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the cross sections of quartz glass tube. The lower half or charge chamber is made by
nesting three close-fitting quartz glass tubes of 1.25 mm wall thickness and appro-
priate internal diameters and fusing the tubes together in vacuum. Quartz glass is
the most suitable composition because of its strength at high temperatures, low ther-
mal coefficient of expansion, and absence of possibly reactive components, whereas
Vycor and Pyrex both contain 3% or more B 2 O 3 in addition to significant quantities
of Al 2 O 3 and the alkalies, and Pyrex softens at about 600 C [27] . Speed and Filice
[6] give the temperature and internal pressure of quartz glass tube failure in
Table 3.8 .
The use of quartz glass autoclaves, however, was not popular until recently
because of the problems associated with higher pressure
temperature conditions.
With the recent discovery of new mineralizers (mixed solvents, high molar acid
solutions, organic solvents, etc.), there is a growing tendency for using the glass or
thick-walled silica tubes in several hydrothermal experiments. The new minerali-
zers, and a better understanding of hydrothermal technology, have brought down
the crystallization temperature with increased solubility for compounds hitherto
obtained only at high pressure
temperature conditions. All these advances
prompted Popolitov [7] to develop and test a hydrothermal autoclave for visual
examination. The schematic diagram of the visual autoclave is given in Figure 3.9 .
The apparatus consists of a cylindrical reactor made of transparent quartz glass
with the following parameters: compression strength
6556 kg/cm 2 , tensile
strength 5 2734 kg/cm 2 , bending strength 5 1134 kg/cm 2 , and shock bending
strength
5
1.08 kg/cm 2 . These data show that the quartz glass is a good material for
a transparent reactor, provided that no shocks will occur during its operation. This
condition requires certain precautions to be followed during charging, discharging,
and sealing of the reactor.
The reactor has been fabricated in the following way: A cylindrical block of
quartz glass of special purity having a diameter of 85
5
1000 mm was drilled using
a diamond bore with the diameter 50
55 mm. Then, the inner and outer walls
were ground to the specified dimensions and polished. The hydrostatic pressure
admissible for this reactor was not calculated before hand, but its strength was
checked. To do so, the reactor was filled with water, sealed by a special valve,
placed into a furnace, and heated. The growing pressure due to the heating was
measured with a manometer. The reactor was heated until the pressure inside it
achieved 100 atm, but the reactor did bear this pressure. The external heating of
steel autoclaves introduces stresses on its walls due to heating. By contrast, quartz
Table 3.8 Temperatures and Internal Pressures of Quartz Glass Tube Failure [6]
Temperature ( C)
Tube
Charge
Pressure (bar)
1
Saturated NaBr solution
543 6 7
146 6 4
2
Saturated NaCl solution
511 6 13
345 6 13
3
Saturated NaCl solution
493 6 13
323 6 12
4
H 2 O
474 6 5
414 6 11
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