• Title/Summary/Keyword: Variable Transport properties

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Near-Extinction Structure of Counterflow Nonpremixed Hydrogen-Air Flames (소염상태 근방에서의 대향류 수소확산화염의 구조)

  • Kim, H.J.;Kim, Y.M.
    • 한국연소학회:학술대회논문집
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    • 1997.06a
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    • pp.77-87
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    • 1997
  • The axisymmetric Navier-Stokes model together with detailed chemical kinetics and variable transport properties has been applied to analyze the effects of the multidimensional flow on the flame characteristics in the nitrogen-diluted hydrogen counterflow nonpremixed flame. Computations are performed for two nozzle exit area-averaged velocities. Effects of multidimensional flow and strain rate on the near-extinction structure of the highly diluted hydrogen flames are discussed in detail.

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Metal-Insulator Transition Induced by Short Range Magnetic Ordering in Mono-layered Manganite

  • Chi, E.O.;Kim, W.S.;Hong, C.S.;Hur, N.H.;Choi, Y.N.
    • Bulletin of the Korean Chemical Society
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    • v.24 no.5
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    • pp.573-578
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    • 2003
  • The structural, magnetic, and transport properties of a mono-layered manganite $La_{0.7}Sr_{1.3}MnO_{4+{\delta}}$ were investigated using variable temperature neutron powder diffraction as well as magnetization and transport measurements. The compound adopts the tetragonal I4/mmm symmetry and exhibits no magnetic reflection in the temperature region of 10 K ≤ T ≤ 300 K. A weak ferromagnetic (FM) transition occurs about 130 K, which almost coincides with the onset of a metal-insulator (M-I) transition. Extra oxygen that occupies the interstitial site between the [(La,Sr)O] layers makes the spacing between the [MnO₂] layers shorten, which enhances the inter-layer coupling and eventually leads to the M-I transition. We also found negative magneto resistance (MR) below the M-I transition temperature, which can be understood on the basis of the percolative transport via FM metallic domains in the antiferromagnetic (AFM) insulating matrix.

Investigation on the Structural, Electrical and Magnetic Properties of Layered Perovskite Manganite La0.5Sr1.5Mn0.5Cr0.5-xFexO4 (x=0.15, 0.3) System (층상 페로브스카이트 구조인 La0.5Sr1.5Mn0.5Cr0.5-xFexO4 (x=0.15, 0.3) 망가나이트의 구조적, 전기적, 자기적 특성의 연구)

  • Singh, Devinder
    • Journal of the Korean Chemical Society
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    • v.55 no.4
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    • pp.697-702
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    • 2011
  • The new layered perovskite manganites $La_{0.5}Sr_{1.5}Mn_{0.5}Cr_{0.5-x}Fe_xO_4$ (x=0.15, 0.3) have been prepared by standard ceramic method. The powder X-ray diffraction studies show that the phases crystallize with tetragonal unit cell in the space group I4/mmm. The electrical transport properties suggest that the phases show insulating behaviour and the electrical conduction in the phases occurs by a 3D variable range hopping mechanism. The magnetic properties suggest that both the phases are antiferromagnetic.

The Study on In-situ Measurement of Hydrogen Permeability through Polymer Electrolyte Membranes for Fuel Cells (연료전지용 고분자전해질막의 실시간 수소 투과도 측정법 연구)

  • Lim, Yoon Jae;Lee, Chang Hyun
    • Membrane Journal
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    • v.26 no.2
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    • pp.141-145
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    • 2016
  • Polymer electrolyte membranes (PEMs) are key components to determine electrochemical fuel cell performances, in addition to electrode materials. The PEMs need to satisfy selective transport behaviors to small molecules including gases and protons; the PEMs have to transport protons as fast as possible, while they should act as hydrogen barriers, since the permeated gas induces the thermal degradation of cathode catalyst, resulting in rapid electrochemical reduction. To date, limited tools have been used to measure how fast hydrogen gas permeates through PEMs (e.g., Constant volume/variable Pressure (time-lag) method). However, most of the measurements are conducted under vacuum where PEMs are fully dried. Otherwise, the obtained hydrogen permeance is easily changeable, which causes the measurement errors to be large. In this study, hydrogen permeation properties through Nafion212 used as a standard PEM are evaluated using an in-situ measurement system in which both temperature and humidity are controlled at the same time.

