Supersaturation nonuniform

As mentioned previously, a CVD reaction may occur in the gas phase instead of at the substrate surface if the supersaturation of the reactive gases and the temperature are sufficiently high. This is generally detrimental because gas-phase precipitated particles, in the form of soot, become incorporated in the deposit, causing nonuniformity in the structure, surface roughness, and poor adhesion. In some cases, gas-phase precipitation is used purposely, such as in the production of extremely fine powders (see Ch. 19). 

Most commonly, factors such as rapid crystal growth and uneven levels of supersaturation across the crystal interface are related to the formation of inclusions. The formation of inclusions in the crystals of ammonium dihydrogen phosphate (NH4H2PO4) and sodium chlorate (NaClOs) crystals have, for example, been correlated with sudden step changes in supersaturation (Brooks et al. 1968). Thus, it appears likely that nonuniform concentration and temperature in industrial crystallizers can also promote the formation of inclusions. 

It turns out that the distributions of a crystallizer s key variables are often not uniform. While it is often possible to achieve uniformity or near uniformity at the laboratory scale, it is almost never achieved at the industrial scale. Therefore, the effect of inhomogeneity in crystallizers can not be underestimated. Let us now consider the effects of nonuniform distribution of supersaturation. 

Electrolytically evolved gas bubbles affect three components of the cell voltage and change the macro- and microscopic current distributions in electrolyzers. Dispersed in the bulk electrolyte, they increase ohmic losses in the cell and, if nonuniformly distributed in the direction parallel to the electrode, they deflect current from regions where they are more concentrated to regions of lower void fraction. Bubbles attached to or located very near the electrodes likewise present ohmic resistance, and also, by making the microscopic current distribution nonuniform, increase the effective current density on the electrode, which adds to the electrode kinetic polarization. Evolution of gas bubbles stirs the electrolyte and thus reduces the supersaturation of product gas at the electrode, thereby lowering the concentration polarization of the electrode. Thus electrolytically evolved gas bubbles affect the electrolyte conductivity, electrode current distribution, and concentration overpotential and the effects depend on the location of the bubbles in the cell. Discussed in this section are the conductivity of bulk dispersions and the electrical effects of bubbles attached to or very near the electrode. Readers interested in the effect of bubbles dispersed in the bulk on the macroscopic current distribution in electrolyzers should see a recent review of Vogt.31... 

Uniform suspension is often the desired process result for process operations where a representative sample of solids is required or a uniform concentration of solids must be achieved. For example, in crystallization, nonuniform solids concentration may lead to unacceptably high local supersaturation levels and subsequent nonuniformity in crystal growth. Also, in as practical a way as possible, a slurry must be fed at a uniform solids concentration to a continuous reactor or to a centrifuge for uniform buildup of solids required for proper filtration and washing of the solid cake. 

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