• Journal of Korean Vacuum Science & Technology
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• v.3 no.1
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• pp.38-42
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• 1999

Using the LOCOS process, we have fabricated the lateral type polysilicon field emission triodes with poly-Si/oxide/Si structure and investigated their current-voltage characteristics for three biasing modes of operation. The fabricated devices exhibit excellent electrical performances such as a relatively low turn-on anode voltage of 14 V at VGC = 0V, a stable and high emission current of 92${\mu}$A/triode over 90 hours, a small gate leakage current of 0.23 ${\mu}$A/triode and an outstanding transconductance of 57${\mu}$S/5triodes at VGC = 5V and VAC = 26V. these superior electrical operation is believed to be due to a large field enhancement effect, which is related to the sharp cathode tips produced by the LOCOS process as well as the high aspect ratio (height /radius ) of the cathode tip end.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.7 s.238
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• pp.784-794
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• 2005

In the very low specific speed range ($n_s=0.24$ < 0.25, non-dimensional), the efficiency of centrifugal pump designed by a conventional method is very low in common. Therefore, positive-displacement pumps have long been used widely. Recently, since the centrifugal pumps are becoming higher in rotational speed and smaller in size, there expects to develop a new centrifugal pump with a high performance to replace the positive-displacement pumps. The purpose of this study is to investigate the internal flow characteristics of a very low specific speed centrifugal pump and to examine the effect of internal flow pattern on pump performance. The results show that the theoretical head definition of semi-open impeller should be revised by the consideration of high slip factor in the semi-open impeller, and the leakage flow through the tip clearance results in a large effect on the impeller internal flow. Strong reverse flow at the outlet of semi-open impeller reduces the absolute tangential velocity considerably, and the decreased absolute tangential velocity increasese the slip factor with the reduction of theoretical head.

• The Plant Pathology Journal
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• v.16 no.3
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• pp.137-141
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• 2000

A causal fungus of turfgrass blight was isolated from the infected leaves of zoysiagrass (Zoysia japonica Steud.) and identified as Nigrospora sphaerica (Sacc.) Mason by using a light misroscope. Its conidia are large (14-20 ${\mu}{\textrm}{m}$ diameter), shiny, black, aseptate, and smooth-walled spheres. The fungus caused typical blighting symptoms on the two turfgrass plants of bermudagrass (Cynodon dactylon (L.) Pers.) and bentgrass (Agrostis palustris Huds.). The fungus was found to produce a phytotoxic subtance to be associated with the pathogenic mechanism. A phytotoxin was isolated from the liquid cultures of N. sphaerica by repeated silica gel column chromatography and its structure was determined to be 5, 6-dihydro-5-hydroxy-6-propenyl-2H-pyr-2-one (T-3 compound). It was not a host-specific toxin showing phytotoxic effects to various plants inclusing turfgrasses in the leaf-wounding assay, the whole plant test, and the cellular leakage test. The compound caused leaf tip dieback symptoms in turfgrass plants similar to those caused by the pathogen. Thus, T-3 compound is thought to be involved in the development of Nigrospora blight.

• Journal of the Korean Society of Propulsion Engineers
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• v.17 no.3
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• pp.1-8
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• 2013

In this study, we performed three-dimensional CFD analysis to investigate the effect of the rotor blade sweep of a partial admission supersonic turbine on the stage performance and the flow field. The computations are conducted for three different sweep cases, No sweep(NSW), Backward sweep(BSW), and Forward sweep(FSW), using flow analysis program, FLUENT 6.3 Parallel. The results of the BSW model show reduced mass flow rates of tip leakage and increased total-to-static efficiency. The strength of leading edge bow shock was decreased a little with BSW model. And the BSW model also shows a good performance around the hub region compared to other models.

• Corrosion Science and Technology
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• v.8 no.4
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• pp.153-156
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• 2009

The water cooling system for VFD (Variable Frequency Drive) of a fossil fuel power plant was reported to be shut down due to a water leak at the metal connection of the heat-sink to the hoses. In order to identify the cause of the failure, the system was visually inspected, and corrosion products were analyzed with SEM equipped with EDX. The failure was observed repeatedly at the nipples of certain location, suggesting galvanic corrosion. In a U-shaped heat sink with two nipples, for inlet and outlet, only one nipple was corrosively damaged at the tip, while the other was not. Most of the corrosion products were observed at the sound nipple and in the filter, identified as $Cu(OH)_2$. Some other corrosion products, composed of mostly $Cu_2O$, were found at the corrosively attacked nipple. A fair amount of Cl was also detected on the surface of the damaged nipple. It was concluded that galvanic corrosion was occurred due to a current leakage over the whole system, and the damage was accelerated by the accumulated chlorine ions in the cooling water.

