Liquid-solid segregation selection

Adsorption is the phenomenon that selectively segregates atoms or molecules between a fluid and a solid. In some ways, it is like absorption, except the liquid-phase absorbent is replaced with a solid-phase ar/sorbent. Sometimes solids could be said to absorb or ar/sorb fluids, i.e., the distinction is blurred. In this chapter, the focus is on selective uptake (and release) by porous solids, especially for process applications. Another subtle distinction occurs when vapor is adsorbed from a gas it is like condensation, except that the driving force is subtle molecular forces rather than a temperamre gradient. 

All voltammetric equations obtained at the moment by solving simple problems of linear semi-infinite diffusion (in particular, the well-known Randles-Sevcik equation) do not take into account the specific characteristics of anodic selective dissolution of a homogeneous alloy solid phase segregation of an alloy components, initial roughness of an electrode, coupled solid-liquid phase transport, displacement of an alloy/solution interface, concentration dependence of the interdiffusion coefficient, presence of the vacancy sinks and relaxation of the non-equilibrium vacancy subsystem. 

One of the observations about Czochralski growth is that the concentration of impurities in the solid is generally much different (and fortunately often much lower) than in the liquid. The ratio of the concentration in the solid to the concentration in the liquid is referred to as the segregation ratio. Typical values for segregation ratios of selected impurities are given in Table 4.1. Some of the more striking values are for O (which actually prefers to be in the solid phase) and Fe, which is in equilibrium when the liquid contains one hundred twenty five thousand times more Fe than does the sohd. The latter is particularly fortunate as Fe is a major problem for semiconductor devices. Detectable degradations in performance may be found at the part-per-billion Fe level. Consequently, effective methods, such as those discussed above, to remove Fe are particularly important. 

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