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Park J.-J.,Joongbu University | Lee J.-Y.,Woosuk University
IEEE Transactions on Dielectrics and Electrical Insulation | Year: 2010

A new electric application method was developed to prepare epoxy/organoclay nanocomposite for the electrical insulation in the ac electric fields and it could be also used in the field of various viscous polymer/organoclay systems. The applied ac electric field condition was as follows; (1) inter-electrode distance: 40 mm, (2) application voltage: 3-11 kV, (3) frequency: 60-1,000 Hz, and (4) application time: 0-60 min. To characterize the epoxy/clay nanocomposite, WAXS and TEM analyses were performed. In order to explain how the organic modifier affects the exfoliation phenomena, a mechanism of the oscillating collision of the quaternary ammonium head was proposed and the effects of the ac voltage and frequency and the organoclay content were studied. © 2010 IEEE.


Lee J.H.,Joongbu University | Kim C.J.,Joongbu University
Hydrological Processes | Year: 2013

In this study, the patterns of past and future drought occurrences in the Seoul region were analysed using observed historical data from the Seoul weather station located in the Korean Peninsula and four different types of general circulation models (GCMs), namely, GFDL:CM2_1, CONS:ECHO-G, MRI:CGCM2_3_2 and UKMO:HADGEM1. To analyse statistical properties such as drought frequency duration and return period, the Standardized Precipitation Index was used to derive the severity-duration-frequency (SDF) curve from the drought frequency analysis. In addition, a drought spell analysis was conducted to estimate the frequency and change of drought duration for each drought classification. The results of the analysis suggested a decrease in the frequency of mild droughts and an increase in the frequency of severe and extreme droughts in the future. Furthermore, the average duration of droughts is expected to increase. A comparison of the SDF relationship derived from the observed data with that derived via the GCMs indicated that the drought severity for each return period was reduced as drought duration increased and that the drought severity derived from the GCMs was severer than the severity obtained using the observed data for the same duration and return period. Furthermore, among the four types of GCMs used in this study, the MRI model predicted the most severe future drought for the Seoul region, and the SDF curve derived using the MRI model also resulted in the highest degree of drought severity compared with the other GCMs. © 2012 John Wiley & Sons, Ltd.


Park J.-J.,Joongbu University
Transactions on Electrical and Electronic Materials | Year: 2013

In order to develop electrical insulation material, organically modified layered silicate was incorporated into an epoxy matrix to prepare nanocomposite. Transmission electron microscopy (TEM) observation showed that organophillic clay was in an exfoliated state, while hydrophilic clay was not dispersed into nanolayers within the epoxy matrix. Epoxy/organophilic clay (2.8 wt%) nanocomposite was mixed and cured at 150°C for 4.5 hr. I-V characteristics, volume resistance and dielectric properties for the cured nanocomposite were estimated. Current density increased with increasing temperature, and volume resistance decreased with increasing temperature, in neat epoxy and epoxy/ organophilic clay (2.8 wt%) nanocomposite. As frequency increased, the dielectric loss value decreased in the two systems. Copyright © 2013 KIEEME. All rights reserved.


Park J.-J.,Joongbu University
Transactions on Electrical and Electronic Materials | Year: 2013

In order to develop high voltage (HV) insulation materials, epoxy/spherical alumina composites with two different particle sizes (in μm) were prepared and a dynamic mechanical analysis (DMA) and electrical insulation breakdown strength test were carried out in sphere-sphere electrodes and the data were estimated using Weibull statistical analysis. Alumina content varied from 50 to 70 wt%. The electrical insulation breakdown strength for epoxy/alumina (50 wt%) was 44.0 kV/1 mm and this value decreased with increasing alumina content. The effects of insulation thickness and alumina particle size on the insulation breakdown strength were also studied. The insulation thickness varied from 1 mm to 3 mm, and the particle sizes were 7.3 or 40.3 μm. © 2013 KIEEME. All rights reserved.


Park J.-J.,Joongbu University
Transactions on Electrical and Electronic Materials | Year: 2013

The effect of the mixing ratio of spherical silica on the electrical insulation breakdown strength in an epoxy/silica composite was studied. Spherical silicas with two average particle sizes of 5 μm and 20 μm were mixed in different mixing ratios, and their total filling content was fixed at 60 wt%. In order to observe the dispersion of the silicas and the interfacial morphology between silica and epoxy matrix, scanning electron microscopy (SEM) was used. The electrical insulation breakdown strength was estimated in sphere-sphere electrodes with different insulation thicknesses of 1, 2, and 3 mm. Electrical insulation breakdown strength decreased with increasing mixing ratio of 5/20 μm and the thickness dependence of the breakdown strength was also observed. © 2013 KIEEME. All rights reserved.


