Vacancies clusters

Typical surfaces observed in Ising model simulations are illustrated in Fig. 2. The size and extent of adatom and vacancy clusters increases with the temperature. Above a transition temperature (T. 62 for the surface illustrated), the clusters percolate. That is, some of the clusters link up to produce a connected network over the entire surface. Above Tj, crystal growth can proceed without two-dimensional nucleation, since large clusters are an inherent part of the interface structure. Finite growth rates are expected at arbitrarily small values of the supersaturation. 

Fig. 5.24 Crystalline metal face (a) ad-atom, (b) ad-atom cluster, (c) a kink (half-crystal position), (d) atom at the step, (e) atom in the kink, (f) vacancy, (g) vacancy cluster. (According to E. Budevski)... 

Figure 3.5 STM images from a movie showing the formation, separation, and annihilation of H-vacancy clusters. The images on the left (3 nm x 2.5 nm) are repeated on the right with annotations, (a) Five vacancies near the center are labeled (A-E) and two triangular vacancy pairs (2V) are marked with dashed triangles for reference, (b) Vacancies A and B have formed a 2V cluster indicated by the triangle containing the number 2 . Vacancies C-E have formed a...

As a continuum approximation, this approach should break down by the atomistic level. For islands it is presumably inappropriate for the small clusters imaged with FIM. More importantly, in many cases the stiffness may not be nearly anisotropic, as we have assumed it to be in our analysis. Then, as perhaps for Ag( 100) islands, new mechanisms may play a role. For vacancy clusters, there can be trapping in corners in systems that might seem to be cases of PD from consideration of vicinal surfaces. 

It is found that the atomic arrangement, or a vacancy network, in a depleted zone in a refractory metal or a dilute alloy of a refractory metal, created by bombardment of an ion can be reconstructed on an atomic scale from which the shape and size of the zone, the radial distribution function of the vacancies, and the fraction of monovacancies and vacancy clusters can be calculated. For example, Wei Seidman108 studied structures of depleted zones in tungsten produced by the bombardment of 30 keV ions of different masses, W+, Mo+ and Cr+. They find the average diameters of the depleted zones created by these ions to be 18,25 and 42 A, respectively. The fractions of isolated monovacancies are, respectively, 0.13,0.19and0.28,andthe fractions of vacancies with more than six nearest neighbor vacancies (or vacancy clusters) are, respect-... 

Fig. 5.19 A helium field ion image of a tungsten tip electroplated with a thick layer of platinum. Many lattice defects such as grain boundaries, twin boundaries, vacancies and vacancy clusters can be seen. From the work of Rendulic ...

As mentioned previously (see Section 1.3.5) the binary M-X system shows a phase separation phenomenon in which the phase decomposes into two phases, having lower and higher concentrations of vacancies, below the critical temperature f, under the condition < 0, i.e. there is an attractive force between vacancies. In Section 1.3.5 it was not possible to refer to the details of those structures, because the model was less than simple. In any case, it can be safely said that if s < 0, vacancies cluster at low temperatures (from a thermodynamic point of view). Here let us briefly review the non-stoichiometry of 3d transition metal monoxides Mj- O, and then discuss the Fej system as a typical example of the clustering of vacancies in detail. 

The conductivity of the sample 4, decreases by the formation of the cation vacancy clusters. In the case of 17.2 mol % Gd2(S0A)3 mixed sample( 5), the oT becomes higher at relatively lower temperature. However, the discontinuity exists at about 543 K and the conductivity becomes lower than that of the samples 1-3. From these results, the samples which maintain potential characteristics for the electrolyte are 1-3. 

Poster 31. Masataka Mizuno, Teruo Kihara, Hideki Araki, Yasuharu Shirai and Takashi Onishi (Osaka University, Kobe Steel Ltd.) Theoretical Calculation of Positron Lifetimes of Vacancy Clusters in Cu... 

Nanoparticle materials are important because they exhibit unique properties due to size effects, quantum tunneling, and quantum confinement. As sizes of embedded particles are reduced to the nanometer scale, the surface-to-bulk ratio increases significantly. Therefore, surface effects can dominate bulk properties and an understanding of nanosurfaces becomes important. In this chapter, we discuss characterization of vacancy clusters that reside on surfaces of embedded nanoparticles as well as studies on the correlation of surface vacancy clusters to the properties of the nanomaterials. 

A new spectroscopic method for the characterization of surface vacancy clusters is a combination of positron lifetime spectroscopy, which determines the size of vacancy clusters, and coincidence Doppler broadening of annihilation radiation, which gives information on where vacancy clusters are located [5, 6]. If these clusters are located on the surface of gold nanoparticles, namely the interface between the particle and host matrix, the surroundings of the clusters should include both particle atoms and the matrix atoms. Doppler broadening of annihilation radiation (DBAR) with two-detector coincidence should be able to reveal these atomic constituents, and therefore elucidate the location of vacancy clusters. 

Vacancy clusters on the surface of gold nanoparticles embedded in MgO... 

Subnanometer vacancy clusters are a special group of quantum anti-dots that have a localized electronic structure. A great deal of literature has been accumulated on anti-dot or quantum void systems in semiconductor or metal—oxide thin films [14], The positron data described above may... 

J. J. GILMAN (General Electric) Prof. Davis and one of his students at Harvard has recently shown that during rapt cooling from high temperatures, vacancy clusters precipitate at the surfaces of metals. Could such vacancy clusters be the nucleatien centers for oxidation ... 

N. CABRERA Vacancy clusters could conceivably be nucleation sites for oxidation however, the number of such clusters at the surface must be very high, in the order of 10 per cm, to account for the observed nuclei density. Furthermore, at the high temperatures of oxidation that I have been discussing, one could question the existence of vacancy clusters at the surface. 

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