# 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 .