Melt growth:
Melt growth is the process of crystallization by fusion and re solidification of the starting materials
The growth from melt is further sub-grouped into various techniques. The mani techniques are
a) Czochralski technique
b) Bridgmen Technique
c) Verneuil method
d) Zone melting technique
The major practical factors is to be considered during growth crystals from melt are
1) Velatiltiy
2) Chemical resistivity
3) Melting point
BRIDGEMAN TECHNIQUE
A
common technique of growth single crystals. It involves selective cooling of
the molten material, so that solidification occurs along a particular crystal
direction.
In
this technique, the melt in a sealed crucible is progressively frozen from one
end. This can be achieved by
Moving
the crucible down the temperature gradient
Moving
the furnace over the crucible
By
keeping both the furnace and the crucible stationery and cooling the furnace so
that freezing isotherm moves steadily through the originally molten charge.
GROWTN PROCESS
Figure
shows a platinum crucible filled with high quality nature growth material, to
which the desired impurities may be added. The crucible is fixed in the upper
furnace until the contents are completely melted. It is then lowered from upper
furnace into the lower furnace with the help of electric moter and reduction
gearing. Since the potential tip enters the lower furnace first, the grown
material starts to crystalline over there.
Sodium
chloride :NaCl
Potassium
Chloride :KCl
Calcium
fluoride : CaF2
Silver
Bromide :AgBr
As
the crucible continues to be lowered crystallization proceeds until al the
melts become solid crystal. A bulk single crystal can be grown in the crucible
at steady rate keeping the temperature constant.
Example
Advantages
Simple
technique
Control
over vapour pressure
Containers
can be evacuated and sealed
Control
of shape and size of growing crystals
Stabilization
of thermal gradients.
Limitations
Confinement
of crytals
Crystal
perfection is not better than that of the seed
No
visibility
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