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81.
Antonio Gasperini Angelo Malachias Gilberto Fabbris Guinther Kellermann Angelo Gobbi Esteban Avendao Gustavo de Medeiros Azevedo 《Journal of Applied Crystallography》2012,45(1):71-84
The formation of GeSi nanoparticles on an SiO2 matrix is studied here by synchrotron‐based techniques. The shape, average diameter and size dispersion were obtained from grazing‐incidence small‐angle X‐ray scattering data. X‐ray diffraction measurements were used to obtain crystallite sizes and composition via resonant (anomalous) measurements. By using these techniques as input for extended X‐ray absorption fine structure analysis, the local composition surrounding the Ge atoms is investigated. Although the results for each of the methods above are commonly analyzed separately, the combination of such techniques leads to an improved understanding of nanoparticle structural and chemical properties. Crucial indirect parameters that cannot be quantified by other means are accessed in this work, such as local strain, the possibility of forming core–shell structures, the fraction of Ge atoms diluted in the matrix (not forming nanoparticles), the amorphous and crystalline Ge fractions, and the relative population of nanoparticles with single and multiple crystalline domains. 相似文献
82.
This work concerns the modelling of stratified two-phase turbulent flows with interfaces. We consider an equation for an intermittency function which denotes the probability of finding an interface at a given time t and a given point . In Wacławczyk and Oberlack (2011) a model for the unclosed terms in this equation was proposed. Here, we investigate the performance of this model by a priori tests, and finally, based on the a priori data discuss its possible modification and improvements. 相似文献
83.
This paper proposes a method for the creation of hybrid meshes with embedded surfaces for viscous flow simulations as an extension of the multiple marching direction approach (AIAA J. 2007; 45 (1):162–167). The multiple marching direction approach enables to place semi‐structured elements around singular points, where valid semi‐structured elements cannot be placed using conventional hybrid mesh generation methods. This feature is discussed first with a couple of examples. Elements sometimes need to be clustered inside a computational domain to obtain more accurate results. For example, solution features, such as shocks, vortex cores and wake regions, can be extracted during the process of adaptive mesh generation. These features can be represented as surface meshes embedded in a computational domain. Semi‐structured elements can be placed around the embedded surface meshes using the multiple marching direction approach with a pretreatment method. Tetrahedral elements can be placed easily instead. A couple of results are presented to demonstrate the capability of the mesh generation method. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
84.
M. Bîrsan 《ZAMM - Journal of Applied Mathematics and Mechanics / Zeitschrift für Angewandte Mathematik und Mechanik》2008,88(1):74-78
This paper is concerned with the linear theory of porous Cosserat thermoelastic shells. We consider anisotropic and inhomogeneous shells and establish the existence and uniqueness of solution to the boundary–initial–value problem associated to the dynamic deformations. To this aim, we employ the semigroup of linear operators theory. 相似文献
85.
W. J. P. Van Enckevort P. Bennema 《Acta Crystallographica. Section A, Foundations and Advances》2004,60(6):532-541
During crystal growth, concentric steps of unit‐layer thickness [= , with the surface's hkl Miller indices corrected according to the selection rules for non‐primitive lattices] are often found to split into lower steps in a regular fashion [Frank (1951). Phil. Mag. 42 , 1014–1021]. These `interlaced' step patterns are introduced by a stacking of two or more growth layers, with different lateral anisotropy in step velocity within each unit layer. In this paper, a general relation between the symmetry of the crystal surface and the configuration of the concentric step patterns thereon is derived and is used to give theoretical shapes of spirals, growth hillocks and etch pits. It is shown that many of the interlaced patterns and their details are imposed by the presence of screw axes and/or glide planes perpendicular to the crystal surface. Finally, the results are compared with the patterns of unit‐layer height and lower steps observed by optical and atomic force microscopy on crystals such as SiC, GaN, potash alum, garnet and NiSO4·6H2O. 相似文献
86.
Markus Winterer Robert Delaplane Robert McGreevy 《Journal of Applied Crystallography》2002,35(4):434-442
Extended X‐ray absorption fine structure (EXAFS) and neutron scattering data from monoclinic zirconia are analysed independently and simultaneously by reverse Monte Carlo (RMC) modelling. X‐ray and neutron powder diffraction data are analysed by Rietveld refinement. The results are compared with respect to the local structure around the zirconium cations. Monoclinic zirconia was chosen as a model system for the comparison of structural information obtained by EXAFS spectroscopy and scattering methods because it is crystalline but also has some local disorder. In the case of zirconia, analysis of EXAFS spectra by RMC modelling results in reliable and accurate information on the local structure, consistent with neutron scattering and diffraction experiments. 相似文献
87.
