Baek-Seok Seong

Effect of wavelength-dependent attenuation on neutron diffraction stress measurements at depth in steels

The wavelength dependence of the maximum feasible penetration depth was studied for neutron diffraction stress measurements in ferritic and austenitic steels. This property was examined with wavelengths from the close vicinity of the Bragg edges, where the neutron total cross section has its local minimum and for which the scattering angles are convenient for stress measurements. These wavelengths (e.g. 2.39 and 2.19 A) are longer than those commonly used in stress measurements (∼1.6 A). By using such wavelengths, configured by a focusing bent perfect crystal Si(111) monochromator, it was observed that the available total beam path length is about 85 mm in both ferritic and austenitic steels. This study provides specific information for choosing the instrument configuration suitable for most strain-scanning experimental tasks.

Structure of a planar electric double layer containing size-asymmetric ions: density functional approach

We have studied the structure of the electrolytes with asymmetries in charge and size near a charged planar electric double layer by a density functional theory. In the present theory, the hard-sphere contribution has been approximated as the direct pair correlation function with the coupling parameter, whereas the electronic contribution has been approximated as the mean-spherical approximation in the bulk phase. This theoretical approach for the size-symmetric and size-asymmetric electrolytes displays a good agreement with the simulation results over a wide range of surface charge densities and electrolyte concentrations. However, the accuracy between the present theory and the simulation results slightly deteriorates for the highly size-asymmetric electrolytes and the multivalent electrolytes. In these cases, the performance of the present theory is comparable to those of the simplified extension of the Poisson–Boltzmann theory and the modified Poisson–Boltzmann theory. The calculated result indicates that the surface charge distribution function, which was introduced as an indicator for studying the charge reversal, layering effect, and surface charge amplification in a planar electric double layer, describes the electronic properties of a planar electric double layer well.

Neutron diffraction study of U–5.4 wt% Mo alloy

Abstract The structure of U–5.4 wt% Mo alloy prepared by the centrifugal atomization method and the decomposition of the alloy at elevated temperature were studied. The single uniform γ-phase was obtained after annealing the synthesized alloy at 700°C for 48 h. The homogenized alloy was annealed at 400°C and 500°C to search for an ordered phase and study the decomposition process. No ordered phase in U–5.4 wt% Mo alloy after annealing was observed. With the result from the Rietveld refinement of the neutron diffraction patterns it was concluded that the b parameter of the α-phase is contracted like metastable α′-phase and phase boundary of the α-phase region at 500°C lies near 2.6 at.% Mo.

SANS study on self-assembled structures of Pluronic F127 triblock copolymer induced by additives and temperature

In the temperature range of 303–333 K, the self-assembled structures of a mixture of Pluronic F127 triblock copolymer [PEO106PPO70PEO106; PEO is poly(ethylene oxide) and PPO is poly(propylene oxide)] and an organic derivative, 5-methyl salicylic acid (5mS), in aqueous solution have been investigated using small-angle neutron scattering (SANS). Above a 5mS concentration of 1.93 g l−1, the F127–5mS mixture solution became cloudy with a blue colour arising from the Tyndall effect, indicating that large polymer aggregates had formed in the mixture solution. SANS measurements showed that the self-assembled structure of the F127–5mS mixture transformed from a spherical to a cylindrical micelle with increasing the concentration of 5mS in the temperature range of 303–323 K. When the 5mS concentration was increased to 3.3 g l−1, the self-assembled structure of the F127–5mS mixture at 333 K underwent an additional phase transition from a cylindrical to a spherical micelle of large size at a 5mS concentration of 2.75 g l−1, although its self-assembled structure changed from a spherical to a cylindrical micelle at a 5mS concentration of 1.93 g l−1 as well. The phase transitions are explained by the variation of the mass fraction of the hydrophilic part of F127 and the coupled effect of the limited solubility and the strong tendency to bind with amphiphilic molecules of 5mS. Using a simple material balance equation and the structural information obtained from SANS model analyses, the numbers of D2O and of 5mS molecules in the core and corona regions are calculated. This result can provide a simple and easy way to prepare various nanostructures using a Pluronic triblock copolymer in aqueous solution and may be very useful for practical applications of a Pluronic polymer such as various nanobuilding blocks or nanotemplates.

Effect of polymer size and chain length on depletion interactions between two colloids.

A density functional theory based on the weighted density has been developed to investigate the depletion interactions between two colloids immersed in a bath of the binary polymer mixtures, where the colloids are modeled as hard spheres and the polymers as freely jointed tangent hard-sphere chain mixtures. The theoretical calculations for the depletion forces between two colloids induced by the polymer are in good agreement with the computer simulations. The effects of polymer packing fraction, degree of polymerization, polymer/polymer size ratio, colloid/polymer size ratio on the depletion interactions, and colloid-colloid second virial coefficient B2 due to polymer-mediated interactions have been studied. With increasing the polymer packing fraction, the depletion interaction becomes more long ranged and the attractive interaction near the colloid becomes deeper. The effect of degree polymerization shows that the long chain gives a more stable dispersion for colloids rather than the short chain. The strong effective colloid-colloid attraction appears for the large colloid/polymer and polymer/polymer size ratio. The location of maximum repulsion Rmax is found to appear Rmax approximately sigmac+Rg2 for the low polymer packing fraction and this is shifted to smaller separation Rmax approximately sigmac+sigmap2 with increasing the polymer packing fraction, where sigmap2 and Rg2 are the small-particle diameter and the radius of gyration of the polymer with the small-particle diameter, respectively.

