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Old 03-01-14, 21:06   #1
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Chẳng là em nhóm em đang nghiên cứu phần định luật fick nhưng vướng đoạn Gradien nồng độ không tài nào dịch được, anh chị giúp đỡ em được không ạ.
Concentration Gradient The concentration gradient shows how the composition of the material varies with distance: c is the difference in concentration over the distance x (Figure 5-9). A concentration gradient may be created when two materials of different composition are placed in contact, when a gas or liquid is in contact with a solid material, when nonequilibrium structures are produced in a material due to processing, and from a host of other sources.
The flux at a particular temperature is constant only if the concentration gradient is also constant—that is, the compositions on each side of the plane in Figure 5-8 remain unchanged. In many practical cases, however, these compositions vary as atoms are redistributed, and thus the flux also changes. Often, we find that the flux is initially high and then gradually decreases as the concentration gradient is reduced by diffusion. The examples that follow illustrate calculations of flux and concentration gradients for diffusion of dopants in semiconductors and ceramics, but only for the case of constant concentration gradient. Later in this chapter, we will consider non-steady state diffusion with
the aid of Fick’s second law.
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