• Proceedings of the Korean Vacuum Society Conference
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• 한국진공학회 2016년도 제50회 동계 정기학술대회 초록집
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• pp.286-286
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• 2016

Hexagonal boron nitride (hBN) is a dielectric insulator with a two-dimensional (2D) layered structure. It is an appealing substrate dielectric for many applications due to its favorable properties, such as a wide band gap energy, chemical inertness and high thermal conductivity[1]. Furthermore, its remarkable mechanical strength renders few-layered hBN a flexible and transparent substrate, ideal for next-generation electronics and optoelectronics in applications. However, the difficulty of preparing high quality large-area hBN films has hindered their widespread use. Generally, large-area hBN layers prepared by chemical vapor deposition (CVD) usually exhibit polycrystalline structures with a typical average grain size of several microns. It has been reported that grain boundaries or dislocations in hBN can degrade its electronic or mechanical properties. Accordingly, large-area single crystalline hBN layers are desired to fully realize the potential advantages of hBN in device applications. In this presentation, we report the growth and transfer of centimeter-sized, nearly single crystal hexagonal boron nitride (hBN) few-layer films using Ni(111) single crystal substrates. The hBN films were grown on Ni(111) substrates using atmospheric pressure chemical vapor deposition (APCVD). The grown films were transferred to arbitrary substrates via an electrochemical delamination technique, and remaining Ni(111) substrates were repeatedly re-used. The crystallinity of the grown films from the atomic to centimeter scale was confirmed based on transmission electron microscopy (TEM) and reflection high energy electron diffraction (RHEED). Careful study of the growth parameters was also carried out. Moreover, various characterizations confirmed that the grown films exhibited typical characteristics of hexagonal boron nitride layers over the entire area. Our results suggest that hBN can be widely used in various applications where large-area, high quality, and single crystalline 2D insulating layers are required.

• Polymer(Korea)
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• 제31권1호
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• pp.58-67
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• 2007

Fully cholesteryloxycarbonated and (8-cholesteryloxycarbonyl) heptanoated disaccharide derivatives were synthesized by reacting cellobiose, maltose, and lactose with cholesteryl chloroformate or 8- cholesteryloxycarbonylheptanoyl chloride, and their thermotropic liquid crystalline properties were investigated. All the cholesteryloxycarbonated derivatives (CH1DSs) formed enantiotropic cholesteric phases, whereas all the (8-cholesteryloxycarbonyl) heptanoated derivatives (CH8DSs) exhibited monotropic cholesteric phases with left-handed helicoidal structures whose optical pitches (${\lambda}m's$) decrease with increasing temperature. All the CH1DSs, contrast with the CH8DSs, did not display reflection colors over the full cholesteric range, suggesting that the helicoidal twisting power of the cholesteryl group highly depends on the length of the spacer joining the cholesteryl group to the disaccharide chain. The thermal stability and degree of order in the mesophase and the temperature dependence of the ${\lambda}m$ observed for EH8DSs were entirely different from those reported for the cholesterol-bearing dimers and triplet and the (8-cholesteryloxycarbonyl) heptanoated polysaccharide derivatives. The results were discussed in terms of the difference in the number of the mesogenic units per mole of repeating unit and the flexibility of the main chain.

• Polymer(Korea)
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• 제31권4호
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• pp.356-367
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• 2007

Three kinds of amylose derivatives such as: cholesteryloxycarbonated amyloses(CAMs) with degree of esterification(DE) ranging from 1.8 to 3, (6-cholesteryloxycarbonyl)pentanoated amyloses(PAMs) with DE ranging from 0.3 to 3, and fully cholesteryloxycarbonated PAMs(CPAMs) were synthesized, and their thermotropic liquid crystalline properties were investigated. CAMs with $DE{\geq}2.6$, PAM with DE=1.6 and all the CPAMs formed enantiotropic cholesteric phases, whereas PAM with $DE{\geq}2.2$ exhibited monotropic cholesteric phases. PAM with $DE{\geq}2.2$ and CPAMs with (6-cholesteryloxycarbonyl)pentanoyl DE (DS) more than 1.0 formed cholesteric phases with left-handed helical structures whose optical pitches (${\lambda}_{m'}s$) decrease with increasing temperature. However, the ${\lambda}_{m'}s$ of these samples decreased with increasing DS at the same temperature. On the other hand, CAMs, PAM with DE=1.6, and CPAM with DS=0.3 did not display reflection colors over the full cholesteric range, suggesting that the helicoidal twisting power of the cholesteryl group highly depends on the length of the spacer joining the cholesteryl group to the main chain and DS. The thermal stability and degree of order in the mesophase observed for the amylose derivatives highly depended on DE or DS. The results were discussed in terms of the difference ul the hydrogen bond, the internal plasticization, and the decoupling of the motion of side group with the main chain.

