Apparatus - Hydrothermal Technology - page 76

worldwide. Hence, for the benefit of the reader, we consider only the standard
autoclaves used worldwide and some important designs from the Russian school.
With the disintegration of the erstwhile Soviet Union, there is not much develop-
ment in the field of design and fabrication of new autoclaves. The Japanese groups
are coming out with new designs of autoclaves for applications in crystal growth
and materials processing. With the recent advances in hydrothermal technology
for nanotechnology, the design and fabrication of new reactors focus mainly on
the rapid production of nanomaterials with a minimum residence time. Although
there is no significant development in designing and fabrication of hydrothermal
autoclaves for crystal growth and bulk materials processing, there is a substantial
progress in the designing of new reactors such as flow reactors, multienergy pro-
cessing system reactors, hydrothermal cells for in situ time-resolved synthesis for
tomographic energy-dispersive diffraction imaging (TEDDI) or neutron diffraction
analyses, and instant hydrothermal systems [1] .
An ideal hydrothermal autoclave should have the following characteristics:

i. Inertness to acids, bases, and oxidizing agents.
ii. Easy to assemble and dissemble.
iii. A sufficient length to obtain a desired temperature gradient.
iv. Leak-proof with unlimited capabilities to the required temperature and pressure.
v. Rugged enough to bear high-pressure and high-temperature experiments for long duration,
so that no machining or treatment is needed after each experimental run.

The most commonly used autoclaves in hydrothermal research are listed in
Table 3.1 [2] . The majority of these autoclaves are externally heated pressure
vessels and their pressure temperature range cannot be extended further due to

lack of suitable refractory alloys. However, internally heated pressure vessels
are now commercially available up to 10 kbar and 1400 C. The list given in

Table 3.1 Autoclaves

Type Characteristic Data

6 bar at 250 C Pyrex tube 5 mm i.d. 2 mm wall thickness

6 bar at 300 C Quartz tube 5 mm i.d. 2 mm wall thickness

Flat-plate seal, Morey type 400 bar at 400 C

Welded Walker Buehler closure 2600 bar at 350 C 2 kbar at 480 C

Delta ring, unsupported area 2.3 kbar at 400 C

Modified Bridgman, unsupported area 3.7 kbar at 500 C

3.7 kbar at 750 C Full Bridgman, unsupported area

5 kbar at 750 C Cold-cone seal, Tuttle Roy type (batch reactors)

40 kbar at 1000 C

Piston cylinder

100 kbar at . 1500 C Belt apparatus

200 kbar at . 1500 C

Opposed anvil

Up to 500 kbar at . 2000 C Opposed diamond anvil

2 kbar at 600 C Continuous flow reactors