Geoscience Reference
In-Depth Information
Figure 9.3 Polycrystalline diamond
[29]
.
autoclave filled with a specially prepared water solution. Natural diamonds of 0.22
and 0.49 g were used as seeds. The solution and seeds were subjected to 170 MPa
and 400
C for 21 days. A thin irregular (15
m) colorless film of polycrystal-
line diamond on the natural (111) faces of the seed crystals was obtained
(
Figure 9.3
). This work shows that hydrothermal synthesis may be an effective way
for homo- and heteroepitaxy of diamond as a continuous layer on seed material sur-
faces of diversified shape and possibly for the growth of large diamonds on seeds.
Recently, Yamaoka and Akaishi
[33]
reported the growth of diamond under HPHT
water conditions. The basic principle used by these authors is that carbon reacts with
high-temperature water (vapor) producing so called “water gas.” It is natural to expect
that graphite reacts with water and precipitates as diamond under HPHT conditions in
the thermodynamically stable region of diamond. The authors have used three types of
capsules to carry out the diamond synthesis under HPHT conditions: (i) Ta cylindrical
containers, (ii) Mo double capsules, and (iii) conventional point-sealed capsules. The
selection of these capsules depends upon the duration of the experiments and the
pressure
14
μ
temperature conditions.
Figure 9.4
shows the diamond growth under HPHT
conditions. This work shows that graphite reacts with HPHT water and precipitates as
diamond in the regions of 1300
2200
C and 5.5
7.7 GPa. The reaction proceeded
more rapidly with increasing pressure and temperature.
Suito and Onodera
[34]
have synthesized diamond using phenolic resins
between 1.6 and 4 GPa and cobalt at 10 GPa with calcium carbonate as catalyst-
solvents. In this case, the pressure and the temperature conditions of diamond
synthesis depended upon the prefiring temperatures. Well-defined single crystals of
0.3
0.7 mm in size were obtained at 4 GPa and 1500
C(
Figure 9.5
).
However, the diamond synthesis in the presence of metal powder, molten high-
molar alkali solution, and water at 800
C and 1.4 kbar pressure range is the most
popular technique. The growth rate is quite low. The crystals obtained are usually
5
10
μ
m in size. Several
types of starting materials have been tried for the
