• Title/Summary/Keyword: Atomistic

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A Molecular Dynamics Simulation Study of Na- and K-birnessite Interlayer Structures (Na-, K-버네사이트 층간 구조에 대한 분자동역학 시뮬레이션 연구)

  • Park, Sujeong;Kwon, Kideok D.
    • Korean Journal of Mineralogy and Petrology
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    • v.33 no.3
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    • pp.143-152
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    • 2020
  • Birnessite is a layered manganese oxide mineral with ~7 Å of d-spacing. Because of its high cation exchange capacity, birnessite greatly impacts the chemical compositions of ground water and fluids in sediment pores. Understanding the cation exchange mechanisms requires atomistic investigations of the crystal structures and coordination environments of hydrated cations in the interlayer. In this study, we conducted classical molecular dynamics (MD) simulations, an atomistic simulation method of computational mineralogy, for triclinic Na-birnessite and K-birnessite whose chemical formula are from previous experiments. We report our MD simulation results of the crystal structures, coordination environments of Na+ and K+, and the polytypes of birnessite and compare them with available experimental results. The simulation results well reproduced experimental lattice parameters and provided atomic level information for the interlayer cation and water molecule sites that are difficult to distinguish in X-ray experiments. We also report that the polytype of the Mn octahedral sheets is identical between Na- and K-birnessite, but the cation positions differ from each other, demonstrating a correlation between the coordination environment of the interlayer cations and the crystal lattice parameters. This study shows that MD simulations are very promising in elucidating ion exchange reactions of birnessite.

Prediction of Adsorption Isotherms and Diffusivity on Activated Carbon for Persistent Organic Pollutant(2,3,7,8-TCDD) (활성탄 위에서 잔류성 유기 오염물질(2,3,7,8-TCDD)의 등온 흡착식 및 확산계수 예측)

  • Lim, Young-Il;Son, Hae-Jeong;Lee, Ohsung;Nam, Kyong-Soo;Yoo, Kyoung-Seun
    • Korean Chemical Engineering Research
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    • v.47 no.6
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    • pp.747-754
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    • 2009
  • In this study, adsorption isotherms of o-DCB(ortho-dichlorobenzene) on an activated carbon heated at $1000^{\circ}C$ for 24 hours were obtained by experiment and were predicted by using molecular simulation. The initial molecular structure of the activated carbon was designed on the basis of its molecular formula and functional groups ratio measured experimentally. Then, the molecular structure was optimized using the COMPASS(condensed-phase optimized molecular potentials for atomistic simulation studies) force field. The particle porosity, specific surface area, and particle density obtained from the optimized molecular structure of activated carbon were compared with those experimental data. The errors between experimental data and simulation results of the particle porosity, specific surface area, and particle density were shown as 7.6, 3.8, and 2.8%, respectively. Adsorption isotherms constants of o-DCB are calculated by the GCMC(grand canonical Monte Carlo) method in the optimized molecular structure of activated carbon. The simulation result of the adsorption isotherms showed an error of under 3%, compared to that of experimental data. Adsorption isotherms, adsorption heat and pore diffusivity of 2,3,7,8-TCDD(tetrachlorodibenzo-p-dioxin) was finally obtained in the same molecular structure of the activated carbon as used for o-DCB. Thus, adsorption characteristics of persistent organic pollutants on activated carbon, which are not easy to experimentally evaluate, are predicted by the molecular simulation.

Ball-milling Induced Changes in the Crystallinity of Quartz and Wear of Milling Media (볼 밀링에 의한 석영의 결정도 변화와 밀링 매체의 마모의 영향)

  • Jin Jung Kweon;Hoon Khim;Sung Keun Lee
    • Korean Journal of Mineralogy and Petrology
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    • v.36 no.2
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    • pp.95-106
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    • 2023
  • Quartz (SiO2) is among the major rock-forming minerals in the earth's crust. The atomistic structures of SiO2 may evolve during diverse frictional processes. The reduction of friction of quartz-rock accompanied by its amorphization, hydration, and formation of silica gel provides mineralogical insights into earthquakes and related phenomena. Ball milling, together with rotary shear experiments have been useful to infer the atomic origins of such processes. In this study, optimal experimental conditions for ball milling for amorphization of SiO2 were determined by taking into account various process variables. The crystallinity of SiO2 gradually decreased and became amorphous as the ball milling time increased at a high milling speed. The degree of wear of the milling media and its effect on the amorphization of SiO2 were analyzed using distinct milling materials (zirconia, stainless steel). The amount of ball wear increased with increasing milling time. Furthermore, the worn stainless steel particles from balls tend to interact with amorphized SiO2 to form Si-O-Cr. These results aid in understanding the process of atomistic structural changes caused by ball milling of divserse materials with relatively high hardness, such as SiO2, and understanding various geological friction processes.

