Necking in a Seed

What happens is that the crystallites melt and fuse into a small tip. If we do this carefully, we will have our seed . The tip s small size limits regrowth of the remelted part to that of a single crystal. Then, returning to the melt, we can initiate the growth of a single crystal, provided that growth-conditions are suitable. 

Another way to obtain a seed is to dip a capillary tube into the melt. Surface tension causes the tube to fill and when it freezes inside, a small seed results. The difficulty with this method is that it is difficult to obtain a tube of proper diameter, made of the proper composition. Glass softens at too low a temperature and quartz melts around 1400-1500 °C. Usually, we are restricted to metals and even then, we must be able to cut the tube to obtain the seed, since it is confined within the tube. Once in a while, we can use the tube directly and obtain growth directly upon the seed, even though it has remained within the tube. 

There is one other method that can be employed to obtain a seed. We use a metal crucible having a small tip and cause a melt to form as shown in 

The tip acts in the same way to form the seed. Nevertheless, we have the same problem as when we use the metal caplUaiy. We need to obtain the seed free from its holder. Thus, the crucible must be sacrificed, or else the whole mass must be extracted from the crucible with the seed in the tip intact. 

If there is a sufficient difference in contraction between the mass and the crucible, perhaps then we can obtain the whole mass intact, including our seed. But this is rarely the case. 

---