Geoscience Reference
In-Depth Information
studies laid a firm foundation for the growth of some of the most complex inor-
ganic single crystals which did not have analogues in nature. During 1960s, there
was a question about the search and growth of hitherto unknown compounds of
photosemiconductors, ferromagnets, lasers, piezoelectrics, and ferrielectrics, so the
hydrothermal method gained greater attention. This also led to the studies concern-
ing the solubility, solvent
solute interaction, kinetics, and thermodynamic aspects
of crystal growth. A large number of groups appeared in Europe, Asia, and North
America, and also the number of crystals obtained by hydrothermal method
increased exponentially. Further, the studies concerning the phase equilibria
paved a way for the emergence of a new technology, viz., ceramics processing
technology
[111]
.
From the mid-1970s, exploration of the advantages of hydrothermal reactions
other than the hydrometallurgical and crystal growth aspects began in Japan, partic-
ularly with reference to ceramic powder processing. A team of researchers from
the Tokyo Institute of Technology, Japan, did pioneering work in ceramic proces-
sing such as powder preparation, reaction sintering, hot isostatic processing, and so
on. Powder prepared by the hydrothermal technique is not agglomerated, but is
fine-grained, highly pure, with controlled morphology, has a narrow size distribu-
tion, and consists of single crystals. The technique has several advantages like high
reaction rate of powders, good dispersion in liquid, almost pollution free, does not
require very expensive and highly sophisticated equipment, energy-saving process,
and many times it produces new phases. These aspects are discussed in the subse-
quent chapters of this topic.
In the last decade, the hydrothermal technique has offered several new advan-
tages like new homogeneous precipitation using metal chelates under hydrothermal
conditions, decomposition of hazardous and/or refractory chemical substances,
monomerization of high polymers like polyethylene terephthalate, and a host of
other environmental engineering and chemical engineering issues dealing with the
recycling of rubbers and plastics instead of burning, and so on. The solvation prop-
erties of supercritical solvents are being extensively used for detoxifying organic
and pharmaceutical wastes and also for replacing toxic solvents commonly used for
chemical synthesis. Similarly, they are used to remove caffeine and other food-
related compounds selectively. In fact, a new term, hydrothermal cooking, is being
used by the food and nutrition experts in recent years
[112]
. These unique proper-
ties take the hydrothermal technique altogether in a new direction toward the
twenty-first century and one can forecast a slow emergence of a new branch of sci-
ence and technology for sustained human development. An understanding of the
structure, dynamics, and reactivity of water and other aqueous electrolyte solutions
with the advancement in instrumentation like neutron diffraction, X-ray, synchro-
tron scattering and diffraction, Raman and NMR (Nuclear Magnetic Resonance)
spectroscopic approaches have greatly contributed to the emergence of this new
branch of science and technology employing hydrothermal techniques. As is well
known, water is environmentally the safest material and the cheapest of all sol-
vents. It can act as a mineralizer or a catalyst under elevated pressure
temperature
conditions. The thermodynamic and transport properties of supercritical water are
