Apparatus - Hydrothermal Technology - page 93

safer. The apparatus can also be used in alkali solutions and hydrofluoric acid
media by providing a thin Teflon coating to the inner walls of the quartz glass.
This quartz vessel can be used to synthesize and grow single crystals at tempera-
tures up to 300 C and pressures up to 100 atm. The greatest advantage is the facil-
ity to observe all the processes taking place inside the autoclave including
dissolution of the mixture, movement of the solution, and growth of the crystal. By
employing the appropriate thermocouple sheath, the temperature distribution in the
autoclave can be measured accurately. It is convenient to study the physicochemi-
cal processes and hydrodynamics of the solution both in the homogeneous and
binary phase “gas

liquid” reagents.

Steel Autoclaves
The success of the earliest experiments in the hydrothermal growth of minerals
using very simple autoclaves during the nineteenth century provided an impetus for
Morey at the Geophysical Laboratory of Carnegie Institute, Washington, to design
a highly versatile autoclave to work in the medium pressure range [28] . This was
further developed by him and his coworkers to operate at 500 bar and 400 C [29] .
Followed by this, several other designs appeared, and today there are many designs
to suit any particular aspect of hydrothermal crystallization. All the conventional
autoclaves can be divided into four types and are popularly known as: (1) Flat-
plate closures developed by Morey and coworkers; (2) cold-cone-seal closures
designed by Tuttle [30] and popularized by Roy and coworkers; (3) welded clo-
sures designed by Walker and Buehler [31] ; and (4) unsupported area closures
developed by Bridgman and modified by several commercial autoclave vendors
[32] . These autoclaves have been discussed by many authors [2,3,33] .

Flat-Plate Closure Autoclave
This is popularly known as a Morey-autoclave since Morey designed this simple
gasketed, sealed steel autoclave of 25 100 ml capacity. It is widely used in hydro-

thermal research and the cross section of a typical Morey-autoclave is shown in
Figure 3.10 . The usual dimensions of a Morey-autoclave are 10

20 cm length and
2.5 cm inner diameter. The problems encountered by Morey and coworkers in the
initial experiments were alleviated in a later design by Morey 1913, where in the
closure is made by a Bridgman unsupported area seal gasket made of copper or sil-
ver or Teflon. Higher pressure

temperature conditions may be obtained with an
Inconel vessel. At low pressures, sealing is achieved by compression when the
main nut or closure nut is tightened. The autoclave generates an autogeneous pres-
sure depending on the degree of filling, the fluid, and the temperature. The auto-
clave is limited to 450 C and 2 kbar in routine use. In the later versions, the

pressure can be directly measured and adjusted during an experiment by providing
an axial hole through the closure nut, but this caused, sometimes, compositional
changes as the material was transported to the cooler region. A thermocouple is
inserted in the well close to the sample, and the vessel

B

is placed inside a