Geoscience Reference
In-Depth Information
done through chemical bonding, and the beads can be recovered easily after the
degradation and used again without losing the photocatalysts and also its effi-
ciency
[340,341]
.
In recent years, functionalized surfaces based on CNTs and polymers, DNA-
assisted dispersion and separation of CNTs, and some special HAp based compo-
sites
for biomedical
and optoelectronic
applications have been prepared
[342
347]
. Hydrothermal technique has been effectively used to synthesize such
nanocomposites. Similarly, the encapsulation of active molecules such as ZnO,
TiO
2
, and zeolites into CNTs has also been achieved.
10.8.5 Related Methods of Hydrothermal Processing of Materials
Modern methods of the hydrothermal processing of materials cover several other
processing techniques related to the hydrothermal method. Although, these new
methods are not used widely in the routine hydrothermal processing, they have spe-
cial applications in the processing of some selected technological materials, like
hydrothermal transformation alteration, recycling, densification, solidification,
strengthening, and sintering. The most commonly used processing techniques
are HHP, HIP, hydrothermal sintering, microwave hydrothermal, hydrothermal
leaching, hydrothermal decomposition of toxic organic materials, amongst
others. These processes have helped in the processing/treatment of materials,
increasing their mechanical properties, enhancing the yield, reaction kinetics,
and so on.
HHP and HIP
HHP is one of the good processing routes for preparing a ceramic body at relatively
low temperature (below 300
C). Yamasaki and coworkers
[348]
have used this
technique popularly to process a wide range of materials. The compression of sam-
ples under hydrothermal conditions accelerates densification of inorganic materials.
This technique is applied to solidify many kinds of materials such as glass, silicate,
titania, calcium aluminate-phosphate cement, calcium carbonate, and HAp sewage
sludge
[344
353]
, and the method is expected to provide energy-effective proces-
sing to fabricate new engineering materials. Calcium alumina reacts with phosphate
to form strong bonding. Using HHP techniques, the strength of solidified bodies
with mixtures of alumina cement or calcium aluminates, sodium phosphates, and
silica fume could be enhanced. Yanagisawa et al.
[354]
have solidified three kinds
of silica powders, crystalline silica, glass, and gel using the HHP method. With the
addition of an alkaline solution, mixtures of low-quartz and other oxides were den-
sified by this method. Reaction products of the mixtures under alkaline hydrother-
mal conditions are bonded to quartz grains. The addition of pure water-densified
silica glass powders containing network-modifying oxides. In this case, a hydrated
reaction layer was produced by the reaction of the glass particles. The silica gel
(5 g) was placed in the autoclave and compressed at 20 MPa. The autoclave was
heated to 300
C at the rate of 2
C/min, while the pressure was kept constant at
