Crystallization
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Main article: Crystallization.
Crystallization - the process of transition from a substance to a crystalline state. Typically, it means the formation of crystals from solutions, melts, gases, volatile substances and amorphous crystalline structures.
Forming crystal seed
Let`s look at some melted substance cooled to the crystallization point or below.
Due to the huge number of molecules in a substance (for example, there are about 6.02 · 1023 (!) atoms in 12g of carbon), at any moment you can find a group of molecules which are close to lining up the right crystal lattice. It is usually unstable because the collision with the more energetic (hotter) molecule lead to deformation or fracture of this lattice. However, when it the lattice a certain size, called the critical, the effect of a collision with such a molecule becomes close to zero as all it energy is quickly absorbed. After collision, the molecule is slowed down enough to become a part of this crystal lattice.
This fragment of the lattice gained the right size is called crystal seed, and the process of forming the seed is called nucleation.
Often the seed forms at the crystal on various defects and impurities. Also mixing the melted substance or disturbing it in any other way helps the nucleation process as they reduce the distance between the some molecules, allowing them to line up properly and form a seed crystal.
If there are no defects or any disturbance, crystallization is not going to, even after cooling substance to a temperature far below the crystallization point. Such supercooled or supersaturated state is metastable as the substance may start to crystallize after a slightest touch.
Crystal growth
As the cooling of the melted substance rapidly decreases the overall speed of of molecules, they tend to line up in a crystal.
After placing the seed to the melt or solution (it does not matter, it is formed independently or was introduced from the outside), molecules are precipitate mainly at this seed.
Depending on the growth speed, the growth direction of the seed can vary.
If it is below a certain limit, the new particles will have time to be align and form the correct crystal structure, giving the seed a polyhedron shape. It is called normal or tangential crystal growth, such growing speed leads to forming single crystals' with great transparency, a few defects and very distinct shape.
If the crystallization process is too fast, particles will form mostly new seeds instead of bonding to existing ones.
When growth is going only at one direction, these seeds are aligned to form of twigs. They are called dendrites, they are polycrystalline bodies with few large seeds.
If growth goes in all directions, then particles are forming a shapeless lumps consisting of seeds of different sizes. This called skeletal growth, the resulting formation is a typical polycrystalline with a lot of defects and impurities inclusions.
Growth factors
The speed of binding molecules into a crystal lattice, and thus crystallization speed depend on many factors such as:
- Molecules concentration
- The presence of seeds
- Substance cooling rate
- Solvent evaporation rate (for growing from the solution)
- The presence of Convection flows
- Stirring of the substance or the movement of seed
- The presence of strong magnetic and electric fields that cause the spatial orientation and motion of particles
Recrystallization
Main article: Methods of compounds purification.
Recrystallization - chemical substance's purification procedure, repeating of crystallization process using crystals from previous step as a raw material.
Substances during crystallization process can capture various impurities, both soluble and insoluble ones, but in very small amounts to its total weight - is much smaller than achieved by many chemical cleaning methods.
Due to this property, crystallization of the substance leads to a significant purification level - while recrystallizing the same substance as single crystals for two or three times you may reduce the number of impurities up to 10-5 %!
The main disadvantages of this method are a large loss of substance and high requirements on the crystallization speed.
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