Apparatus - Hydrothermal Technology - page 126

It is obvious that the grinding and mechanical mixing and subsequent fracture
generate fresh, highly reactive surfaces, and lead to substantial improvement in the
reaction kinetics—especially in the conversion of highly stable oxides to make
clays. This was made possible by a simple innovation in the apparatus. In effect,
the Morey-autoclaves were converted into ball mills or rod mills [123,124] .The
autoclave is rotated around its long axis inside the furnace. An increase in the rate
of reaction by two or three orders of magnitude can be attained at a given pressure
and temperature. It is not clear whether this should be ascribed to strain energy
stored in the lattice or merely to breakage of bonds.
Boldyrev et al. [124] , through his extensive work with mechanochemical effects,
concludes that for many reactions it is possible to increase the kinetics by about
two orders of magnitude.
During the early 1980s, a considerable number of publications have appeared in
the field of sonochemical breakdown of liquid phases. Suslick [125] gives an excel-
lent review on this topic and shows that metallic phases may be melted and/or
corroded in aqueous suspension at “room temperature.” The general understanding
is that collapsing bubbles during cavitation could generate temperatures of the
order of 5000 K and modest local pressures. Roy and his group [126] attempted to
synthesize novel materials or combination reactions, which could be accelerated at
least by two orders of magnitude by using AWS. These AWS techniques have an
interesting effect on size. In another case, ultrasonic energy was used to accelerate
the formation of hydroxylapatite [127] . Abu-Samra et al. [128] and Kingston and
Jassie [129] reported that all inorganic mineral phases can be dissolved faster in
(dilute) acids by several orders of magnitude when a microwave field is superim-
posed upon mild ( 200 C, 10 20 bar) hydrothermal conditions. This technique

is used worldwide today for chemical analysis as well as for dissolving samples.
Komarneni and Roy [130,131] , in a series of recent papers, have been applying
hydrothermal-plus-microwave conditions for material synthesis, and their study
enhances, by two orders of magnitude, the reaction kinetics of many reactions from
the precipitation of metals from polyols to the synthesis of ferrites, phosphors, and
so on. There is also some advantage in morphology control and it sometimes leads
to the crystallization of new phases like hydrate of Al 2 O [132] . Recently,
Komarneni [133] has reviewed the importance of this. Figure 3.39 shows a sche-
matic diagram of a typical hydrothermal-plus-microwave setup [132] . As the
popularity of this technique is growing with the new results poured into literature,
there is already a commercial production of these autoclaves.
There are several other designs of hydrothermal autoclaves to suit a specific
purpose. For example, Strubel (1975) has designed a Teflon-lined autoclave with a
stirrer [137] . Similarly, Shternberg [52] has designed a double autoclave (exoclave)
in order to avoid having a mechanical compression system for producing a desired
pressure in the reaction chamber. Also, this permits independent selection of tem-
perature and pressure. The exoclave is designed for use at temperatures up to
700 C and pressures up to 3000 kPa/cm 2 .
In recent years, there is an increasing tendency for in situ time-resolved
hydrothermal synthesis for TEDDI using synchrotron beam and similarly in situ

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