Geoscience Reference
In-Depth Information
synthesis of diamond. The most important aspects are the behavior of carbon under
HPHT conditions, the presence of a matching substrate, then preferential oxidation
of sp-bonded C by water. Thermodynamically diamond is stable at temperatures
,
800
C. Graphitization of the diamond surface should be prevented. The most
important thing is to bring carbon into the high-energy state under hydrothermal
conditions and arrest it.
The growth of diamond crystals is still a developing area of research, and an
extensive research is being carried out on the synthesis of diamonds for a highly
cost-effective method to produce diamond crystals with sufficiently larger size
without noticeable defects. Dunin-Barkovsky et al.
[35,36]
have carried out hydro-
thermal growth of diamond at seedings at atmospheric pressure and temperatures
below 100
C and later carried out low-temperature growth of diamond seedings in
acid solutions of nitric, sulfuric and acetic acids with graphite as the source of
carbon. These authors also proposed an oxidative dissolution of graphite under
hydrothermal conditions with the formation of various intermediate carbon com-
pounds, and subsequent transfer of carbon into the growth zone and the growth of
diamond on the seedings. There are several other reports on the diamond synthesis
in the wet systems, but the success is limited towards the growth of bigger diamond
crystals of several millimeters in size, and much of the work is on the nanoforms
of carbon including nanodiamond.
On the whole, the growth of diamond crystals is a challenging task because of
the lack of knowledge of the pH of the growth media, halogen problem, very slow
growth rates, lack of solubility data, the solvent chemistry, and so on. Therefore,
the current situation in diamond synthesis can be compared to that of quartz growth
some one hundred years ago. By 2020 the future of diamond research will become
clear.
9.3 Hydrothermal Synthesis of Hydroxides
The hydrothermal method is popularly used in the synthesis of single crystals of
hydroxides of several divalent and trivalent metals. Usually, the field of stability is
expanded by the presence of surplus water during the transition reaction oxide
"
hydroxide and temperature of the system. In this regard, the alkali metal hydroxide
solutions are more effective.
Table 9.1
gives the list of the metal hydroxides
obtained by hydrothermal methods and their experimental conditions
[37]
. The
experimental
temperatures and pressures vary from 100 to 600
C and 6 to
4000 atm.
Hydrothermal synthesis of Mn(OH)
2
, Cd(OH)
2
, Co(OH)
2
, and Ni(OH)
2
crystals
depends upon the partial pressure of hydrogen
ð
Þ:
Some of the
hydroxide crystals like Mg(OH)
2
, Mn(OH)
2
, and Co(OH)
2
form fairly large-size
crystals of 5
P
H
2
O
.
10 atm
10 mm size with a thickness of 0.1 mm. The other hydroxide
crystals are small (0.1 mm). The double-distilled water is usually used in the syn-
thesis of hydroxides. The temperature gradient method gives single crystals of
10
3
5
