Facets Form

The anodic dissolution of metals on surfaces without defects occurs in the half-crystal positions. Similarly to nucleation, the dissolution of metals involves the formation of empty nuclei (atomic vacancies). Screw dislocations have the same significance. Dissolution often leads to the formation of continuous crystal faces with lower Miller indices on the metal. This process, termed facetting, forms the basis of metallographic etching. 

The geometry of the pore tips is pyramidal, with facets formed by (111) planes, for Jap < JPS. This is the case when the current density is limited by the applied bias, as is the case for the samples shown in Fig. 9.13 b and c. If the bias is increased, dissolution at the pore tip occurs partially in the isotropic electropolishing regime (/tip=/ps)- This reduces the tendency to form facets and the tip geometry becomes almost hemispherical, as shown in Fig. 9.13d. 

The rate of the reaction varied with the face exposed. The nature of the facets formed and the relative activities of different faces varied rapidly with changes in the experimental conditions. 

Au(lll) facets formed on a single crystal bead were produced by annealing Au wire in a H2-O2 flame to form a droplet, which was quenched in pure water saturated with H. The bead was transferred into a STM cell... 

Fig. 3.5. STM image at the atomic scale of a 4-nm Pd particle supported on a graphite (0001) substrate. The STM image reveals unambiguously that this particle reveals (100) facets forming a regular tetrahedron [17]...

Figure 1 shows the current-potential curve for a clean and an iodine-coated Pd(lll) facet, formed on a single-crystal bead, in halide-free 0.05 M H2SO4. The exceedingly large anodic peak at about 1.2 V represents the Pd (s)-to-Pd (aq) anodic stripping that occurs only when interfacial iodine is present [4, 5]. 

On the other hand, in the reaction of H2 and O2 on a copper single crystal, the reaction rate depends on the orientation of the facets formed by rearrangement of the surface. The facets, however, are rather large—of the order of microns—and these results do not conflict with the idea that very small facets would be less important to the surface properties than the over-all orientation. 

A. T. Gwathmey University of Virginia) I should like to ask Professor Farnsworth if in his experiments he has any information (1) on the smoothness of his surfaces on an atomic scale or (2) on any facets formed during the treatment of his surfaces. 

Fig. 3-25a shows the surface of Ce02 colloid particles, 3-25b shows a Ce02 particle annealed at 500 "C and finally 3-25c shows a Tb02 particle. The surface of all of them is build from lll -facets forming steps on a flat surface. This step surface exhibits a high surface area, which might be of interest for the reactivity of catalysts. 

Pyramid Indenters. The most common pyramid indenters are the Vickers, Knoop, and Berkovich indenters. The Vickers indenter consists of a square-based diamond pyramid with included angles (a) of 136° between nonadjacent faces. The Knoop test uses a rhombic-based diamond with included angles of 172° and 130° between opposite edges. The Berkovich indenter is a diamond trigonal pyramid whose facets form an angle of 65.3° with respect to the normal to the base. 

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