Effect of Pore Geometry on Gas Adsorption: Grand Canonical Monte Carlo Simulation Studies

  • Lee, Eon-Ji;Chang, Rak-Woo;Han, Ji-Hyung;Chung, Taek-Dong
    • Bulletin of the Korean Chemical Society
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    • v.33 no.3
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    • pp.901-905
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    • 2012
  • In this study, we investigated the pure geometrical effect of porous materials in gas adsorption using the grand canonical Monte Carlo simulations of primitive gas-pore models with various pore geometries such as planar, cylindrical, and random pore geometries. Although the model does not possess atomistic level details of porous materials, our simulation results provided many insightful information in the effect of pore geometry on the adsorption behavior of gas molecules. First, the surface curvature of porous materials plays a significant role in the amount of adsorbed gas molecules: the concave surface such as in cylindrical pores induces more attraction between gas molecules and pore, which results in the enhanced gas adsorption. On the contrary, the convex surface of random pores gives the opposite effect. Second, this geometrical effect shows a nonmonotonic dependence on the gas-pore interaction strength and length. Third, as the external gas pressure is increased, the change in the gas adsorption due to pore geometry is reduced. Finally, the pore geometry also affects the collision dynamics of gas molecules. Since our model is based on primitive description of fluid molecules, our conclusion can be applied to any fluidic systems including reactant-electrode systems.

A molecular dynamics simulation on the defect structure in silicon under indentation (분자동력학 해석을 이용한 인덴테이션시 실리콘 내부의 결함구조에 관한 연구)

  • Trandinh, Long;Ryu, Yong-Moon;Kang, Woo-Jong;Cheon, Seong-Sik
    • Composites Research
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    • v.24 no.2
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    • pp.9-13
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    • 2011
  • ,In this paper, the symmetric axis parameter method, which was proposed to identify defects, dislocations and stacking fault, with perfect structures in the zinc-blende materials, was introduced as a way to distinguish between elastic and plastic deformation. LAMMPS, a molecular dynamics programme of Sandia National Laboratories, was used to perform nanoindentation simulation on silicon, a zinc-blende material. Defects in silicon (111) under spherical indentation showed the threefold pattern and the slip system in the form of ring crack. Also simulation results show good agreement with experimental results and existing theoretical analyses.

Load and Stiffness Dependence of Atomistic Sliding Friction (원자스케일 마찰의 하중 및 강성 의존성)

  • Sung, In-Ha
    • Tribology and Lubricants
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    • v.23 no.1
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    • pp.9-13
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    • 2007
  • Despite numerous researches on atomic-scale friction have been carried out for understanding the origin of friction, lots of questions about sliding friction still remain. It is known that friction at atomic-scale always shows unique phenomena called 'stick-slips' which reflect atomic lattice of a scanned surface. In this work, experimental study on the effects of system stiffnesses and load on the atomic-scale stick-slip friction of graphite was performed by using an Atomic Force Microscope and various cantilevers/tips. The objective of this research is to figure out the dependency of atomic-scale friction on the nanomechanical properties in sliding contact such as load, stiffness and contact materials systematically. From this work, the experimental observation of transitions in atomic-scale friction from smooth sliding to multiple stick-slips in air was first made, according to the lateral cantilever stiffness and applied normal load. The superlubricity of graphite could be verified from friction vs. load experiments. Based on the results, the relationship between the stickslip behaviors and contact stiffness was carefully discussed in this work. The results or this work indicate that the atomic-scale stick-slip behaviors can be controlled by adjusting the system stiffnesses and contact materials.