Park J.-J.,Joongbu University
Transactions on Electrical and Electronic Materials | Year: 2013

Effects of ambient temperature on the ac electrical treeing and breakdown behaviors in an epoxy/layered silicate (1 wt%) were carried out in needle-plate electrode geometry. A layered silicate was exfoliated in an epoxy base resin, using our ac electric field apparatus. To measure the treeing initiation and propagation, and the breakdown rate, constant alternating current (ac) of 10 kV (60 Hz) was applied to the specimen in a needle-plate electrode arrangement, at 30°C, 90°C or 130°C of insulating oil bath. At 30°C, the treeing initiation time and the breakdown time in the epoxy/layered silicate (1 wt%) system were 1.4 times higher than those of the neat epoxy resin. At 90°C (lower than Tg), electrical treeing was initiated in 55 min, and propagated until 1,390 min at the speed of 0.35×10-3 mm/min, which was 4.4 times higher than that at 30°C; however, there was almost no further treeing propagation after 1,390 min. At 130°C (higher than Tg), electrical treeing was initiated in 44 min, and propagated until 2,000 min at the speed of 0.96×10-3 mm/min. Typical branch type electrical treeing was obtained from the neat epoxy and epoxy/layered silicate at 30°C, while bush type treeing was observed out from the needle tip at 90°C and 130°C. © 2013 KIEEME. All rights reserved.


Park J.-J.,Joongbu University
Transactions on Electrical and Electronic Materials | Year: 2013

The effects of particle size on the mechanical and electrical properties of epoxy/spherical silica composites were studied. The silica particle sizes were varied from 5 to 30 μm and the filler content was fixed to 60 wt%. Tensile and flexural tests were carried out and the interfacial morphology was observed by scanning electron microscopy (SEM). The electrical insulation breakdown strength was estimated using sphere-sphere electrodes with different insulation thicknesses of 1, 2 and 3 mm. The tensile strength and flexural strength increased with decreasing particle size, while electrical insulation breakdown strength increased with increasing particle size. Copyright © 2013 KIEEME. All rights reserved.


Kim N.-Y.,Joongbu University
Journal of the Korean Society of Food Science and Nutrition | Year: 2011

The study investigated the quality of pound cake added with various concentrations of perilla leaves (Perilla frutescens var. japonica HARA) powder. Pound cake were prepared by addition of 0, 5, 10 and 15% powder to the flour of basic formulation. The pH of the batter and cake decreased with increasing perilla leaves powder concentration. The baking loss rate of the pound cake tended to increase by 7.13~7.42%. The height of pound cake added with perilla leaves powder decreased with increasing powder concentration. The lightness, redness, and yellowness values were significantly decreased with increases in perilla leaves powder except for redness of pound cake crumb. The pound cakes containing 5% perilla leaves powder had acceptable sensory evaluation, such as flavor, taste, moistureness, and overall preference. The hardness and gumminess tended to increase, while springiness and cohesiveness decreased with increases in perilla leaves powder. The retrogradation degree of pound cake prepared with perilla leaves powder for 7 days at 25oC was higher than the control but there were no significant differences. For the antioxidative activity measured by DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging activity, pound cake added with 10% and 15% added perilla leaves powder showed high antioxidant activities. The results exhibited that adding the perilla leaves powder into the pound cake increased antioxidant activity with the highest quality improvement obtained by incorporating 5% (w/w) of perilla leaves powder into the pound cake formula.


Park J.-J.,Joongbu University
Transactions on Electrical and Electronic Materials | Year: 2013

The effects of electric field frequency on the AC electrical treeing phenomena in an epoxy/layered silicate (1.5 wt%) were investigated in a needle-plate electrode arrangement. A layered silicate was exfoliated in an epoxy-base resin with AC electric field apparatus. To measure the treeing initiation and propagationand the breakdown rate, a constant alternating current (AC) of 10 kV with three different electric field frequencies (60, 500, and 1,000 Hz) was applied to the specimen in the needle-plate electrode specimen in an insulating oil bath at130°. At 60 Hz, the treeing initiation time was 12 min, the propagation rate was 0.24 × 10-3 mm/min, and the morphology was a dense branch type. As the electric field frequency increased, the treeing initiation time decreased and the propagation rate increased. At 1,000 Hz, the treeing initiation time was 5 min, the propagation rate was 0.30 × 10-3 mm/min, and the morphology was a dense bush type. © 2013 KIEEME. All rights reserved.


Lee S.-Y.,Joongbu University
Composites Science and Technology | Year: 2010

The finite element dynamic stability analysis of laminated composite skew structures subjected to in-plane pulsating forces is carried out based on the higher-order shear deformation theory (HSDT). The two boundaries of the instability regions are determined using the method proposed by Bolotin. The numerical results obtained for square and skew plates with or without central cutout are in good agreement with those reported by other investigators. The new results for laminated skew plate structures containing cutout in this study mainly show the effect of the interactions between the skew angle and other various parameters, for example, cutout size, the fiber angle of layer and thickness-to-length ratio. The effect of the magnitude of the periodic in-plane load on the dynamic instability index is also investigated. © 2010 Elsevier Ltd.

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