• Journal of the Korean Ceramic Society
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• v.28 no.8
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• pp.585-592
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• 1991

ZnO films were deposited onto Corning glass 7059 substrate in the temperature range from $200^{\circ}C$ to $450^{\circ}C$ by chemical vapor deposition technique using the hydrolysis of Diet ylzinc (DEZ). As the deposition temperature increased from $200^{\circ}C$ to $350^{\circ}C$, the deposition rate increased with the apparent activation energy of ∼23kJ/mole. Further increase of the deposition temperature above $400^{\circ}C$, however, resulted in a reduction of the rate. It was found that ZnO film grew with a strong C-axis preferred orientation at the temperature of $400^{\circ}C$. As the deposition temperature increased, the film resistivity decreased down to ∼0.2 $\Omega$cm at $450^{\circ}C$. The electrical resistivity was governed more likely by electron concentration rather than by electron mobility. Average optical transmission of the films in the optical wavelength range of 400 nm to 900 nm was over 90% and the optical energy band gap of 3.28∼3.32 eV was obtained from the direct transition.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2004.07a
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• pp.13-16
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• 2004

The reliability of 1.550m-wavelength InGaAs mesa waveguide photodiodes(WGPDs), which developed for 40-Gbps optical receiver applications, fabricated by metal organic chemical vapor deposition is investigated. Reliability is examined by both high-temperature storage tests and the accelerated life tests by monitoring dark current and breakdown voltage. The median device lifetime and the activation energy of the degradation mechanism are computed for WGPD test structures. From the accelerated life test results, the activation energy of the degradation mechanism and median lifetime of these devices in room temperature are extracted from the log-normal failure model by using average lifetime and the standard deviation of that lifetime in each test temperature. It is found that the WGPD structure yields devices with the median lifetime of much longer than $10^6$ h at practical use conditions. Consequently, this WGPD structure has sufficient characteristics for practical 40-Gbps optical receiver modules.

• Korean Journal of Metals and Materials
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• v.47 no.5
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• pp.316-321
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• 2009

Co nanoparticles on silica substrates were fabricated by inducing a thin-film dewetting through two different processes-furnace annealing and pulsed-laser annealing. The effects of annealing temperature, film thickness and laser energy density on dewetting morphology and mechanism were investigated. Co thinfilms with thicknesses between 3 to 15 nm were deposited using ion-beam sputtering, and then, in order to induce dewetting, thermally annealed in furnace at temperatures between 600 and $900^{\circ}C$. Some as-deposited films were irradiated using a Nd-YAG pulsed-laser of 266 nm wavelength to induce dewetting in liquid-state. Films annealed in furnace agglomerated to form nanoparticles above $700^{\circ}C$, and those average particle size and spacing were increased with an increase of film thickness. On the laser annealing process, above the energy density of $100mJ/cm^2$, metal films were completely dewetted and the agglomerated particles exhibited greater size uniformity than those on the furnace annealing process. A detailed dewetting mechanism underlaying both processes were discussed.

• Current Optics and Photonics
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• v.6 no.4
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• pp.407-412
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• 2022

We report on a diode-laser-pumped mode-locked Yb:KGW laser system, which delivers ultrashort pulses down to 89 fs at a repetition rate of 63 MHz, with an average power of up to 5.6 W. A fiber-coupled diode laser at 981 nm, operated with a compact driver, is used to optically pump the gain crystal via an off-axis parabolic mirror. A semiconductor saturable-absorber mirror is used to initiate the pulsed operation. Laser characteristics such as the pulse duration, spectrum bandwidth, and output power are investigated by varying the intracavity dispersions via changing the number of bounces between negative-dispersive mirrors within the cavity. Short pulses with a duration of 89 fs, a center wavelength of 1,027 nm, and 3.6 W of output power are produced at a group-velocity dispersion (GVD) of -3,300 fs². As the negative GVD increases, the pulse duration lengthens but the output power at the single-pulse condition can be enhanced, reaching 5.6 W at a GVD of -6,600 fs². Because of pulse broadening at high negative GVDs, the highest peak intensity is achievable at a moderate GVD with our system.

