3.1.1 Sol-Gel Preparation Techniques: Advanced Ceramic
3.1.1 Sol-Gel Preparation Techniques: Advanced Ceramic
3.1.1 Sol-Gel Preparation Techniques: Advanced Ceramic
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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 (20C) 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
80C 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 (80C) 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
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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 ~80C 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
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