Bridgeman Technique

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

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