Impurity concentration gradient

Several theories have been proposed to explain secondary nucieation. These theories fall into two categories—one traces the origin of the secondary nuclei to the parent crystal—that include (1) initial or dust breeding (2) needle breeding and (3) collision breeding. Secondary nuclei can also originate from the solute in the liquid phase and the theories that take this into account include (1) impurity concentration gradient nucieation and (2) nucieation due to fluid shear. 

The impurity concentration gradient theory assumes that the solution is more structured in the presence of a crystal. This increases the local supersaturation of the fluid near the crystal, which is the source of crystal nuclie. Changes in the structure of the solution near the crystal surface have been observed experimentally. Dissolved impurities in the solution are known to inhibit nucleation rates. Some of the impurities are incorporated into the crystal surface. Thus, a concentration gradient is formed that enhances the probability of nucleation. Experimental evidence of the theory was presented for the nucleation of potassium chloride in the presence of lead impurities. As expected, stirring the solution causes the impurity concentration gradient to disappear and hence, lower the nucleation rates (Denk 1970). 

Botsaris, G.D., Denk, E.G. and Chua, J. (1972) Nucleation in an impurity concentration gradient - a new mechanism of secondary nucleation. AlChE Symposium Series No. 121, 68, 21-30. 

In systems where the admixture can easily be incorporated into the growing crystals lattice, the so-called impurity concentration gradient can be effective [22,611. Nucleatlon in the bulk of solution is hindered due to presence of the admixture at high concentration. Incorporation of the admixture into the crystal lattice leads to a decrease of Its concentration close to the surface so that spontaneous nucleatlon in the Intermediate layer becomes possible again. Presence of growth-restralners also exhibits an effect on nucleatlon [126 (they enlarge the metastable zone width [210]). 

Figure 1 shows the particulate loading of a pipe containing gas and particulates where the nonuniformity induced by a disturbance, ie, a 90° bend, is obvious (2). A profile of concentration gradients in a long, straight, horizontal pipe containing suspended soHds is shown in Figure 2. Segregation occurs as a result of particle mass. Certain impurities, eg, metal-rich particulates, however, occur near the bottom of the pipe others, eg, oily flocculates, occur near the top (3). Moreover, the distribution may be affected by Hquid-velocity disturbances and pipe roughness.

Component Separation by Progressive Freezing When the distribution coefficient is less than I, the first solid which ciystaUizes contains less solute than the liquid from which it was formed. As the frac tion which is frozen increases, the concentration of the impurity in the remaining liquid is increased and hence the concentration of impurity in the sohd phase increases (for k < 1). The concentration gradient is reversed for k > 1. Consequently, in the absence of diffusion in the solid phase a concentration gradient is estabhshed in the frozen ingot. 

One of us (RES) suggested over two years ago[6] that the resolution to this question lies in the electric field inherent to the arc plasma. As argued then, neither thermal nor concentration gradients are close to the magnitudes required to influence tip annealing, and trace impurities such as hydrogen, which might keep the tip open, should have almost no chemisorption residence time at 3000°C. The fact that w ell-formed nanotubes are found only in the cathode deposit, where... 

In general, it is fair to state that one of the major difficulties in interpreting, and consequently in establishing definitive tests of, corrosion phenomena in fused metal or salt environments is the large influence of very small, and therefore not easily controlled, variations in solubility, impurity concentration, temperature gradient, etc. . For example, the solubility of iron in liquid mercury is of the order of 5 x 10 at 649°C, and static tests show iron and steel to be practically unaltered by exposure to mercury. Nevertheless, in mercury boiler service, severe operating difficulties were encountered owing to the mass transfer of iron from the hot to the cold portions of the unit. Another minute variation was found substantially to alleviate the problem the presence of 10 ppm of titanium in the mercury reduced the rate of attack to an inappreciable value at 650°C as little as 1 ppm of titanium was similarly effective at 454°C . 

Diffusion in general, not only in the case of thin films, is a thermodynamically irreversible self-driven process. It is best defined in simple terms, such as the tendency of two gases to mix when separated by a porous partition. It drives toward an equilibrium maximum-entropy state of a system. It does so by eliminating concentration gradients of, for example, impurity atoms or vacancies in a solid or between physically connected thin films. In the case of two gases separated by a porous partition, it leads eventually to perfect mixing of the two. 

Unless carried out very carefully, data from flow reactors may be influenced by experimental uncertainties. Potential problems with the flow reactor technique include imperfect mixing of reactants, radial gradients of concentration and temperature, and catalytic effects on reactor walls. Uncertainties in induction times, introduced by finite rate mixing of reactants, presence of impurities, or catalytic effects, may require interpretation of the data in terms of concentration gradients, rather than just exhaust composition [442]. 

Fig. 13.27. Calculated concentration gradients of impurities in silicon melt...

If there is a change in morphology, this is a possible effect of impurities or high concentration gradients in the film and at the growing surface. 

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