• The KSFM Journal of Fluid Machinery
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• v.9 no.1 s.34
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• pp.9-18
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• 2006

In the range of very low specific speed ($n_s<0.25$, non-dimensional), the performance of a centrifugal pump is much different from that of a centrifugal pump of normal ns and the efficiency of the pump drops rapidly with the decrease of $n_s$. In order to examine the reason of unstable performance characteristics of the very low $n_s$- centrifugal pump, the internal flow of the pump with a semi-open impeller is measured by a PTV(Particle Tracking Velocimetry) system. The purpose of this study is to make clear the internal flow characteristics and to obtain basic knowledge of the pump performance. The results show that the leakage flow through tip clearance give a strong effect on the flow pattern of impeller passage. A large vortex in the impeller passage and a strong reverse flow at impeller outlet are formed in the range of small flow rates, and the vortex and the reverse flow together reduce the absolute tangential velocity at the impeller outlet and cause the performance instability.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.5
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• pp.1044-1051
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• 1989

A numerical calculation is carried out for the analysis of 3-dimensional compressible flow field in axial-flow rotating blades by using finite element method. The calculation of flow in impellers plays a dominant role in the theoretical research and design of turbomachines. Three-dimensional flow fields can be obtained by the quasi-three-dimensional iterative calculation of the flows both on blade-to-blade stream surfaces and hub-to-shroud stream surfaces with the introduction of viscous loss model in order to consider a loss due to viscosity of fluid. In devising the loss model, four primary sources of losses were identified: (1) blade profile loss (2) end wall loss (3) secondary flow loss (4) tip-leakage loss. For the consideration of an axially parabolic distribution of loss, the results of present calcullation are well agreed with the results by experiment, thus the introduction of loss model is proved to be valid.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.11
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• pp.1376-1383
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• 2004

In the present work, a series of experiments have been performed on electro-hydrodynamic atomization of non-conducting liquid using a charge injection type nozzle. Effects of liquid flow rate, input voltage, and distance between the needle and the ground electrode (nozzle-embedded metal plate) have been examined. For fixed electrode distances, total and spray currents increase with the increase of liquid flow rate and input voltage. When the distance between the needle tip and the ground electrode becomes closer, the total, leakage and spray currents increase, while the onset voltage for the dielectric breakdown decreases. When the electric field strength of the liquid jet exceeds that for the air breakdown, a portion of the electric charges in the liquid jet is dissipated into the ambient air, and the charge density shows a limiting value. Atomization quality can be improved by increasing the liquid flow rate due to the higher charge density and the enhanced aerodynamic effect.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2012.05a
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• pp.173-179
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• 2012

Steady and unsteady three-dimensional RANS simulations have been performed on partial admission supersonic axial turbine having backward/forward sweep angles(${\pm}15^{\circ}$) and the results are compared with each other. The objective of this paper is to study the effect of unsteadiness on turbine flow characteristics and performances. The all results indicated that the losses of unsteady simulations were greater than those of steady cases. It was also shown that BSW model give the effect on the reducing of mass flow rates of tip leakage. In unsteady simulation, the increase of t-to-s efficiency at Rotor Out plane was observed more clearly.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.54 no.10
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• pp.438-442
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• 2005

Si nanocrystal (Si NC) memory device has several advantages such as better retention, lower operating voltage, reduced punch-through and consequently a smaller cell area, suppressed leakage current. However, the physical and electrical reasons for this behavior are not completely understood but could be related to interface states of Si NCs. In order to find out this effect, we characterized electrical properties of Si NCs embedded in a SiO$_{2}$ layer by scanning probe microscopy (SPM). The Si NCs were generated by the laser ablation method with compressed Si powder and followed by a sharpening oxidation. In this step Si NCs are capped with a thin oxide layer with the thickness of 1$\~$2 nm for isolation and the size control. The size of 51 NCs is in the range of 10$\~$50 m and the density around 10$^{11}$/cm$^{2}$ It also affects the interface states of Si NCs, resulting in the change of electrical properties. Using a conducting tip, the charge was injected directly into each Si NC, and the image contrast change and dC/dV curve shift due to the trapped charges were monitored. The results were compared with C-V characteristics of the conventional MOS capacitor structure.