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Arthur J Freeman - One of the best experts on this subject based on the ideXlab platform.
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co2mnx x si ge sn heusler compounds an ab initio study of their structural electronic and magnetic properties at zero and Elevated Pressure
Physical Review B, 2002Co-Authors: Silvia Picozzi, A Continenza, Arthur J FreemanAbstract:The structural, electronic, and magnetic properties of Co 2 MnX (X = Si, Ge, Sn) Heusler compounds have been determined by means of all-electron full-potential linearized augmented plane wave (FLAPW) calculations. We focuson the effects on the electronic and magnetic properties induced by: (i) substitution of the X atom, (ii) applied Pressure, and (iii) the use of the local spin density approximation (LSDA) vs the generalized gradient approximation (GGA) in density functional theory. A comparison between LSDA and GGA for the exchange-correlation functional shows that GGA is essential for an accurate description of the equilibrium volumes and of the electronic and magnetic properties of these systems. We find that both the energy gap and the spin gap increase as the X atomic number decreases. As a result of the semiconducting (metallic) character found in the minority (majority) spin band structure, the Si and Ge based alloys are predicted to be half-metallic. In contrast, Co 2 MnSn is found to be a "nearly half-metallic" compound, since the minority valence band maximum crosses the Fermi level. The calculated total magnetization of 5 μ B is in excellent agreement with recent experiments. By including a fully self-consistent treatment of spin-orbit coupling, the GGA calculated orbital moments are shown to be very small (about 0.008 μ B for Mn and about 0.02 μ B for Co), showing that the quenching of the orbital magnetic moment is nearly complete. The calculated hyperfine fields, both at zero and Elevated Pressure, are compared with available experimental data, and show general agreement, except for Mn. Finally, the calculated Mn 2p exchange splittings, found to be in good agreement with experiment, are proportional to the Mn magnetic moments, suggesting a localized nature of ferromagnetism in these Heusler compounds.
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co2mnx x si ge sn heusler compounds an ab initio study of their structural electronic and magnetic properties at zero and Elevated Pressure
Physical Review B, 2002Co-Authors: Silvia Picozzi, A Continenza, Arthur J FreemanAbstract:The structural, electronic, and magnetic properties of ${\mathrm{Co}}_{2}\mathrm{Mn}X$ $(X=\mathrm{Si},$ Ge, Sn) Heusler compounds have been determined by means of all-electron full-potential linearized augmented plane wave (FLAPW) calculations. We focus on the effects on the electronic and magnetic properties induced by: (i) substitution of the X atom, (ii) applied Pressure, and (iii) the use of the local spin density approximation (LSDA) vs the generalized gradient approximation (GGA) in density functional theory. A comparison between LSDA and GGA for the exchange-correlation functional shows that GGA is essential for an accurate description of the equilibrium volumes and of the electronic and magnetic properties of these systems. We find that both the energy gap and the spin gap increase as the X atomic number decreases. As a result of the semiconducting (metallic) character found in the minority (majority) spin band structure, the Si and Ge based alloys are predicted to be half-metallic. In contrast, ${\mathrm{Co}}_{2}\mathrm{MnSn}$ is found to be a ``nearly half-metallic'' compound, since the minority valence band maximum crosses the Fermi level. The calculated total magnetization of $5{\ensuremath{\mu}}_{B}$ is in excellent agreement with recent experiments. By including a fully self-consistent treatment of spin-orbit coupling, the GGA calculated orbital moments are shown to be very small (about $0.008{\ensuremath{\mu}}_{B}$ for Mn and about $0.02{\ensuremath{\mu}}_{B}$ for Co), showing that the quenching of the orbital magnetic moment is nearly complete. The calculated hyperfine fields, both at zero and Elevated Pressure, are compared with available experimental data, and show general agreement, except for Mn. Finally, the calculated Mn $2p$ exchange splittings, found to be in good agreement with experiment, are proportional to the Mn magnetic moments, suggesting a localized nature of ferromagnetism in these Heusler compounds.
