Environmental Engineering Reference
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molecular mixture, which ensures the atomic-scale mixing (i.e., ultra
homogenization) of the components in the sol-gel technique. h e liquid
phase for the vast majority of the oxide gels is short-chain alcohols or
water, and the solutes may be inorganic nitrates, inorganic chlorides, or a
wide variety of metal organic molecules.
Step II: h is step is the key step in the sol-gel process. It involves for-
mation of a sol and conversion of it to a gel so that it retains the chemical
homogeneity of the samples during the desiccation. Conversion to a sol
is accompanied by adjusting the activity of some species, H + and OH - and
other ions, which results in the formation of a dispersed solid phase. In
principle, the pH, ionic strength and temperature of the precursor mixture
controls the gelation of the sol. Manipulation of these parameters is an
empirical procedure and it must be worked out independently for each
composition.
Step III: h e last step of the process involves desiccation and heat treat-
ment of the gels to ceramic powders. h e particle sizes in this process have
been reported to vary from 20 to 300 nm according to the experimental
conditions.
h erefore, the most convenient process is Step II in which a wet gel will
form when the sol is cast into a mold, and the wet gel is converted into a
dense ceramic with further drying and heat treatment. A highly porous
and extremely low-density material called an aerogel is obtained if the
solvent in a wet gel is removed under a supercritical condition. Ceramic
i bers can be drawn from the sol when the viscosity of a sol is adjusted
into a proper viscosity range. Ultrai ne and uniform ceramic powders are
formed by precipitation, spray pyrolysis or emulsion techniques. Under
proper conditions, nanomaterials can be obtained. h e controlled hydro-
lysis of dif erent alkoxides, acetates, nitrates and sub-nitrates are normally
used for the preparation of transition metal-doped TiO 2 or coupled oxide
sol-gel process [97-102]. h e alkoxides Ta/Nb/Ti-ethoxides/isopropoxide/
propoxide/butoxide, etc., are used as the precursor material. Powders pro-
duced in this manner are uniform and well dispersed.
h e TiO 2 nanomaterials have been synthesized with the sol-gel method
from hydrolysis of a titanium precusor [103-119]. h is process normally
proceeds via an acid-catalyzed hydrolysis step of titanium(IV) alkoxide fol-
lowed by condensation [103-119]. h e development of Ti-O-Ti chains is
favored with low content of water, low hydrolysis rates, and excess titanium
alkoxide in the reaction mixture. h ree-dimensional polymeric skeletons
with close packing result from the development of Ti-O-Ti chains. h e
formation of Ti(OH) 4 is favored with high hydrolysis rates for a medium
amount of water. h e presence of a large quantity of Ti-OH and insui cient
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