Geoscience Reference
In-Depth Information
9.4.4 Hydrothermal Growth of Corundum
The hydrothermal synthesis of corundum began during the 1890s. Corundum is
stable under hydrothermal conditions at T
400
C
[66,67]
and is soluble in alkali
and carbonate solutions
[67a]
. Corundum shows a lower temperature coefficient of
solubility in alkaline solutions and that is the reason for the unsuccessful growth of
corundum in these solutions
[68]
. Although, hydrothermally grown corundum is
commercially available at present, as gemstones it has not been reported. The
hydrothermal growth of corundum is achieved using a metastable-phase technique
because of its very low solubility. Gibbsite (Al(OH)
3
) is used as a nutrient,
α
.
-Al
2
O
3
as a seed, and an alkaline solution as a solvent. A more rapid growth rate
is obtained in KOH or K
2
CO
3
solution than in NaOH or Na
2
CO
3
solution. The sol-
ubility is lower in KOH or K
2
CO
3
, but the interaction between the impurity
absorbed beforehand onto the crystal surface and K
1
ion existing in the solution
acts as a factor to augment the growth rate. The lower the pH level in the solution,
the thicker the growth layer along the c-axis. The growth of corundum in extremely
low pH 6 N HCl has been reported. In this case, the nutrient is dissolved at the
lower temperature side and is deposited on the seed crystal at the higher tempera-
ture side, because the enthalpy in the
α
-Al
2
O
3
a
HCl
reaction is negative.
Isothermal studies of the system Al
2
O
3
a
H
2
O have been carried out by many work-
ers and have shown that corundum could be formed from diaspore, boehmite, or
gibbsite in situ at
450
C. The pressure varied from 15,000 to
30,000 psi.
Figure 9.10
shows the phase relations in Al
2
O
3
systems
[68]
.
Kashkurov et al.
[69]
have studied the growth of large corundum crystals at pres-
sures up to 2000 atm and at temperatures up to 550
C in alkali solutions of various
concentrations. Crystals weighing up to 1 kg were prepared by the hydrothermal
technique, and the imperfect state of these crystals was apparently associated with
internal stresses and the mosaic structure of seed crystals which were prepared by
temperatures
.
Figure 9.9 View down c-axis of (a) 3 in. ZnO crystal and (b) 3 in. (0001) ZnO wafer
(Cytidine 5-monophosphate). Scale bar is 80 mm.
Source: Photograph courtesy of Dirk Ehrentraut
[65a]
.