Silvia Picozzi - One of the best experts on this subject based on the ideXlab platform.
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co2mnx x si ge sn heusler compounds an ab initio study of their structural electronic and magnetic properties at zero and Elevated Pressure
Physical Review B, 2002Co-Authors: Silvia Picozzi, A Continenza, Arthur J FreemanAbstract:The structural, electronic, and magnetic properties of Co 2 MnX (X = Si, Ge, Sn) Heusler compounds have been determined by means of all-electron full-potential linearized augmented plane wave (FLAPW) calculations. We focuson the effects on the electronic and magnetic properties induced by: (i) substitution of the X atom, (ii) applied Pressure, and (iii) the use of the local spin density approximation (LSDA) vs the generalized gradient approximation (GGA) in density functional theory. A comparison between LSDA and GGA for the exchange-correlation functional shows that GGA is essential for an accurate description of the equilibrium volumes and of the electronic and magnetic properties of these systems. We find that both the energy gap and the spin gap increase as the X atomic number decreases. As a result of the semiconducting (metallic) character found in the minority (majority) spin band structure, the Si and Ge based alloys are predicted to be half-metallic. In contrast, Co 2 MnSn is found to be a "nearly half-metallic" compound, since the minority valence band maximum crosses the Fermi level. The calculated total magnetization of 5 μ B is in excellent agreement with recent experiments. By including a fully self-consistent treatment of spin-orbit coupling, the GGA calculated orbital moments are shown to be very small (about 0.008 μ B for Mn and about 0.02 μ B for Co), showing that the quenching of the orbital magnetic moment is nearly complete. The calculated hyperfine fields, both at zero and Elevated Pressure, are compared with available experimental data, and show general agreement, except for Mn. Finally, the calculated Mn 2p exchange splittings, found to be in good agreement with experiment, are proportional to the Mn magnetic moments, suggesting a localized nature of ferromagnetism in these Heusler compounds.
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co2mnx x si ge sn heusler compounds an ab initio study of their structural electronic and magnetic properties at zero and Elevated Pressure
Physical Review B, 2002Co-Authors: Silvia Picozzi, A Continenza, Arthur J FreemanAbstract:The structural, electronic, and magnetic properties of ${\mathrm{Co}}_{2}\mathrm{Mn}X$ $(X=\mathrm{Si},$ Ge, Sn) Heusler compounds have been determined by means of all-electron full-potential linearized augmented plane wave (FLAPW) calculations. We focus on the effects on the electronic and magnetic properties induced by: (i) substitution of the X atom, (ii) applied Pressure, and (iii) the use of the local spin density approximation (LSDA) vs the generalized gradient approximation (GGA) in density functional theory. A comparison between LSDA and GGA for the exchange-correlation functional shows that GGA is essential for an accurate description of the equilibrium volumes and of the electronic and magnetic properties of these systems. We find that both the energy gap and the spin gap increase as the X atomic number decreases. As a result of the semiconducting (metallic) character found in the minority (majority) spin band structure, the Si and Ge based alloys are predicted to be half-metallic. In contrast, ${\mathrm{Co}}_{2}\mathrm{MnSn}$ is found to be a ``nearly half-metallic'' compound, since the minority valence band maximum crosses the Fermi level. The calculated total magnetization of $5{\ensuremath{\mu}}_{B}$ is in excellent agreement with recent experiments. By including a fully self-consistent treatment of spin-orbit coupling, the GGA calculated orbital moments are shown to be very small (about $0.008{\ensuremath{\mu}}_{B}$ for Mn and about $0.02{\ensuremath{\mu}}_{B}$ for Co), showing that the quenching of the orbital magnetic moment is nearly complete. The calculated hyperfine fields, both at zero and Elevated Pressure, are compared with available experimental data, and show general agreement, except for Mn. Finally, the calculated Mn $2p$ exchange splittings, found to be in good agreement with experiment, are proportional to the Mn magnetic moments, suggesting a localized nature of ferromagnetism in these Heusler compounds.
