Geoscience Reference
In-Depth Information
2
μ
m
2
μ
m
(a)
(b)
Figure 10.81 SEM fracture surface of high-density hydrothermal reaction sintered Cr
2
O
3
[368]
.
or so-called zone sintering. During fast sintering, a powder compact undergoes a
rapid high-temperature thermal treatment by being brought to a hot zone within
a very short time. In this way, the densification process (e.g., lattice diffusion)
can be enhanced while limiting the grain growth process (e.g., surface diffusion).
With proper control of time and temperature, it can result in a high-density
ceramic with a fine grain size.
Figure 10.81
shows the typical SEM micrographs
of
fast-sintered BaTiO
3
samples for 5 min at: (a) 1250
Cand(b)1300
C, of the
fast-sintered samples for 15 min at different temperatures.
Table 10.8
and
Figure 10.82
give density, grain size, and dielectric properties of the sintered
BaTiO
3
samples.
Similarly, sintering effects on HAp bioceramics have been studied in detail by
several workers
[369,370]
. Thus, hydrothermal sintering reactions find interesting
applications in ceramic processing.
Multienergy Hydrothermal Processing of Advanced Materials
Earlier in Chapter 3, Section 3.5.2, Novel Autoclaves, we have treated the equip-
ment used in the multienergy hydrothermal processing of advanced materials. The
multienergy covers the combination of hydrothermal, solvothermal reactions
in combination with microwave, ultrasonic, mechanochemical, photochemical,
electrochemical, or the presence of biomolecules, and this takes hydrothermal tech-
nology to a new direction and opens an enormous potential which is yet to be
explored by the humankind. The experimental duration is being reduced by at least
4
6 orders of magnitude which in turn makes the technique more economic.
Also it evolves a new concept like chemistry at the speed of light. This has also
led to the other concepts called Instant Hydrothermal System and Automatic
Hydrothermal Vending Machine to synthesize particles instantly.
The use of microwaves in solid-state materials processing is at least 25 years
old and is presently being employed very widely to process a large variety of inor-
ganic materials
[371
373]
. For the past few years, a number of groups have dem-
onstrated that the kinetics of organic and inorganic chemical synthesis may be
significantly accelerated using 2.45 GHz microwaves. The acceleration of chemical
reactions by microwave dielectric heating has been demonstrated for organic