Carbon Nanotube Oscillator Operated by Thermal Expansion of Encapsulated Gases (삽입 가스의 부피 팽창을 이용한 탄소나노튜브 진동기)

  • Kwon, Oh-Keun
    • Journal of the Korean Institute of Electrical and Electronic Material Engineers
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    • v.18 no.12
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    • pp.1092-1100
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    • 2005
  • We investigated a carbon nanotube (CNT) oscillator controlled by the thermal gas expansion using classical molecular dynamics simulations. When the temperature rapidly increased, the force on the CNT oscillator induced by the thermal gas expansion rapidly increased and pushed out the CNT oscillator. As the CNT oscillator extruded from the outer nanotube, the suction force on the CNT oscillator increased by the excess van der Waals(vdW) energy. When the CNT oscillator reached at the maximum extrusion point, the CNT oscillator was encapsulated into the outer nanotube by the suction force. Therefore, the CNT oscillator could be oscillated by both the gas expansion and the excess vdW interaction. As the temperature increased, the amplitude of the CNT oscillator increased. At the high temperatures, the CNT oscillator escaped from the outer nanotube, because the force on the CNT oscillator due to the thermal gas expansion was higher than the suction force due to the excess vdW energy. By the appropriate temperature controls, such as the maximum temperature, the heating rate, and the cooling rate, the CNT oscillator could be operated.

Random Dopant Fluctuation Effects of Tunneling Field-Effect Transistors (TFETs) (터널링 전계효과 트랜지스터의 불순물 분포 변동 효과)

  • Jang, Jung-Shik;Lee, Hyun Kook;Choi, Woo Young
    • Journal of the Institute of Electronics and Information Engineers
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    • v.49 no.12
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    • pp.179-183
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    • 2012
  • The random dopant fluctuation (RDF) effects of tunneling field-effect transistors (TFETs) have been observed by using atomistic 3-D device simulation. Due to extremely low body doping concentration, the RDF effects of TFETs have not been seriously investigated. However, in this paper, it has been found that the randomly generated and distributed source dopants increase the variation of threshold voltage ($V_{th}$), drain induced current enhancement (DICE) and subthreshold slope (SS) of TFETs. Also, some ways of relieving the RDF effects of TFETs have been presented.

Multi-scale simulation of drying process for porous materials using molecular dynamics (part 2: material properties) (분자동역학을 이용한 다공성 물질 건조공정 멀티스케일 시뮬레이션(2부: 미시 물성))

  • Baik S.M.;Keum Y.T.
    • Journal of the Korean Crystal Growth and Crystal Technology
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    • v.15 no.4
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    • pp.162-167
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    • 2005
  • As the properties of porous materials during the drying process relate to the atomistic defects of heterogeneous materials such as dislocation, grain, grain boundary, pore, etc., the knowledge of nano-scale analysis is needed in order to accurately analyze the drying process for porous materials. In this study, the atomic behavior of porous materials Is statically predicted by using the molecular dynamics simulation and the nano-scale material properties are computed. The elastic modulus, thermal expansion coefficient, and volumetric heat capacity numerically found from the molecular dynamics simulation are compared with those of experiment and theory and proved the accuracy.

Effect of structural voids on mesoscale mechanics of epoxy-based materials

  • Tam, Lik-ho;Lau, Denvid
    • Coupled systems mechanics
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    • v.5 no.4
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    • pp.355-369
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    • 2016
  • Changes in chemical structure have profound effects on the physical properties of epoxy-based materials, and eventually affect the durability of the entire system. Microscopic structural voids generally existing in the epoxy cross-linked networks have a detrimental influence on the epoxy mechanical properties, but the relation remains elusive, which is hindered by the complex structure of epoxy-based materials. In this paper, we investigate the effect of structural voids on the epoxy-based materials by using our developed mesoscale model equipped with the concept of multiscale modeling, and SU-8 photoresist is used as a representative of epoxy-based materials. Developed from the results of full atomistic simulations, the mesoscopic model is validated against experimental measurements, which is suitable to describe the elastic deformation of epoxy-based materials over several orders of magnitude in time- and length scales. After that, a certain quantity of the structure voids is incorporated in the mesoscale model. It is found that the existence of structural voids reduces the tensile stiffness of the mesoscale epoxy network, when compared with the case without any voids in the model. In addition, it is noticed that a certain number of the structural voids have an insignificant effect on the epoxy elastic properties, and the mesoscale model containing structural voids is close to those found in real systems.