Geoscience Reference
In-Depth Information
reaction medium is highly acidic or highly basic, very small particles along with
the large particles are formed leading to a broader size distribution. It seems that
because of redissolving of nanoparticles at very high and low pH, Ostwald repining
occurs. Therefore, in surface modification, pH of the medium, isoelectric point
(iep), and another important parameter, namely, the dissociation constant (pK
a
)of
the modifiers 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 posi-
tive charges (major) and hydroxylic groups (minor). Under these conditions, there is
no chemical reaction occurring between the modifier and the metal oxide nanoparti-
cle surface, but it is only through a strong hydrogen bonding the modifier can attach
to the nanoparticles surface. On the contrary, at higher pH than pKa, dissociation of
modifier takes place and results in a chemical reaction between dissociated part of
modifier and OH
2
from particles' surface. Thus, by dehydration reaction, the modi-
fier attaches to the surface of the particles. By considering the and chemical reac-
tions, mass balances, and charge balance in the actual system, pH and the modifier
can be fixed for most of the systems.
10.4 Hydrothermal Processing of Fine Particles
Processing of fine particles under hydrothermal conditions has been known ever
since hydrothermal technology was born. During the late-nineteenth and mid-
twentieth centuries, lots of such experiments were carried out on the synthesis of
fine particles of zeolites, clays, some silicates, and hydroxides
[21,22]
. When Barrer
reported the hydrothermal synthesis of fine particles of zeolites during 1940s, it
opened a new branch of science, namely, molecular sieve technology. During the
late 1960s and 1970s, attempts were made to synthesize fine ceramic particles, espe-
cially metal oxides using hydrothermal method. It was a most popular field of
research under hydrothermal technology
[23
26]
. A great variety of ceramic materi-
als were synthesized, and the significance of the hydrothermal technique was real-
ized in the processing of highly crystalline fine ceramic particles
[27]
. This also
showed the advantages of hydrothermal technique over other conventional techni-
ques like firing, heat treatment, molding, and hot pressing. The hydrothermal
research during 1990s marks the beginning of the work on the processing of fine
ultrafine particles with a controlled size and morphology. Today, it has evolved as
one of the most efficient methods of soft chemistry in processing the advanced mate-
rials like fine nanomaterials with a controlled size and shape. Also, evident from
Table 1.7, the hydrothermal technique is ideal for the processing of very fine pow-
ders having high purity, controlled stoichiometry, high quality, narrow particle size
distribution, controlled morphology, uniformity, less defects, dense particles, high
crystallinity, excellent reproducibility, controlling of microstructure, high reactivity/
sinterability, and so on.
Figure 10.5
shows the major differences in the products
obtained by ball milling or sintering or firing, and hydrothermal methods
[1]
.
Currently, the annual market value of electronic ceramics is over a billion dollar,
and the market for nanoparticles processing in 2002 was 120 billion dollars and is
