Geoscience Reference
In-Depth Information
Table 1.2 Different Terminologies Under Novel Solution Routes
[27]
Conventional
Hydrothermal
Solvothermal Supercritical
Hydrothermal
Related
Terminologies
Multienergy
Hydrothermal
Aqueous
solvent
Nonaqueous
solvents
Critical to
supercritical
conditions
Ammonothermal,
glycothermal,
lyothermal,
alcothermal,
carbonothermal,
etc. depending
upon the
specific solvent
used
Hydrothermal in
combination with
extra energy like
microwave,
electrochemical,
sonar,
mechanochemical,
biomolecular,
sol
gel, etc.
Refers to
conditions
above the
atmospheric
temperature
and
pressure.
Suitable for
high-quality
bulk, fine
nanocrystals
Low to high
temperature
conditions.
Suitable for
good-
quality
bulk, fine
nanocrystals
Both aqueous
and
nonaqueous
solvents
Suitable for
fine and
nanocrystals,
rapid
Spray pyrolysis
Extremely efficient
for thin films,
fine to
nanocrystals.
Epitaxy, etc.
Suitable for fine
to nanocrystals
and thin films
Very fast processing
1.3 Mineralizers
In any hydrothermal system or reaction confined to any one of the processes
described in
Section 1.1
, the role played by the solvent under the action of tempera-
ture and pressure is very important. In case of hydrothermal processing, the solvent
plays multiple roles and its role has to be understood clearly in order to work out a
mechanism for materials processing under hydrothermal conditions. It has been
interpreted in various ways by many workers. Yoshimura and Suda
[20]
have
described these processes to understand the action of solvent, for example, water
on solid substances under elevated pressure and temperature conditions. This pro-
cess is represented in
Table 1.3
and
Figure 1.1
.
Figure 1.1
shows the actions of
hydrothermal fluids on the solid substances from physical action as the P, T
medium to chemical action as the reactant. Accordingly, we can expect the solvent
to work as a medium providing the suitable environment, also it can act as an
absorbent, as a solvent and reactant. Each action has a specific role to play in
hydrothermal reactions. Depending upon the action, the application and apparatus
are selected for the hydrothermal processing of materials.
Through proper interpretation of the above-listed processes, one can easily
develop a required hydrothermal process corresponding to the material synthesis or
crystal growth or materials process using a suitable solvent to increase the solubil-
ity of the desired compound. Water is the most important solvent and it was popu-
larly used as a hydrothermal mineralizer in all the earlier experiments. However,
several compounds do not show high solubility for water even at supercritical
