• Title/Summary/Keyword: gas diffusion

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Development of Novel Techniques for Determining the Oxygen Tracer Diffusion Coefficients in Oxides II - Measurements of the Depth Profiles of $^{18}O$ Concentration in the solid Samples by Raman Spectroscopy (산화물에서의 산소추적자확산계수를 결정하는 새로운 방법의 개발 II - 라만분광법에 의한 고체시료 중의 시간에 따른 $^{18}O$ 농도변화 측정 -)

  • 김병국;마하구찌히로오;박순자
    • Journal of the Korean Ceramic Society
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    • v.31 no.12
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    • pp.1459-1466
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    • 1994
  • A novel technique for determining the oxygen tracer diffusion coefficients in oxides was developed. After the 16O-18O solid-gas exchange reactions between 16O in the oxides and 18O in the ambient gas, Raman spectra of the cross sections of oxide samples were measured in a spatial resolution of 5 ${\mu}{\textrm}{m}$. From thus obtained Raman spectra, depth profiles of 18O concentration in the oxide samples were calculated. The oxygen tracer diffusion coefficients and the surface exchange coefficients were determined under the assumptions that samples are semi-infinite slab and that the surface exchange reactionsare not negligible. The oxygen tracer diffusion coefficient of 2.8 mol% Y2O3-containing tetragonal ZrO2 polycrystals, 8 mol% Y2O3-containing ZrO2 polycrystals, and 10 mol% Y2O3-containing cubic ZrO2 single crystals (along the a axis) are as follows.

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Kinetic Analysis of Diffusion Aluminide Coating (확산 알루미나이드 코팅의 속도론적 해석)

  • 손희식;김문일
    • Journal of the Korean institute of surface engineering
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    • v.28 no.3
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    • pp.152-163
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    • 1995
  • A theoretical model which combines gaseous transport and solid state diffusion with the multi-component equilibrium at the gas/pack and gas/coating interfaces was used to study the kinetics of diffusion aluminide coating. The diffusion aluminide coatings were applied by pack cementation with Ni substrate under argon atmosphere in the high activity and the low activity pack containing $NH_4CL$ or $AlF_3$ activator. On the basis of the process conditions, the suggested model allows the surface composition, the growth rate of coating layers and the aluminium concentration profiles in coatings to be calculated. In the case of $NH_4$Cl activator, careful consideration was required in the analysis, because activator contains nitrogen and hydrogen as well as halogen element to activate the pack. A good agreement is obtained between the theoretical predictions and the experimental results.

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A Study on the Mass Flow Effects to the Performance of PEMFC (고분자 전해질형 연료전지내의 질량유동이 성능에 미치는 영향)

  • Park, Chang-Kwon;Jo, In-Su;Oh, Byeong-Soo
    • Transactions of the Korean hydrogen and new energy society
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    • v.18 no.4
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    • pp.422-431
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    • 2007
  • Polymer electrolyte membrane fuel cell(PEMFC) is very interesting power source due to high power density, simple construction and operation at low temperature. But it has problems such as high cost, improvement of performance and effect of temperature. These problems can be approached to be solved by using mathematical models which are useful tools for analysis and optimization of fuel cell performance and for heat and water management. In this paper, the present work is to develop an electrochemical model to examine the electrochemical process inside PEM fuel cell. A complete set of considerations of mass, momentum, species and charge is developed and solved numerically with proper account of electrochemical kinetics. When depth of gas channel becomes thinner, diffusion of reactant makes well into gas diffusion layer(GDL) and the performance increases. Although at low current region there is little voltage difference between experimental data of PEM fuel cell and numerical data. When the porosity size of gas diffusion layer for PEM fuel cell is bigger, oxygen diffusion occurs well and oxygen mass fraction appears high in catalyst layer.

