process. Hydrolysis of aluminum alkoxides by hot water (80�C) results in the formation of boehmite, which is relatively unaffected by aging. Using aluminum alkoxides as the starting material, the production of alumina by the colloidal gel route involves the following main steps:

1. Hydrolysis of the alkoxide to precipitate a hydroxide

2. Peptization of the precipitated hydroxide (e.g., by the addition of acids) to form a clear sol

3. Gelation (e.g., by evaporation of solvent)

4. Drying of the gel

5. Sintering of the dried gel

�� The formation of the sol can be a critical part of the process. While boehmite and the amorphous hydroxide prepared by cold water hydrolysis can be peptized to a clear sol, bayerite will not form a sol and its formation during hydrolysis should therefore be avoided. In addition, the nature of the acid has a significant effect on the peptization step. The results are similar when aluminum isopropoxide is used. It appears that only strong or fairly strong acids, which do not form chemical complexes (or form only very weak complexes) with aluminum ions, are effective for achieving peptization.

�� For these acids, the concentration of the acid also has an effect. Peptization requires the addition of at least 0.03 mole of acid per mole of alkoxide (followed by heating at ~80�C for a sufficient time). The amount of acid used in the peptization step also has a significant influence on the gelation of the sol and on the properties of the fabricated aluminum oxide. There is a critical acid concentration at which the volume of the gel is a minimum. For nitric acid, this critical concentration is~0.07 mol/mol of alkoxide. At this minimum volume, the gel contains an equivalent of 25 wt% of Al2O3. Deviation from the critical acid concentration, to higher or lower values, causes a sharp increase in the volume of the gel. At higher acid concentration, the gels may contain an equivalent of only 2 to 3 wt % Al2O3. Because of the large shrinkages that occur, gels containing an equivalent of less than~4 wt% of Al2O3 do not retain their integrity after drying and firing.

Multicomponent Gels

In the case of multicomponent particulate gels, a primary concern is the prevention of segregation of the individual components so that uniform mixing may be achieved. Various routes have been used for their preparation, including (1) coprecipitation of mixed oxides or hydroxides, (2) mixing of sols of different oxides or hydroxides, and (3) mixing of sols and solutions. In the co-precipitation technique, the general approach is to mix different salt solutions or alkoxide solutions to give the required composition, followed by hydrolysis with water. The precipitated material is usually referred to as a gel but unlike the gels produced from dispersions of colloidal particles, it is not normally dispersible in water.

�� The success of the method depends on controlling the concentration of the reactants and the pH and temperature of the solution to produce mixed products with the desired chemical homogeneity. When gels are formed by mixing sols of different oxides or hydroxides, the uniformity of mixing is, at best, of the order of the colloidal particle size. The chemical homogeneity will therefore be worse than that obtained in the coprecipitation method assuming ideal coprecipitation (i.e., without aggregation).

�� An example of this technique is the preparation of alumino-silicate gels, for example one with the mullite composition (3Al2O3.2SiO2) by the mixing of boehmite sol and silica sol. Although these sols can be prepared in the laboratory (by, for example, the hydrolysis of alkoxides), they are also available commercially. In the pH range of~2.5–8, the surfaces of the boehmite particles are positively charged while those of the silica particles are negatively charged. If the mixture of the two sols is gelled within this pH range, then a fairly homogeneous colloidal gel can be obtained because of the attraction and intimate contact between the oppositely charged boehmite and silica particles.

The alumino-silicate system can also be used to illustrate the third method of mixing sols and solutions. In one case, boehmite sol is mixed with a solution of TEOS in ethanol. Gelling is achieved by heating the mixture to evaporate some solvent. Alternatively, silica sol is mixed with a solution of aluminum nitrate and the mixture is gelled by heating.

3.1.1.2 Polymeric Gels

For single-component gels such as silica gel, we have considered in detail the conditions that lead to the formation of polymeric gels. Turning now to the preparation of multicomponent gels, further considerations must be taken into account. As an example, consider the formation of silica-titania glasses. A convenient starting point for the preparation of the gel is the hydrolysis and condensation of a mixed solution of a silicon alkoxide (e.g., TEOS) and a titanium alkoxide (e.g., titanium tetraethoxide). From our earlier discussion of the properties of alkoxides, we would expect the hydrolysis of the titanium alkoxide to be much faster than that of the silicon alkoxide. Uncontrolled additions of water to the mixture of the two alkoxides would lead to vigorous hydrolysis of the titanium alkoxide and the formation of precipitates that are useless for polymerization.

The problem of mismatched hydrolysis rates must therefore be considered when gels with good chemical homogeneity are required. In general, five different approaches can be used to prepare multicomponent gels:

1. Use of double alkoxides.

2. Partial hydrolysis of the slowest reacting alkoxide.

3. Use of a mixture of alkoxides and metal salts.

4. Slow addition of small amounts of water.

5. Matching the hydrolysis rates of the individual alkoxides.

Of these, methods (2), (3), and (4) are more commonly used.

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