The nucleation is a key process in order to understand the thermal process of the polymers , alloys and some ceramics . In the branches of chemistry and biology, it can even refer to the formation of multimers, which are defined as those that participate as intermediaries in polymerization processes . This is the best model that exists for the processes of crystallization and amyloidogenesis . In the branch of molecular biology, this process is used to end the critical phase in the assembly of polymeric structures such as microfilaments.. The nucleation process also presents two contributions that determine the stability of the nuclei formed: the free energy of solidification, and the free energy of formation of the new surfaces of the nucleus, which is unfavorable. A clear sample is, for example, when pure water freezes at 0 ° C, but can “cool down” to -42 ° C1 without freezing, if there are no nucleators for ice formation. Therefore, nucleators are important in meteorology , because there are often few nucleators present within the upper atmosphere .
Nucleation is the appearance of a new stable phase . It is an important and necessary process to understand the thermal process of polymers . It can also be defined as the formation of multimers, nucleation being the best way to intermediate in the polymerization processes, helping in the stability of the nuclei .
- Nucleation characteristics
- Mechanics of nucleation
Characteristics of nucleation
Nucleation is a process that generally occurs with more difficulty within a uniform substance . It is characterized because the thermal activation that it possesses will provide the necessary energy to be able to shape a stable nucleus . It has two different types, the homogeneous and the heterogeneous . This process can be called the appearance of a new phase.
Mechanics of nucleation
It can occur with difficulty within a substance that has characteristics of uniformity , and this is done through a process called homogeneous nucleation . Liquids that are cooled below the maximum heterogeneous nucleation temperature , but which are above the homogeneous nucleation temperature are said to be supercooled. In the heterogeneous, a certain amount of energy is released through the destruction of a part of the previous interface. At the end of the process, thermal activation will give us enough energy to create a stable nucleus . These can then grow until thermodynamic equilibrium is restored.
The transformation of liquids to solids occurs through two stages. The first of them is the nucleation of the solid phase in the middle of the liquid phase, where the generation of a solid-liquid surface that has a surface energy (energy per unit area) takes place and, the second of them during the solidification, both solid and liquid phases coexist.
The solidification process of metals is based on three different and important stages; formation of stable nuclei in the melt, the growth of the nucleus to give rise to crystals and the formation of a granular structure. The appearance of each of these grains after the solidification of the metal occurs will depend on numerous factors, one of the most important being thermal gradients . There are two different types regarding the solidification of metals and they are the following:
- Homogeneous nucleation : This type occurs in the molten liquid when the metal itself gives the atoms to achieve the formation of nuclei. When a pure liquid metal is cooled below its solidification equilibrium temperature correctly and in sufficient quantity, homogeneous nuclei are formed by the slow movement of atoms which tend to stay grouped. To achieve this requires a high degree of subcooling , which can even be hundreds of degrees Celsius for metals. In order for a stable nucleus to become a crystal, it must reach a critical size. The set of atoms that are intertwined with each other is called an embryo. The group with the largest critical size is called the nucleus .
- Heterogeneous nucleation : This is what happens in a liquid on the surface of the container that contains impurities that are insoluble, or some other structural materials that decrease free energy in order to form a nucleus that is stable. For this type of nucleation to occur, the solid nucleation agent must be wetted by the liquid metal . It takes place on the nucleating agent because the surface energy required to form a stable nucleus on the material is lower than if the nucleus were created on its own nucleus.