• Journal of the Korean Vacuum Society
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• 제21권1호
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• pp.62-68
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• 2012

The conversion efficiency of solar cells depending on incident angle of light is important for building-integrated photovoltaics (BIPV) applications. The quantum efficiency is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the solar cell. The analysis of angle dependence of quantum efficiencies give more information upon the variation of power output of a solar cell by the incident angle of light. The variations in power output of solar cells with increasing angle of incidence is different for the type of cell structures. In this study we present the results of the quantum efficiency measurement of single-crystalline silicon solar cells and a-Si:H thin-film solar cells with the angle of incidence of light. As a result, as the angle of incidence increases in single-crystalline silicon solar cells, quantum efficiency at all wavelength (300~1,100 nm) of light were reduced. But in case of a-Si:H thin-film solar cells, quantum efficiency was increased or maintained at the angle of incidence from 0 degree to about 40 degrees and dramatically decrease at more than 40 degrees in the range of visible light. This results of quantum efficiency with increasing incident angle were caused by haze and interference effects in thin-film structure. Thus, the structural optimization considering incident angle dependence of solar cells is expected to benefit BIPV.

• Composites Research
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• 제29권4호
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• pp.145-150
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• 2016

Polyvinylidene fluoride (PVDF) as a representative polymer with the piezoelectric property has been studied since the 1960s. Crystalline structure of poly(vinylidene fluoride) polymer is composed of five different crystal structure of the polymer as a semi-crystalline. Among the various crystal structures, ${\beta}-type$ crystal exhibits a piezoelectricity because the permanent dipoles are aligned in one direction. Generally ${\beta}-form$ crystal structure can be obtained through the transformation of the ${\alpha}-form$ crystal structure by the stretching and it can increase the amount through the after treatment as poling process after stretching. ${\beta}-form$ crystal structure the PVDF fibers produced by wet spinning is formed through a diffusion mechanism of a polar solvent in the coagulation bath. However, it has a disadvantage that the diffusion path of the solvent remains as pores in the fiber because the fiber solidification occurs simultaneously with the diffusion of the polar solvent. These pores play a role in reducing effect of poling process owing to effect of disturbances acting on the polarization by the electric field. In this work, the drying method using the microwave was introduced to remove more effectively the residual solvent and the pore within PVDF fibers produced through wet-spinning process and piezoelectric PVDF fibers was produced by transformation of the remaining ${\alpha}$ form crystal structure into ${\beta}-crystal$ structure through the stretching process.

• Journal of the Microelectronics and Packaging Society
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• 제10권2호
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• pp.7-12
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• 2003

The $Bi_{3.3}La_{0.7}O_{12}$(BLT) capacitors with Metal-Ferroelectric-Insulator-Silicon structure were prepared on $SiO_2/Si$ substrates by using sol-gel method. The BLT thin films annealed at $650^{\circ}C$ and $700^{\circ}C$ showed randomly oriented perovskite crystalline structures. The full with at half maximum (FWHM) of the (117) main peak was decreased from $0.65^{\circ}$ to $0.53^{\circ}$ with increasing the annealing temperature from $650^{\circ}C$ to $700^{\circ}C$, indicating the improvement in the crystalline quality of the film. In addition, the grain size and $R_rms$ , values were increased with increasing the annealing temperatures, showing the rough film surface at higher annealing temperatures. From the capacitance-voltage (C-V) measurements, the memory window voltage of the BLT film annealed at $700^{\circ}C$ was found to be about 0.7 V at an applied voltage of 5 V. The leakage current density of the BLT film annealed at $700^{\circ}C$ was about $3.1{\times}10^{-8}A/cm^2$.

• Journal of the Korea Academia-Industrial cooperation Society
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• 제18권10호
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• pp.69-74
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• 2017

In this study, cholesteryl 4-n-alkoxybenzoates (Chol-n), with alkyl groups used for controlling the temperature of transition to the liquid crystal phase, were synthesized, and the effects of the length of the alkyl groups on the physical properties of the liquid crystal compounds were investigated. The chemical structures and thermal and liquid crystalline properties of the synthesized compounds were investigated by Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance spectroscopy ($^1H$-NMR), differential scanning calorimetry (DSC), and polarizing optical microscopy (POM). The synthesized compounds showed melting transition temperatures ($T_m$) in the range of $103^{\circ}C$ to $143^{\circ}C$ and all of the compounds except Chol-6 exhibited a wide liquid crystal phase temperature range of about $60^{\circ}C$ to $100^{\circ}C$. No correlation between the number of carbon atoms in the molecule and the thermal properties of the compounds was found. All of the synthesized compounds showed an enantiotropic cholesteric phase, which was accompanied by a chiral smectic phase in the compounds Chol-6, Chol-8, Chol-9, and Chol-10. All of the compounds exhibited thermochromism in the liquid crystal state, and their color changed from red to blue as the temperature was increased.

