Geoscience Reference
In-Depth Information
Figure 5.20 Autoclave used in the berlinite
synthesis
[83]
.
Cap
Plunger
Teflongasket
Nutrient
Steel casing
85%H
3
PO
4
Teflon liner
Crystal
Autoclave
Growth with reverse temperature gradient. This is a more popular and widely
used method in the growth of berlinite crystals. Here, the nutrient is kept in a cas-
ket at the upper portion of the autoclave, which is cooler. The nutrient is nothing but
the crystalline powder or fine grains of AlPO
4
obtained by a slow heating method. The
temperature of the bottom of the liner is kept at a slightly higher temperature and this is
the zone of crystallization, where berlinite can be crystallized either spontaneously or
on seeds
[95]
.
Crystal growth on seeds. Spontaneously nucleated seeds and oriented seeds cut
from grown crystals are mounted on a platinum frame which is placed in the bot-
tom (hotter) region of an autoclave, and the nutrient (
60 mesh particle size
prepared as described earlier) is placed above in a platinum gasket in the upper
region (or in a Teflon gasket). Thus, the seeds are in the warmer supersaturated
region at the bottom of the autoclave and the temperature gradient achieved by
cooling the top allowed proper convection. The schematic diagram showing the
position of seeds, nutrient, and the baffle is represented in
Figure 5.21
.
In the growth of berlinite on seeds, the orientation of the seed is very important
because the growth rate and perfection depend upon the seed orientation.
According to Laudise (1985)
[85]
, the growth rates are shown for seeds whose
faces are indicated below:
B
(000
1
) (basal plane)
0.25
0.50 mm/day
ð
102
1
Þ
(X-cut)
0.23
0.30 mm/day
ð
1010
Þ
(
Y-cut)
0.12
0.15 mm/day
ð
011
1
Þ
(minor rhombohedral face)
0.12 mm/day
ð
1011
Þ
(major rhombohedral face)
0.15 mm/day
