• Structural Engineering and Mechanics
• /
• 제30권2호
• /
• pp.177-190
• /
• 2008

Material failure behavior is generally dependent on loading rate. Especially in brittle and quasi-brittle materials, rate dependent material behavior can be significant. Empirical formulations are often used to predict the rate dependency, but such methods depend on extensive experimental works and are limited by practical constraints of physical testing. Numerical simulation can be an effective means for extracting knowledge about rate dependent behavior and for complementing the results obtained by testing. In this paper, the failure behavior of a brittle material under different loading rates is simulated by molecular dynamics analysis. A notched specimen is modeled by sub-million particles with a normalization scheme. Lennard-Jones potential is used to describe the interparticle force. Numerical simulations are performed with six different loading rates in a direct tensile test, where the loading velocity is normalized to the ratio of the pseudo-sonic speed. As a consequence, dynamic features are achieved from the numerical experiments. Remarkable failure characteristics, such as crack surface interaction/crack arrest, branching, and void nucleation, vary in case of the six loading cases. These characteristics are interpreted by the energy concept approach. This study provides insight into the change in dynamic failure mechanism under different loading rates.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2008년도 추계학술대회A
• /
• pp.1885-1890
• /
• 2008

3-D heterogeneous nucleation was simulated by classical molecular dynamics (MD), where the Lennard-Jones (LJ) gas and solid cluster-seed molecules have argon and aluminum properties, respectively. There are three shapes of cluster-seeds, cube, rod, and sphere, and three classes of masses and the simulation took place under nine supersaturation ratios, making a total of 81 calculations. Results show that the dimension of the cluster-seed highly affects the rates of cluster development. In order to analyze heterogeneous nucleation above and below the critical supersaturation ratio, growth rate and liquefaction rate were separately defined to supplement the investigation. Design of experiments (DOE) was used for analysis which displayed that the shape and mass of the cluster-seed are prominent for the growth rate, while the supersaturation ratio is most significant followed by the mass for liquefaction rate. The significance of the supersaturation ratio for overall liquefaction suggests that thermal diffusion is more dominant than mass interactions for this system.

• 한국진공학회:학술대회논문집
• /
• 한국진공학회 2014년도 제46회 동계 정기학술대회 초록집
• /
• pp.160.2-160.2
• /
• 2014

Recently silicon has attracted intense interest as a promising anode material of lithium-ion batteries due to its extremely high capacity of 4200 mA/g (for Li4.2Si) that is much higher than 372 mAh/g (for LiC6) of graphite. However, it seriously suffers from large volume change (even up to 300%) of the electrode upon lithiation, leading to its pulverization or mechanical failure during lithiation/delithiation processes and the rapid capacity fading. To overcome this problem, Si nanowires have been considered. Use of such Si nanowires provides their facile relaxation during lithiation/delithiation without mechanical breaking. To design better Si electrodes, a study to unveil atomic-scale mechanisms involving the volume expansion and the phase transformation upon lithiation is critical. In order to investigate the lithiation mechanism in Si nanowires, we have developed a reactive force field (ReaxFF) for Si-Li systems based on density functional theory calculations. The ReaxFF method provides a highly transferable simulation method for atomistic scale simulation on chemical reactions at the nanosecond and nanometer scale. Molecular dynamics (MD) simulations with the ReaxFF reproduces well experimental anisotropic volume expansion of Si nanowires during lithiation and diffusion behaviors of lithium atoms, indicating that it would be definitely helpful to investigate lithiation mechanism of Si electrodes and then design new Si electrodes.

• Tribology and Lubricants
• /
• 제21권4호
• /
• pp.177-184
• /
• 2005

The molecular dynamics simulation of nanoimprint lithography (NIL) using $SiO_2$ stamp and amorphous poly-(methylmethacrylate) (PNMA) film is performed to study pattern transfer in NIL. Force fields including bond, angle, torsion, van der Waals and electrostatic potential are used to describe the intermolecular and intramolecular force of PMMA molecules and $SiO_2$ stamp. Nose-Hoover thermostat is used to control the system temperature and cell multipole method is adopted to treat long range interactions. The deformation of PMMA film is observed during pattern transfer in the NIL process. For the detail analysis of deformation characteristics, the distributions of density and stress in PMHA film are calculated. The adhesion and friction forces are obtained by dividing the PMMA film into subregions and calculating the interacting force between subregion and stamp. Their effects on the pattern transfer are also discussed as varying the indentation depth and speed.

• 한국진공학회:학술대회논문집
• /
• 한국진공학회 2011년도 제40회 동계학술대회 초록집
• /
• pp.325-325
• /
• 2011

Very initial stage of oxidation process of Si (001) surface at room temperature (300 K) and high temperature (1200 K) was investigated using large scale molecular dynamics simulation. Reactive force field potential [1] was used for the simulation owing to its ability to handle charge variation as well as breaking and forming of bonds associated with the oxidation reaction. The results show that oxygen molecules adsorb dissociatively or otherwise leave the silicon surface. Initial position and orientation of oxygen molecule above the surface play important role in determining final state and time needed to dissociate. At 300 K, continuous transformation of ion $Si^+$ (or suboxide Si2O) to $Si2^+$ (SiO), $Si3^+$ (Si2O3) and finally to $Si4^+$ (SiO2) clearly observed. High temperature silicon surface provide heat energy that enable oxygen atom to penetrate into deeper silicon surface. The heat energy also retards adsorption process. As a result, transformation of ion $Si^+$ is impeded at 1200 K.

