• 한국신재생에너지학회:학술대회논문집
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• 2008.05a
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• pp.1-4
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• 2008

Fuel cells have the potential of providing several times higher energy storage densities than those possible using current state-of-the-art lithium-ion batteries, but current energy density of fuel cell system is not better than that of lithium-ion batteries. To achieve the high energy density, volume and weight of fuel cell system need to be reduced by miniaturizing system components such as stack, fuel tank, and balance-of-plant. In this paper, the thin flexible PCB (Printed circuit board) is used as a current collector to reduce the stack volume. Two end plates are made from light weight aluminum alloy plate. The plate surface is wholly oxidized through the anodizing treatment for electrical insulation. The opening rate of cathode plate hole is optimized through unit cell performance measurement of various opening rates. The performances are measured at room temperature and ambient pressure condition without any repulsive air supply. The active area of MEA is 10.08 $cm^2$ and active area per a unit cell is 1.68 $cm^2$. The peak power density is about 210 mW/$cm^2$ and the air-breathing planar stack of 2 Wis achieved as a small volume of 18 cc.

• 한국신재생에너지학회:학술대회논문집
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• 2009.06a
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• pp.357-360
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• 2009

The present study focused on the segmented cell which has the similar performance to unaltered (not segmented) cell in real operating condition. Many literatures have been made the segmented cell to observe the behavior of local current density distribution in the single cell. However, it has been lack of scheme to segment the cell in that the detailed interpretation of segmenting in analytic point of view was insufficient. Hence, the basic idea of segmenting was introduced to determine the component to be segmented in anode side of unit cell. The electrical contact/bulk resistance was measured by using four wire/probe method through each part of cell components including MEA, GDL, Bipolar Plate and Current Collector. Electron transport mechanism was predicted by comparing resistance values which were obtained from the experiment. As a result, this offered a great benefit to segment the cell efficiently. With this method further experiments would be conducted in research areas which require current density distribution at the same operating condition as unaltered cell.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.16 no.1
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• pp.75-79
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• 2007

The cathode design is one of the most important parts in order to enhance the performance of fuel cells. A 3-D model of the porous oxygen reducing cathode with perforated current collectors is analysed for the enhanced design in fuel cells. Simulation is performed using equations of electric potential balance, momentum balance, and mass balance. The gas concentrations are quite large and are significantly affected by the reactions that take place. The weight fraction of oxygen, velocity field for the gas phase, and local overvoltage are illustrated in the porous reactive cathode layer. The current density is also analysed and the result shows the distribution and variation are stated in a wide range. It is found that the rate of reaction and the current production is higher beneath the orifice, and decreases as the distance to the gas inlet increases. The significance of the results is discussed in the viewpoint of the mass transportation phenomena, which is inferred that the mass transport of reactants dictates the efficiency of the electrode in this design and at these conditions.

• Journal of the Korean Institute of Telematics and Electronics A
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• v.31A no.9
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• pp.104-113
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• 1994

We have fabricated n-p-n HBTs using 3-inchAlgaAs/GaAs hetero structure epi-wafers grown by MBE. DC and AC characteristics of HBT devices were measured and analyzed. For HBT epi-structure, Al composition of emitter was graded in the region between emitter cap and emitter. And base layer was designed with concentration of 1${\times}10^{19}/cm^{3}$ and thickness of 50nm, and Be was used as the p-type dopant. Principal processes for device fabrication consist of photolithography using i-line stepper, wet mesa etching, and lift-off of each ohmic metal. The PECVD SiN film was used as the inslator for the metal interconnection. HBT device with emitter size of 3${\times}10{\mu}m^{2}$ resulted in cut-off frequency of 35GHz, maximum oscillation frequency of 21GHz, and current gain of 60. The distribution of the ideality factor of collector and base current was very uniform, and the average values of off-set voltage and current was very uniform, and the average values of off-set voltage and current gain were 0.32V and 32 within a 3-inch wafer.

• Proceedings of the KIPE Conference
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• 2013.07a
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• pp.244-245
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• 2013

This paper proposes a control algorithm for permanent magnet synchronous generator with a back-to-back three-level neutral-point clamped voltage source converter in a medium-voltage offshore wind power system under unbalanced grid conditions. The proposed control algorithm particularly compensates for the unbalanced grid voltage at the point of common coupling in a collector bus of offshore wind power system. This control algorithm has been formulated based on the symmetrical components in positive and negative rotating synchronous reference frames under generalized unbalanced operating conditions. Instantaneous active and reactive power are described in terms of symmetrical components of measured grid input voltages and currents. Negative sequential component of ac input current is injected to the point of common coupling in the proposed control strategy. The amplitude of negative sequential component is calculated to minimize the negative sequential component of grid voltage under the limitation of current capability in a voltage source converter. The proposed control algorithm makes it possible to provide a balanced voltage at the point of common coupling resulting in the generated power of high quality from offshore wind power system under unbalanced network conditions.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.61 no.7
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• pp.1001-1006
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• 2012

