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drastically reducing the temperature and the pressure conditions of processing.
Similarly, the solvothermal and supercritical processing which used a variety of
other solvents like organic, organometallic complexes in materials processing,
thereby taking this technology toward Green Chemistry. Table 10.1 gives the trends
in hydrothermal processing of materials, and these trends take hydrothermal tech-
nology toward green technology for sustained human development. Hydrothermal
technology consumes less energy with no or little solid waste, or liquid or gas
waste, or no further treatment to recover materials, without involving any hazard-
ous materials to process, and has a high selectivity to process materials in a closed
system [1] . The important subjects of technology in the twenty-first century are
predicted to be the balance of environmental and resource and or energy problems.
This has led to the development of a new concept related to the processing of
advanced materials in the twenty-first century, namely, industrial ecology or the
science of sustainability [3] . Hydrothermal chemistry has to be understood pre-
cisely in order to process the materials under soft and environmentally benign con-
ditions. The behavior of the solvents under hydrothermal conditions dealing with
aspects like structure at critical, supercritical, and subcritical conditions, dielectric
constant, pH variation, viscosity, coefficient of expansion, and density are to be
understood with respect to pressure and temperature. Today, much of the hydro-
thermal research is done based on the intelligent modeling of the hydrothermal
reactions prior to the actual experiments. This greatly helps in predicting the exper-
imental conditions to obtain a desired phase with a controlled shape and size [4,5] .
10.3 New Concepts in Hydrothermal Technology
More recently, the addition of external energy like microwave energy, sonar,
mechanochemical, electrical, and magnetic into hydrothermal
technology has
Table 10.1 Current Trends in Hydrothermal Technology
Compound
Earlier Work
Byrappa's work
Li 2 B 4 O 7
T 5 500 700 C
T 5 240 C
P 5 500 1500 bar
P 5, 100 bar
Li 3 B 5 O 8 (OH) 2
T 5 450 C
T 5 240 C
P 5
1000 bar
P 5
80 bar
900 C
200 C
NaR(WO 4 ) 2
T 5
700
T 5
R 5 La,Ce,Nd
P 5 2000 3000 bar
P 5, 100 bar
Melting point . 1800 C
T 5 100 C
R:MVO 4
R 5 Nd,Eu,Tm;M 5 Y,Gd
P 5, 30 bar
Synthesized at . 1200 C
T 5, 120 C
P 5, 40 bar
LaPO 4
T . 1000 C
T , 800 C
Diamond
P 5 10 Kbar
P 5, 3 Kbar
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