Technology & Engineering
Nucleation
Nucleation is the process by which a new phase or structure forms within a material. In engineering, it is crucial for understanding the formation of crystals in metals, the initiation of bubbles in liquids, and the development of new phases in materials. Nucleation plays a significant role in the design and control of various technological processes, such as crystallization and phase transformations.
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3 Key excerpts on "Nucleation"
- Donald Askeland, Wendelin Wright(Authors)
- 2018(Publication Date)
- Cengage Learning EMEA(Publisher)
307 9-2 Nucleation 9-2 Nucleation In the context of solidification, the term Nucleation refers to the formation of the first nanocrystallites from molten material. For example, as water begins to freeze, nanocrys-tals, known as nuclei , form first. In a broader sense, the term Nucleation refers to the initial stage of formation of one phase from another phase. When a vapor condenses into liquid, the nanoscale-sized drops of liquid that appear when the condensation begins are referred to as nuclei. Later, we will also see that there are many systems in which the nuclei of a solid ( b ) will form from a second solid material ( a ) (i.e., an a - to b -phase transformation). What is interesting about these transformations is that, in most engineered materials, many of them occur while the material is in the solid state (i.e., there is no melting involved). Therefore, although we discuss Nucleation from a solidifica-tion perspective, it is important to note that the phenomenon of Nucleation is general and is associated with phase transformations. We expect a material to solidify when the liquid cools to just below its freezing (or melting) temperature because the energy associated with the crystalline structure of the solid is then less than the energy of the liquid. This energy difference between the liquid and the solid is the free energy per unit volume D G v and is the driving force for solidification. When the solid forms, however, a solid-liquid interface is created [Figure 9-1(a)]. A surface free energy s sl is associated with this interface.
- Donald Askeland, Wendelin Wright, Donald Askeland(Authors)
- 2020(Publication Date)
- Cengage Learning EMEA(Publisher)
Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 307 9-2 Nucleation 9-2 Nucleation In the context of solidification, the term Nucleation refers to the formation of the first nanocrystallites from molten material. For example, as water begins to freeze, nanocrys- tals, known as nuclei, form first. In a broader sense, the term Nucleation refers to the initial stage of formation of one phase from another phase. When a vapor condenses into liquid, the nanoscale-sized drops of liquid that appear when the condensation begins are referred to as nuclei. Later, we will also see that there are many systems in which the nuclei of a solid (b) will form from a second solid material (a) (i.e., an a- to b-phase transformation). What is interesting about these transformations is that, in most engineered materials, many of them occur while the material is in the solid state (i.e., there is no melting involved). Therefore, although we discuss Nucleation from a solidifica- tion perspective, it is important to note that the phenomenon of Nucleation is general and is associated with phase transformations. We expect a material to solidify when the liquid cools to just below its freezing (or melting) temperature because the energy associated with the crystalline structure of the solid is then less than the energy of the liquid. This energy difference between the liquid and the solid is the free energy per unit volume DG v and is the driving force for solidification. When the solid forms, however, a solid-liquid interface is created [Figure 9-1(a)]. A surface free energy s sl is associated with this interface.
eBook - PDF- J W Mullin(Author)
- 2001(Publication Date)
- Butterworth-Heinemann(Publisher)
At the present time there is no general agreement on Nucleation nomenclat-ure so to avoid confusion the terminology to be used in this and subsequent chapters will be defined here. The term `primary' will be reserved for all cases of Nucleation in systems that do not contain crystalline matter. On the other hand, nuclei are often generated in the vicinity of crystals present in a supersaturated system; this behaviour will be referred to as `secondary' Nucleation. Thus we may consider a simple scheme: N U C LEATI ON HO M O GE N E O US ( spontaneous ) H ETER O GE N E O US ( induced by foreign particles ) SE CON DARY ( induced by crystals ) PRIMARY 5.1 Primary Nucleation 5.1.1 Homogeneous Nucleation Exactly how a stable crystal nucleus is formed within a homogeneous fluid is not known with any degree of certainty. To take a simple example, the con-densation of a supersaturated vapour to the liquid phase is only possible after the appearance of microscopic droplets, called condensation nuclei, on the condensing surface. However, as the vapour pressure at the surface of these minute droplets is exceedingly high, they evaporate rapidly even though the surrounding vapour is supersaturated. New nuclei form while old ones evap-orate, until eventually stable droplets are formed either by coagulation or under conditions of very high vapour supersaturation. The formation of crystal nuclei is an even more difficult process to envisage. Not only have the constituent molecules to coagulate, resisting the tendency to redissolve (section 3.7), but they also have to become orientated into a fixed lattice. The number of molecules in a stable crystal nucleus can vary from about ten to several thousand: water (ice) nuclei, for instance, may contain about 100 molecules. However, a stable nucleus could hardly result from the simultaneous collision of the required number of molecules since this would constitute an extremely rare event.
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