K. A. Friedrich - One of the best experts on this subject based on the ideXlab platform.
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A validated multi‐scale model of a SOFC stack at Elevated Pressure
Fuel Cells, 2013Co-Authors: Moritz Henke, Caroline Willich, Christina Westner, Florian Leucht, Josef Kallo, Wolfgang G. Bessler, K. A. FriedrichAbstract:This paper presents a multi-scale model of a solid oxide fuel cell (SOFC) stack consisting of five anode-supported cells. A two-dimensional isothermal elementary kinetic model is used to calculate the performance of single cells. Several of these models are thermally coupled to form the stack model. Simulations can be carried out at steady-state as well as dynamic operation. The model is validated over a wide range of operating conditions including variation of temperature, gas composition (both on anode and cathode side), and Pressure. Validation is carried out using polarization curves and impedance spectra. The model is then used to explain the Pressure-induced performance increase measured at constant fuel utilization of 40%. Results show that activation and concentration overpotentials are reduced with increasing Pressure.
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A validated multi-scale model of a SOFC stack at Elevated Pressure
Fuel Cells, 2013Co-Authors: Moritz Henke, Caroline Willich, Christina Westner, Florian Leucht, Josef Kallo, Wolfgang G. Bessler, K. A. FriedrichAbstract:This paper presents a multi-scale model of a solid oxide fuel\ud cell (SOFC) stack consisting of five anode-supported cells. A\ud two-dimensional isothermal elementary kinetic model is\ud used to calculate the performance of single cells. Several of\ud these models are thermally coupled to form the stack model.\ud Simulations can be carried out at steady-state as well as\ud dynamic operation. The model is validated over a wide\ud range of operating conditions including variation of temperature, gas composition (both on anode and cathode side),\ud and Pressure. Validation is carried out using polarization\ud curves and impedance spectra. The model is then used to\ud explain the Pressure-induced performance increase measured\ud at constant fuel utilization of 40%. Results show that\ud activation and concentration overpotentials are reduced\ud with increasing Pressure
Chih-jen Sung - One of the best experts on this subject based on the ideXlab platform.
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Acetone photophysics at 282 nm excitation at Elevated Pressure and temperature. II: Fluorescence modeling
Applied Physics B, 2017Co-Authors: Jason Hartwig, Mandhapati Raju, Chih-jen SungAbstract:This is the second in a series of two papers that presents an updated fluorescence model and compares with the new experimental data reported in the first paper, as well as the available literature data, to extend the range of acetone photophysics to Elevated Pressure and temperature conditions. This work elucidates the complete acetone photophysical model in terms of each and every competing radiative and non-radiative rate. The acetone fluorescence model is then thoroughly examined and optimized based on disparity with recently conducted Elevated Pressure and temperature photophysical calibration experiments. The current work offers insight into the competition between non-radiative and vibrational energy decay rates at Elevated temperature and Pressure and proposes a global optimization of model parameters from the photophysical model developed by Thurber (Acetone Laser-Induced Fluorescence for Temperature and Multiparameter Imaging in Gaseous Flows. PhD thesis, Stanford University Mechanical Engineering Department, 1999). The collisional constants of proportionality, which govern vibrational relaxation, are shown to be temperature dependent at Elevated Pressures. A new oxygen quenching rate is proposed which takes into account collisions with oxygen as well as the oxygen-assisted intersystem crossing component. Additionally, global trends in ketone photophysics are presented and discussed.
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Acetone photophysics at 282 nm excitation at Elevated Pressure and temperature. II: Fluorescence modeling
Applied Physics B, 2017Co-Authors: Jason Hartwig, Mandhapati Raju, Chih-jen SungAbstract:This is the second in a series of two papers that presents an updated fluorescence model and compares with the new experimental data reported in the first paper, as well as the available literature data, to extend the range of acetone photophysics to Elevated Pressure and temperature conditions. This work elucidates the complete acetone photophysical model in terms of each and every competing radiative and non-radiative rate. The acetone fluorescence model is then thoroughly examined and optimized based on disparity with recently conducted Elevated Pressure and temperature photophysical calibration experiments. The current work offers insight into the competition between non-radiative and vibrational energy decay rates at Elevated temperature and Pressure and proposes a global optimization of model parameters from the photophysical model developed by Thurber (Acetone Laser-Induced Fluorescence for Temperature and Multiparameter Imaging in Gaseous Flows. PhD thesis, Stanford University Mechanical Engineering Department, 1999). The collisional constants of proportionality, which govern vibrational relaxation, are shown to be temperature dependent at Elevated Pressures. A new oxygen quenching rate is proposed which takes into account collisions with oxygen as well as the oxygen-assisted intersystem crossing component. Additionally, global trends in ketone photophysics are presented and discussed.