Depletion interactions in two-dimensional colloid–polymer mixtures: molecular dynamics simulations

The depletion interactions acting between two hard colloids immersed in a bath of polymers, in which the interaction potentials include the soft repulsion/attraction, are extensively studied by using the molecular dynamics simulations. The collision frequencies and collision angle distributions for both incidental and reflection conditions are computed to study the dynamic properties of the colloidal mixtures. The depletion effect induced by the polymer-polymer and colloid-polymer interactions are investigated as well as the size ratio of the colloid and polymer. The simulated results show that the strong depletion interaction between two hard colloids appears for the highly asymmetric hard-disc mixtures. The attractive depletion force at contact becomes deeper and the repulsive barrier becomes wider as the asymmetry in size ratio increases. The strong polymer-polymer attraction leads to the purely attractive depletion interaction between two hard colloids, whereas the purely repulsive depletion interaction is induced by the strong colloid-polymer attraction.

Thermally responsive vesicles based on a mixture of cationic surfactant and organic derivative below the CMC

Surfactants have attracted great attention in a wide range of potential applications such as nano-building blocks or drug delivery. However, the morphology of an aggregate formed from the surfactant in a fairly low concentration including below the critical micelle concentration (CMC), which is key information to easily control the morphology of surfactant aggregates at various concentrations, has not been exploited yet. Herein we report a thermally responsive and spontaneous surfactant vesicle by mixing a cationic surfactant and an organic derivative below as well as above the CMC of the surfactant. Depending on the concentration of the additive organic derivative and temperature, the spontaneous surfactant vesicles are easily transformed into the micelle or isotropic free surfactant phase. Furthermore, the vesicle to cylindrical micelle transition temperature of the prepared surfactant vesicle could be easily controlled by the concentration of the additive organic derivative that provides the thermally responsive surfactant vesicles with a variety of transition temperatures, allowing the surfactant vesicle to be used in gene or drug delivery.

Double bent crystal dispersive arrangement for high resolution diffractometry

In this paper some properties of the dispersive double-bent-crystal setting are described. Thanks to an easy manipulation with focusing of the monochromatic beam, high reflection probability of both bent crystals and very narrow (1-3 mm) obtained monochromatic beam such an arrangement could be attractive for an employment. We tested the properties of the double-bent-crystal setting of the bent Si(111) + bent Si(220) slabs on the diffractometer at the neutron wavelength of 0.163 nm for various curvatures of the Si(220) slabs. By using a standard polycrystalline sample of Fe it has been found that besides an excellent resolution the neutron current is sufficient even for powder diffraction experiments.

Effect of precipitate size and dispersion on recrystallization behavior in Ti-added ultra low carbon steels

The effect of coiling temperature on precipitates and solid solution was investigated in P-free and P-added Ti-stabilized ultra low carbon steels. The volume fractions of the fine precipitates smaller than 60 nm were evaluated by using small-angle neutron scattering technique. The solute P was quantified from the lattice parameters obtained from neutron diffraction patterns. In the P-free steels, TiC and Ti4C2S2 with various sizes ranging from 5~60 nm were observed. In addition to these precipitates, FeTiP precipitates of a size exceeding 50 nm were also observed in the P-added steel. The amount of fine precipitates smaller than 10 nm and the concentration of solute P was higher in the samples coiled at low temperature. The recrystallization temperature increases if the coiling temperature decreases in both, P-free and P-added steels. The recrystallization temperature of P-free steels is lower than that of P-added steels. In the P-free steels, the pinning effect of fine precipitates played a key role for the retardation of the recrystallization. In the P-added steels, the retardation of recrystallization is due to both, the pinning effect of fine precipitates and the solute drag effect of P in solid solution.

Structure and thermodynamics of hard-core Yukawa fluids: Thermodynamic perturbation approaches

The thermodynamic perturbation theories, which are based on the power series of a coupling constant (λ-expansion), have been proposed for studying the structural and thermodynamic properties of a hard-core Yukawa (HCY) fluid: one (A1-approximation) is the perturbation theory based on the hard-sphere repulsion as a reference system. The other (A2-approximation) is the perturbation theory based on the reference system which incorporates both the repulsive and short-range attractive interactions. The first-order mean-spherical approximation (FMSA) provided by Tang and Lu [J. Chem. Phys. 99, 9828 (1993)] has been employed for investigating the thermodynamic properties of a HCY fluid using the alternative method via the direct correlation function. The calculated results show that (i) the A1 and A2 approximations are in excellent agreements with previous computer simulation results in the literature and compare with the semi-empirical works of Shukla including the higher-order free energy terms, (ii) the A1 and A2 approximations are better than the FMSA and the mean-spherical approximation, (iii) the A2-approximation compares with the A1-approximation, even though the perturbation effect of an A2-approximation is much smaller than that of an A1-approximation, and that (iv) the FMSA study is particularly of advantage in providing the structure and thermodynamics in a simple and analytic manner.