Impurity drag

Fig. 8 reveals that under the assumption of reasonable values for the adjustable parameters in Eq. (7) the theoretical predictions compare well to the experimental data. The very different behaviour of special and non-special boundaries reflects an influence of grain boundary structure on grain boundary migration mechanism. In particular, impurities not only may have an effect on migration by impurity drag, but also by changing grain boundary structure itself. This was shown recently by Udler and Seidman in a Monte Carlo... 

The effect of solute atoms on grain boundary migration cannot adequately be described by standard impurity drag theories. A more satisfactory agreement is obtained by taking an interaction ofthe impurities in the boundary into account. 

J. W. Cahn,The impurity-drag effect iu graiu boimdary motion, Acta Metall. 10 789 (1962). 

J.W. Cahn. The impurity-drag effect in grain boundary motion. Acta Metall., 10(9) 789—798, 1962. 

When we have an impure sin e phase system, the impurities can give rise to an impurity drag effect which impedes boundary motion... 

Drag substances <0.2-1.0% of each specified impurity Drag products <0.2-1.0% of each specified degradant (Total impurity/degradant limits are also specified) Accuracy three levels in triplicate (LOQ - 1%), <+10-20% of spike values Precision six replicate at LOQ, <10-20% RSD Specificity resolve key specified impurities and degradants... 

There are essentially two strategies that can be employed to prevent pore breakaway, namely, reduce grain boundary mobility and/or enhance pore mobility. An example of how slowing grain boundary mobility enhances the final density is shown in Fig. 10.6g, where the grain size versus density trajectories for two aluminas, one pure and the other doped with 250 ppm MgO, are compared. It is obvious from the results that the doped alumina achieves higher density — the reason is believed to be the result of impurity drag on the boundary by the MgO. 

On the other hand, potential measurements at the free surface of purified water have shown50 that the value for a flowing surface differs by about 0.3 V from that for a quiescent surface, as a result of adsorption of surface-active residual impurities in the solution (probably also coming from the gas phase). Since emersed electrodes drag off the surface layer of the solution as they come out of the liquid phase, the liquid layer attached to emersed solid surfaces might also be contaminated. 

Contaminated plating baths, for example, carbonate buildup in cyanide baths, can increase drag-out as much as 50% by increasing the viscosity of the bath. Excessive impurities also make the application of recovery technology difficult, if not impossible. 

In impure metals, dislocation motion ocures in a stick-slip mode. Between impurities (or other point defects) slip occurs, that is, fast motion limited only by viscous drag. At impurities, which are usually bound internally and to the surrounding matrix by covalent bonds, dislocations get stuck. At low temperatures, they can only become freed by a quantum mechanical tunneling process driven by stress. Thus this part of the process is mechanically, not thermally, driven. The description of the tunneling rate has the form of Equation (4.3). Overall, the motion has two parts the viscous part and the tunneling part. 

N. J. Haskins, C. Eckers, A. J. Organ, M. F. Dunk and B. E. Winger, The use of electrospray ionization with Fourier transform ion cyclotron resonance mass spectrometry in the analysis of trace impurities in a drag substance. Rapid Communications in Mass Spectrometry, 1995,9(11), 1027-1030. 

As for other types of fluid particle, the internal circulation of water drops in air depends on the accumulation of surface-active impurities at the interface (H9). Observed internal velocities are of order 1% of the terminal velocity (G4, P5), too small to affect drag detectably. Ryan (R6) examined the effect of surface tension reduction by surface-active agents on falling water drops. 

The lifetime of a plating solution is limited by the accumulation of impurities and/or by depletion of constituents due to drag-out. Drag-out reduction is examined later in Section 8.0. The impurities come from six sources racks, dropped parts, anodes, drag-in, water... 

The plating bath can also contain impurities which cannot be removed by ordinary filtration, such as solubilized metals dragged in from a previous etching bath (Foecke 1989). Ion exchange (IE) treatment can often be used for removing solubilized metals, although IE systems are fairly expensive compared to standard filtration technologies. 

The non-ionic surfactants have an advantage over all the drag-reducing additives studied so far. They are both mechanically and chemically stable, do not precipitate in the presence of calcium, and hence can be used in all impure waters, sea water, brackish water, or concentrated brine solutions. They have an excellent potential for drag reduction at high temperatures. 

If motion is unaffected by drag effects due to impurity atoms, it is called intrinsic. 

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