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.
Initial particle system requirements
The transition system of particles to a clear periodic structure is only possible when:
- The final crystalline state has a lower energy than the original non-crystal
- The current system has enough energy to overcome the energy "jumps" during the transition between the crystal lattices
The crystallization process starts when the two conditions above are satisfied.
For example, for any substance melt the first condition is achieved by cooling to a crystallization temperature, when for the transforming from the orthorhombic sulfur to the monoclinic one the second condition is achieved by heating to a transforming point.
Forming crystal seed
Let`s look at an array of chaotically moving particles (such as melt, solution, etc), cooled to the crystallization point or below.
Due to the huge number of these particles (for example, there are about 6.02 · 1023 (!) atoms in 12g of carbon) any moment you can find a group of particles which are close to lining up the right crystal lattice. This set has a lower energy than the surrounding particles but is usually unstable because the collision with the hotter particles lead to deformation or fracture. However, when it reaches a certain size, called the critical, the effect of a collision with a particle becomes close to zero as all it energy is readily absorbed by the lattice. At the same time that particle lowers its speed and is added to the lattice, because this state is energetically favorable for it.
This fragment of the lattice gains the right sizes and is called crystal seed, when the process of forming the seed is called nucleation.
Often the seed forms at the crystal surfaces with various defects and impurities as the defective lattice have more energy and therefore it captures particles that have speed loss to very small amount. Also, mechanical vibrations of the environment such noise or mechanic hit helps the nucleation process as they briefly reduce the distance between the particles, allowing them to line up in the seed crystal.
If there are no defects and vibrations crystallization can`t start for a long time even after cooling substance to temperature that is lower than crystallization point. Such supercooled or supersaturated state is unstable as the substance may start to crystallize by the slightest deformation or vessel vibrations.
As the cooling of the melt rapidly increases the number of particles which have speed loss, they increasingly tend to line up in a crystal.
After the adding of the seed to the melt or solution (it does not matter, it is formed independently or was introduced from the outside), the cooled particle`s precipitation occurs mainly at seed, sometimes forming small additional seeds at defects in its structure.
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 inserted to the lattice notes and form the correct structure, giving the seed a polyhedron shape. It is called normal crystal growth, such growing speed is used while forming single crystals' with great transparency, a few defects and correct form.
If the crystallization process is going too fast, the particles will generally form new seeds, combining to an increasingly large and irregular objects.
When growth is going only at one direction, these formations take the form of twigs. They are called dendrites, they are polycrystalline bodies with large seeds, connected in one direction.
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.
The particles settling on a seed speed and thus the crystallization speed dependent in a complicated manner on many factors such as:
- Particles concentration
- Seeds presence
- Substance cooling rate
- Solvent evaporation rate (for growing from the solution)
- Convection flows presence
- 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
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 - while growing 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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