A Study on the Improvement of the Electron Transport Properties in $SF_{6+}Ar$ Mixtures Gas ($SF_{6+}Ar$혼합기체의 전자수송특성 개선에 관한 연구)

  • 하성철;김상남;유회영;서상현;임상원;전병훈
    • Journal of the Korean Institute of Electrical and Electronic Material Engineers
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    • v.11 no.1
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    • pp.67-73
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    • 1998
  • In this paper, the electron swarm parameters in the 0.5% and 0.2% SF\ulcorner+Ar mixtures are measured by time of flight method over the E/N(Td) range from 30 to 300(Td). The measurements have been carried out by the double shutter drift tube with variable drift distance from the cathod. A two-term approximation of the boltzmann equation analysis and Monte Carlo simulation have been also used to study electron transport coefficients. We have calculated W, $ND_L,\;ND_T,\;\alpha,\;\eta,\;\alpha-\eta$, and the limiting breakdown electric field to gas mixtures ratio in pure $SF_6$+Ar mixtures. The electron energy distribution function has been analysed in $SF_6$+Ar mixtures at E/N : 200(Td) for a case of the equilibrium region in the mean electron energy. The measured results and the calculated results have been compared each other.

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Effects of micromechanical models on the dynamics of functionally graded nanoplate

  • Tao Hai;A. Yvaz;Mujahid Ali;Stanislav Strashnov;Mohamed Hechmi El Ouni;Mohammad Alkhedher;Arameh Eyvazian
    • Steel and Composite Structures
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    • v.48 no.2
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    • pp.191-206
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    • 2023
  • The present research investigates how micromechanical models affect the behavior of Functionally Graded (FG) plates under different boundary conditions. The study employs diverse micromechanical models to assess the effective material properties of a two-phase particle composite featuring a volume fraction of particles that continuously varies throughout the thickness of the plate. Specifically, the research examines the vibrational response of the plate on a Winkler-Pasternak elastic foundation, considering different boundary conditions. To achieve this, the governing differential equations and boundary conditions are derived using Hamilton's principle, which is based on a four-variable shear deformation refined plate theory. Additionally, the Galerkin method is utilized to compute the plate's natural frequencies. The study explores how the plate's natural frequencies are influenced by various micromechanical models, such as Voigt, Reuss, Hashin-Shtrikman bounds, and Tamura, as well as factors such as boundary conditions, elastic foundation parameters, length-to-thickness ratio, and aspect ratio. The research results can provide valuable insights for future analyses of FG plates with different boundaries, utilizing different micromechanical models.

Recent Developments in Ion-Exchange Nanocomposite Membranes for Energy Applications (에너지용 이온 교환 복합막 최근 연구 개발 동향)

  • Hwang, Doo Sung;Chung, Tiffany;Wang, Tongshuai;Kim, Sangil
    • Membrane Journal
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    • v.26 no.6
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    • pp.432-448
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    • 2016
  • In the last decade, various types of energy harvesting and conversion systems based on ion exchange membranes (IEMs) have been developed for eco-friendly power generation and energy-grid systems. In these membrane-based energy systems, high ion selectivity and conductivity properties of IEMs are critical parameters to improve efficiency of the systems such as proton exchange membrane fuel cells, anion exchange membrane fuel cells, redox flow batteries, water electrodialysis for hydrogen production, and reverse electrodialysis. This article suggests variable approaches to overcome trade-off limitation of polymeric membrane ion transport properties by reviewing various types of composite ion-exchange membranes including novel inorganic-organic nanocomposite membrane, surface modified membranes, cross-linked and pore-filled membranes.

