Search results for "amorphous"

Computer simulations of SiO2 and GeO2

2004

Classical Molecular Dynamics (MD) simulations are used to study structural and dynamic properties of amorphous germania (GeO2) in comparison to those of silica (SiO2). The total structure factor, as obtained from these simulations, is in very good agreement with that of neutron scattering experiments, both for germania and silica. The tetrahedral network structure in silica and germania leads to a prepeak in the structure factor that appears at slightly smaller wavenumbers in GeO2 than in SiO2. At high temperatures the diffusion constants are very similar in both systems whereas at low temperatures diffusion is significantly faster in germania than in silica. We also outline the strategy fo…

Specific Heat of Amorphous Silica within the Harmonic Approximation

1999

We investigate to what extent the specific heat of amorphous silica can be calculated within the harmonic approximation. For this we use molecular dynamics computer simulations to calculate, for a simple silica model (the BKS potential), the velocity autocorrelation function and hence an effective density of states g(ν). We find that the harmonic approximation is valid for temperatures below 300 K but starts to break down at higher temperatures. We show that, to obtain a reliable description of the low-frequency part of g(ν), i.e., where the boson peak is observed, it is essential to use large systems for the simulations and small cooling rates to quench the samples. We find that the calcul…

The Mixed Akali Effect in Ternary Alkali Silicate Melts: Insight from Molecular Dynamics Computer simulations

2005

Large scale molecular dynamics (MD) computer simulations are used to study the amorphous alkali silicates (Li2O)(2·SiO2) [LS2], (K2O)(2·SiO2) [KS2], and (0.5·Li2O)(0.5·K2O)(2·SiO2) [LKS2]. These systems are characterized by a fast alkali ion motion in a relatively immobile Si-0 matrix. We investigate the so-called mixed alkali effect (MAE) which is reflected as a significant decrease of the alkali ion diffusion constants in LKS2 as compared to the corresponding binary systems LS2 and KS2. We show that the subtle interplay between the structure on intermediate length scales and the alkali diffusion is important to understand the microscopic origin of the MAE.

Water-Controlled Crystallization of CaCO3, SrCO3, and MnCO3 from Amorphous Precursors

2018

Calcium carbonate is the most abundant biomineral, whose amorphous form is stabilized in nature by a variety of organic additives and water. It is used to manipulate the morphology of new materials and to make strong inorganic/organic hybrid materials. However, the crystallization pathways (e.g., nucleation and growth, two-step nucleation pathways involving disordered, amorphous, or dense liquid states preceding the appearance of crystalline phases) remain often unclear. We have synthesized three amorphous carbonates, CaCO3 (ACC), SrCO3 (ASC), and MnCO3 (AMnC), that do not require any stabilization by additives to study their crystallization kinetics and mechanisms in the presence of water.…

On the nature of morphological features in phase-separated (PEO)nNaSCN mixtures: a SAXS investigation

2000

Abstract The SAXS technique is used to throw light on the morphology of phase-separated poly(ethylene oxide)n–sodium thiocyanate [(PEO)nNaSCN] mixtures. A temperature-dependent investigation is reported for (PEO)4.5NaSCN. (PEO)nNaSCN mixtures show a complex phase behaviour. At the investigated composition, a crystalline complex (CC) is formed between PEO and NaSCN. The considered composition is representative of thermodynamic states where three different phases coexist simultaneously: crystalline PEO (CPEO), amorphous PEO (APEO) and crystalline complex (CC). Various SAXS data analysis approaches are presented to understand the nature of these coexisting phases. Invariant analysis shows a bi…

Use of Mott-Schottky Plots to Characterise the Amorphous Passive Film/Electrolyte Junction

2010

A hydrated crystalline calcium carbonate phase: Calcium carbonate hemihydrate.

2019

Hydrous CaCO 3 gets a new structure Calcium carbonate (CaCO 3 ) forms important minerals on Earth and is a model system for understanding crystal nucleation. Three different structures of CaCO 3 are known, along with two structures that are hydrated. Zou et al. found a third hydrated CaCO 3 structure formed from amorphous CaCO 3 in the presence of magnesium ions. The discovery illustrates the importance of amorphous precursors for producing new materials. Science , this issue p. 396

Field-driven domain wall motion under a bias current in the creep andflow regimes in Pt/[CoSiB/Pt](N) nanowires

2016

AbstractThe dynamics of magnetic domain wall (DW) in perpendicular magnetic anisotropy Pt/[CoSiB/Pt]N nanowires was studied by measuring the DW velocity under a magnetic field (H) and an electric current (J) in two extreme regimes of DW creep and flow. Two important findings are addressed. One is that the field-driven DW velocity increases with increasing N in the flow regime, whereas the trend is inverted in the creep regime. The other is that the sign of spin current-induced effective field is gradually reversed with increasing N in both DW creep and flow regimes. To reveal the underlying mechanism of new findings, we performed further experiment and micromagnetic simulation, from which w…

Fast photoconduction in the highly ordered columnar phase of a discotic liquid crystal

1994

THE search for organic materials suitable for electronic applica-tions dates back to the early 1950s. But the only organic systems known so far to show electronic charge-carrier mobilities comparable to the amorphous inorganic semiconductors that are the main-stay of the microelectronics industry are zone-refined organic single crystals1–4. Single crystals are difficult and costly to process, however, and are not suitable for device applications. Here we show that a highly ordered columnar (stacked) phase of disk-like organic molecules can exhibit high mobilities for photoinduced charge carriers, of the order of 0.1 cm2 V-1 s-1—higher than for any organic material other than single-crystal …

Multiscale microstructural characterization of particulate-reinforced composite with non-destructive X-ray micro- and nanotomography

2018

Abstract Methods based on X-ray tomography are developed to study the relevant statistical quantities describing the microstructural inhomogeneity of particulate reinforced composites. The developed methods are applied in estimating microstructural inhomogeneity parameters of composites containing metallic glass particles in metal matrix, extruded in varying pressure loads. This study indicates that the critical characteristics with regard to the effect of particle clustering are cluster size and shape, local volume fraction of particles in the cluster and the distance between clusters. The results demonstrate that the spatial distribution of reinforcement is very uneven and the amount of p…