• 한국신재생에너지학회:학술대회논문집
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• 2010.06a
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• pp.90.1-90.1
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• 2010

Due to the rapidly diminishing energy sources and higher energy production cost, the interest in dye-sensitized solar cells (DSSCs) has been increasing dramatically in recent years. A typical DSSC is constructed of wide band gap semiconductor electrode such as $TiO_2$ or ZnO that are anchored by light-harvesting sensitizer dyes and surrounded by a liquid electrolyte with a iodide ion/triiodide ion redox couple. DSSCs based on one-dimensional nano-structures, such as ZnO nanorods, have been recently attracting increasing attention due to their excellent electrical conductivity, high optical transmittance, diverse and abundant configurations, direct band gap, absence of toxicity, large exiton binding energy, etc. However, solar-to-electrical conversion performances of DSSCs composed of ZnO n-type photo electrode compared with that of $TiO_2$ are not satisfactory. An important reason for the low photovoltaic performance is the dissolution of $Zn^{2+}$ by the adsorption of acidic dye followed by the formation of agglomerates with dye molecules which could block the I-diffusion pathway into the dye molecule on the ZnO surface. In this paper, we prepared the DSSC with the ZnO electrode using the chemical bath deposition (CBD) method under low temperature condition (< $100^{\circ}C$). It was demonstrated that the ZnO seed layers played an important role on the formation of the ZnO nanostructures using CBD. To achieve truly low-temperature growth of the ZnO nanostructures on the substrates, a two-step method was developed and optimized in the present work. Firstly, ZnO seed layer was prepared on the FTO substrate through the spin-coating method. Secondly, the deposited ZnO seed substrate was immersed into an aqueous solution of 0.25M zinc nitrate hexahydrate and 0.25M hexamethylenetetramine at $90^{\circ}C$ for hydrothermal reaction several times.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.21 no.12
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• pp.1077-1082
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• 2008

To fabricate a multi-layered piezoelectrics/electrodes structure, the piezoelectrics should be sintered at the temperature lower than $950^{\circ}C$ to use the silver electrode, which is cheaper than the electrodes containing noble metals such as Pd and Pt. Therefore, in this study, we modified the composition of $Pb(Zr,Ti)O_3$-based material as $(Pb_{0.98}Cd_{0.02})(Ni_{1/3}Nb_{2/3})_{0.25}Zr_{0.35}Ti_{0.4}O_3$ to lower the sintering temperature and to improve the piezoelectric properties. Small amount of $MnCO_3$, $SiO_2$, and $Pb_3O_4$ were also added to lower the sintering temperature of the ceramic. The prepared raw powders were mixed by using a ball mill for 24 hours. And then the mixed powders were calcinated for 2 hours at $800^{\circ}C$. The calcinated powders were again crushed with the ball mill for 72 hours. The final powders were pressed for making the shape of ${\emptyset}15\;mm$ disk. The disk-type samples were sintered at temperature range of $850{\sim}950^{\circ}C$. The crystal phases of the sintered specimens were perovskite structure without secondary phases. All of the measured electrical properties such as electromechanical coupling coefficients ($k_p$), mechanical quality factors ($Q_m$), and piezoelectric charge constants ($d_{33}$) were decreased with decreasing the sintering temperatures. The electrical properties measured at the sample sintered at $950^{\circ}C$ were 54% of $k_p$, 503 of $Q_m$, and 390 pC/N of $d_{33}$, respectively. These properties were considered to be fairly good for the application of multi-layered piezoelectric generators or actuators.

• JSTS:Journal of Semiconductor Technology and Science
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• v.8 no.1
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• pp.66-79
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• 2008

Several oxides have recently been reported to have resistance-switching characteristics for nonvolatile memory (NVM) applications. Both binary and ternary oxides demonstrated great potential as resistive-switching memory elements. However, the switching mechanisms have not yet been clearly understood, and the uniformity and reproducibility of devices have not been sufficient for gigabit-NVM applications. The primary requirements for oxides in memory applications are scalability, fast switching speed, good memory retention, a reasonable resistive window, and constant working voltage. In this paper, we discuss several materials that are resistive-switching elements and also focus on their switching mechanisms. We evaluated non-stoichiometric polycrystalline oxides ($Nb_2O_5$, and $ZrO_x$) and subsequently the resistive switching of $Cu_xO$ and heavily Cu-doped $MoO_x$ film for their compatibility with modem transistor-process cycles. Single-crystalline Nb-doped $SrTiO_3$ (NbSTO) was also investigated, and we found a Pt/single-crystal NbSTO Schottky junction had excellent memory characteristics. Epitaxial NbSTO film was grown on an Si substrate using conducting TiN as a buffer layer to introduce single-crystal NbSTO into the CMOS process and preserve its excellent electrical characteristics.