The characteristics of laminar diffusion flame impinging on the wall (벽면 충돌 층류 확산화염의 특성)

  • Park,Yong-Yeol;Kim, Ho-Yeong
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.20 no.3
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    • pp.979-987
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    • 1996
  • A theoretical study for the laminar round jet diffusion flame impinging on the wall was carried out to predict the characteristics and structure of impinging jet flame and heat transfer to the wall. Finite chemistry via Arrhenius equation was adopted as the combustion model. All the transport properties were considered as the variable depending on the temperature and composition. For the parametric study, the distance from nozzle to perpendicular wall and Reynolds number at nozzle exit were chosen as the major parameters. As the results of the present study, the characteristics of flow field and the distributions of temperature, density and each chemical species were obtained. The heat transfer rate from flame to the wall and the effective heating area were calculated to investigate the influence of the major parameters on the heat transfer characteristics.

Characteristics of Heptane Droplet Vaporization in High-Pressure and Temperature Flow Field (고온 고압 유동장에서 햅탄 액적의 기화 특성)

  • Ko, Jung-Bin;Koo, Ja-Ye
    • Journal of ILASS-Korea
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    • v.9 no.4
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    • pp.83-89
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    • 2004
  • Vaporization characteristics of a liquid heptane droplet in high-pressure and temperature flow field are numerically studied. Variable thermodynamic and transport properties and high-pressure effects are taken into account in order to consider real gas effects. Droplet Vaporization in convective environments was investigated on the basis of droplet vaporization in quiescent and convective environment. In quiescent environments, droplet lifetime is directly proportional to pressure at the subcritical temperature range but it is inversely proportional to pressure at the supercritical temperature range. In convective environment, droplet deformation becomes stronger by increasing Reynolds number due to increase of velocity while droplet deformation is relatively weak at a higher pressure for the same Reynolds number cases.

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Magnetic and Electric Transport Properties of MnTe Thin Film Grown by Molecular Beam Epitaxy (분자선 증착법에 의해 성장한 MnTe 박막의 자기적 및 전기수송 특성)

  • Kim, Woo-Chul;Bae, Sung-Whan;Kim, Sam-Jin;Kim, Chul-Sung;Kim, Kwang-Joo;Yoon, Jung-Bum;Jung, Myung-Hwa
    • Journal of the Korean Magnetics Society
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    • v.17 no.2
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    • pp.81-85
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    • 2007
  • MnTe layers of high crystalline quality were successfully grown on Si(100) : B and Si(111) substrates by molecular beam epitaxy (MBE). Under tellurium-rich condition and the substrate temperature around $400^{\circ}C$, a layer thickness of $700{\AA}$ could be easily obtained with the growth rate of $1.1 {\AA}/s$. We investigated the structural, magnetic and transport properties of MnTe layers by using x-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometry, and physical properties measurement system (PPMS). Characterization of MnTe layers on Si(100) : B and Si(111) substrates by XRD revealed a hexagonal structure of polycrystals with lattice parameters, ${\alpha}=4.143{\pm}0.001{\AA}\;and\;c=6.707{\pm}0.001{\AA}$. Investigation of magnetic and transport properties of MnTe films showed anomalies unlike antiferromagnetic powder MnTe. The temperature dependence of the magnetization data taken in zero-field-tooling (ZFC) and field-cooling (FC) conditions indicates three magnetic transitions at around 21, 49, and 210 K as well as the great irreversibility between ZFC and FC magnetization in the films. These anomalies are attributable to a magnetic-elastic coupling in the films. Magnetization measurements indicate ferromagnetic behaviour with hysteresis loops at 5 and 300 K for MnTe polycrystalline film. The coercivity ($H_c$) values at 5 and 300 K are 55 and 44 Oe, respectively. In electro-transport measurements, the temperature dependence of resistivity revealed a noticeable semiconducting behaviours and showed conduction via Mott variable range hopping at low temperatures.