Cottrell atmosphere

Although this line of reasoning shows one of the principal features of heterogeneous nucleation, the real situation of nucleation near a dislocation line is much more complex [S. Q. Xiao, P. Haasen (1989)]. The stress field of the dislocation changes the composition of both the matrix and the precipitate, which in turn influences both yp and Agp. In view of this fact, one has to determine whether nucleation near the dislocation occurs before or after the Cottrell atmosphere around the dislocation had sufficient time to form. 

When all the SE s of a solid with non-hydrostatic (deviatoric) stresses are immobile, no chemical potential of the solid exists, although transport between differently stressed surfaces takes place provided external transport paths are available. Attention should be given to crystals with immobile SE s which contain an (equilibrium) network of mobile dislocations. In these crystals, no bulk diffusion takes place although there may be gradients of the chemical free energy density and, in multicomponent systems, composition gradients (e.g., Cottrell atmospheres [A.H. Cottrell (1953)]). 

For a > 1, nucleation is barrierless—i.e., the transformation is controlled solely by growth kinetics. However, for a < 1, a barrier exists. The local minimum of AQ (r) at point A in the plot corresponds to a metastable cylinder of /3 of radius r0 forming along the dislocation line. (In a sense, this is analogous to the Cottrell atmosphere described in Section 3.5.2.) In Eq. 19.54, the metastable cylinder s radius is... 

It may be summarized that Stroh [52] assumed that the dislocation sources in the grain adjacent to the pUe-up are locked by Cottrell atmospheres and, thus. 

Thompson, K., Flaitz, P.L., Ronsheim, P, Larson, D.J., Kelly,TF. (2007) Imaging of arsenic Cottrell atmospheres around silicon defects by three-dimensional atom probe tomography. Science, 317,1370-1374. 

Imaging lattice defects (dislocations, stacking faults) and dopant striations that actively influence the diffusion length, for example, by a Cottrell atmosphere of dopant atoms... 

The non-monotonous dependence of surface layer microhardness on deformation degree results from different mechanisms of nitrogen diffusion in deformed material. In our point of view, under the deformations of 3-8 and 20-30 % the greatest number of mobile dislocations, capable to provide the additional transfer of nitrogen interstitial atoms with Cottrell s atmospheres by the dislocation-dynamic mechanism [6-8], can be formed. 

Cottrell, Frederick G. (1877-1948). American scientist, inventor of an electrostatic precipitator, now known as Cottrell Precipitator, for smoke, dust fumes. Among other inventions are the pebble bed furnace, boiling point apparatus the Cottrell-Daniels process for fixation of atmospheric nitrogen. Cottrell was Director of US Bureau of Mines Director of the Fixed Nitrogen Research Laboratory, and founder of the Research Corporation, a nonprofit organization... 

Typical curves are shown in Fig. 22 a and b for current and potential steps respectively. For the short times (trate determining step is due to a concentration polarization [78], The variation of the transient current versus ]fi is linear, described satisfactorily by the classical Cottrel-type relation (see Fig. 23). After a certain time which depends on the experimental conditions, mainly the oxygen partial pressure in the surrounding atmosphere, a depolarization effect is observed the V (t) curve shows an overshoot while the I(t) curve shows a net enhancement of current. 

In at least one instance, in an urban area, selective discard of catalyst fines collected by a Cottrell precipitator has been practiced in order to decrease the per cent fines in the circulating catalyst and thereby minimize dust losses to the atmosphere (288a). 

Figure 4.51 indicates that the yielding in sapphire undergoing basal slip is a consequence of dislocation multiplication and is not due to the unpinning of a Cottrell-type atmosphere, where dislocation pinning results from impurities. The study of two types of sapphires, with different initial dislocation densities, was meant to point out the difference in their surface dislocation densities and the consequent differences in their yield phenomena. 

In the Cottrel precipitator, colloidal solids are removed from gaseous smokestack wastes before they are released into the atmosphere. The precipitator contains a number of highly charged plates or electrodes. As smoke passes over the charged surfaces, the colloid particles lose their charges. The particles then coalesce into larger particles that settle out and are collected for disposal. 

Apart from its function as a point defects trap, Ti diffuses towards structure dislocations to form Cottrell-type atmospheres that can block the restoration and the rise of the initial lattice. Weertman and Green [41] demonstrated that these dislocations, thus decorated by dense clusters of large solutes such as Ti, become neutral sinks that cause unbiased elimination of point defects and therefore an increase in the resistance to swelling. Furthermore, by screening dislocations from the arrival of point defects, these clusters are probably conducive to mutual recombinations close to the decorated dislocations which will also tend to increase the resistance to swelling ... 

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