• 한국진공학회:학술대회논문집
• /
• 한국진공학회 2011년도 제40회 동계학술대회 초록집
• /
• pp.310-310
• /
• 2011

Morphological evolution of amorphous carbon film is investigated by molecular dynamics simulation. Here, energetic carbon atoms (75 eV) are deposited on the diamond (001) substrate to find effect of incidence angles. At normal and near-normal incidences ($0^{\circ}{\sim}30^{\circ}$) atomically smooth surfaces are observed during their growth. However, rough surfaces emerge and develop into a ripple structure at grazing incidences ($60^{\circ}{\sim}70^{\circ}$). The different growth modes according to the incidence angles can be described by impact-induced displacements of atoms. Downhill transport along any sloped surfaces is predominant for the case of normal incidence. As the incidence angles become grazing, uphill transport is allowed along the surfaces, which have smaller slopes than incidence angle, so the surface features can be amplified. Impact-induced transport and self-shadowing effect can be responsible to the initial growth of seeding structures at a grazing incidence, which would be grown up as tilted columnar structures in further depositions.

• 한국진공학회:학술대회논문집
• /
• 한국진공학회 2011년도 제40회 동계학술대회 초록집
• /
• pp.433-433
• /
• 2011

Since the concept of graphene was established, it has been intensively investigated by researchers. The unique characteristics of graphene have been reported, the graphene attracted a lot of attention for material overcomes the limitations of existing semiconductor materials. Because of these trends, economical fabrication technique is becoming more and more important topic. Especially, the epitaxial growth method by sublimating the silicon atoms on Silicon carbide (SiC) substrate have been reported on the mass production of high quality graphene sheets. Although SiC exists in a variety of polytypes, the 3C-SiC polytypes is the only polytype that grows directly on Si substrate. To practical use of graphene for electronic devices, the technique, forming the graphene on 3C-SiC(111)/Si structure, is much helpful technique. In this paper, we report on the growth of graphene on 3C-SiC(111) surface. To investigate the morphology of formed graphene on the 3C-SiC(111) surface, the radial distribution function (RDF) was calculated using molecular dynamics (MD) simulation. Through the comparison between the kinetic energies and the diffusion energy barrier of surface carbon atoms, we successfully determined that the graphitization strongly depends on temperature. This graphitization occurs above the annealing temperature of 1500K, and is also closely related to the behavior of carbon atoms on SiC surface. By analyzing the results, we found that the diffusion energy barrier is the key parameter of graphene growth on SiC surface.

• 한국재료학회지
• /
• 제16권2호
• /
• pp.92-98
• /
• 2006

Molecular dynamics simulations were performed to study interface structures between an $Al_2O_3$ crystalline phase and a interface phase of $CaAl_2Si_2O_8$. We calculated atomic structures and excess interface energies in systems with different thicknesses of the interface film. It was found that excess interface energies at first readily decreased with increasing film thickness, but increased for larger thicknesses of more than 2 nm. The excess energies of $Al_2O_3/CaAl_2Si_2O_8$ interfaces exhibit a minimum at a thickness around 1 nm. In this range of film thicknesses, the atoms in the interface film show a short-range ordered structure and slow diffusion rather than the random structure and rapid diffusion expected to an observation of an equilibrium thickness for interface films in ceramics.

• 열처리공학회지
• /
• 제13권4호
• /
• pp.259-264
• /
• 2000

In this study, we quantitatively measure the ion ranges of arsenic with energies ranging from 10 KeV to 100 KeV, implanted at $3^{\circ}$, $9^{\circ}$ $15^{\circ}$ the (100) plane, and the damage created during ion implantation. To obtain detailed information of ion range and damage distributions in low energy region where elastic collisions dominate the slowing down process, molecular dynamics computer simulation was performed and compared to the existing results. The effects of implant energy and degree on damage generation are present. The number of vacancy were calculated from the deposited energy using Kinchin-Pease equation. In the energy range 10 keV-100 keV, simulations show that the number of Frenckel pairs produced by As-ion bimbardment is 9 and incident angle dependence of the vacancy was the same but defects were distributed at different depth.

• 대한의용생체공학회:의공학회지
• /
• 제24권5호
• /
• pp.447-451
• /
• 2003

준분자 동력학 기법을 적용한 새로운 골-재형성 해석 방법을 제안 하였다. 본 연구에서는 기존의 연구에서 고려하지 못 하였던 해변골 내의 유체 압력이 골-재형성에 미치는 영향 까지를 고려한 해석 모델이 제안 되었다. 골-재형성의 대상 모델로 추체 시상면상의 2-D 모델을 개발 하였으며, 약 2kN의 수직 압축력에 해당되는 1.8564mm의 일정 변위 조건에 따른 모델내의 준분자들의 재배치 과정을 해석 하였다. 수렴된 피질골 및 해면골 모델의 준 분자의 형상은 실제 추체의 모양과 일치된 형태를 보이고 있었으며 계산된 수핵 압력 변화도 기존의 in-Vitro 실험 결과와 비교적 일치 하여 본 모델의 타당성을 입증 할 수 있었다.