88.
An attempt is made to go beyond the distorted‐wave Born approximation addressed to the grazing‐incidence small‐angle X‐ray (GISAX) scattering from a random rough surface. The integral wave equation adjusted with the Green function formalism is applied. To find out an asymptotic solution of the non‐averaged integral wave equation in terms of the Green function formalism, the theoretical approach based on a self‐consistent approximation for the X‐ray wavefunction is elaborated. Such an asymptotic solution allows one to describe the reflected X‐ray wavefield everywhere in the scattering (θ, ϕ) angular range, in particular below the critical angle θcr for total external reflection (θ is the grazing scattering angle with the surface, ϕ is the azimuth scattering angle; θ0 is the grazing incidence angle). Analytical expressions for the reflected GISAX specular and diffuse scattering waves are obtained using the statistical model of a random Gaussian surface in terms of the r.m.s. roughness and two‐point cumulant correlation function. For specular scattering the conventional Fresnel expression multiplied by the Debye–Waller factor is obtained. For the reflected GISAX diffuse scattering the intensity of the Rdif(θ, ϕ) scan is written in terms of the statistical scattering factor and Fourier transform of the two‐point cumulant correlation function. To be specific for isotropic solid surfaces, the statistical scattering factor and Fourier transform of the two‐point cumulant correlation function parametrically depend on the root‐mean‐square roughness σ [ = 0 for σ = 0] and cumulant correlation length , respectively. The reflected Rdif(θ, ϕ) scans are numerically simulated for the typical‐valued {θ0, σ, ℓ} parameters array. 相似文献
89.
Leandro M. Acua Diego G. Lamas Rodolfo O. Fuentes Ismael O. Fbregas Mrcia C. A. Fantini Aldo F. Craievich Rogrio J. Prado 《Journal of Applied Crystallography》2010,43(2):227-236
The local atomic structures around the Zr atom of pure (undoped) ZrO2 nanopowders with different average crystallite sizes, ranging from 7 to 40 nm, have been investigated. The nanopowders were synthesized by different wet‐chemical routes, but all exhibit the high‐temperature tetragonal phase stabilized at room temperature, as established by synchrotron radiation X‐ray diffraction. The extended X‐ray absorption fine structure (EXAFS) technique was applied to analyze the local structure around the Zr atoms. Several authors have studied this system using the EXAFS technique without obtaining a good agreement between crystallographic and EXAFS data. In this work, it is shown that the local structure of ZrO2 nanopowders can be described by a model consisting of two oxygen subshells (4 + 4 atoms) with different Zr—O distances, in agreement with those independently determined by X‐ray diffraction. However, the EXAFS study shows that the second oxygen subshell exhibits a Debye–Waller (DW) parameter much higher than that of the first oxygen subshell, a result that cannot be explained by the crystallographic model accepted for the tetragonal phase of zirconia‐based materials. However, as proposed by other authors, the difference in the DW parameters between the two oxygen subshells around the Zr atoms can be explained by the existence of oxygen displacements perpendicular to the z direction; these mainly affect the second oxygen subshell because of the directional character of the EXAFS DW parameter, in contradiction to the crystallographic value. It is also established that this model is similar to another model having three oxygen subshells, with a 4 + 2 + 2 distribution of atoms, with only one DW parameter for all oxygen subshells. Both models are in good agreement with the crystal structure determined by X‐ray diffraction experiments. 相似文献
90.
Owing to the relatively low intensity of diffraction and resulting low contrast topographic images, and also the richly detailed domain structure, it can be difficult to extract individual mosaic domains (size and shape) for protein crystals. Here, orthogonal projection was tested and found to be a superior approach over previous methods to extract mosaic domain information from a fine‐sliced topographic sequence of images. The topographic sequence was collected at room temperature from a lysozyme crystal with a low mosaicity. Orthogonal projection can be applied to image sequences that are spatially invariant and composed of linearly additive image formation processes. The domains were determined by using a particle swarm optimizer to fit Gaussians to the integrated intensity profile of the reflection as a function of angle, although any basis function could have been used. This optimization method was more amenable to automation and converged to the global minimum more efficiently. The number of domains can normally be determined by visually inspecting the integrated intensity profile; this only needs to be done once for each crystal, because the number of domains remains constant during collection, and not for each reflection. The results can be used to provide a picture of the internal structure of the crystal and may be useful to aid in the improvement of crystal growth techniques. 相似文献