Apparatus - Hydrothermal Technology - page 83

primary interest was only in synthesizing a product resembling some natural min-
eral. It was von Chroustshoff who first proposed the gold lining steel autoclaves to
prevent corrosion. By doing so, he could attain a temperature of 350 C to synthe-
size tridymite phase of quartz and increased experimental duration. Subsequently,
the use of steel autoclave and noble metal lining became very popular, and the
tendency to reach higher pressure temperature conditions also started. Bruhns [12]

used steel bombs with a platinum lining and cover held down by bolts and made
tight by means of a copper washer, which was protected from the action of the min-
eralizer by platinum.
Doetler [13] used a nickeled gun-barrel or a silver-lined steel tube in his experi-
ments. The quartz glass tube liners of 15 mm diameter can easily withstand the
outside pressure of 3 kbar at 500 C. However, the internal pressure has to be kept a
little below than that of the outside value. Rabenau and Rau [14] have described
this technique in detail. For larger autoclaves, like the Morey flat and the modified
Bridgman closures, noble metal or other suitable metal lining is provided. The
main difficulty in such a design is that the lining should be free of cracks, pinholes,
or ridges, and it should be honed near to a mirror finish. For Tuttle cold-cone-seal
autoclaves, capsules made of noble metals are used. The capsule tube, charged
with a desired amount of nutrient, is clamped tightly at the top and is welded shut.
During the experiment the capsule collapses but does not rupture. The vessel is
then heated to the desired temperature at constant pressure. When the desired tem-
perature is reached, the capsule expands to its original volume and is not subjected
to tension and virtually no capsule failures occur. For some experiments, the
capsule needs only to be pressed together, the capsule length being shorter for
phase-equilibria studies and longer where a temperature gradient for crystal growth
is desired. In large diameter vessels, platinum liners are usually used. These liners
have silver caps to avoid platinum seizing.
The liner is filled to a given percent and closed, and the space between the liner
and the inner wall of the autoclave is filled to the same percent or slightly higher;
this provides a pressure balance and the liner supports virtually no pressure.
Pressure imbalance can cause rupture of the liner. For the growth of high-purity
crystals in solution in highly corrosive media, Teflon beakers or liners are used.
Similarly, for special studies pertaining to reaction kinetics, solubility and materials
processing under mild hydrothermal conditions or pressure temperature conditions

below 250 bar and 300 C, Teflon is the most popularly used lining material.
Several new autoclave designs with Teflon lining or coating for such studies have
been reported in the literature. The Teflon liner or beaker should sit exactly inside
the autoclave without leaving any gap, pressure balancing technique can be used
alternatively. A slightly higher pressure of nearly 1 2% of fill outside the Teflon

liner helps in the sealing of Teflon liners as the cap and liner cannot be welded.
In addition, in some cases where the experiment is carried out for longer duration,
effective sealing of the Teflon liner can be obtained with a sharp ringlike mecha-
nism on the liner as shown in Figure 3.2 . As the temperature rises, the Teflon
expands and hermitic sealing can be obtained. The greatest disadvantage with liners
or cans is the difficulty associated with the measuring of the actual temperature