• Tunnel and Underground Space
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• 제19권4호
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• pp.304-317
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• 2009

Most crystalline rocks have much higher compressive strength than tensile strength and show brittle failure. In-situ rock mass, strong enough in general sense, often fails in brittle manner when subjected to high stress exceeding strength in due of geometrically induced stress concentration or of high initial stress. Therefore, it is necessary to verify the brittle failure characteristics of rock and rock mass for proper stability assessment of underground structures excavated in great depths. In this study, damage controlled tests were conducted on biotite-granite and granitic gneiss, which are the two major crystalline rock types in Korea, to obtain the strain dependency characteristics of the cohesion and friction angle. A Cohesion-Weakening Friction-Strengthening (CWFS hereafter) model for each rock type was constructed and a series of compression tests were carried out numerically while varying confining pressures. The same tests were also conducted assuming the rock is Mohr-Coulomb material and results were compared.

• Journal of the Korean institute of surface engineering
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• 제50권4호
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• pp.282-288
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• 2017

$Eu^{3+}$- or $Sm^{3+}$-doped $La_2MoO_6$ phosphors were synthesized with different concentrations of activator ions via a solid-state reaction. The X-ray diffraction patterns exhibited that crystalline structures of all the phosphors were tetragonal systems with the dominant peak occurring at (103) plane, irrespective of the concentration and the type of activator ions. The crystallites showed the pebble-like crystalline shapes and the average crystallite size increased with a tendency to agglomerate as the concentration of $Eu^{3+}$ ions increased. The excitation spectra of $Eu^{3+}$-doped $La_2MoO_6$ phosphors contained an intense charge transfer band centered at 331 nm in the range of 250-370 nm and three weak peaks at 381, 394, and 415 nm, respectively, due to the $^7F_0{\rightarrow}^5L_7$, $^7F_0{\rightarrow}^5L_6$, and $^7F_0{\rightarrow}^5D_3$ transitions of $Eu^{3+}$ ions. The emission spectra under excitation at 331 nm exhibited a strong red band centered at 620 nm and two weak bands at 593 and 704 nm. As the concentration of $Eu^{3+}$ increased from 1 to 20 mol%, the intensities of all the emission bands gradually increased. For the $Sm^{3+}$-doped $La_2MoO_6$ phosphors, the emission spectra consisted of an intense emission band at 607 nm arising from the $^4G_{5/2}{\rightarrow}^6H_{7/2}$ transition and three relatively small bands at 565, 648, and 707 nm originating from the $^4G_{5/2}{\rightarrow}^6H_{5/2}$, $^4G_{5/2}{\rightarrow}^6H_{9/2}$, and $^4G_{5/2}{\rightarrow}^6H_{11/2}$ transitions of $Sm^{3+}$, respectively. The intensities of all the emission bands approached maxima when concentration of $Sm^{3+}$ ions was 5 mol%. These results indicate that the optimum concentrations for highly-luminescent red and orange emission are 20 mol% of $Eu^{3+}$ and 5 mol% of $Sm^{3+}$ ions, respectively.

• Composites Research
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• 제24권6호
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• pp.56-63
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• 2011

The objective of this study is to investigate appropriate bonding methods of solar cells in order to apply solar cells, which have been receiving particular attention as a renewable energy due to fossil energy depletion and environment issues, to composite structures. Back-contact solar cells with approximately 24.2% energy conversion efficiency were used in this study. Since silicon-based solar cells are mechanically fragile, the secondary-bonding methods using adhesive were examined in this study. The experiment was conducted with three kinds of bonding materials such as EVA film, Resin film and elastic adhesive. The performance of solar cells for three types of adhesives under mechanical loading on test specimens is conducted. In addition, the measuring equipment was designed to evaluate the performance of the solar cells under mechanical loading in real time and the fracture characteristics depending on bonding materials were evaluated. The reason decreasing solar cells efficiency were analyzed and considered by Fractography. The results show that the solar cell performance is largely affected by bonding techniques. Moreover, the bonding method using elastic adhesive shows best solar cell efficiency.