Increase Of Mean-square Displacement Research Articles

A Three-Dimensional Random Walk Algorithm for Estimating the Chloride Diffusivity of Concrete.

The chloride diffusivity of concrete is an important parameter for assessing the long-term durability of coastal concrete structures. The purpose of this paper is to present a three-dimensional random walk algorithm (RWA) for estimating the chloride diffusivity of concrete. By analyzing the size distribution of aggregates, the equivalent interfacial transition zone (ITZ) thickness is derived in an analytical manner. Each aggregate is combined with the surrounding ITZ to construct an equivalent aggregate model (EAM) and the chloride diffusivity is formulated. It is found that the equivalent ITZ thickness decreases with the increase of practical ITZ thickness and aggregate volume fraction. The aggregate gradation influences the equivalent ITZ thickness to a certain extent. The relative chloride diffusivity of the equivalent aggregate is almost directly and inversely proportional to the equivalent ITZ thickness and the aggregate radius, respectively. The numerical results show that, when the EAM is adopted, the computational time is greatly reduced. With the EAM, concrete can be modeled as a two-phase material and the chloride diffusivity is estimated by applying the RWA. It is shown that, with the increase of mean square displacement and number of Brownian particles, the average chloride diffusivity of concrete approaches a stable value. Finally, through comparison with experimental data, the validation of the RWA is preliminarily verified.

X-ray diffraction study of TiB2, W2B4, and CrB2 at high temperatures

This study investigates thermal expansion and thermal vibration behavior of borides using HT-XRD. In the previous research, it has been known that the vibrational behavior directly expresses the behavior of materials at high temperatures. In this work, the high temperature X-ray diffraction (HT-XRD) measurements of TiB2, W2B4, and CrB2 were conducted at the high temperature ranges; up to 1500 K. From the generated results, the study calculates thermal expansion coefficients, which coincides the previous research. Rather, the study calculates the mean square displacements of borides for evaluating the thermal vibration behaviors at elevated temperatures, demonstrating that: (1) the application of a formula into HT-XRD data could determine Debye-Waller factors of these borides; (2) a term of “agglomeration” could contribute to defining the mean square displacement u2; (3) both the Debye-Waller factor and the thermal expansion may function to design new boride-based materials at the high temperature; and (4) CrB2 has a quick increase in mean square displacement and is remarkably large value at elevated temperatures.

Mixing of the fluid phase in slowly sheared particle suspensions of cylinders

We introduce a finite element model for neutrally buoyant particle suspensions of cylinders at zero Reynolds number and infinite Péclet number in the purely hydrodynamic limit, which allows us to access a high-accuracy fluid velocity field at any time during the simulation. We use the diffusive strip method to characterize the development of the concentration field in the fluid phase of sheared suspensions from initial thin filaments, and characterize the structures that form with their fractal dimension. We find that the growth of the fractal dimension of the filaments scales with the increase of mean square displacement in the fluid phase. Further, we measure the concentration distribution of tracers in the fluid phase, as well as the shear-induced self-diffusion coefficient in both the solid phase and the fluid phase. We demonstrate that the shear-induced self-diffusion coefficient is slightly larger in the fluid phase at infinite Péclet number. Finally, we investigate enhanced mass diffusivity in the fluid phase by systematic measurements of the shear-induced self-diffusion coefficient in the fluid phase for a wide range of fluid tracer Péclet numbers. We find that the functional dependence $D_{s}/D=1+\unicode[STIX]{x1D6FD}\unicode[STIX]{x1D719}^{\unicode[STIX]{x1D6FC}}Pe^{\unicode[STIX]{x1D701}}$ (where $D_{s}$ is the shear-induced self-diffusion coefficient, $D$ is the molecular diffusivity and $\unicode[STIX]{x1D719}$ is the particle volume fraction) fits the observations fairly well. We measure the constants $\unicode[STIX]{x1D6FD}=2.98\pm 0.39$, $\unicode[STIX]{x1D6FC}=1.61\pm 0.26$ and $\unicode[STIX]{x1D701}=0.900\pm 0.031$.

Temperature-dependent dynamics of dry and hydrated β-casein studied by quasielastic neutron scattering.

β-Casein is a component of casein micelle with amphillic nature and is recognized as a "natively disordered" protein that lacks secondary structures. In this study, the temperature and hydration effects on the dynamics of β-casein are explored by quasielastic neutron scattering (QENS). An upturn in the mean square displacement (MSD) of hydrated β-casein indicates an increase of protein flexibility at a temperature of ~225 K. Another increase in MSD at ~100 K, observed in both dry and hydrated β-casein, is ascribed to the methyl group rotations, which are not sensitive to hydration. QENS analysis in the energy domain reveals that the fraction of hydrogen atoms participating in motion in a sphere of diffusion is highly hydration dependent and increases with temperature. In the time domain analysis, a logarithmic-like decay is observed in the range of picosecond to nanosecond (β-relaxation time) in the dynamics of hydrated β-casein. This dynamical behavior has been observed in hydrated globular and oligomeric proteins. Our temperature-dependent QENS experiments provide evidence that lack of a secondary structure in β-casein results in higher flexibility in its dynamics and easier reversible thermal unfolding compared to other rigid biomolecules.

A study of disorder effects at ferroelectric phase transition in BaTiO 3

The total structure factor of BaTiO 3 obtained by neutron powder diffraction in the temperature range 360–430 K in the vicinity of ferroelectric phase transition has been modeled by the reverse Monte Carlo method. A presence of disordered displacements of Ti atoms due to dynamical tetragonal elongations of unit cells and off-centered sites of Ti atoms in paraelectric phase in the vicinity of the ferroelectric phase transition was observed. Both features result in the anomalous increase of mean square displacements of Ti atoms and reflect the coexistence of displacive and order–disorder components in the ferroelectric phase transition.

Segmental Motion of Supercooled Random Copolyester Studied by Neutron Polarization Analysis

We investigated the segmental motion of supercooled random copolyester by elastic neutron scattering. Neutron polarization analysis was applied in order to decompose the total scattering into the coherent and incoherent contributions, which plays an essential role in the careful discussion of the scattering vector dependence of the elastic incoherent scattering. The study on highly oriented glassy random copolyester revealed the anisotropic features of segmental motions in a wide temperature region from glassy to liquid states. In this paper, we report in detail (i) the anharmonic increase of mean-square displacement, (ii) the non-Gaussian parameter A(T), and (iii) the elastic incoherent structure factor for the rotational segmental motion.