• Bulletin of the Korean Chemical Society
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• v.35 no.10
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• pp.3052-3058
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• 2014

In this article, we have designed and synthesized a novel donor-${\pi}$-acceptor (D-${\pi}$-A) type porphyrin-based sensitizer (denoted UI-5), in which a carboxyl anchoring group and a 9,9-dimethyl fluorene were introduced at the meso-positions of porphyrin ring via phenylethynyl and ethynyl bridging units, respectively. Long alkoxy chains in ortho-positions of the phenyls were supposed to reduce the degree of dye aggregation, which tends to affect electron injection yield in a photovoltaic cell. The cyclic voltammetry was employed to determine the band gap of UI-5 to be 1.41 eV based on the HOMO and LUMO energy levels, which were estimated by the onset oxidation and reduction potentials. The incident monochromatic photon-to-current conversion efficiency of the UI-5 DSSC assembled with double-layer (20 nm-sized $TiO_2$/400 nm-sized $TiO_2$) film electrodes appeared lower upon overall ranges of the excitation wavelengths, but exhibited a higher value over the NIR ranges (${\lambda}$ = 650-700 nm) compared to the common reference sensitizer N719. The UI-5-sensitized cell yielded a relatively poor device performance with an overall conversion efficiency of 0.74% with a short circuit photocurrent density of $3.05mA/cm^2$, an open circuit voltage of 0.54 mV and a fill factor of 0.44 under the standard global air mass (AM 1.5) solar conditions. However, our report about the synthesis and the photovoltaic characteristics of a porphyrin-based sensitizer in a D-${\pi}$-A structure demonstrated a significant complex relationship between the sensitizer structure and the cell performance.

• Korean Journal of Materials Research
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• v.28 no.9
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• pp.489-494
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• 2018

Lead free $(1-x)(0.675BiFeO_3-0.325BaTiO_3)-xLiTaO_3$ (BFBTLT, x = 0, 0.01, 0.02, and 0.03, with 0.6 mol% $MnO_2$ and 0.4 mol% CuO) were prepared by a solid state reaction method, followed by air quenching and their crystalline phase, morphology, dielectric, ferroelectric and piezoelectric properties were explored. An X-ray diffraction study indicates that lithium (Li) and tantalum (Ta) were fully incorporated in the BFBT materials with the absence of any secondary phases. Dense ceramic samples (> 92 %) with a wide range of grain sizes from $3.70{\mu}m$ to $1.82{\mu}m$ were obtained in the selected compositions ($0{\leq}x{\leq}0.03$) of BFBTLT system. The maximum temperatures ($T_{max}$) were mostly higher than $420^{\circ}C$ in the studied composition range. The maximum values of maximum polarization ($P_{max}{\approx}31.01{\mu}C/cm^2$), remnant polarization ($P_{rem}{\approx}22.82{\mu}C/cm^2$) and static piezoelectric constant ($d_{33}{\approx}145pC/N$) were obtained at BFBT-0.01LT composition with 0.6 mol% $MnO_2$ and 0.4 mol% CuO. This study demonstrates that the high $T_{max}$ and $d_{33}$ for BFBTLT ceramics are favorable for industrial applications.

• Korean Journal of Materials Research
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• v.10 no.2
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• pp.160-165
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• 2000

We have investigated the electrical and diffusion barrier properties of MgO produced on the surface of Cu (Mg) alloy. Also the diffusion barrier property of the interfacial MgO between Cu alloy and $SiO_2$ has been examined. The results show that the $150\;{\AA}$-MgO layer on the surface remains stable up to $700^{\circ}C$, preventing the interdiffusion of C Cu and Si in Si/MgO/Cu(Mg) structure. It also has the breakdown voltage of 4.5V and leakage current density of $10^{-7}A/\textrm{cm}^2/$. In addition, the combined structure of $Si_3N4(100{\AA})/MgO(100{\AA})$ increases the breakdown voltage up to lOV and reduces the leakage current density to $8{\tiems}10^{-7}A/\textrm{cm}^2$. Furthermore, the interfacial MgO formed by the chemical reac­t tion of Mg and $SiO_2$ reduces the diffusion of copper into $SiO_2$ substrate. Consequently, Cu(Mg) alloy can be applied as a g gate electrode in TFT /LCDs, reducing the process steps.

