• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회B
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• pp.2227-2230
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• 2008

Crossover of nitrogen from cathode to anode is inevitable in typical membranes used in PEM fuel cells. This crossovered nitrogen accumulates in anode recirculation system and excessive buildup of nitrogen in the recirculating anode gas lowers the hydrogen concentration and finally affects the performance of fuel cell stacks. In this study, characterization of nitrogen gas crossover was investigated in PEM fuel cell stacks. The mass spectroscopy (MS) has been applied to measure the amount of the crossovered nitrogen at the exit of anode. Results show that anode and cathode stoichiometric number ($SR_c$) have a big effect of nitrogen crossover.

• 한국결정성장학회지
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• 제13권4호
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• pp.187-198
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• 2003

Zinc selenide (ZnSe) single crystals hold promise for many electro-optics, acousto-optic and green laser generation applications. This material is prepared in closed ampoules by the physical vapor transport (PVT) growth method based on the dissociative sublimation. We investigate the effects of diffusive-convection on the crystal growth rate of ZnSe with a low vapor pressure system in a horizontal configuration. Our results show that for the ratios of partial pressures, s=0.2 and 2.9, the growth rate increases with the Peclet number and the temperature differences between the source and crystal. As the ratio of partial pressures approaches the stoichiometric value of 2, the rate increases. The mass fluk based on one dimensional (1D model) flow for low vapor pressure system fall within the range of the predictions (2D model) obtained by solving the coupled set of conservation equations, which indicates the flow fields would be advective-diffusive. Therefore, the rate and the flow fields are independent of gravity acceleration levels.

• 한국자동차공학회논문집
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• 제11권5호
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• pp.15-21
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• 2003

The liquid phase LPG injection engine is a new technology to make good use of LPG as a clean energy. However, it is difficult to precisely control air/fuel ratio in the system because of variation of fuel composition, change of temperature and flash boiling injection mechanism. This study newly suggests an injector control logic for liquid phase LPG injection systems. This logic compensates a number of effects such as variations of density, stoichiometric air/fuel ratio, injection delay time, injection pressure, release pressure which is formed by flash boiling of fuel at nozzle exit. This logic can precisely control air/fuel ratio with only two parameters of intake air flow rate and injection pressure without considering fuel composition, fuel temperature.

• 한국연소학회지
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• 제6권1호
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• pp.7-13
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• 2001

Propagation characteristics of tribrachial flames have been studied numerically in a two-dimensional fuel/oxidizer mixing layer. A flame is initiated by imposing a high temperature ignition source. Subsequent propagation of a tribrachial flame is traced. The flow redirection effect at the leading edge of a tribrachial flame increases the propagation speed beyond the corresponding stoichiometric laminar burning velocity. The effect of mixture fraction gradient on the propagation speed of a tribrachial flame is analyzed in a mixing layer considering that mixture fraction gradient increases as a tribrachial flame propagates toward upstream. As the flame curvature at the leading edge increases with decreasing mixture fraction gradient, the flow redirection effect becomes more pronounced on the flame propagation speed. As a result, the propagation speed of a tribrachial flame increases with decreasing mixture fraction gradient.

• 대한기계학회논문집B
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• 제30권5호
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• pp.397-404
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• 2006

Effects of surface defect distribution on flame instability during flame-surface interaction are experimentally investigated. To examine chemical quenching phenomenon which is caused by radical adsorption and recombination processes on the surface, thermally grown silicon oxide plates with well-defined defect density were prepared. ion implantation technique was used to control the number of defects, i.e. oxygen vacancies. In an attempt to preferentially remove oxygen atoms from silicon dioxide surface, argon ions with low energy level from 3keV to 5keV were irradiated at the incident angle of $60^{\circ}$. Compositional and structural modification of $SiO_2$ induced by low-energy $Ar^+$ ion irradiation has been characterized by Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). It has been found that as the ion energy is increased, the number of structural defect is also increased and non-stoichiometric condition of $SiO_x({\le}2)$ is enhanced.

• Environmental Engineering Research
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• 제12권4호
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• pp.148-154
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• 2007

