Geoscience Reference
In-Depth Information
reaction in the presence of a solvent (whether aqueous or nonaqueous) above room
temperature and at pressure greater than 1 atm in a closed system.” However, with
an ever-increasing application of hydrothermal technology in nanotechnology, the
definition of hydrothermal needs a major revision; since in nanomaterials prepara-
tion most often solution-based precursors and gels are employed to obtain high-
quality nanomaterials under hydrothermal conditions. Thus, the hydrothermal can
be defined as: “any heterogeneous or homogeneous chemical reaction in the pres-
ence of a solvent (whether aqueous or nonaqueous) above room temperature and at
pressure greater than 1 atm in a closed system.”
[21]
In addition to this nonunani-
mity, there is also a lot of confusion with regard to the very usage of the term
hydrothermal. For example, chemists prefer to use a broader term, viz., solvother-
mal, meaning any chemical reaction in the presence of a solvent in supercritical or
near-supercritical conditions
[22]
. However, this term has been introduced recently,
and in fact, the early work in this direction was carried out by geologists using
CO
2
[23]
. Several international conferences and symposia have already been orga-
nized in different countries
[24
26]
. Similarly, there are several other terms like
glycothermal, alcothermal, ammonothermal, lyothermal, carbonothermal, and so
on, depending upon the type of solvent used in such chemical reactions
[27]
.
Table 1.2
gives different terminologies under hydrothermal and related processes
involved.
However, the purpose behind using these different solvents in the chemical reac-
tions is essentially to bring down the pressure
temperature conditions. In this con-
text, Yoshimura
[28]
has proposed a new term, soft solution processing, for
processes in which the pressure and the temperature conditions reach near or just
above ambient conditions. Though this term has a broader meaning, it covers only
a portion of the hydrothermal research and refers mainly to any solution processing
at or near ambient conditions. Thus, in the present topic, the authors retain a broad-
er term, hydrothermal, throughout the text and use other terms only when such
occasion arises.
As mentioned above, under hydrothermal conditions, the reactants which are
otherwise difficult to dissolve go into solution as complexes under the action of
mineralizers or solvents; hence, one can expect the conditions of chemical transport
reactions. Therefore, some workers even define hydrothermal reactions as special
cases of chemical transport reactions. Owing to the specific physical properties,
particularly the high solvation power, high compressibility, and mass transport of
these solvents, one can also expect the occurrence of different types of reactions
like:
i. Synthesis of new phases or stabilization of new complexes.
ii. Crystal growth of several inorganic compounds.
iii. Preparation of finely divided materials and microcrystallites with well-defined size and
morphology for specific applications.
iv. In situ fabrication of materials with desired size, shape, and also dispersibility in case of
nanomaterials.
v. Leaching of ores in metal extraction.
vi. Decomposition, alteration, corrosion, and etching.
