• Corrosion Science and Technology
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• v.16 no.2
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• pp.64-68
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• 2017

The interactions between corrosion pits on stainless steel under loading conditions are studied by using a cellular automata model coupled with finite element method at a mesoscopic scale. The cellular automata model focuses on a metal/film/electrolyte system, including anodic dissolution, passivation, diffusion of hydrogen ions and salt film hydrolysis. The Chopard block algorithm is used to improve the diffusion simulation efficiency. The finite element method is used to calculate the stress concentration on the pit surface during pit growth, and the effect of local stress and strain on anodic current is obtained by using the Gutman model, which is used as the boundary conditions of the cellular automata model. The transient current characteristics of the interactions between corrosion pits under different simulation factors including the breakdown of the passive film at the pit mouth and the diffusion of hydrogen ions are analyzed. The analysis of the pit stability product shows that the simulation results are close to the experimental conclusions.

• Interaction and multiscale mechanics
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• v.7 no.1
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• pp.543-561
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• 2014

We present a full energy and force formulation of the quasicontinuum method with non-local and local transition elements. Non-local transition elements are developed to transmit inhomogeneity from the atomistic to the continuum regions. Local transition elements are developed to resolve the mathematical mismatch between non-local atoms and the local continuum. The rationale behind these transition elements is provided by analyzing the energy and force transitions between atoms and continuum under the Cauchy-Born rule. We show that breakdown of the Cauchy-Born rule occurs for slaved atoms of local elements within the cutoff of non-local atoms. The inadequacy of the Cauchy-Born rule at the transition region naturally leads to the need of atomistic treatment of transition slaved and transition representative atoms. Such an atomistic treatment together with a full or cutoff sampling allows non-local transition elements containing these transition entities to transmit inhomogeneity. Different force formulations for transition representative atoms and pure local representative atoms allow the local transition elements to resolve non-local and local mismatches. The method presented herein is validated by force calculations in an unstressed perfect crystal as well as an unrelaxed grain boundary model. A nanoindentation simulation in 3D is conducted to demonstrate the accuracy and efficiency of the proposed method.

• Journal of the Korean Institute of Educational Facilities
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• v.26 no.5
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• pp.17-23
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• 2019

The classroom is a learning space where students live mainly in school, and it is the most important space within the entire educational space of the school. Although our educational system has been trying to revise and change the curriculum many times, it still does not provide space for the educational concept of modern society. The impact of the abolished 'school facility standard design' in 1992 is still evident in the design of school facilities at present. Specifically, the uniformity of the educational space, the rigid boundary of the classroom unit, the blockage between the school facility and the outside, and the separation due to the break of the inner and outer spaces. In the future, we need a flexible space that can contain the contents of the future education, and it is necessary to study the composition and type of educational space that can escape uniformity and spatial breakdown. In this paper, we analyze the successful cases of Irvine school facilities and examine the type and composition of classroom space, and it will be a task to find the direction and change of thinking about our educational space.

• Tribology and Lubricants
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• v.11 no.5
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• pp.128-135
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• 1995

Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales, ranging from atomic and molecular to microscales. These techniques have been used to study surface roughness, adhesion, friction, scratching/wear, indentation, detection of material transfer, and boundary lubrication and for nanofabrication/nanomachining purposes. Micro/nanotribological studies of single-crystal silicon, natural diamond, magnetic media (magnetic tapes and disks) and magnetic heads have been conducted. Commonly measured roughness parameters are found to be scale dependent, requiring the need of scale-independent fractal parameters to characterize surface roughness. Measurements of atomic-scale friction of a freshly-cleaved highly-oriented pyrolytic graphite exhibited the same periodicity as that of corresponding topography. However, the peaks in friction and those in corresponding topography were displaced relative to each other. Variations in atomic-scale friction and the observed displacement has been explained by the variations in interatomic forces in the normal and lateral directions. Local variation in microscale friction is found to correspond to the local slope suggesting that a ratchet mechanism is responsible for this variation. Directionality in the friction is observed on both micro- and macro scales which results from the surface preparation and anisotropy in surface roughness. Microscale friction is generally found to be smaller than the macrofriction as there is less ploughing contribution in microscale measurements. Microscale friction is load dependent and friction values increase with an increase in the normal load approaching to the macrofriction at contact stresses higher than the hardness of the softer material. Wear rate for single-crystal silicon is approximately constant for various loads and test durations. However, for magnetic disks with a multilayered thin-film structure, the wear of the diamond like carbon overcoat is catastrophic. Breakdown of thin films can be detected with AFM. Evolution of the wear has also been studied using AFM. Wear is found to be initiated at nono scratches. AFM has been modified to obtain load-displacement curves and for nanoindentation hardness measurements with depth of indentation as low as 1 mm. Scratching and indentation on nanoscales are the powerful ways to screen for adhesion and resistance to deformation of ultrathin fdms. Detection of material transfer on a nanoscale is possible with AFM. Boundary lubrication studies and measurement of lubricant-film thichness with a lateral resolution on a nanoscale have been conducted using AFM. Self-assembled monolyers and chemically-bonded lubricant films with a mobile fraction are superior in wear resistance. Finally, AFM has also shown to be useful for nanofabrication/nanomachining. Friction and wear on micro-and nanoscales have been found to be generally smaller compared to that at macroscales. Therefore, micro/nanotribological studies may help def'me the regimes for ultra-low friction and near zero wear.

