Geoscience Reference
In-Depth Information
easily prepared. This technique becomes handy for the solidification of radioactive
wastes
[131]
and sludge ashes
[132]
.
Corrosion is a major problem in industries and power stations. Corrosion afflicts
all metals, alloys, and even advanced ceramics. The commonly observed problems
of corrosion include exfoliation, stress-corrosion cracking, intergranular corrosion,
and dealloying. From the perspective of engineering, the prediction, monitoring, and
mitigating of the corrosion problem is highly cumbersome. Once the attack is initi-
ated, it makes localized corrosion a major problem in industries, power plants, and
reactors. In this respect, the supercritical solvents oxidation, particularly of water,
can be of great help to solve many of the problems associated with corrosion and
erosion. In these systems, the toxic and corrosive ions are treated and made to
form a solid crystalline coating of the metal surfaces as oxides or as oxychlorides.
Several aqueous and nonaqueous solvents have been tried on a variety of metals,
alloys, and ceramics, which greatly contribute toward solving the major industrial
corrosion problem. The reader can get additional information from the works in
Refs
[133
140]
.
Hydrothermal processing has become a most powerful tool, especially from
1990s, to transform various inorganic compounds and also to treat the raw materi-
als for technological applications. The hydrothermally treated raw materials
become very highly reactive as a function of solvent used in the preparation of
advanced materials. One can find an extensive literature data available on this topic
covering a wide range of oxides and hydroxides, carbosils, and so on. Similarly,
the hydrothermal transformation, for example, montmorillonite and other variety
of clays into the most useful zeolites with larger pore volumes, is a subject of
great interest, because of its efficiency and economy
[117]
. In the last couple of
years, the hydrothermal technique has been widely employed for metal interca-
lated nanocomposites preparation. Several varieties of clay minerals which have
two-dimensional layer structures
[140]
like montmorillonite, beidellite, hectorite,
saponite, nontronite, and so on, are attractive materials because of their properties;
like cation exchange and swelling
shrinking in the interlayers. They readily pro-
duce pillared clays upon hydrothermal treatment. The preparation of pillared clays
involves essentially the cation exchange with hydroxy polymeric cations followed
by dehydration and dehydroxylation of the polymeric species to prop the layers
apart with ceramic oxide pillars in the interlayers
[141]
. These pillared layers are
successfully intercalated with transitional metals. Likewise, the hydrothermal tech-
nique is very useful for stabilizing the metastable phases. Also, for example, the
water-rich natural phyllosilicates, which have lower thermal stability, can be made
more stable chemically and thermally through the replacement of (OH)
2
groups by
oxygen atoms
[142]
. By these means, several phyllosiloxides have been prepared
by many workers
[143,144]
.
All the above information on hydrothermal technology provides enough support
for the growing popularity of this technique for materials synthesis, crystal growth,
and materials processing.
Table 1.6
lists all the advantages of hydrothermal tech-
nique in comparison with other popular techniques frequently employed in the pro-
cessing of nanomaterials
[21]
.
