arXiv subjects

Un-Song Hwang

Publications and source records attributed to Un-Song Hwang.

3 recordsLinked to original sources

arXiv2020

Influence of M/A substitution on material properties of intermetallic compounds MSn$_2$ (M = Fe, Co; A = Li, Na): A first-principles study

Iron and cobalt distannides \ce{MSn2} (M = Fe, Co) are regarded as a promising conversion-type anode material for lithium- and sodium-ion batteries, but their properties are not well understood. In this work, we report a first-principles study of alkali metal (A = Li, Na) substitutional effect on the structural, mechanical, lattice vibrational, electronic and defect properties of these distannides. Special attention is paid to systematic comparison between \ce{FeSn2} and \ce{CoSn2}. Our calculations reveal that M/A substitution induces a lattice expansion and decrease of elastic constants, which is more announced with Na substitution than Li, and moreover changes the elastic property of \ce{FeSn2} from ductile to brittle whereas preserves the ductility of \ce{CoSn2}. An imaginary phonon frequency mode appears only for \ce{FeSn2} and \ce{FeNaSn2}, and M/A substitution provokes a definite gap between high and low frequency regions. We perform a careful analysis of electronic density of states, band structures and Fermi surface, providing an insight into difference of electronic structures between \ce{FeSn2} and \ce{CoSn2}. With further calculation of defect formation energies and alkali ion diffusion barriers, we believe this work can be useful to design conversion-type anode materials for alkali-ion batteries.

cond-mat.mtrl-sci↗

arXiv2019

Thermal conductivity of porous jennite by molecular dynamics method

The thermal conductivity of porous jennite, as the major component of cement paste, and its porosity and temperature dependences are simulated by molecular dynamics methods using ClayFF force field. The porous jennite models with different porosities are created by removing atoms within the sphere from bulk jennite model. The thermal conductivity elements of bulk jennite calculated by equilibrium Green-Kubo method are almost identical to those by non-equilibrium Müller-Plathe (MP) method at 300 K. The volumetric thermal conductivity of porous jennite is found to decrease from 1.141 W/m$\cdot$K to 0.144 W/m$\cdot$K as increasing the porosity from 0\% to 72.22\% at 300 K, following the empirical coherent potential model when the pore is assumed to be filled with air. As increasing temperature, the thermal conductivity is observed to increase from 240 K to 560 K and gradually decrease until 1100 K for porous jennites with the porosities of 0\%, 15.99\% and 32.79\%.

cond-mat.mtrl-sci↗

arXiv2019

Formation and characterization of ceramic coating from alumino silicate mineral powders in the matrix of cement composite on the concrete wall

Enhancement of thermal performance of concrete wall is nowadays of great importance in reducing the operational energy demand of buildings. We developed a new kind of inorganic coating material based on \ce{SiO2}-\ce{Al2O3}-rich minerals and Portland cement (PC) powder. The finely pulverized mineral powder with the particle size distribution (PSD) of 0.4-40 $μ$m was mixed with the vehicle solvent containing some agents, cement powder with PSD of 2-100 $μ$m, and water in the certain weight ratio, producing the colloid solution. After application within 2 hours to the plaster layer of concrete wall and sufficient long hardening period of over three months, the coating layer of 0.6-1.0 mm thickness was observed to become a densified ceramic. Powder X-ray diffraction (XRD) experiments were performed to identify the crystalline components of minerals, cement and ceramic coating powders. Three- and two-dimensional surface morphologies and chemical compositions of coating material were obtained with the optical interferometer and scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDX). These XRD and SEM/EDX analyses demonstrated obviously that the coating layer is mainly composed of the calcium-silicate-hydrate (C-S-H) and the calcium-aluminate-hydrate (C-A-H) ceramics with the relatively small number of closed pores (10\% porosity) compared with the cement mortar and concrete layers. Two-step hydrations of cement and subsequently \ce{SiO2}-\ce{Al2O3} promoted by the alkali product \ce{Ca(OH)2} were proposed as the main mechanism of ceramic formation.

cond-mat.mtrl-sci↗