Dissolution cluster

The microstmcture appeared well mixed although co-continuity of the phases was not obvious. The blends appeared to have a continuous PP phase containing extended, yet isolated, SBR components as shown in Figure 11.17. It appeared to be similar to the microstmcture of the TPV-based on nylon and EPDM. The presence of entrapped air or mumal dissolution was not observed. As the fraction of PP increased, the microstmctures became clustered into larger PP and SBR single phases, with lower SBR-PP interface area. Both the materials were shear thinning. There is a large decrease in the viscosity of the composites at small shear rate. The viscosity values of the phases followed the equation... 

The reader is invited to examine this phenomenon by running the models described above, by varying these two sets of parameters. The solute is modeled as a 10 X 10 block of 100 cells in the center of a 55 x 55 cell grid. The water content of the grid is 69% of the spaces around the solute block, randomly placed at the beginning of each run. The water temperature (WW), solute-solute afiinity (SS), and hydropathic character of the solute (WS) are presented in the parameter setup for Example 4.4. The extent of dissolution as a function of the rules and time (5000 iterations) is recorded as the fo and the average cluster size of the solute (S). 

A remarkable feature of the clusters generated by the present procedure was their unusual stability. In fact, it was found that Cu nanoclusters generated on Au(lll) surfaces presented the amazing property of remaining stable at potentials above the reversible dissolution potential for bulk Cu (Kolb et ah, 2002). 

This is not easy to understand, since on thermodynamic grounds clusters should be less stable than the bulk material. It is possible that clusters generated by electrochemical nanostructuring may undergo a certain degree of alloying with the material of the surface that could increase their stability. Indeed, Monte Carlo simulations show that if achievable, such alloying will improve the stability of the clusters toward dissolution. 

Experiments and simulations show that the characteristics of the nanostructures generated by this procedure are basically given by live parameters the distance between the STM and the substrate, the quantity of material loaded on the tip, the maximum ion current density for the dissolution of the material on the tip, the potential of the substrate, and the diameter of the STM apex. The controlled variation of these five parameters allows tailoring of the diameter and height of the clusters. 

The second procedure is different from the previous one in several aspects. First, the metallic substrate employed is Au, which does not show a remarkable dissolution under the experimental conditions chosen, so that no faradaic processes are involved at either the substrate or the tip. Second, the tip is polarized negatively with respect to the surface. Third, the potential bias between the tip and the substrate must be extremely small (e.g., -2 mV) otherwise, no nanocavity formation is observed. Fourth, the potential of the substrate must be in a region where reconstruction of the Au(lll) surface occurs. Thus, when the bias potential is stepped from a significant positive value (typically, 200 mV) to a small negative value and kept there for a period of several seconds, individual pits of about 40 nm result, with a depth of two to four atomic layers. According to the authors, this nanostructuring procedure is initiated by an important electronic (but not mechanical) contact between tip and substrate. As a consequence of this interaction, and stimulated by an enhanced local reconstruction of the surface, some Au atoms are mobilized from the Au surface to the tip, where they are adhered. When the tip is pulled out of the surface, a pit with a mound beside it is left on the surface. The formation of the connecting neck between the tip and surface is similar to the TILMD technique described above but with a different hnal result a hole instead of a cluster on the surface (Chi et al., 2000). 

In situ SAXS investigations of a variety of sol-gel-derived silicates are consistent with the above predictions. For example, silicate species formed by hydrolysis of TEOS at pH 11.5 and H20/Si = 12, conditions in which we expect monomers to be continually produced by dissolution, are dense, uniform particles with well defined interfaces as determined in SAXS experiments by the Porod slope of -4 (non-fractal) (Brinker, C. J., Hurd, A. J. and Ward, K. D., in press). By comparison, silicate polymers formed by hydrolysis at pH 2 and H20/Si = 5, conditions in which we expect reaction-limited cluster-cluster aggregation with an absence of monomer due to the hydrolytic stability of siloxane bonds, are fractal structures characterized by D - 1.9 (Porod slope — -1.9) (29-30). 

Figure 5.17 STM image of H-terminated n-Si(l 1 1) in 0.1 M HCI04 + 1 mM Pb(CI04)2, onto which two Pb clusters have been deposited by a burst-like dissolution of Pb from the STM tip. (Reproduced with permission from Ref. [77].)...

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