Sequences in Particle Growth

We have already shown that embryo formation leads to nucleation, and that this nucleation precedes any solid state reaction or change of state. 

In a like manner, nuelei must form in order for any precipitation process to proceed. Once formed, these nuclei then grow until impingement of the growing particles occurs. Impingement implies that all of the nutrient supplying the particle growth has been used up. This mechanism applies to both solid state reaction and precipitation processes to form product particles. Then a process known as Ostwald Ripening takes over. This process occurs even at room temperature. 

Eimbryo formation Nucleation Nuclei Growth Impingement 

We have already covered the first three in some detail. Impingement involves the point where the growing particles actually touch each other and have used up all of the nutrient which had originally caused them to start growing in the first place. This mechanism occurs in both precipitation and solid state reaction mechanisms. It is for this reason that we did not discuss precipitation until we examined solid state reaction mechanisms. 

Ostwald ripening differs jBrom nuclei growth in that the relative size and numbers of particles change, whereas in nuclei growth, the numbers of particles growing from nuclei do not change. Sintering, on the other hand, is an entirely different process, and usualty occurs when external heat is applied to the particles. 

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Although we have covered mechanisms relating to solid state reactions, the formation and growth of nuclei and the rate of their growth in both heterogeneous and homogeneous solids, and the diffusion processes thereby associated, there exist still other processes zifter the particles have formed. These include sequences in particle growth, once the particles have formed. Such sequences include ... 

These methods involve the formation of nuclei and the rate of their growth. We will examine such mechanisms as they relate to both solid state reactions and precipitation as methods for obtaining discrete particles (crystallites). Still other processes exist after particles have formed, including sequences in particle growth like sintering. 

The size of each cluster in the sequence as expressed through the number N of primary particles it contains is 5, 25, 125, and 625 for the four examples shown in Fig. 6.10. Thus, N = 5 where n = 1, 2,. .., denotes the stage of cluster growth in the sequence. In the same vein the characteristic dimension, L = 3nd0, where n = 1, 2,. . . , once again. The relationship between these two properties, size and dimension, can be expressed mathematically by 43... 

Finally, bioinspired techniques to control shape of metal nanocrystals are briefly described. Proteins, peptides or nucleic acids invariably possess sulfide, amide or carbonyl functions, while polypeptide sequences may initiate Au particle growth, resulting in thin platelets [263]. The shape control of AgNPs has also been observed with phage-display peptides [264]. 

Figure 6. Soot particle growth in a sequence VE test...

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