Cinética de la cristalización

Abstract

Liquids crystallize in three steps. First, the liquid should be metastable. A good example would be a liquid that has been supercooled beyond its temperature of coexistence. This means that its temperature and pressure should correspond to the crystalline phase in thephase diagram of the substance. Thus, the crystal would be more stable and one should expect the liquid to transform into it. However, this condition is not enough to observe the phase transition spontaneously. This is due to the fact that this is an activated process which may take long depending on the thermodynamic conditions 1. In this kind of process there is a transition between states that are separated by a free energy barrier. In this particular case, the free energy barrier appears due to the competition between thermodynamic stability and the energy cost of creating solid-liquid interface. The interfacial free energy is akey variable in the process since it is the one that goes against the transition allowing a metastable liquid to survive long times. When crystallization originates within a pure substance,the process is known as homogeneous nucleation but, in the presence of impurities or substrates, it is known as heterogeneous nucleation. In both cases, local uctuations are responsible for triggering the phase transition although whereas in homogeneous nucleationthey occur in the bulk of the metastable liquid, in the heterogeneous case they occur also near an interface which facilitates the nucleation 24...