Geoscience Reference
In-Depth Information
Figure 3.35 Schematic diagram of the
autoclave with transparent windows
[103]
.
(1) Window, (2) body of autoclave, (3)
copper casing, (4) thermal insulation, (5)
measuring thermocouples, (6) resistance
thermometer, (7) thermocouples, (8)heater,
(9) seal, (10) camera, (11) device for
automatic single frame filming, (12) three
channels proportional regulator, (13) 12-
point EPR-09-Rd apparatus, and (14)
pressure gauge.
14
9
8
1
10
2
7
3
13
11
12
5
4
6
solution. The cause of leakage was either the cracking of sapphire windows or the
solution squeezing out through the sleeve sapphire seal. At temperatures
300
C,
the surfaces of sapphire cylinders in contact with the solution were etched and
their transparency was impaired.
In recent years, several improvements have been made in the designs, and
special provisions have been made to perform optical/spectroscopic measurements.
For example, the Laser Raman and other vibrational spectroscopic cells have been
fabricated in some of the recent autoclave designs
[105]
. It is worth mentioning
here that the recent advances in the instrumentation, particularly to study the opti-
cal/vibrational spectroscopic measurements under hydrothermal conditions, have
contributed greatly to understanding the nature or behavior of hydrothermal solu-
tions during the crystallization process and the complexation processes taking place
inside the autoclave. Thus, the hydrothermal autoclave is no longer a “black box.”
Further, such studies have greatly helped in reducing the pressure
.
temperature
conditions of growth for a large variety of inorganic compounds. With the reduc-
tion in the pressure
temperature conditions of growth, one can use thick glass-
walled or thick silica glass autoclaves, which permit visual examination.
Hydrothermal Hot Pressing
HHP technology is a method by which hard solid bodies from powders can be pro-
duced in a short
time and at a relatively low temperature (approximately
300
C) under saturated vapor pressure
[106]
. Using the HHP technique, one
200
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