• Korean Journal of Materials Research
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• v.1 no.3
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• pp.125-131
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• 1991

Boron penetration phenomenon of $p^{+}$ silicon gate with as-deposited amorphous or polycrystalline Si upon high temperature annealing was investigated using high frequency C-V (Capacitance-Volt-age) analysis, CCST(Constant Current Stress Test), TEM(Transmission Electron Microscopy) and SIMS(Secondary Ion Mass Spectroscopy), C-V analysis showed that an as-deposited amorphous Si gate resulted in smaller positive shifts in flatband voltage compared wish a polycrystalline Si gate, thus giving 60-80 percent higher charge-to-breakdown of gate oxides. The reduced boron penetration of amorphous Si gate may be attributed to the fewer grain boundaries available for boron diffusion into the gate oxide and the shallower projected range of $BF_2$ implantation. The relation between electron trapping rate and flatband voltage shift was also discussed.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.42 no.7
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• pp.535-543
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• 2014

A computational simulation for a nonslender BWB UCAV configuration with rounded leading edge and span of 1.0m was performed to analyze its aerodynamic characteristics. The freestream is 50m/s over -4 to 26 degree A.o.A.s. Reynolds number based on the mean chord length is $1.25{\times}10^6$. 3D multi block hexahedral grids are used which allow good grid quality and ease to capture boundary layer. ${\gamma}-Re_{\theta}$ model as well as $k-{\omega}$ SST model is employed to assess the effect of transition for flow behavior. Drag and lift of the UCAV were well predicted while $C_M$ is under predicted at high angle of attacks and influenced by the turbulence models strongly. After assessing pressure distribution, skin friction lines and velocity field around the UCAV configuration, it was found that transition effect should be considered to enhance the prediction of aerodynamic behavior by a vortical flowfield.

• Tunnel and Underground Space
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• v.12 no.4
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• pp.312-320
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• 2002

It is well-known that when single rock fractures undergo shear displacement, they are influenced by the boundary conditions and fracture roughness. In this case, aperture geometry will change by means of dilation due to the shear displacement. As fractures become the flow paths, fluid flow through rock fractures is affected by the void geometry. In this study, therefore, the influence of roughness on shear behavior of fractures has been investigated, and the resulting hydraulic behavior has been analyzed. In order for this study, a statistical method has been used to generate rough fractures, and they have been adopted into new conceptual models fur fracture shearing and flow calculations. The main contributions of this study are as follows: firstly, fracture shear behavior becomes less brittle with decreasing fracture roughness and increasing normal stress. Then, the characteristics of aperture distribution becomes those of roughness of fractures indicating its hydraulic significance. Finally, it is observed that with decreasing fracture roughness the breakdown of channel flow occurs more slowly.

• Korean Journal of Materials Research
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• v.5 no.1
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• pp.3-11
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• 1995

$(TiO_{2})$ thin films were deposited on p-Si(100) substrate by APMOCVD using titanium isopropoxide as a source material. The deposition mechanism was well explained by the simple boundary layer theory and the apparent activation energy of the chemical reaction controlled process was 18.2kcal /mol. The asdeposited films were polycrystalline anatase phase and were transformed into rutile phase after postannealing. The postannealing time and the film thikness as well as the postannealing temperature also affected the phase transition. The C-V plot exhibited typical charateristics of MOS diode, from which the dielectric constant of about 80 was obtained. The capacitance of the annealed film was decreased but those of the Nb or Sr doped films were not changed. I-V characteristics revealed that the conduction mechanism was hopping conduction. The postannealing and the doping of Nb or Sr cause to decrease the leakage current and to increase the breakdown voltage.

• Journal of the Korean Ceramic Society
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• v.34 no.8
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• pp.803-810
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• 1997

Ba0.5Sr0.5TiO3 thin films were deposited by RF magnetron sputliring method in order to clarify the anneal condition and doping effect on loakage current Nb and Al were selected as electron donor and acceptor dopants respectively, in the BST films because they have been known to have nearly same ionic radii as Ti and thought to substitute Ti sites to influence the charge carrier and the acceptor state adjacent to the gram boundary. BST thin films prepared in-situ at elevated temperature showed selatively high leakage current density and low breakdown voltage. In order to achieve smooth surface and to improve electrical properties, BST thin films were deposited at room temperature and annealed at elevated temperature. Post-annealed BST thin films showed smoother surface morphology and lower leakage current density than in-situ prepared thin films. The leakage current density of Al doped thin films was measured to be around 10-8A/cm2, which is much lower than those of undoped and Nb doped BST films. The result clearly demonstrates that higher Schottky barrier and lower mobile charge carrier concentration achieved by annealing in the oxygen atmosphere and by Al doping are desirable for reducing leakage current density in BST thin films.

• Korean Journal of Materials Research
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• v.31 no.10
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• pp.539-545
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• 2021

The effect of Sm₂O₃ doping on the microstructure and electrical properties of the ZPCCA-based varistors is comprehensively investigated. The increase of doping content of Sm₂O₃ results in better densification (from 5.70 to 5.82 g/cm³) and smaller mean grain size (from 7.8 to 4.1 ㎛). The breakdown electric field increases significantly from 2568 to 6800 V/cm as the doping content of Sm₂O₃ increases. The doping of Sm₂O₃ remarkably improves the nonlinear properties (increasing from 23.9 to 91 in the nonlinear coefficient and decreasing from 35.2 to 0.2 µA/cm² in the leakage current density). Meanwhile, the doping of Sm₂O₃ reduces the donor concentration (the range of 2.73 × 10¹⁸ to 1.18 × 10¹⁸ cm^-3) of bulk grain and increases the barrier height (the range of 1.10 to 1.49 eV) at the grain boundary. The density of the interface states decreases in the range of of 5.31 × 10¹² to 4.08 × 10¹² cm^-2 with the increase of doping content of Sm₂O₃. The dielectric constant decreases from 1594.8 to 507.5 with the increase of doping content of Sm₂O₃.