Nucleation of a New Solid Phase

People usually admit that the transformation producing a new solid phase occurs via two processes, which are con )lex in themselves but are of quite different nature, nucleation and growth. 

Nucleation is the creation, starting from the initial phases, of small amounts of the final solid phase called nuclei . 

These two processes are generally thought to be basically different because nucleation is done in the absence, locally, of the final phase, whereas during growth, we have to consider the final phase that some species must cross either for the continuation of the reaction or to be eliminated. 

Nucleation of a solid phase starting from another solid phase, the first inevitable stage, plays a part that is very important but is very difficult to study. General information of the laws of nucleation, which were first established for the condensation of a vapor, but for which we can observe that they also apply to crystallization starting from the liquid solutions or the manufacture of a solid from another one, justifies the position of this chapter. We can often argue on the case of the appearance of the solid phase starting from a fluid and make the transpositions with the case, which interests us here, of the nucleation of a solid starting from another sohd. 

Chapter written in collaboration with Patrice Nortier. 

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The pseudo-steady state modes for a reaction, if the whole of the reaction is held in only a single zone or several zones whose dimensions always remain equal to each other, even if these sizes vary with time (we will encounter this case in the process of nucleation of a new solid phase on the surface of an initial solid phase, see chapter 8, the mode being a pseudo-steady state one). 

Nucleation of a New Solid Phase 263 The Gibbs energy of condensation at the interface is thus... 

There are obviously two steps involved in the preparation of crystal matter from a solution, the crystals must first form and then grow. The formation of a new solid phase either on an inert particle in the solution or in the solution itself is called nucleation. The increase in size of this nucleus with a layer-by-layer addition of solute is called crystal growth. Both nucleation and crystal growth have supersaturation as a common driving force. Unless a solution is supersaturated, crystals can neither form nor grow. The particle-size distribution of this weight, however, will depend on the relationship between the two processes of nucleation and growth. 

In the case of a very fast nucleation of the new solid phase of AC around each contact and if each grain is surrounded in the same way, this expression is tme at any given time or whatever the fractional extent. 

The second part (Chapters 7 to 11) presents the modeling of the reactions of solids by the introduction of the general concepts with the installation of the mechanisms and their resolutions in a single process (Chapter 7), the study of the nucleation process of a new solid phase (Chapter 8), the growth of the nucleus (Chapter 9), and the superposition of the two processes of nucleation and growth (Chapter 10). This part finishes with Chapter 11 which makes it possible to connect the concepts introduced by modeling to the experimental data. This part is largely devoted to space function. 

The presence of a crystalline solid phase in a supersaturated solution often causes new nuclei to form at appreciably lower levels of supersaturation than is required for primary nucleation events. Several mechanisms are responsible for this [6] ... 

An important consideration of reactions in which a new solid phase appears is the number, source and location of nucleation sites and the form of the new phase. A needle-like new phase can result in a marked apparent expansion of the material leading to particle disintegration in packed beds. 

Nucleation of a new phase in the solid state is more complicated than that of nucleation in freezing. Volume difference between the new and old phases causes an elastic misfit term that increases AG. Destruction of existing grain boundaries reduces AG. An expression for the free energy change during nucleation of 3 in a matrix of a is... 

Chemisorption reactions in soils, which are two-dimensional surface processes, can rarely be separated experimentally from the three-dimensional nucleation and precipitation reactions. It is perhaps best to view the removal of adsorbate ions from solution, broadly termed sorption, as a continuous process that ranges from chemisorption (at the low end of solubility) to precipitation (at the high end of solubility). Unless a new solid phase can be detected, the onset of precipitation and termination of chemisorption during sorption is usually not recognized by experimentalists. For this reason, an understanding of sorption necessitates some knowledge of precipitation reactions, which will be outlined here. 

The diagram in Figure 4.15 emphasizes the continuity of precipitation and coprecipitation processes with chemisorption, both in time and space. Low levels of adsorbate (whether metal cations or anions) are usually bound by chemisorption, higher levels by the formation of sohd solutions or by the nucleation of small adsorbate clusters at surfaces. The highest levels of adsorbate lead to precipitation of separate mineral phases, a process that can be viewed as an extension of cluster growth that allows a new solid phase to become detectable. 

Factors Affecting Secondary Nucleation. The rate of secondary nucleation is governed by three processes (1) the generation of secondary nuclei on or near a solid phase (2) removal of the clusters and (3) growth to form a new solid phase. Several factors... 

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