• Journal of the Korean Crystal Growth and Crystal Technology
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• v.5 no.1
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• pp.1-10
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• 1995

Congruent $LiNbO_3$ single crystals and $LiNbO_3$ : Mg single crystals, having high resistance to optical damage, doped with MgO to the levels of 2.0, 5.0 and 7.0 mol% were grown successfully by CZ method and optical damage of each crystal was measured by compen. sation method. With doping level reaching about 5 mol%, there was an abrupt change in the features of optical absorption edge and $OH^-$ absorption band. From these data, we confirmed indirectly the threshold in MgO doping level. When the MgO doping amount reaches about 5 mol% in the melt, $Mg^{2+}$ occupies Nb site and becomes $Mg_{Nb}^{2+}$, resulting in the sharp increase of optical damage resistance. The optical damage resistance of $LiNbO_3$ : Mg was improved more than three times when MgO amount in the melt reaches 5 mol%.

• The Journal of Korean Academy of Prosthodontics
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• v.18 no.1
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• pp.37-47
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• 1980

The purpose of this study was to investigate the stresses in different proximal margins and to measure, quantitatively, the effect of different modifications in the design of preparations on the stresses using two-dimensional photoelasticity. Photoelastic stress analysis is based on the phenomenon, exhibited by most transparent solids, of becoming birefringent, or doubly refracting, when strained. Two birefringent materials were used in this study, PSM-1 and PSM-5 in .standard sheet ($10'{\times}10'{\times}\frac{1}{4}'$ thickness), PSM-1(polyester) was used for constructing the substructure, and PSM-5(epoxy resin) was used in making the restorations to be investigated. Two birefringent materials were used in the construction of composite photoelastic model. Seven variable models were constructed. The peripheral dimensions of all model were constant and the models represent an occlusomesial section of a lower posterior molar. Model 1 represents the knife edge margin (shoulderless), Model 2 represents the chamfer, Model 3 represents a rounded shoulder(no sharp angle between the axial wall and gingival floor), Model 4 represents a flat shoulder (axial wall is a $90^{\circ}$ angle to the gingival wall), Model 5 represents $+15^{\circ}$ angulation, Model 6 has a $-15^{\circ}$ angulation, and Model 7 is the same as Model 4 except that it has a $45^{\circ}$ bevel. Improved artificial stone was used to represent dental cement in luting the composite photoelastic model. Static loading procedures(100 pounds) were used at preplanned sites. The results were as follows; 1. The stresses in the proximal portion of all tested models were compressive in nature when the proximal shoulders were loaded vertically on the same proximal marginal ridge. 2. The round and chamfered preparations were the optimum designs in proximoocclusal restorations. They showed the lowest stress concentration factor, i.e. 2.16 and 2.23, respectively. The knife edged shoulder had the highest value, K=5.39. Round type shoulder geometry experiments reduced the stress concentration factor (S.C.F.) 3. The gingival portion of proximal shoulder geometry was a critical location for stress concentration.

• International Journal of Fluid Machinery and Systems
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• v.9 no.1
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• pp.66-74
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• 2016

Concerning the numerical simulation of high-speed water jet with intensive cavitation this paper presents a practical compressible mixture flow method by coupling a simplified estimation of bubble cavitation and a compressible mixture flow computation. The mean flow of two-phase mixture is calculated by URANS for compressible fluid. The intensity of cavitation in a local field is evaluated by the volume fraction of gas phase varying with the mean flow, and the effect of cavitation on the flow turbulence is considered by applying a density correction to the evaluation of eddy viscosity. High-speed submerged water jets issuing from a sheathed sharp-edge orifice nozzle are treated when the cavitation number, ${\sigma}=0.1$, and the computation result is compared with experimental data The result reveals that cavitation occurs initially at the entrance of orifice and bubble cloud develops gradually while flowing downstream along the shear layer. Developed bubble cloud breaks up and then sheds downstream periodically near the sheath exit. The pattern of cavitation cloud shedding evaluated by simulation agrees experimental one, and the possibility to capture the unsteadily shedding of cavitation clouds is demonstrated. The decay of core velocity in cavitating jet is delayed greatly compared to that in no-activation jet, and the effect of the nozzle sheath is demonstrated.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2011.10a
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• pp.930-933
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• 2011