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Acetone photophysics at 282 nm excitation at Elevated Pressure and temperature. II: Fluorescence modeling
Applied Physics B, 2017Co-Authors: Jason Hartwig, Mandhapati Raju, Chih-jen SungAbstract:This is the second in a series of two papers that presents an updated fluorescence model and compares with the new experimental data reported in the first paper, as well as the available literature data, to extend the range of acetone photophysics to Elevated Pressure and temperature conditions. This work elucidates the complete acetone photophysical model in terms of each and every competing radiative and non-radiative rate. The acetone fluorescence model is then thoroughly examined and optimized based on disparity with recently conducted Elevated Pressure and temperature photophysical calibration experiments. The current work offers insight into the competition between non-radiative and vibrational energy decay rates at Elevated temperature and Pressure and proposes a global optimization of model parameters from the photophysical model developed by Thurber (Acetone Laser-Induced Fluorescence for Temperature and Multiparameter Imaging in Gaseous Flows. PhD thesis, Stanford University Mechanical Engineering Department, 1999). The collisional constants of proportionality, which govern vibrational relaxation, are shown to be temperature dependent at Elevated Pressures. A new oxygen quenching rate is proposed which takes into account collisions with oxygen as well as the oxygen-assisted intersystem crossing component. Additionally, global trends in ketone photophysics are presented and discussed.
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A Rapid Compression Study of the Butanol Isomers at Elevated Pressure
arXiv: Chemical Physics, 2017Co-Authors: Bryan W Weber, Chih-jen SungAbstract:Investigation of the autoignition delay of the butanol isomers has been performed at Elevated Pressure of 15 bar and low to intermediate temperatures of 725-870 K. Stoichiometric mixtures made in nitrogen/oxygen air were studied. For the temperature and Pressure conditions in this study, no NTC or two-stage ignition behavior were observed. The reactivity of the isomers of butanol, in terms of inverse ignition delay, was ranked as n-butanol > sec-butanol ~ iso-butanol > tert-butanol. Predictions of the ignition delay by several kinetic mechanisms available in the literature generally over-predict the ignition delays.
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autoignition of n butanol at Elevated Pressure and low to intermediate temperature
Combustion and Flame, 2011Co-Authors: Bryan W Weber, Kamal Kumar, Yu Zhang, Chih-jen SungAbstract:Abstract Autoignition experiments for n -butanol have been performed using a heated rapid compression machine at compressed Pressures of 15 and 30 bar, in the compressed temperature range of 675–925 K, and for equivalence ratios of 0.5, 1.0, and 2.0. Over the conditions studied, the ignition delay decreases monotonically as temperature increases, and the autoignition response exhibits single-stage characteristics. A non-linear fit to the experimental data is performed and the reactivity, in terms of the inverse of ignition delay, shows nearly second order dependence on the initial oxygen mole fraction and slightly greater than first order dependence on initial fuel mole fraction and compressed Pressure. Experimentally measured ignition delays are also compared to simulations using several reaction mechanisms available in the literature. Agreement between simulated and experimental ignition delay is found to be unsatisfactory. Sensitivity analysis is performed on one recent mechanism and indicates that uncertainties in the rate coefficients of parent fuel decomposition reactions play a major role in causing the poor agreement. Path analysis of the fuel decomposition reactions supports this conclusion and also highlights the particular importance of certain pathways. Further experimental investigations of the fuel decomposition, including speciation measurements, are required.