• Journal of Environmental Science International
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• v.32 no.1
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• pp.25-35
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• 2023

The purpose of this study was to determine the optimal sound absorption conditions by comparing the sound absorption characteristics of fresh and air-dried leaves of Quercus glauca, the main species of evergreen broadleaf trees (EBLT) in southern Korea. The sound absorption coefficients (SACs) obtained under 18 conditions were comparatively analyzed. The SAC of air-dried leaves improved significantly with increasing leaf layer thickness. The highest average SAC in the fresh leaf group was 0.617, which was observed under the condition of a leaf specimen size of 0.5 × 0.5 cm² and a leaf layer thickness of 1.75 cm. In a group of air-dried leaves, this was 0.615 under the condition of a leaf specimen size of 0.5 × 0.5 cm² and a leaf layer thickness of 2.50 cm. The maximum value of SAC for each wavelength was observed under the condition of a leaf layer thickness of 2.50 cm consisting of 0.5 × 0.5 cm² leaf specimens, ranging from 1,400 Hz to 1,500 Hz.

• Journal of Ceramic Processing Research
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• v.13 no.spc2
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• pp.407-410
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• 2012

The properties of Al-doped ZnO (AZO) films were investigated as a function of H2/(Ar + H2) gas ratio using an AZO (2 wt% Al2O3) ceramic target in a radio frequency (RF) magnetron sputtering system. The deposition process was done at 200 ℃ and in 2 × 10-2Torr working pressure and with various ratios of H2/(Ar + H2) gas. During the AZO film deposition process, partial H2 gas affected the AZO film characteristics. The electron resistivity (~ 9.21 × 10-4 Ωcm) was lowest and mobility (~17.8 ㎠/Vs) was highest in AZO films when the H2/(Ar + H2) gas ratio was 2.5%. When the H2/(Ar + H2) gas ratio was increased above 2.5%, the electron resistivity increased and mobility decreased with increasing H2/(Ar + H2) gas ratio in AZO films. The carrier concentration increased with increasing H2/(Ar + H2) gas ratio from 0% to 7.5%. This phenomenon was explained by reaction of hydrogen and oxygen and additional formation of oxygen vacancy. The average optical transmission in the visible light wavelength region over 90% and an orientation of the deposition was [002] orientation for AZO films grown with all H2/(Ar + H2) gas ratios.

• Journal of Powder Materials
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• v.30 no.5
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• pp.424-430
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• 2023

YAG phosphor powders were fabricated by the atmospheric plasma spraying method with the spray-dried spherical YAG precursor. The YAG precursor slurry for the spray drying process was prepared by the PVA solution chemical processing utilizing a domestic easy-sintered aluminum oxide (Al₂O₃) powder as a seed. The homogenous and viscous slurry resulted in dense granules, not hollow or porous particles. The synthesized phosphor powders demonstrated a stable YAG phase, and excellent fluorescence properties of approximately 115% compared with commercial YAG:Ce³⁺ powder. The microstructure of the phosphor powder had a perfect spherical shape and an average particle size of approx imately 30 ㎛. As a result of the PKG test of the YAG phosphor powder, the synthesized phosphor powders exhibited an outstanding luminous intensity, and a peak wavelength was observed at 531 nm.

• Food Science and Preservation
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• v.16 no.6
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• pp.789-796
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• 2009