Heat Energy Diffusion Analysis in the Gas Sensor Body with the Variation of Drain-Source Electrode Distance (드레인-소스 전극 간극의 변화에 따른 Gas Sensor의 열에너지 확산 해석)

  • Jang, Kyung-Uk
    • Journal of the Korean Institute of Electrical and Electronic Material Engineers
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    • v.30 no.9
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    • pp.589-595
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    • 2017
  • MOS-FET structured gas sensors were manufactured using MWCNTs for application as NOx gas sensors. As the gas sensors need to be heated to facilitate desorption of the gas molecules, heat dispersion plays a key role in boosting the degree of uniformity of molecular desorption. We report the desorption of gas molecules from the sensor at $150^{\circ}C$ for different sensor electrode gaps (30, 60, and $90{\mu}m$). The COMSOL analysis program was used to verify the process of heat dispersion. For heat analysis, structure of FET gas sensor modeling was proceeded. In addition, a property value of the material was used for two-dimensional modeling. To ascertain the degree of heat dispersion by FEM, the governing equations were presented as partial differential equations. The heat analysis revealed that although a large electrode gap is advantageous for effective gas adsorption, consideration of the heat dispersion gradient indicated that the optimal electrode gap for the sensor is $60{\mu}m$.

The Characteristics of Compound Layers Formed during Plasma Nitrocarburising in Pure Iron (플라즈마 침질탄화처리된 순철의 화합물층 특성)

  • Cho, H.S.;Lee, S.Y.;Bell, T.
    • Journal of the Korean Society for Heat Treatment
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    • v.13 no.3
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    • pp.143-150
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    • 2000
  • Ferritic plasma nitrocarburising was performed on pure iron using a modified DC plasma unit. This investigation was carried out with various gas compositions which consisted of nitrogen, hydrogen and carbon monoxide gases, and various gas pressures for 3 hours at $570^{\circ}C$. After treatment, the different cooling rates(slow cooling and fast cooling) were used to investigate its effect on the structure of the compound layer. The ${\varepsilon}$ phase occupied the outer part of the compound layer and ${\gamma}^{\prime}$ phase existed between the ${\varepsilon}$ phase and the diffusion zone. The gas composition of the atmosphere influenced the constitution of the compound layer produced, i.e. high nitrogen contents were essential for the production of ${\varepsilon}$ phase compound layer. It was found that with increasing carbon content in the gas mixture the compound layer thickness increased up to 10%. In the gas pressure around 3 mbar, the compound layer characteristics were slightly effected by gas pressure. However, in the low gas pressure and high gas pressure, the compound layer characteristics were significantly changed. The constitution of the compound layer was altered by varying the cooling rate. A large amount of ${\gamma}^{\prime}$ phase was transformed from the ${\varepsilon}$ phase during slow cooling.

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High-Temperature Tensile Strengths of Alloy 617 Diffusion Weldment (Alloy 617 확산용접재의 고온 인장강도)

  • Sah, Injin;Hwang, Jong-Bae;Kim, Eung-Seon
    • Transactions of the Korean Society of Pressure Vessels and Piping
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    • v.14 no.1
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    • pp.15-23
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    • 2018
  • A compact heat exchanger is one of critical components in a very high temperature gas-cooled reactor (VHTR). Alloy 617 (Ni-Cr-Co-Mo) is considered as one of leading candidates for this application due to its excellent thermal stability and strengths in anticipated operating conditions. On the basis of current ASME code requirements, sixty sheets of this alloy are prepared for diffusion welding, which is the key technology to have a reliable compact heat exchanger. Optical microscopic analysis show that there are no cracks, incomplete bond, and porosity at/near the interface of diffusion weldment, but Cr-rich carbides and Al-rich oxides are identified through high resolution electron microscopic analysis. In high-temperature tensile testing, superior yield strengths of the diffusion weldment compared to the code requirement are obtained up to 1223 K ($950^{\circ}C$). However, both tensile strength and ductility drop rapidly at higher temperature due to the insufficient grain boundary migration across the interface of diffusion weldment. Best fit curves for minimum yield strength and average tensile strength are drawn from the experimental tensile results of this study.

Numerical Simulation of Water Transport in a Gas Diffusion Layer with Microchannels in PEMFC (마이크로채널이 적용된 고분자 전해질 연료전지 가스확산층의 물 이송에 대한 전산해석 연구)

  • Woo, Ahyoung;Cha, Dowon;Kim, Bosung;Kim, Yongchan
    • Journal of the Korean Electrochemical Society
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    • v.16 no.1
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    • pp.39-45
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    • 2013
  • The water management is one of the key issues in low operating temperature proton exchange membrane fuel cells (PEMFCs). The gas diffusion layer (GDL) allows the reactant gases flow to the reaction sites of the catalyst layer (CL). At high current density, generated water forms droplets because the normal operating temperature is $60{\sim}80^{\circ}C$. If liquid water is not evacuated properly, the pores in the GDL will be blocked and the performance will be reduced severely. In this study, the microchannel GDL was proposed to solve the flooding problem. The liquid water transport through 3-D constructed conventional GDL and microchannel GDL was analyzed varying air velocity, water velocity, and contact angle. The simulation results showed that the liquid water was evacuated rapidly through the microchannel GDL because of the lower flow resistance. Therefore, the microchannel GDL was efficient to remove liquid water in the GDL and gas channels.

