Search results for "amorphous"
showing 10 items of 790 documents
Understanding the glass transition and the amorphous state of matter: can computer simulation solve the challenge?
1999
The glass transition of supercooled fluids is one of the big puzzles of condensed matter physics, because there occurs a dramatic slowing down (the viscosity η can increase from about η = 1 Poise at the melting transition to η 10 13 Poise at the glass transition temperature T g ), but one hardly sees any accompanying change in the static structure. Theoretical concepts are very controversial - e.g., the Gibbs-di Marzio theory attributes glassy freezing to an underlying entropy catastrophe (the entropy of the supercooled fluid would fall below the crystal entropy at the Kauzmann temperature T 0 T g . Computer simulations offer the advantage that atomistically detailed information on structur…
Multiscale Computer Simulations in Physics, Chemistry, and Biology: The Example Of Silica
2002
We show to what extent molecular dynamics simulations (MD) can explore struc-tural and dynamic properties of atomic systems whereby the system under consideration is amorphous silica (SiO2). Two studies are presented: (i) a large scale simulation of the dynam-ics of a SiO2 melt and (ii) the investigation of free silica surfaces where a mixture of a classical MD and a Car-Parrinello molecular dynamics is used.
EXAFS study of mixed nickel molybdenum oxide thin films at the Ni and Mo K-edges
2004
Mixed nickel molybdenum oxide thin films were produced by DC magnetron co-sputtering technique with the nickel content about 8, 16 and 25 at%. X-ray absorption spectroscopy at the Ni and Mo K-edges was used to study the local atomic structure in the films. The best-fit analysis of the EXAFS signals suggests that (i) the films are amorphous, except for the highest nickel content (25 at%), at which a segregation of NiO phase was observed; (ii) nickel and molybdenum atoms are octahedrally coordinated by oxygen atoms. Opposite to the NiO6 octahedra, the MoO6 octahedra are strongly distorted, that results in an existence of two groups of oxygen atoms—four nearest at B1.76 ( A and two distant at …
Surface relief grating formation in amorphous As40S15Se45 and As2S3 films under 0.532 μm wavelength illumination
2014
In this research we have studied the influence of λ = 0.532 μm laser light illumination on the formation and properties of surface relief gratings in amorphous As2S3 and As40Se15Se45 films, as well as a bulk As2S3 sample. The surface relief grating formation was studied for a wide range of grating periods (∼1 – ∼80 μm) for the variable thicknesses of the samples. The examination of the relief depth, Δh, and its dependence on the grating period at constant exposures shows that the impact of sample thickness could manifest itself as a shift in the optimal grating period at holographic surface relief formation (As2S3) and affect the process itself.
The dynamics of sodium in sodium disilicate: Channel relaxation and sodium diffusion
2001
We use molecular dynamics computer simulations to study the dynamics of amorphous (Na_2O)2(SiO_2). We find that the Na ions move in channels embedded in a SiO_2 matrix. The characteristic distance between these channels gives rise to a prepeak in the structure factor at around q=0.95 A^-1. The dynamics of sodium is given by a fast process which can be seen in the incoherent scattering function and a slow process which is seen in the coherent function. The relaxation time of the latter coincides with the alpha-relaxation time of the matrix. The Kohlrausch exponent of the fast process for q>1.6 A^1 is the same as the von Schweidler exponent for the slow one, demonstrating that the two proc…
Continuum constitutive laws to describe acoustic attenuation in glasses
2020
International audience; Nowadays metamaterials are at the focus of an intense research as promising for thermal and acoustic engineering. However, the computational cost associated to the large system size required for correctly simulating them imposes the use of finite-elements simulations, developing continuum models, able to grasp the physics at play without entering in the atomistic details. Still, a correct description should be able to reproduce not only the extrinsic scattering sources on waves propagation, as introduced by the metamaterial microstructure, but also the intrinsic wave attenuation of the material itself. This becomes dramatically important when the metamaterial is made…
Isotropic Chiral Acoustic Phonons in 3D Quasicrystalline Metamaterials.
2020
International audience; The elastic properties of three-dimensional (3D) crystalline mechanical metamaterials, unlike those of amorphous structures, are generally strongly anisotropic—even in the long-wavelength limit and for highly symmetric crystals. Aiming at isotropic linear elastic wave propagation, we therefore study 3D periodic approximants of 3D icosahedral quasicrystalline mechanical metamaterials consisting of uniaxial chiral metarods. Considering the increasing order of the approximants, we approach nearly isotropic effective speeds of sound and isotropic acoustical activity. The latter is directly connected to circularly polarized 3D metamaterial chiral acoustic phonons—for all …
Sound attenuation and anharmonic damping in solids with correlated disorder
2010
We study via self-consistent Born approximation a model for sound waves in a disordered environment, in which the local fluctuations of the shear modulus G are spatially correlated with a certain correlation length The theory predicts an enhancement of the density of states over Debye's omega(2) law (boson peak) whose intensity increases for increasing correlation length, and whose frequency position is shifted downwards as lg. Moreover, the predicted disorder-induced sound attenuation coefficient r(k) obeys a universal scaling law F(k) = f (ke) for a given variance of G. Finally, the inclusion of the lowest-order contribution to the anharmonic sound damping into the theory allows us to rec…
Spherulite formation in a ?noncrystalline? two-dimensional hydrogen-bond assembly
1996
4-Urazoyl benzoic acid groups 1 are attached as terminal end group to an oligoisobutene. According to wide-angle X-ray scattering the material is amorphous. Differential scanning calorimetry, small-angle X-ray scattering, transmission electron microscopy, and polarizing microscopy show that the polar head groups form ordered two-dimensional assemblies which organize into spherulites. Temperature-dependent SAXS measurements and the d.s.c. analysis give a close insight into the order-disorder (“melting”) process of the two-dimensional clusters which form macroscopic spherulites. The order-disorder transition of the clusters is closely related to the dynamics and order of the polyisobutene cha…
Size dependent light absorption modulation and enhanced carrier transport in germanium quantum dots devices
2015
Quantum confinement in closely packed arrays of Ge quantum dots (QDs) was studied for energy applications. In this work, we report an efficient tuning mechanism of the light harvesting and detection of Ge QDs. Thin films of SiGeO alloys, produced by rf-magnetron sputtering, were annealed at 600 degrees C in N-2 to induce precipitation of small amorphous Ge QDs into the oxide matrix. Varying the Ge content, the QD size was tailored between 2 and 4 nm, as measured by high resolution transmission electron microscopy. X-ray photoelectron spectroscopy (XPS) measurements indicate the formation of pure SiO2, as well as the presence of a sub-stoichiometric Ge oxide shell at the QD interface. Light …