• Journal of Welding and Joining
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• v.25 no.5
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• pp.45-50
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• 2007

The Ni-base superalloy GTD111 DS is used in the first stage blade of high power land-based gas turbines. Advanced repair technologies of the blade have been introduced to the gas turbine industry over recent years. The effect of the filler metal powder on Transient Liquid Phase bonding phenomenon and tensile mechanical properties was investigated on the GTD111 DS superalloy. At the filler metal powder N series, the base metal powders fully melted at the initial time and a large amount of the base metal near the bonded interlayer was dissolved by liquid inter metal. Liquid filler metal powder was eliminated by isothermal solidification which was controlled by the diffusion of B into the base metal. The solids in the bonded interlayer grew from the base metal near the bonded interlayer inward the insert metal during the isothermal solidification. The bond strength of N series filler metal powder was over 1000 MPa. and ${\gamma}'$ phase size of N series TLP bonded region was similar with base metal by influence of Ti, Al elements. At the insert metal powder M series, the Si element fluidity of the filler metal was good but microstructure irregularity on bonded region because of excessive Si element. Nuclear of solids formed not only from the base metal near the bonded interlayer but also from the remained filler metal powder in the bonded interlayer. When the isothermal solidification was finished, the content of the elements in the boned interlayer was approximately equal to that of the base metal. But boride and silicide formed in the base metal near the bonded interlayer. And these boride decreased with the increasing of holding time. The bond strength of M series filler metal powder was about 400 MPa.

• The korean journal of orthodontics
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• v.35 no.5 s.112
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• pp.381-387
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• 2005

The aim of this study was to compare the clinical performance of 4 types of orthodontic wires, indicated for initial tooth alignment: stainless steel, multistranded steel, superelastic and thermoactivated nickel-titanium. A prospective randomized clinical trial was conducted on a sample of 45 patients, at the Dental School of the State University of Rio do Janeiro, Brazil. Fixed appliances were fitted and study casts were obtained from each patient. Randomly, the wires were allocated as follows: 26 dental arches for superelastic NiTi wires, 22 for stainless steel, 22 for multistranded and 20 for thermoactivated archwires. After 8 weeks, the archwires were removed and impressions for study casts were taken again. Using a 3D digitization technique of defined anatomical points on the study cast crowns, a Dental Irregularity Index (DII) was created for each study cast. The difference between DII before and after the archwire insertion expressed the aligning effect of the wires. ANOVA tests were employed to evaluate the anatomical point approximation (positive DII) and separation (negative DII), for each area of the dental arches: upper and lower whole arch and anterior arch. Results showed no significant difference between the different archwires.

• Proceedings of the Optical Society of Korea Conference
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• 2008.02a
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• pp.429-430
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• 2008

We demonstrate coherent x-ray diffraction imaging using table-top x-ray laser at a wavelength of 13.9nm driven by 10-Hz ti:Sapphire laser system at the Advanced Photonics Research Institute in Korea. Since the flux of x-ray photons reaches as high as $10^9$ photons/pulse in a $20{\times}20{\mu}m^2$ field of view, we measured a ingle-pulse diffraction pattern of a micrometer-scale object with high dynamic range of diffraction intensities and successfully reconstructed to the image using phase retrieval algorithm with an oversampling ratio of 1:6. the imaging resolution is $^{\sim}150$ nm, while that is much improved by stacking the many diffraction patterns. This demonstration can be extended to the biological sample with the diffraction limited resolution.

• Journal of the Korean Ceramic Society
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• v.38 no.10
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• pp.942-947
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• 2001

Thickness dependence of orientation on piezoelectric and electrical properties was investigated by PZT (52/48) films by diol based sol-gel method. The thickness of each layer by spinning at one time was $0.2{\mu}m$ and crack-free films could be successfully deposited on 4 inches Pt/Ti/$SiO_2$/Si substrates by 0.5 mol solutions in the range from $0.2{\mu}m$ to $3.8{\mu}m$. Excellent P-E hysteresis curves were achieved, which were attributed to the well-densified PZT films and columnar grain without pores or any defects between interlayers. The (111) preferred orientation of films were shown in the range of thickness below $1{\mu}m$. As the thickness increased, the (111) preferred orientation disappeared from $1{\mu}m$ to $3{\mu}m$ region, and the orientation of films became random above $3{\mu}m$. Dielectric constants and longitudinal piezoelectric coefficient, $d_{33}$, measured by pneumatic method were saturated around the value of about 1400 and 300 pC/N respectively above the thickness of $1{\mu}m$.