Seung Hwangbo

Direct Correlation between Space Charge and Conduction Characteristics of Low-density Polyethylene at Various Temperature

The space charge distribution and current in low-density polyethylene (LDPE) have been measured by employing the pulsed-electro-acoustic (PEA) method. The measurement has been performed simultaneously under the temperature of 20 to 70°C for electric fields up to 50 MV/m in order to clarify the direct correlation between dynamic and pre-formed space charges and the conduction characteristics of LDPE. It was found that the conduction at high electric fields should be explained by different conduction models as temperature varied. At higher temperatures than 55°C, the conduction could be explained by diffusion-limited electrode injection current, but at 20°C by a kind of space-charge-limited current (SCLC). The power-law index of 4 was in reasonable agreement with the diffusion-limited electrode injection current and was influenced by pre-formed space charge. In addition, the numerical calculation about the experimental current and space charge distribution has been performed in order to correlate them. It was found that the hopping conduction of injected carriers had a deep relation with the formation of homo-space charge and the change of cathode field due to dynamic space charge played an important role in determining the time characteristics of the experimental current at high electric fields.

In-line Variable Optical Attenuator Based on the Bending of the Tapered Single Mode Fiber

We propose a simple in-line variable optical attenuator (VOA) based on the bending effect of tapered single mode fibers. The influence of the taper structure and the reflective index of the external medium surrounding the taper region on the bending loss of the tapered fiber have been investigated experimentally. An attenuation range exceeding 35 dB and a very low excess loss of < 0.2 dB at 1550 nm were achieved. The measured polarization dependent loss of the present VOA at the attenuation level of 10 dB, 20 dB, and 30 dB were 0.1 dB, 0.2 dB, and 0.5 dB, respectively.

Wide-angle catadioptric lens with a rectilinear projection scheme

A catadioptric wide-angle lens having a rectilinear projection scheme has been developed with a view to possible applications in the security-surveillance area. The lens has been designed for a miniature camera with a video graphics array-grade 1/3 in. color CCD sensor. The field of view of the lens is over 151 degrees , and still distortion is under 1%. Furthermore, the modulation transfer function is better than 0.3 at 70 line pairs/mm over the whole active area of the image sensor.

The space charge effect on the discharge current in cross-linked polyethylene under high AC voltages

The space charge distributions in solid dielectrics have been usually investigated by means of the pulsed electroacoustic (PEA) method. However, most previous studies have been limited to the phenomenological analysis under DC voltages. In our study, the space charge distribution in cross-linked polyethylene (XLPE) has been measured using AC voltages by means of the modified PEA method. Simultaneously, the streamer discharges in an air gap have been measured in order to investigate the relationship between space charge and discharge current, and the relationship has been adapted to the case of dielectric barrier discharge. At high AC voltages, discharge current increases to the critical point, but no further increase is exhibited over the critical voltage and the discharge pattern is resolved by the space charge. This result indicates that the frequency effect and space charge characteristics of dielectric materials are preferred to the voltage effect in the adaptation to dielectric barrier discharge. The results well explain the space charge effect on partial discharge and the dielectric barrier discharge phenomenon.

Low-temperature synthesis and luminescence properties of alkaline-earth stannate phosphors prepared by chemical solution process

In this work, we present a Ca2SnO4:Eu3+ phosphor prepared by the sol-gel method at a low temperature using a soluble inorganic salt. The mixed solution was pyrolyzed at 500°C for 180 min in air, and final annealing was performed at 800°C for 180 min in air. The crystal structure and surface morphology were confirmed through x-ray diffraction analysis and field emission — scanning electron microscopy, and the photoluminescence spectra were measured with a fluorescence spectrophotometer. The sample annealed at 800°C exhibited an orthorhombic structure and showed a red emission at 613 nm.

Investigation on the optical properties of side-polished fiber contacted with planar waveguide including a thin metal film

We investigate the characteristics of an asymmetric directional coupler made of side-polished fiber and multimode planar waveguide incorporating a thin metal film. The influence of the metal film thickness and the refractive index of the superstrate on the spectral and polarization properties of the coupler is measured and explained based on the weakly coupled model. It is shown that the coupler behaves like a band-rejection filter or a polarizer, depending on the device parameters. The application possibilities of the sensors to measure the thickness of thin metal film and the refractive index of the liquid are also observed.

Combination of Light Emitting Diode at 375 nm and Photo-reactive TiO2 Layer Prepared by Electrostatic Spraying for Sterilization

The objective of this work was to increase the efficiency of ultraviolet-light emitting diodes at 375 nm for sterilization. Since TiO2 had antibacterial properties, which were attributed to the appearance of hydroxyl radicals and superoxide radical anions on the surface species under ultra violet radiation at about 387 nm, photo-reactive layers such as Ag-doped TiO2 were coated on aluminum substrates by electrostatic spraying. Crystallinity and surface morphology of the coating layer were examined by X-ray diffraction θ-2θ scan and field emission-scanning electron microscope, respectively. In an antibacterial test, we observed above 99% reduction of Escherichia coli populations on 3 or 5 mol% Ag-doped TiO2 layers after irradiation for 2 hrs at 375 nm, while very low inactivation on bare aluminum substrates occurred after irradiation as the same condition.

The space charge effect on PD and dielectric barrier discharge in XLPE under high AC voltages

We have measured the space charge distribution in XLPE under AC voltages by the modified PEA method in order to suggest an improved technique of space charge measurement in air-gaps and to show that the space charge formation dominantly affects the PD pattern in the dielectric-barrier structure. The streamer discharges on the XLPE have been measured simultaneously in order to investigate the relation between the space charge and discharge current. Under high AC voltages, the discharge increases to the critical point, but no more increase is shown over the critical point. The voltage and discharge pattern is resolved by space charge. This result indicates that the frequency effect and space charge characteristics of the dielectric material are dominant in the dielectric-barrier-discharge.

Effects of space charge by DC strength test on AC insulating characteristics of XLPE

The purpose of this paper is to evaluate the effects of pre-formed space charge by /spl plusmn/DC application on the AC insulating characteristics in XLPE. In this paper, we have presented a new measurements method which enables the direct correlation between space charge distribution and PD patterns in electrode configurations including air gaps.

Analysis of Pulse Propagation Characteristics for Detection of Water Trees in Power Cables

In this paper, characteristics of propagated pulse signals are analyzed for detection of water trees in power cables using simulationbased TDR (Time Domain Reflectometry). COMSOL Multiphysics software is used for time-domain simulations of propagated pulse signals with high frequency components. The water trees with various penetration depths and lengths are modeled by axis-symmetry or 3D geometry, depending on the shapes of the water trees considered. The reflected waveforms and RMS values of the applied diagnostic pulse signals are analyzed according to the shapes and lengths of the water tree models and outer conductor conditions. Also, the propagation velocity of the pulse signal is calculated considering the material property of XLPE insulator in the cable, and the location of the water tree is obtained by checking the starting point of the reflected pulse. Furthermore, positional errors with respect to the actual positions of the water trees are calculated.