Advantages of Single Crystal
- Single crystals have better optical and electronic properties than the Polycrystals. The table below shows the improvement of piezoelectric properties of single crystals vis-a-vis the conventional polycrystals. However the properties are very much dependent on the direction of crystal cut.
Material |
Cut |
free-strain ε |
d31 pC/N |
Pb (Zn1/3 Nb2/3)O3
|
111 |
900 |
83 |
Pb (Zn1/3 Nb2/3)O3
|
001 |
3600 |
1100 |
PMN - PT |
001 |
3100 |
1240 |
PMN-PT (Polycrystal) |
|
|
690 |
| |
|
560 |
11 |
PZT (Polycrystal) |
|
1130 |
119 |
It is not always possible to develop single crystals, and even if it is possible, single crystals are generally quite expensive to develop.
These are mostly carried out by cooling the material from liqud phase using the following techniques.
- by first freezing the liquid phase at the lowest point of the melt;
- or by freezing the upper surface of a melt
- by using flame-fusion method
A popular technique in this direction is known as Crystal Pulling technique, also termed as Czochralski method. In this technique the melt is kept at a temperature just above the freezing point. A seed crystal is lowered down with the help of a rotating tube. Air is passed through the tube to extract heat in a controlled way. The tube is slowly withdrawn and upto 90% of a melt can be converted into a single crystal form.
Often superalloys are casted in the form of single crystal to increase the creep and oxidation resistance. In a superalloy of Ni-Al-Ti system, two types of crystal structures develop a so called 'y' phase in which each element forms its own face centred cubic lattice and a 'y/ ' phase in which Aluminium or Titanium atoms remain at the corner of the cubes, whereas the Nickel atoms occupy the centres of the faces. The presence of both 'y' and 'y/' phases in an usual polycrystalline superalloy makes it creep sensitive. For a single crystal structure, the 'y/' phase is absent rendering a very high crrep resistance to the material
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