Apparatus - Hydrothermal Technology - page 125

organic inorganic hybrid nanoparticles. There are several variants in the supercrit-

ical fluid technology like static supercritical fluid (SSF) process, rapid expansion of
supercritical solutions (RESS), particles from gas-saturated solutions (PGSS), gas
antisolvent (GAS) process, precipitation from compressed antisolvent (PCA), aero-
sol solvent extraction system (ASES), supercritical antisolvent (SAS) process,
solution-enhanced dispersion by supercritical fluids (SEDS), supercritical antisol-
vent process with enhanced mass transfer (SAS-EM), depressurization of an
expanded liquid organic solution (DELOS), supercritical assisted atomization
(SAA), hydrothermal synthesis under supercritical conditions via flow reactor
(HTSSF), hydrothermal synthesis under supercritical conditions via batch reactor
(HTSSB), supercritical fluids drying (SCFD), and supercritical fluid extraction
emulsions (SFEE) [117] . Accordingly different designs are available for each vari-
ant of supercritical fluid processing of materials. Here, only the characteristic and
general SCF flow reactors are shown in Figure 3.38a and b . An aqueous metal salt
solution is first prepared and fed into the apparatus in one stream. In another
stream, distilled water is pressurized and then heated to a temperature above the
desired reaction temperature. The pressurized metal salt solution stream and the
pure scH 2 O stream are then combined at a mixing point, which leads to rapid heat-
ing and subsequent reactions in the reactor. After the solution leaves the reactor, it
is rapidly quenched. In-line filters are used to remove larger particles. Pressure is
controlled with a back-pressure regulator. Fine particles are collected in the efflu-
ent. By this rapid heating method, the effect of the heating period on the hydrother-
mal synthesis is eliminated: thus specific features of supercritical hydrothermal
synthesis can be elucidated. There are several advantages in supercritical hydro-
thermal flow reactors, which provide nanoparticles with desired shape, size, and
composition in a shortest possible residence time. Hence, this technique has an
edge over the conventional methods in synthesizing a wide range of inorganic
nanoparticles and also hybrid organic

inorganic nanoparticles within a very short
time or shortest residence time. Figure 3.38b shows the commercial flow reactor
prepared jointly by Adschiri's Laboratory and ITEC Company, Japan [118,119] .

Autoclaves for Accelerating the Kinetics of Hydrothermal Reactions
There is a growing interest in recent years in enhancing the hydrothermal reaction
kinetics through superimposing electric fields or mechanical forces, or mechano-
chemically including high uniaxial pressure with or without shear mechanical
grinding, sonochemically (or acoustic wave stimulation, AWS), with microwave
fields, and so on [120] . These techniques are being popularly used to synthesize a
great variety of materials and also materials processing. Burns and Bredig [121]
had established the remarkable effect that grinding calcite in a mortar converts it
into aragonite. Dachille and Roy [122] generalized the finding to many other
phases such as PbO 2 and MnF 2 showing that mechanochemical effects produce:
1. High-pressure phases stable above or even about 10 15 kbar at room temperature.

2. Enhanced kinetics by nearly two orders of magnitude in similar high-pressure solid-state
reactions, when shearing stresses are superimposed on pressures up to 100,000 atm.

2