Experimental Study on Carbon Corrosion of Gas Diffusion Layer in PEM Fuel Cell (고분자전해질형 연료전지 가스확산층의 탄소 부식에 관한 실험적 분석)

  • Ha, Taehun;Cho, Junhyun;Park, Jaeman;Min, Kyoungdoug;Lee, Eunsook;Jyoung, Jy-Young
    • 한국신재생에너지학회:학술대회논문집
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    • 2010.11a
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    • pp.76.1-76.1
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    • 2010
  • Recently, many efforts to solve the durability problem of PEM fuel cell are carried on constantly. However, despite this attention, durability researches of gas diffusion layer (GDL) are not much reported yet. Generally, GDL of PEM fuel cell experiences three external attacks, which are dissolution of water, erosion of gas flow, corrosion of electric potential. In this study, among these degradation factors, carbon corrosion of electric potential was focused and investigated with accelerated carbon corrosion test. Through the test, it is confirmed that carbon corrosion occurred at GDL, and corroded GDL decreased a performance of operating fuel cell. The property changes of GDL were measured with various methods such as air permeability meter, pore distribution analyzer, thermo gravimetric analyzer, and tensile stress test to discover the effects of carbon corrosion. Carbon corrosion caused not only loss of weight and thickness, but also degradation of mechanical strength of GDL. In addition, to analysis the reason of GDL property changes, a surface and a cross section of GDL were observed with scanning electron microscope. After 100 hours test, the GDL showed serious damage in center of layer.

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Prediction of Bypass Flow Rate through Gas Diffusion Layer in PEMFC with Serpentine Flow Channels (사행 유로를 갖는 고분자 전해질 연료전지의 기체확산층 내부에서의 우회 유동 예측)

  • Jeon, Se-Gye;Kim, Kuoung-Youn
    • Transactions of the Korean hydrogen and new energy society
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    • v.23 no.4
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    • pp.293-299
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    • 2012
  • The serpentine flow channel is widely used in polymer electrolyte membrane fuel cells (PEMFCs) to prevent flooding phenomena because it effectively removes liquid water in the flow channel. The pressure drop between inlet and outlet increases as compared with straight channels due to minor losses associated with the corners of the turning configurations. This results in a strong pressure gradient between adjacent channels in specific regions, where some amount of reactant gas can be delivered to catalyst layers by convection through a gas diffusion layer (GDL). The enhancement of the convective flow in the GDL, so-called bypass flow, affects fuel cell performance since the bypass flow influences the reactant transport and thus its concentration over the active area. In the present paper, for the bipolar plate design, a simple analytic model has been proposed to predict the bypass flow in the serpentine type flow channels and validated with three-dimensional numerical simulation results.

Fabrication of Gas Diffusion Layer for Fuel Cells Using Heat treatment Slurry Coating Method (열처리 슬러리코팅법을 이용한 연료전지 가스확산층의 제조)

  • Kim, Sungjin;Park, Sung Bum;Park, Yong-Il
    • Journal of the Korean Society for Heat Treatment
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    • v.25 no.2
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    • pp.65-73
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    • 2012
  • The Gas Diffusion Layer (GDL) of fuel cell, are required to provide both delivery of reactant gases to the catalyst layer and removal of water in either vapor or liquid form in typical PEMFCs. In this study, the fabrication of GDL containing Micro Porous Layer (MPL) made of the slurry of PVDF mixed with carbon black is investigated in detail. Physical properties of GDL containing MPL, such as electrical resistance, gas permeability and microstructure were examined, and the performance of the cell using developed GDL with MPL was evaluated. The results show that MPL with PVDF binder demonstrated uniformly distributed microstructure without large cracks and pores, which resulted in better electrical conductivity. The fuel cell performance test demonstrates that the developed GDL with MPL has a great potential due to enhanced mass transport property due to its porous structure and small pore size.