Crystal growth
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Crystal growth is the major stage of a crystallization process, after the nucleation stage. It occurs from the addition of new atoms, ions, or polymer strings into the characteristic arrangement, or lattice, of a crystal.
In the nucleation stage, a small nucleus containing the newly forming crystal is created. Nucleation occurs relatively slowly as the initial crystal components must "bump" into each other in the correct orientation and placement for them to adhere and form the crystal. After crystal nucleation, the second stage, growth, rapidly ensues. Crystal growth spreads outwards from the nucleating site. In this faster process, the elements which form the motif add to the growing crystal in a prearranged system, the crystal lattice, started in crystal nucleation. As first pointed out by Charles Frank in 1951, perfect crystals would only grow exceedingly slowly; real crystals grow comparatively rapidly because they contain screw dislocations, which provide the necessary growth points.
Nucleation can be either homogeneous, without the influence of foreign particles, or heterogeneous, with the influence of foreign particles. Generally, heterogeneous nucleation takes place more quickly since the foreign particles act as a scaffold for the crystal to grow on.
Heterogeneous nucleation can take place by several methods. Some of the most typical are small inclusions, or cuts, in the container the crystal is being grown on. This includes scratches on the sides and bottom of glassware. Other nucleating sites can be the dust, dandruff, and random other particles which are found in air. A common practice in crystal growing is to add a foreign substance, such as a string or a rock, to the solution, thereby providing a nucleating site for the project and speeding up the time it will take to grow a crystal.
The number of nucleating sites can also be controlled in this manner. If a brand-new piece of glassware or a plastic container is used, crystals may not form because the container surface is too smooth to allow heterogeneous nucleation. On the other hand, a badly scratched container will result in many lines of small crystals. To achieve a moderate number of medium sized crystals, a container which has a few scratches works best. Likewise, adding small previously made crystals, or seed crystals, to a crystal growing project will provide nucleating sites to the solution. The addition of only one seed crystal should result in a larger single crystal.
Some important features during growth are the arrangement, the origin of growth, the interface form (important for the driving force), and the final size. When origin of growth is only in one direction for all the crystals, it can result in the material becoming very anisotropic (different properties in different directions). The interface form determines the additional free energy for each volume of crystal growth.
Lattice arrangement in metals often takes the structure of body centered cubic, face centered cubic, or hexagonal close packed. The final size of the crystal is important for mechanical properties of materials (in metals it is widely acknowledged that large crystals can stretch further due to the longer deformation path and thus lower internal stresses).
