Crystallization nucleus

Crystallization nuclei or cores are finely dispersed or macroscopic , solid particles in a fluid phase . They facilitate crystallization , i.e. the formation of crystals that would otherwise only be possible from oversaturated media. A distinction can be made between solidification , resublimation or precipitation processes.

Crystallization from solutions

Crystallization from solutions should begin as soon as the solubility product of the dissolved substances is exceeded. However, this is mostly not observed. When setting up a new phase, it must be noted that the associated change in free enthalpy is made up of two competing terms: ${\displaystyle \Delta G}$

${\displaystyle \Delta G=\Delta G_{V}+\Delta G_{\sigma }}$

• the thermodynamically favored volume fraction , which can be visualized as an increase in chemical bonds , ${\displaystyle \Delta G_{V}}$
• the work that must be done to create a new surface . ${\displaystyle \Delta G_{\sigma }}$

In the case of larger crystals, the volume fraction predominates, for very small crystals, on the other hand, the surface term, which is why one speaks of kinetically inhibited crystal growth . This is circumvented by the presence of crystal nuclei.

Crystallization from melts

In melts , crystallization nuclei arise at the beginning of a solidification process of crystalline elements , alloys and compounds . They form the starting point for their further crystallization. A group of atoms that happened to be in an arrangement in the crystal system of the substance in question when the solidification temperature was reached , remains in this position, releasing thermal energy .