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A seed crystal is a small piece of single crystal / polycrystal material from which a large crystal of the same material typically is to be grown. The large crystal can be grown by dipping the seed into a supersaturated solution, into molten material that is then cooled, or by growth on the seed face by passing vapor of the material to be grown over it.
The theory behind this effect is thought to derive from the physical intermolecular interaction that occurs between compounds in a supersaturated solution (or possibly vapor). In solution, liberated (soluble) molecules (solute) are free to move about in random flow. This random flow permits for the possibility of two or more molecular compounds to interact. This interaction can potentiate intermolecular forces between the separate molecules and form a basis for a crystal lattice. The placement of a seed crystal into solution allows the recrystallization process to expedite by eliminating the need for random molecular collision / interaction. By introducing an already pre-formed basis of the target crystal to act upon, the intermolecular interactions are formed much more easily / readily than relying on random flow. Often, this phase transition from solute in a solution to a crystal lattice will be referred to as nucleation. Seeding is therefore said to decrease the necessary amount of time needed for nucleation to occur in a recrystallization process.
One example where a seed crystal is used to grow large boules or ingots of a single crystal is the semiconductor industry where methods such as the Czochralski process or Bridgman technique are employed.