Geoscience Reference
In-Depth Information
are discussed in great detail in Chapter 3. The knowledge acquired through the use
of several new mineralizers has helped to implement this hydrothermal technique
as an effective one in preparative chemistry.
Table 1.4
shows the use of hydrother-
mal processing in various fields of materials synthesis, crystal growth, and materi-
als processing
[20]
. Before going into the details of this, it is appropriate to discuss
the natural hydrothermal systems.
1.4 Surfactants
These are usually organic fatty acids, usually amphiphilic, containing a blend of
surface active agent that is both hydrophobic and hydrophilic. The alignment and
aggregation of surfactant molecules at the surface acts to alter the surface proper-
ties of water at the water
oil interface. The use of surfactants, che-
lates, and other organic cappings is becoming very popular in the processing of
advanced materials to control the nucleation of a desired phase, such that phase
homogeneity, size, shape, and dispersibility could be achieved during the crystalli-
zation of the fine particles. The surfaces of the materials could be altered to hydro-
phobic or hydrophilic depending upon the applications
[30]
. Hence, surfactants are
also called surface modifiers. Today, this approach is playing a key role in nano-
technology to prepare highly dispersed, oriented, and self-assembled particles of
even complex and multicomponent materials. Also, in crystal growth, the use of
surfactants helps in modifying the crystal size and morphology. Thermodynamics
of the surfactant systems are of great importance, theoretically and practically
[31]
.
This is because surfactant systems represent systems between ordered and disor-
dered states of matter. Surfactant solutions may contain an ordered phase (micelles)
and a disordered phase (free surfactant molecules and/or ions in the solution). In
surface modification, the pH of the medium, isoelectric point (iep), and dissociation
constant (pK
a
) of the surfactants are very important. At pH below pK
a
, the modifier
does not dissociate. Moreover, below iep, the surface of metal oxide nanoparticles
is surrounded by positive charges (major) and hydroxylic groups (minor). Under
air or water
Table 1.4 Development of Hydrothermal Processing
1. Crystal synthesis and growth:
Oxide, sulfide, fluoride
...
(1978
)
2. Preparation of fine to nanocrystals with
controlled composition, size, and shape:
PZT, ZrO
2
, BaTiO
3
, HAp, metal oxides,
ferrite (1978
)
3. Crystallized thin/thick films:
BaTiO
3
, SrTiO
3
, LiNbO
3
(1989
)
4. Etching and corrosion:
Oxide, nitride, and carbide
5. Polishing and machining:
Oxide, nitride, and carbide
6. Combined with electrical, photo-, radio-, and
mechanoprocessing:
Synthesis, modification, and coating
=
;
7.
Organic and biomaterials
Nonaqueous solution
Continuous system
Hydrolysis and extraction
8.
:
Polymerization and synthesis
9.
Decomposition and wet combustion
