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Metallic Nanoglasses Investigated by Molecular Dynamics Simulations
Published in Klaus D. Sattler, st Century Nanoscience – A Handbook, 2019
Metallic glasses possess interesting properties in comparison with the crystalline state. Due to the amorphous structure (absence of long-range order), the displacement of atoms, e.g., to accommodate a dislocation is obstructed. With no crystal defects, metallic glasses show special properties such as high strength (twice that of steel, but lighter), increased hardness, high toughness and elasticity [9,10]. Also, some metallic glasses have high corrosion resistance [11] and interesting magnetic properties [12]. Holding these unique characteristics, metallic glasses are interesting candidates for structural and functional applications. However, the current limits of these materials are tied to the poor plasticity at room temperature. The lack of plastic strain is a consequence of shear softening originating from shear-induced dilatation that causes plastic strain to be highly localized in shear bands [13]. The detrimental nature of these defects will promote premature fracture of BMGs. In order to avoid the formation of critical shear bands and consequently to stabilize the glass against catastrophic failure, soft heterogeneities, ranging from intrinsic structural fluctuations [14,15] to secondary phases [16,17], must be generated into the structure. In general, the role of heterogeneities is to introduce strain concentrations which act as initiation sites for shear bands and effectively distribute the applied strain. Consequently, the generation of a high density of shear bands which in turn interact in a very complex way with each other or other heterogeneities would hinder the early failure of BMGs.
Crystal Structure
Published in Alan Owens, Semiconductor Radiation Detectors, 2019
The majority of semiconductors solidify into regular periodic crystallographic patterns (Fig. 3.12(a)). However, they can also form amorphous12,13 solid solutions in which the arrangement of the atoms exhibits no periodicity or long-range order at all (Fig. 3.12(c)). The presence or absence of this long-range order has a profound influence on the electronic and photonic properties of a material and in fact, carrier mobilities and diffusion lengths degrade as the order is reduced. Even amongst crystalline materials, we can distinguish between single-crystal and polycrystalline materials. Polycrystalline materials are solids that are comprised of a collection of crystallites (grains) connected to each other with random orientations and separated from one another by areas of relative disorder known as grain boundaries (see Fig. 3.12(b)). In this case, order can only be maintained within the grains.
Coded Aperture X-Ray Diffraction Tomography
Published in Joel Greenberg, Krzysztof Iniewski, X-Ray Diffraction Imaging, 2018
We next consider non-crystalline materials, which include polymers lacking long-range order, glasses, and liquids. While such materials do not contain any long-range order, there does exist short range, local order due to the shape and interactions of the constituent atoms and molecules. Rather than having well-defined lattice spacings, amorphous materials are instead described by a radial distribution function that characterizes the probability of different interatomic (or intermolecular) separations being present in the material.18 These radial distribution functions can contain peaks (which correspond to peaks in the corresponding XRD signal) but are generally much broader and usually contain only one or two local maxima. Examples of the form factors for materials in this class are shown in Figure 1.3a: Delrin has sparse, sharp peaks due to the organization of the long polymer chain, whereas water and ethanol have broad peaks due to the minimal degree of molecular organization.19 Just as in the case of the polycrystalline samples discussed above, amorphous materials’ lack of crystalline orientation results in the XRD signal being well-described in terms of azimuthally symmetric Debye cones. Figure 1.3c shows an example of the scatter signal obtained for a rod of Delrin, which produces symmetric rings with angular extents at each energy that are well-described by Eq. (1.2).
Research progress of spectra and properties of ultrahard carbon materials at high pressure and high temperature
Published in Functional Diamond, 2022
Zhiqiang Hou, Haikuo Wang, Yao Tang, Jiakun Wu, Chao Wang, Zhicai Zhang, Xiaoping Ouyang
Solid matter in nature can usually be divided into crystalline and non-crystalline states, depending on whether lattice periodicity in the spatial organization of the constituting atoms exists in the material [1–4]. The crystalline state generally exhibits long-range order in the atomic arrangement. However, amorphous state inherits disorder of molecular orientation and short-medium range order in the corresponding crystal, which is characterized by disorder (orientational or/and spatial) for such structure, namely it can be said as a ‘glassy’ state [5]. As the fourth most abundant element in the Universe, carbon possesses numerous allotropes with diverse bonding character (sp1-, sp2- and sp3-hybridized bonds) and structural motif of the constituting atoms, resulting in dramatically different physical and chemical properties [6–9].
Nano-objects - sculpting and shape in molecular material design (The Pierre Gilles de Gennes ILCS prize lecture)
Published in Liquid Crystals, 2019
May be an alternative is to use structural definitions introduced by Leadbetter [67] and to extend his ideas fully to three dimensions. He defined three levels of positional ordering; long-range, quasi long-range, and short-range. For long-range positional order, the X-ray diffraction pattern exhibits sharp peaks due to the long-range order, but due to thermal motion, the amplitude of the peaks decreases as the order of the peak increases.For quasi-long-range order, which is found for hexatic phases, the positional order decays algebraically, and is proportional to r−η, where r is the distance from a reference point and η is a temperature-dependent constant. The diffraction pattern then contains broadened, cusp-like peaks instead of very narrow ones. The hexatic phase also has long-range orientational ordering of the packing array of the molecules.For short-range order found in typical smectic and nematic phases, the positional order decays exponentially, meaning it is proportional to e−r/ξ, where ξ is a temperature-dependent constant, then the diffraction pattern contains even broader peaks.
The short-range clustering in Fe–Cr alloys enhanced by severe plastic deformation and electron irradiation
Published in Philosophical Magazine, 2020
V. A. Shabashov, K. A. Kozlov, A. L. Nikolaev, A. E. Zamatovskii, V. V. Sagaradze, E. G. Novikov, K. A. Lyashkov
The cause of the structural and phase transitions observed consists in that that the moving dislocations, on the one hand, destruct any atomic correlations (i.e. short- or long-range order) and, on the other hand, generate point defects, migration of which can enhance the processes of the short- or long-range order formation. As a result, the state of atomic order obtained after the initial thermal treatment can change and the character of these changes depends on the competition between the order destruction and formation by moving dislocations and migrating point defects, respectively.