Nucleation, crystal tertiary

Three basic types of nucleation are considered at various dimensions primary nucleation, secondary nucleation, and tertiary nucleation. Primary nucleation is a three-dimensional nucleus newly formed by thermal fluctuations, with six extra nucleus surfaces if the nucleus is considered to be cubic. Secondary nucleation is two-dimensional nucleation on the advancing surface of nucleus, with four extra surfaces produced. Secondary nucleation is much easier than primary nucleation as its free energy barrier is lower, making crystallization kinetics like a self-acceleration process. Tertiary nucleation is one-dimensional nucleation at the step edge of the spreading layer on the advancing smooth surface of nucleus, with only two extra surfaces produced. Tertiary nucleation is so fast that it can rarely be observed. The schematic pictures of different types of nucleation are shown in Figure 13.8. 

Naturally, one can also propose a tertiary nucleation step for starting growth on a smooth ledge as seen at position 2 in Fig. 3.68. Its nucleation barrier would be even smaller than for secondary nucleation and even less likely to slow the crystal growth significantly. 

Fig. 10.23 Illustration of three basic situations of crystal nucleation. From left to right are primary nucleation in the bulk polymer phase, secondary nucleation on the smooth growth front, and tertiary nucleation at the terrace of the growth front...

Crystal growth occurs by secondary and tertiary nucleation (Fig. 8.2). The initial step is the formation of a secondary nucleus, which is followed by a series of tertiary nucleation events. 

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