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Chapter II Solution Growth….
2.9 Agitation or Stirring
In a motionless solution the delivery of the substance is carried out by
diffusion and the growing crystals exhibit all the defects of growth in the
diffusion regime. In a pure diffusion regime the super saturation changes over
different areas of the faces. In order to reduce this non-uniformity of the super
saturation and nutrition of different areas of the faces, motion of the crystal
and solution relative to one another must be required. This is usually achieved
in two ways; (i) by free convection of the solution as a result of temperature
difference in different zones of the crystallizer and (ii) by stirring the solution.
Even in the case of free convection of solution the improvement in nutrition
supply to the growing crystal is observed. Notwithstanding, it is often
insufficient because in the steady state of free convection the flow of the
solution near the crystal may be non -uniform. Moreover, the solution velocity
here can only be increased by increasing the temperature difference between
the zone where the growing crystals is placed and the remaining volume of
the solution and this changes the super saturation.
There are several methods adopted for stirring and crystal motion,
which are elaborately discussed by Chernov [37]. The simplest method is
unidirectional central rotation of the crystal on the holder. It is, however,
avoided because the solution flow leads to the formation of cavities in the
crystal and as a result promotes the trapping of inclusions. The faces
receiving more nutrition grow more rapidly, whereas, the others suffering from
malnutrition grow slowly and acquire various defects. In the case of recessive
rotation the reversal period may last several minutes. The assigned rotation
speed depends on the length and shape of the crystal support, the viscosity
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