Benzoic acid (BA) and sorbic acid (SA) are the preservatives most commonly used in food. Although BA and SA are generally safe, some previous studies have shown that consumption of excessive amounts of these food additives can be a health hazard. The aim of this study was to determine the amounts of BA and SA in processed foods in Korea. Different brands of fruit juice, yogurt, cheese, dried fruits, jam, and margarine were purchased at a local market in Daejeon, Korea. Samples were analyzed by high-performance liquid chromatography (HPLC) using a UV detector. Chromatographic separation was achieved with a C18 column. Methanol acetate buffer (pH 4.4) at a 35:65 v/v ratio was used as the initial mobile phase to elute BA and SA. The detector wavelength was set at 254 nm. The average test results observed for BA concentrations in fruit juice, yogurt, cheese, dried fruits, jam, and margarine were $40.26{\pm}0.02$, $2.07{\pm}0.06$, $0.02{\pm}0.09$, $0.36{\pm}08$, $265.30{\pm}0.02$, and $27.34{\pm}0.08\;mg/kg$, respectively. Average concentrations of SA in these samples were $0.92{\pm}0.06$, $1.06{\pm}0.07$, $7.30{\pm}0.01$, $14.14{\pm}0.08$, $25.65{\pm}0.06$, and $4.81{\pm}0.07\;mg/kg$, respectively. Thus, the average levels of BA and SA in the studied food items were lower than the KFDA-permitted limits. Moreover, the estimated daily intake of both BA and SA by a typical consumer were below the maximum recommended daily values.

• Korean Journal of Optics and Photonics
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• v.13 no.4
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• pp.347-355
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• 2002

We have constructed a high efficiency and broad tunable cw Ti:sapphire laser with a four-mirror symmetric Z-type laser cavity to increase the laser usability. From theoretical analyses and experimental data for a symmetric Z-type laser cavity containing a Kerr medium, the cavity mode size and the Kerr-lens mode-locking (KLM) strength for KLM lasers can be confirmed as function of the position in the cavity, the intracavity laser power, and the stability parameter. As a result, the slope efficiency and the maximum average output power of cw Ti:sapphire laser at 5 W pumping power of Ar-ion laser were 31.3% and 1420 ㎽ respectively. The tunablility was ranged from 730 ㎚ to 908 ㎚ with average output power above 700 ㎽. We obtained the KLM operation easily by self-aperturing effect in the Kerr medium and the slope efficiency and the maximum average output power of KLM Ti:sapphire laser was 16% and 550 ㎽ respectively. The spectral bandwidth was 33 ㎚ at the center wavelength of 807 ㎚ and the pulse width was 27 fs with a repetition rate of 82 ㎒.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.14 no.6
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• pp.528-535
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• 2004

Ultrasonic Vibrator is designed to achieve the maximum vibration amplitude at 30 kHz by in-cluding a horn (diameter, 40 mm), mechanical vibration amplifier at the top of the ultrasonic vibrator in the system and making the complete system resonate. In addition, it is experimentally visualized by particle imaging velocimetry (PIV) that the acoustic streaming velocity in the gap is at maximum when the gap between the ultrasonic vibrator and stationary plate agrees with the multiples of half-wavelength of the ultrasonic wave. This fact results from the resonance of the sound wave and the theoretical analysis of that is also accomplished and verified by experiment. It is observed that the magnitude of the acoustic streaming dependent upon the gap between the ultrasonic vibrator and stationary plate possibly changes due to the measurement of the average velocity fields of the acoustic streaming induced by the ultrasonic vibration at resonance and non-resonance. There exists extremely small average velocity at non-resonant gaps while the relatively large average velocity exists at resonant gaps compared with non-resonant gaps. It also reveals that there should be larger axial turbulent intensity at the hub region of the vibrator and at the edge of it in the resonant gap where the air streaming velocity is maximized and the flow phenomena is conspicuous than that at the other region. Because the variation of the acoustic streaming velocity at resonant gap is more distinctive than that at non-resonant gap, shear stress increases more in the resonant gap and is also maximized at the center region of the vibrator except the local position of center (r〓0). At the non-resonant gap there should be low values of vorticity distribution, but in contrast to the non-resonant gap, high and negative values of it exist at the center region of the vibrator with respect to the radial direction and in the vicinity of the middle region with respect to the axial direction. Acoustic streaming is noise-free due to the ultrasonic vibration and maintenance-free because of the absence of moving parts. Moreover, the proposed method by acoustic streaming can be utilized to the nano and micro-electro mechanical systems as a driving mechanism in addition to the augmentation of the streaming velocity.