Geoscience Reference
In-Depth Information
grown by carrying out the crystallization in the welded liners directly, and colorless
crystals which were essentially pure have been prepared by the use of a silver tube
[74]
. The overall features are similar to the growth of corundum crystals. Recently,
there have been several reports on the growth of crystals of ruby, sapphire and,
α
-alumina for various applications. Transition metal (chromium and titanium) doped
α
-alumina crystals have attracted attention for many years not only as gemstones
but also due to their application as high power laser host media
[76]
. Bliss et al.
[77]
have grown Ti
3
1
:Al
2
O
3
(titanium-doped sapphire) single crystals using solvother-
mal method for laser applications. The growth experiments were carried out with
T
550
C and P
430
15
25 kpsi. Several workers have carried out the growth
5
5
of
temperature conditions using organic solvents
like 1.4-butendiol and other glycol-based solvents
[78
α
-alumina under milder pressure
81]
.
9.4.5 Hydrothermal Growth of Oxides of Ti, Zr, and Hf
The oxides of titanium, zirconium, and hafnium belong to a group of chemically
stable compounds with high melting points. These properties make them very impor-
tant technological materials, but at the same time restrict the possibility of growing
them in the form of single crystals. Both ZrO
2
and HfO
2
are popular as solid oxide
fuel cells, high K gate dielectrics, thermal barrier coatings, nuclear waste storage
materials, and so on. Among these three compounds, TiO
2
melts easily (1850
C)
and can be grown by direct melting (Verneuil method), but analogous methods have
not yet found successful practical application for ZrO
2
and HfO
2
, since the melting
points are rather high, 2700
C and 2780
C, respectively. Hence, Li
2
MoO
4
, PbF
2
,
and Na
2
B
2
O
4
fluxes are used to grow these crystals, as their high melting points
make them difficult to grow directly from the melt. Although the crystallization of
rutile by the Verneuil method is frequently employed, the method often yields
crystals of a nonstoichiometric nature. Kuznetsov
[82]
has reviewed the growth of
oxides of the titanium subgroup metals in detail. Anikin et al.
[83]
have obtained
TiO
2
at 550
C and 1000 atm. Subsequently, Harville and Roy
[84]
have obtained
TiO
2
crystals in H
2
SO
4
solutions at 700
C and 1000
4000 atm. However, the
growth rate was very low. The commonly used solvents are solutions of alkalis, lith-
ium, potassium, sodium, and ammonium carbonates and bicarbonates, sodium sul-
fide, lithium, potassium and ammonium chlorides, sulfates (Na
2
SO
4
,K
2
SO
4
), borax,
boric acid, and fluorides (KF, NaF, NH
4
F). The concentrations of the solutions were
5
40%.
TiO
2
Kuznetsov and coworkers
[82,85,86]
have worked out the growth technology of
TiO
2
(rutile phase) under hydrothermal conditions using 7
10% KF or NaF and
550
C and pressures of
5
10% NH
4
F solutions at
temperatures over 500
500
800 atm. The amount of material transported increases with the increasing
temperature of the experiment. Crystals are prismatic with dark-brown or black
color. When a small amount of KCIO
3
is added to the nutrient, crystals become
