Fick's First Law
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where J is the flux of an atomic species, D is the diffusion coefficient, C is the concentration of the species and x is the distance. Fick's First Law is used to study the movement of atoms (i.e. diffusion) within a material. Diffusion is very important in many material processes. Diffusion is also the controlling step in a large number of solid state transformations. Metallurgists and materials engineering try to control these transformation reactions in order to produce various structures, giving rise to desirable properties.
More details about Fick's First Law and other kinetics equations can be discoved in MTGN352 Metallurgical and Materials Kinetics taught by Prof. O'Hayre.