This paper presents a carbon nanotube (CNT) triode-structure field emitter as an ion source in a micro time-of-flight mass spectrometer(TOF-MS). In the ion source by field emission, the electrons emitted from cathodes under an electric field accelerated to the anode and ionize gas molecules by impact before arriving the anode. The generated positive ions are to be accelerated to the ion collector. Whereas most of ions are drawn to the cathodes in diode field emitters, a grid in the triode field emitter prevents the ions from being drawn to the cathodes. The triode field emitter is fabricated by micromachining. The cathode is composed of six CNT cylinders. The total size of the fabricated device is $8.0{\times}7.3{\times}1.9mm^3$. The anode and the grid current of the fabricated CNT field emitter were measured for various anode and grid voltages. When the anode and the grid voltages are 1000 V and 990 V, respectively, the emission current passing through the ionization region is 8.6 ${\mu}A$, which is a sufficient emission current for ionization and mass spectrometry.

• Journal of the Korean Vacuum Society
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• v.14 no.4
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• pp.229-237
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• 2005

The degradation phenomena in SiGe hetero-junction bipolar transistors(SiGe HBTs) induced by bias stress are investigated in this review. If SiGe HBTs are stressed over a specific time interval, the device parameters deviate from their nominal values due to the internal changes in the devices. Reverse-bias stress on emitter-base(EB) junctions causes base current increase and current gain decrease because carriers accelerated by the electrical field generate recombination centers. When forward-bias current stress is conducted at an ambient temperature above $140^{\circ}C$ , hot carriers produced by Auger recombination or avalanche multiplication induce current gain fluctuation. Mixed-mode stressing, where high emitter current and high collector-base voltage are simultaneously applied to the device, provokes base current rise as EB reverse-bias stressing does.

• Journal of the Korean Electrochemical Society
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• v.13 no.4
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• pp.256-263
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• 2010

A Crofer 22 APU mesh coated with a conductive ceramic material was developed as an alternative cathode current collector to Ag-based materials for solid oxide fuel cells. $(La_{0.80}Sr_{0.20})_{0.98}MnO_3$ (LSM) layer was deposited onto the Crofer mesh using a spray-coating technique, in an attempt to mitigate the degradation of electrical properties due to surface oxidation at high temperatures. The oxidation experiments at $800^{\circ}C$ in air indicated that the areaspecific resistance (ASR) of the LSM-coated Crofer mesh was strongly dependent on the wire diameter and the contact morphology between mesh and cell. In addition, the post-heat-treatment in $H_2/N_2$ resulted in a reduced thickness of Cr-containing oxide scales at the interface between Crofer mesh and LSM layer, leading to a decreased ASR.

• Journal of Ecology and Environment
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• v.34 no.2
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• pp.175-191
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• 2011

Colonization patterns and community changes in benthic macroinvertebrates in the Cheonggye Stream, a functionally restored stream in downtown Seoul, Korea, were studied from November 2005 to November 2007. Benthic macroinvertebrates were quantitatively sampled 15 times from five sites in the stream section. Taxa richness (59 species in total) increased gradually over the first year, whereas the density revealed seasonal differences with significantly lower values in the winter season and after flood events. The benthic macroinvertebrate fauna may have drifted from the upstream reaches during floods and from the Han River, arrived aerially, or hitchhiked on artificially planted aquatic plants. Oligochaeta, Chironommidae, Psychodidae, and Hydropsychidae were identified as major community structure contributors in the stream. Swimmers and clingers colonized relatively earlier in the upper and middle reaches, whereas burrowers dominated particularly in the lower reaches. Collector-gatherers colonized at a relatively early period throughout the stream reaches, and collector-filterers, such as the net-spinning caddisfly (Cheumatopyche brevilineata), predominated in the upper and middle reaches after a 1-year time period. Cluster analyses and multi-response permutation procedures demonstrated that the Cheonggye Stream shares more similarities with the Jungnang Stream than with the Gapyeong Stream. Detrended correspondence analysis and nonmetric multidimensional scaling demonstrated that physical environmental factors (depth, current velocity, dissolved oxygen, and pH) as well as nutrients (total nitrogen and total phosphorous), water temperature, and conductivity could affect the distribution of benthic macroinvertebrates in the study streams.

• Journal of the Korean Society of Marine Environment & Safety
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• v.30 no.4
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• pp.317-323
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• 2024

According to the section A-VI/3 of the International Convention on Standards of Training, Certification and Watchkeeping for Seafarers (STCW), Water-based firefighting training center is mandatory to obtain onboard certificates. This space, being similar to fire situations on ships requires that safety measures be quantified to ensure occupant safety and establish operational standards. For fire safety evaluation, cases were designed based on the presence or absence of smoke control equipment using Pyrosim based on Fire Dynamics Simulation (FDS). Vector analysis was performed to evaluate flow of smoke and heat. Available safe escape time / required safe escpae time (ASET/RSET) analysis was conducted to evaluate safety by comparing the interpreted numerical results through Pathfinder. During safety evaluation of the current operational condition, the appropriateness of the function of each smoke control equipment was numerically and visually indicated. The emergency situation with dust collector stopped was expressed by each evacuation time and safety margin of 111.2 seconds, suggesting that be used as a standard of evacuation time.