Apparatus - Hydrothermal Technology - page 100

Table 3.9 Historical Development of Large Autoclaves [42]

Installation 1959 1963 1965 1973 1984

Sealing type Flange Modified Bridgman Grey-Loc

Inside diameter (mm) 120 180 300 400 650

Depth (m) 2 3 5 8 14

Volume (l) 22 76 353 1005 4650

Output (kg) 7 25 120 450 3000

Most of the above closures, which are called self-energized closures, have sealing
gaskets generally made of plastic materials and, most frequently, made of annealed
copper. At temperatures below 300 C Teflon can be used. If the closure is provided
with a cooling system, rubber gaskets are used and the sealing of such autoclaves
does not require as much mechanical effort as with a copper gasket. In fact, much of
the success in hydrothermal growth experiments is due to the equipment. A dramatic
advance in upgrading the capacity of the autoclave (e.g., as in the case of quartz
growth) is the result of new sealing systems from the original flange-type seals to the
modified Bridgman seal which in turn has been superseded by Grey-Loc seals [41] .
Large-capacity autoclaves of 1000 5000 l volume are in use in Japan with this type

of sealing system. Table 3.9 shows the evolution of sealing with reference to the
growth of quartz crystals [42] . In most hydrothermal experiments, corrosive solu-
tions are used and special protective inserts are used nowadays to prevent contamina-
tion of the growth zone by corrosion products. A floating-type insert or variable-
volume insert has been developed at the Institute of Crystallography, Moscow. Such
an insert occupies only a part of the autoclave interior while the remaining space is
filled with water. These inserts are usually made of carbon-free iron, copper, gold
(for alkaline media), titanium, platinum, various grades of glass, molten quartz (for
acid media), Teflon, and so on.
Laudise et al. [43] have proposed a pressure balance under hydrothermal condi-
tions. The pressure temperature relations in the can and in the space between the

can and the autoclave system are critical. The water fill is chosen so that the can
is usually under compression when the autoclave is heated to the desired tempera-
ture since the can is less likely to fail under compression. The goal is to have
pressure balance at the operating temperature (or to have the can under slight
compression). An unbalanced pressures result in can cause dimples, bubbles, and
often ultimate failure. Dimples can restrict flow, crush crystals, and cause imper-
fections in crystals if they grow against the dimpled wall. To understand the bal-
ance strategy examination of H 2 O and mineralizer solution, PVT data is essential.
The authors have studied this while growing K(TiO)PO 4 , a nonlinear optical mate-
rial, in pressure-balanced gold cans and suggest several strategies for pressure bal-
ance in order to avoid dimple formation. Figure 3.16 shows the minimized
volume between the autoclave and noble metal lining. Usually, autoclave bottoms