7. Classical and Nonclassical Theories of Crystal Growth

In this chapter, we discuss classical and nonclassical concepts of crystal growth that coexist in the literature as explanations for the formation of both mono- and polycrystalline particles, often of the same substances. Crystalline particles with intraparticle nanosized subunits, nanoparticulate surface features, and complex morphologies have led to the development of new nonclassical theories of crystal growth based on the aggregation of nanocrystals in solution. At the same time, similar morphologies are explained by monomer incorporation at conditions of stress incorporation, which results in nucleation at the growth front and accompanying branching at the nanoscale. The two mechanisms are differently affected by important process variables like supersaturation, temperature, or additives and are analyzed with respect to their capability of predicting crystal growth rates. A quantitative description of the formation kinetics of the solid phases is essential for the design and operation of industrial precipitation and crystallization processes and for the understanding of fundamental principles in material design and biomineralization processes. In this chapter, we emphasize the importance of supersaturation in order to account for the extensive nanoparticle formation required to build micron-sized particles by nano-aggregative growth, as well as the accompanying change in the population density.