This paper addresses an adaptive postprocessing method applied in the spatial domain for block-based discrete cosine transform (BDCT) coded images. The proposed algorithm is designed by a serial concatenation of a 1D simple smoothing filter and a 2D directional filter. The 1D smoothing filter is applied according to the block type, which is determined by an adaptive threshold. It depends on local statistical properties, and updates block types appropriately by a simple rule, which affects the performance of deblocking processes. In addition, the 2D directional filter is introduced to suppress the ringing effects at the sharp edges and the block discontinuities while preserving true edges and textural information. Comprehensive experiments indicate that the proposed algorithm outperforms many deblocking methods in the literature, in terms of PSNR and subjective visual quality evaluated by GBIM.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2002.08a
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• pp.97-100
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• 2002

$CdGaInS_{4}:Er^{3+}$ single crystal crystallized in the rhombohedral. with lattice constants a = 3.899 $\AA$ and c = 36.970 $\AA$ for $CdGaInS_{4}:Er^{3+}$. The optical absorption measured near the fundamental band edge showed that the optical energy band structure of this compound had a direct and indirect band gaps. the direct and indirect energy gaps are found to be 2.665 and 2.479eV for $CdGaInS_{4}:Er^{3+}$ at 10 K. The photoluminescence spectra of $CdGaInS_{4}:Er^{3+}$ measured in the wavelength ranges of 500 nm~900 nm and 1500~1600 nm at 10 K. Eight sharp emission peaks due to $Er^{3+}$ ion are observed in the regions of 549.5~560.0nm. 661.3~676.5nm. 811.1~ 834.1 nm and 1528.2~1556.0 nm in $CdGaInS_{4}:Er^{3+}$ single crystal. These PL peaks were attributed to the radiative transitions between the split electron energy levels of the $Er^{3+}$ ions occupied at $C_{2v}$ symmetry of the $CdGaInS_4$ single crystals host lattice.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.203.1-203.1
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• 2013

Zinc oxide (ZnO) nanocrystalline thin films on various growth temperatures for active layer and different buffer layer thickness were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on Si substrates. The ZnO active layer were grown with various growth temperature from 500 to $800^{\circ}C$ and the ZnO buffer layer were grown for different time from 5 to 40 minutes. To investigate the structural and optical properties of the ZnO thin films, scanning electron microscope (SEM), X-ray diffractometer (XRD), and photoluminescence (PL) spectroscopy were used, respectively. In the SEM images, the ZnO thin films have high densification of grains and good roughness and uniformity at $800^{\circ}C$ for active layer growth temperature and 20 minutes for buffer layer growth time, respectively. The PL spectra of ZnO buffer layers and active layers display sharp near band edge (NBE) emissions in UV range and broad deep level emissions (DLE) in visible range. The intensity of NBE peaks for the ZnO thin films significantly increase with increase in the active layer growth temperature. In addition, the NBE peak at 20 minutes for buffer layer growth time has the largest emission intensity and the intensity of DLE peaks decrease with increase in the growth time.

• Korean Journal of Materials Research
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• v.31 no.9
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• pp.481-487
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• 2021

ZnO particles are successfully synthesized at 150 ℃ for 30 min using zinc acetate as the Zn source and 1,4-butanediol as solvent using a relatively facile and convenient glycol process. The effect of ammonium hydroxide amounts on the growth behavior and the morphological evolution of ZnO particles are investigated. The prepared ZnO nanoparticle with hexagonal structure exhibits a quasi-spherical shape with an average crystallite size of approximately 30 nm. It is also demonstrated that the morphology of ZnO particles can be controlled by 1,4-butanediol with an additive of ammonium hydroxide. The morphologies of ZnO particles are changed sequentially from a quasi-spherical shape to a rod-like shape and a hexagonal rod shape with a truncated pyramidal tip, exhibiting preferential growth along the [001] direction with increasing ammonium hydroxide amounts. It is demonstrated that much higher OH^- amounts can produce a nano-tip shape grown along the [001] direction at the corners and center of the (001) top polar plane, and a flat hexagonal symmetry shape of the bottom polar plane on ZnO hexagonal prisms. The results indicate that the presence of NH⁴⁺ and OH^- ions in the solution greatly affects the growth behaviors of ZnO particles. A sharp near-band-edge (NBE) emission peak centered at 383 nm in the UV region and a weak broad peak in the visible region between 450 nm and 700 nm are shown in the PL spectra of the ZnO synthesized using the glycol process, regardless of adding ammonium hydroxide. Although the broad peak of the deep-level-emission (DLE) increases with the addition of ammonium hydroxide, it is suggested that the prominent NBE emission peaks indicate that ZnO nanoparticles with good crystallization are obtained under these conditions.

