Geoscience Reference
In-Depth Information
Figure 3.2 Sealing of the Teflon liner.
inside. Certainly, temperature gradients are appreciably lower in the autoclaves
with liners than in the autoclaves without liners. This difference becomes more
prominent when Teflon liners are used. The greatest disadvantages of Teflon lining
is that beyond 300
C, it cannot be used because Teflon dissociates, affecting the
pH of unbuffered, near-neutral solutions
[15]
. This coating tends to tear and gener-
ally must be reapplied after one or two experiments. Despite all these difficulties,
single crystal growth and materials processing using Teflon are being carried out
extensively at various laboratories.
Corrosion resistance can easily be improved at modest cost by coating the
interior of the reaction vessels by painting, electroplating, or vapor deposition. As
mentioned earlier, chromium and gold linings are often applied to stainless steel
vessels to retard corrosion. Such linings are not permanent because of the intrinsic
permeability of the thin linings along crystal boundaries or cracks that develop dur-
ing temperature cycling owing to the differing thermal expansion of the lining and
the vessel. When more acidic solutions are used, most of the alloys corrode readily
and they must be provided with titanium lining, high-strength titanium alloys,
coated alloys, or other alternatives.
Let us consider briefly the internal arrangement of liners in some typical hydrother-
mal crystal growth experiments. In the growth of quartz crystals, the liner has an
assembly as shown in
Figure 3.3 [16]
. The lower portion of the liner is filled with the
required nutrient material. In the upper portion, a seed holder is placed and this is the
growth zone (cooler zone). In between the growth zone and the nutrient zone, a baffle
having a definite percent of opening is placed at an appropriate position. The position-
ing of the baffle and the percent of opening together play an important role in the
growth kinetics. This is the most popular liner assembly in hydrothermal crystal
growth. In contrast to this, in the hydrothermal growth of berlinite, the internal assem-
bly of the liner differs from that of quartz and is shown in
Figure 3.4
. The liner
material is usually Teflon. As berlinite has a negative temperature coefficient of solu-
bility, the nutrient material is placed in the upper portion (cooler zone) and the seed
holder is placed at the lower portion (hotter zone). In between the two zones, a baffle
with a desired percent of opening is placed in addition to the vertical positioning of
these liners. There are several modifications, especially in the horizontally mounted
autoclaves. Jumas et al.
[17]
proposed the liner internal assembly as shown in
Figure 3.5
for the composite gradient method of growing berlinite crystals.
Ashby et al.
[18]
proposed a variable-volume silver liner for high-pressure hydro-
thermal crystal growth. The schematic diagram of the variable-volume silver liner is
