Crystallization

Crystallization is a solid-state phase change in metals or crystalline solids where the atomic structure of the material is altered through heating and cooling. Recrystallization is the phase change in which the material is broken down and restructured into its crystalline structure in order to improve the material’s properties.

To recrystallize a material, it is heated, either on its own or mixed into a solvent, to break down the structure of the crystal. As heating ceases, the atoms of the material form amorphous nanoclusters that become the crystal nuclei. The remaining atoms position themselves around the nuclei to create a new crystal lattice, dependent on the rate at which the crystal is cooled.

In the presence of atmosphere, the crystal forms with thousands of trapped air pockets in it. Atmospheric oxygen, hydrogen, and nitrogen work their way into the lattice by dissolving into the solvent or by finding its way in during formation. Hydrogen is especially tendent of dissolving between the grain boundaries, significantly reducing the structural integrity of the crystal. For this reason, vacuum is applied to the crystallization chamber.

Vacuum provides one more key advantage: the lower the pressure in the chamber, the easier it is to melt the crystal or evaporate the solvent. When the chamber’s pressure drops, the boiling point of all materials within drop with it. This is especially useful for crystallizing heat-sensitive materials, as the heat energy required to melt or boil decreases, preserving the crystal.