Geoscience Reference
In-Depth Information
commonly used mineralizers. The solubility change with temperature is smaller in
NaOH and slightly larger in Na
2
CO
3
. The temperature of the autoclave at the nutri-
ent zone is usually kept at 355
369
C and, in the growth zone, it is kept at 350
C.
The addition of lithium improves the growth rate and small amounts of Li salts are
routinely added to the solution
[22]
. The solubility is also, to some extent, a function
of increasing pressure. The pressure is controlled by the percentage fill in the auto-
clave, and it is usually about 80% for hydroxyl mineralizer (20,000 psi internal pres-
sure). In most of the experiments, the percent opening of the baffle is 20%; even
lower percentages are used by several workers. However, the actual percent opening
of baffle area and its geometry are not disclosed, especially by commercial growers.
The optimum growth conditions for synthesis of quartz based on the work in
Bell Laboratories are
[24,25]
:
425
C
Dissolution temperature
375
C
Growth temperature
Pressure
15,000
25,000 psi
Mineralizer concentration
0.5
1.0 M NaOH
50
C
Temperature gradient (
Δ
T)
% Fill
78
85%
Growth rate in (0001)
1.0
1.25 mm/day
The quality of the grown crystals is also a function of the seed orientation and its
quality. Strained seeds generally produce a strained growth region
[26]
. The seeds are
polished to a very fine finish before use. Most high-quality crystals are grown using
seeds with surfaces perpendicular to the Z-direction since the Z-growth region is the
lowest in aluminum concentration. Though the main part of quartz production consists
of Y-bar crystals, i.e., small crystals (Z
25 mm, 64 mm seed) capable of several
Y-bars per crystal, the pure Z-bars are also produced, representing 10
20
5
20% of this
production. In medium- and high-quality grades, we notice a rise in demand for crys-
tals of very large dimensions and upper-medium quality, especially in the United
States, for manufacturing wafers used in surface wave applications
[11]
. Earlier, most
of the seeds used were natural quartz cut into a definite orientation, but in recent years
this practice is only used when a high-quality crystal is desired.
The growth of quartz crystals has been understood precisely with reference to the
growth temperature, temperature gradient, percent of fill, solubility, percent of baffle
open, orientation and nature of seed, and type of nutrient. Also, many kinetic studies
have been carried out
[15,16]
.
Figures 5.2 and 5.3
show the solubility of quartz with
temperature, growth rate as a function of seed orientation, and growth rate as a func-
tion of percent of fill.
Figure 5.4
shows hydrothermally grown quartz crystals.
The type of crystal to be grown depends on the application, as different properties
are required in each case. For optical use, high uniformity, low strain, and low inclu-
sion counts are needed since all of these can affect the transparency. For surface
acoustic wave devices, large pieces are needed which can take a very high-quality
surface finish. The quality of the material required for resonators used in time and
frequency devices varies with the application. The more precise the need, the more