A Continenza - One of the best experts on this subject based on the ideXlab platform.
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co2mnx x si ge sn heusler compounds an ab initio study of their structural electronic and magnetic properties at zero and Elevated Pressure
Physical Review B, 2002Co-Authors: Silvia Picozzi, A Continenza, Arthur J FreemanAbstract:The structural, electronic, and magnetic properties of Co 2 MnX (X = Si, Ge, Sn) Heusler compounds have been determined by means of all-electron full-potential linearized augmented plane wave (FLAPW) calculations. We focuson the effects on the electronic and magnetic properties induced by: (i) substitution of the X atom, (ii) applied Pressure, and (iii) the use of the local spin density approximation (LSDA) vs the generalized gradient approximation (GGA) in density functional theory. A comparison between LSDA and GGA for the exchange-correlation functional shows that GGA is essential for an accurate description of the equilibrium volumes and of the electronic and magnetic properties of these systems. We find that both the energy gap and the spin gap increase as the X atomic number decreases. As a result of the semiconducting (metallic) character found in the minority (majority) spin band structure, the Si and Ge based alloys are predicted to be half-metallic. In contrast, Co 2 MnSn is found to be a "nearly half-metallic" compound, since the minority valence band maximum crosses the Fermi level. The calculated total magnetization of 5 μ B is in excellent agreement with recent experiments. By including a fully self-consistent treatment of spin-orbit coupling, the GGA calculated orbital moments are shown to be very small (about 0.008 μ B for Mn and about 0.02 μ B for Co), showing that the quenching of the orbital magnetic moment is nearly complete. The calculated hyperfine fields, both at zero and Elevated Pressure, are compared with available experimental data, and show general agreement, except for Mn. Finally, the calculated Mn 2p exchange splittings, found to be in good agreement with experiment, are proportional to the Mn magnetic moments, suggesting a localized nature of ferromagnetism in these Heusler compounds.
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co2mnx x si ge sn heusler compounds an ab initio study of their structural electronic and magnetic properties at zero and Elevated Pressure
Physical Review B, 2002Co-Authors: Silvia Picozzi, A Continenza, Arthur J FreemanAbstract:The structural, electronic, and magnetic properties of ${\mathrm{Co}}_{2}\mathrm{Mn}X$ $(X=\mathrm{Si},$ Ge, Sn) Heusler compounds have been determined by means of all-electron full-potential linearized augmented plane wave (FLAPW) calculations. We focus on the effects on the electronic and magnetic properties induced by: (i) substitution of the X atom, (ii) applied Pressure, and (iii) the use of the local spin density approximation (LSDA) vs the generalized gradient approximation (GGA) in density functional theory. A comparison between LSDA and GGA for the exchange-correlation functional shows that GGA is essential for an accurate description of the equilibrium volumes and of the electronic and magnetic properties of these systems. We find that both the energy gap and the spin gap increase as the X atomic number decreases. As a result of the semiconducting (metallic) character found in the minority (majority) spin band structure, the Si and Ge based alloys are predicted to be half-metallic. In contrast, ${\mathrm{Co}}_{2}\mathrm{MnSn}$ is found to be a ``nearly half-metallic'' compound, since the minority valence band maximum crosses the Fermi level. The calculated total magnetization of $5{\ensuremath{\mu}}_{B}$ is in excellent agreement with recent experiments. By including a fully self-consistent treatment of spin-orbit coupling, the GGA calculated orbital moments are shown to be very small (about $0.008{\ensuremath{\mu}}_{B}$ for Mn and about $0.02{\ensuremath{\mu}}_{B}$ for Co), showing that the quenching of the orbital magnetic moment is nearly complete. The calculated hyperfine fields, both at zero and Elevated Pressure, are compared with available experimental data, and show general agreement, except for Mn. Finally, the calculated Mn $2p$ exchange splittings, found to be in good agreement with experiment, are proportional to the Mn magnetic moments, suggesting a localized nature of ferromagnetism in these Heusler compounds.