• Journal of the Korean Society of Visualization
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• v.20 no.3
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• pp.94-101
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• 2022

The evaluation of the drag and lift as the aerodynamic performance of airfoils is essential. In addition, the analysis of the velocity and pressure fields is needed to support the physical mechanism of the force coefficients of the airfoil. Thus, the present study aims at establishing two different deep learning models to predict force coefficients and flow fields of the airfoil. One is the convolutional neural network (CNN) model to predict drag and lift coefficients of airfoil. Another is the Encoder-Decoder (ED) model to predict pressure distribution and velocity vector field. The images of airfoil section are applied as the input data of both models. Thus, the computational fluid dynamics (CFD) is adopted to form the dataset to training and test of both CNN models. The models are established by the convergence performance for the various hyperparameters. The prediction capability of the established CNN model and ED model is evaluated for the various NACA sections by comparing the true results obtained by the CFD, resulting in the high accurate prediction. It is noted that the predicted results near the leading edge, where the velocity has sharp gradient, reveal relatively lower accuracies. Therefore, the more and high resolved dataset are required to improve the highly nonlinear flow fields.

• Composites Research
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• v.36 no.4
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• pp.259-263
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• 2023

This study investigated the open hole tensile (OHT) properties of carbon fiber/epoxy composites and compared them to bolt joint tensile (BJT) properties. The net nominal modulus and strength (1376 MPa) were found to be higher than the gross nominal strength (1041 MPa), likely due to increasing hole size. The OHT and BJT specimens exhibited similar stiffness, as expected without bolt rotation causing secondary bending. OHT specimens experienced a sharp drop in stress indicating unstable crack propagation, delamination, and catastrophic failure. BJT specimens failed through shear out on the bolt side and bearing failure on the nut side, involving fiber kinking, matrix splitting, and delamination, resulting in lower strength compared to OHT specimens. The strength retention of carbon fiber/epoxy composites with open holes was 66%. Delamination initiation at the hole's edge caused a reduction in the stress concentration factor. Filling the hole with a bolt suppressed this relieving mechanism, leading to lower strength in BJT specimens compared to OHT specimens. Bolt joint efficiency was calculated as 15%. The reduction in strength in bolted joints was attributed to fiber-matrix splitting and delamination, aligning with Hart Smith's bolted joint efficiency diagram. These findings contribute to materials selection and structural reliability estimation for carbon fiber/epoxy composites. They highlight the behavior of open hole and bolt joint configurations under tensile loading, providing valuable insights for engineering applications.

• Composites Research
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• v.21 no.3
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• pp.9-17
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• 2008

In this paper, the low velocity impact characteristics of filament winding CFRP pressure vessel was investigated using numerical and experimental methods. The cylinder part of CFRP vessel was impacted using triangular shape impactor which simulated the sharp edge of dropping tools and impact response behavior of CFRP was reviewed. The mechanical behavior, such as deformation and stress distribution, were also predicted by explicit finite element method and the validity of the model was investigated. For the quantitative evaluation of the residual strength of the pressure vessel after impact, a series of the ring specimens was cut from the impacted vessel and its burst pressure was measured by hydraulic pressure hoop tension test. As the results, the relationship between the residual strength degradation and the impact energy was successively obtained and a useful methodology to evaluate quantitatively the impact damage tolerance of CFRP pressure vessel was established.