The purpose of this research was to investigate the effects of low concentration of oxygen on reduction of perchlorate, especially low perchlorate influent concentrations in a biofilm reactor, as well as the effect of flow pattern in a biofilm reactor. Dissolved oxygen averaging 1 mg/L did not inhibit reduction of influent perchlorate from 23 to $426\;{\mu}g/L$ in the biofilm reactors when sufficient acetate was added, probably due to limitation of oxygen diffusion into the biofilm. Influent perchlorate ranging from 23 to $426\;{\mu}g/L$ was reduced to below detection level ($4\;{\mu}g/L$) in the presence of 1 mg/L dissolved oxygen (DO). Chloride was produced in a ratio of $0.37gCl^-/g{ClO_4}^-$ and $0.35gCl^-/g{ClO_4}^-$ in plug flow and recirculation biofilm reactor which is similar to stoichiometric amount ($0.36gCl^-/g{ClO_4}^-$) indicating complete perchlorate reduction at $426\;{\mu}g/L$ of ${ClO_4}^-$ feeding. At $23\;{\mu}g/L$L influent perchlorate, total biomass solids were 3.18 g and 2.81 g in the plug flow and recirculation biofilm reactors. The most probable number(MPN) analysis for perchlorate-reducing bacteria showed $10^4$ to $10^5\;cells/cm^2$ in both biofilm reactors throughout the experiments. The effluent perchlorate concentrations were not significantly different in the two different flow regimes, plug flow and recirculation biofilm reactors.

• 한국자동차공학회논문집
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• 제8권6호
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• pp.31-39
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• 2000

In this paper, characteristics of a port injection type LPG fuel system were investigated to adopt the system to a spark ignition engine through rig test. Engine combustion characteristics for limited conditions and the precise control method of LPG fuel supply were also studied. As a basic experiment, the effects and the relationships of parameters such as orifice area, fuel delivery pressure, fuel temperature and flow coefficient were established. From this, one dimensional compressible flow equation can be applied to control gaseous fuel flow rate by setting pressure difference between vaporizer and manifold to a certain range, for example about 1.2 bar in a naturally aspirated engine. The combustion analysis results of LPG engine were also compared with those of gasoline engine according to spark timing and load change. At part load and stoichiometric condition, the MBT spark timing of LPG fueled engine is retarded by 2$^{\circ}$ - 4$^{\circ}$CA compared to that of gasoline engine. On the contrary, the spark timing of LPG fueled engine can be advanced by 5$^{\circ}$- 10$^{\circ}$ CA at WOT, which results from higher Octane Number and burned fraction of LPG fuel compared to gasoline.

• 한국재료학회지
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• 제13권7호
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• pp.442-446
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• 2003

Tin dioxide ($SnO _2$) thin films were deposited at $375^{\circ}C$ on alumina substrate by metal-organic chemical vapor deposition. A few hillocks like a cauliflower were observed and the number of hillock on thin film surface increased with annealing temperature in air atmosphere. The oxygen content and the binding energy during air annealing at$ 500^{\circ}C$ came to close the stoichiometric $SnO_2$. The cauliflower hillocks seem to be the result of the continuous migration of the tiny grains to release the stress of an expanded grain. Sensitivity of CO gas depended on annealing temperature and increased with increasing annealing temperature.

• 한국세라믹학회지
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• 제26권1호
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• pp.59-66
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• 1989

Single crystals of the superconducting Y-Ba-Cu oxide were grown by a flux technique. Stoichiometric mixture with excess BaO and CuOcontent was melted at 130$0^{\circ}C$, followed by fast cooling to 108$0^{\circ}C$ to prevent crystallizing of CuO. It was then reheated to 110$0^{\circ}C$ followed by being held at this temperature to control the number of nuclei, and cooled by 5$^{\circ}C$/hr to 80$0^{\circ}C$. This procedure was repeated with various compositions and crystals were obtained in cavity. Molten solution was seperated to two parts : the upper part of almost CuO and the rest of different composition. Appropriate composition of flux was 30mol% BaO and 70mol% CuO due to the seperation of molten solution. Single crystals have the habit of thin plate with good developed (001) crystal face, XRD, EDAX and single crystal X-ray investigations were carried out. The grown crystals have tetragonal structure with the lattice parameters a=b=3.84$\AA$, c=11.7$\AA$, and the space group P4/mmm. The crystal surface were observed by SEM.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 1998년도 춘계학술대회 논문집
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• pp.395-398
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• 1998

Lithium insertion has been studied in a number of vanadium oxides with special regard to their application as the active materials in rechargeable lithium cells. Very high stoichiometric energy densities for lithium insertion are found for several of these materials. Some vanadium oxides, e.g. V$_2$ $O_{5}$ and V$_{6}$ $O_{13}$, are now being used in commercially developed rechargeable Li batteries. Another material which is receiving remarkable attention for this kind of cells is LiV$_3$ $O_{8}$. In variety of ternary lithium-vanadium oxides, the lithium content can be varied between certain limits without major changes in the vanadium oxygen lattice. In our worts, the oxides which do net form these thermodynamically stable bronzes can still accommodate large amounts of lithium at ambient temperature, forming kinetically stable insertion compounds. These compounds owe their existence to the whereas lithium is easily introduced into these open structures. The oxides investigated are rather poor electronic conductors; the conductivity decrease with increase in the lithium content. Improvements in the electrode fabrication technique are needed to alleviate this Problem.oblem.