Geoscience Reference
In-Depth Information
5.2.3 Nutrient Effect
The quality of the nutrient has a profound effect on the purity of the grown crystal. It
appears that most of the impurities incorporated into the crystal come from the nutri-
ent and not from the autoclave walls. The search for new sources of nutrient is not a
new field of research. In the nineteenth century, earlier workers tried several varieties
of nutrient materials to obtain
-quartz under hydrothermal conditions. However, the
results were not satisfactory owing to the lack of knowledge on the solubility data for
quartz, which resulted in a very low growth rate. In this respect, some serious efforts
were initiated by Kolb et al. during 1976
[35]
. They examined new sources of nutrient
to replace Brazilian
α
-quartz for hydrothermal crystallization. It was found that the
acoustic Q strain depends upon the source of the nutrient and also its geographic
region/location. Both vein and appropriately chosen pegmatite quartz can be used as a
nutrient. High purity sand can be used as the nutrient provided process conditions are
altered so as to compensate for its effective lower surface area. Non-
α
-quartz nutrients
such as silica glass and silica gel produce higher supersaturation initially and have fast
growth rates leading to poor quality of crystal growth. However, alterations in process
conditions can be made to reduce initial growth rates and prepare reasonable quality
crystals. Therefore, in the growth of high-quality quartz crystals, Z-growth material
and recrystallized glass is used as the nutrient material. If future requirements become
more stringent, particularly for radiation damage, it will be necessary to reduce the
aluminum content even further. In this case, recrystallized glass will be the leading
candidate, as higher purity glass is available. This generally requires two runs, since
the glass must first be converted to
α
-quartz.
There are several laboratories throughout the world working to find a
suitable nutrient for high-quality quartz growth. Hosaka and Miyata (1991)
[36]
have obtained high-quality
α
-quartz crystals using cristobalite as the nutrient. The
authors have used high-purity
α
α
-cristobalite powder compacted into grains of
0.5
1.00 mm or into lumps of approximately 1 cm in diameter for the hydrothermal
synthesis of
-quartz.
Alpha-cristobalite is a polymorphic form of silica as are quartz and tridymite
and has a higher solubility than quartz
[37]
. Alpha-cristobalite powder
[38]
has the
following advantages: It can be obtained in a high-purity state with low contents of
Al and alkaline metallic ions, and can be prepared with relative ease as particles of
uniform size. As compared with the conventional growth of synthetic quartz crystals
using Brazilian lascas, the use of
α
α
-cristobalite powder may be expected to allow:
i. Synthesis of high-purity large quartz crystals.
ii. Synthesis of microquartz crystals having uniform grain size.
Hosaka (1991)
[38]
had attempted to crystallize microquartz crystals by HHP
method using
α
-cristobalite powder as a source material.
Figure 5.8
shows
α
-quartz
crystals grown at high (left) and low (right) fillings using
-cristobalite as the nutrient.
The percentage fill or pressure greatly influences the growth rate. However, a consid-
erable amount of work has to be carried out for the industrial production of
α
α
-quartz
using cristobalite as the nutrient.
