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The Cornerstone Fall 2006, Department Of Civil & Environmental Engineering
The Cornerstone Fall 2006, Department Of Civil & Environmental Engineering
The Cornerstone
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Effect Of Disorder On The Thermal Transport And Elastic Properties In Thermoelectric Zn4Sb3, S. Bhattacharya, R.P. Hermann, Veerle Keppens, T. M. Tritt, G. J. Snyder
Effect Of Disorder On The Thermal Transport And Elastic Properties In Thermoelectric Zn4Sb3, S. Bhattacharya, R.P. Hermann, Veerle Keppens, T. M. Tritt, G. J. Snyder
Faculty Publications and Other Works -- Materials Science & Engineering
Zn4Sb3 undergoes a phase transition from α to β phase at T1≈250 K. The high temperature β-Zn4Sb3 phase has been widely investigated as a potential state-of-the-art thermoelectric (TE) material, due to its remarkably low thermal conductivity. We have performed electronic and thermal transport measurements exploring the structural phase transition at 250 K. The α to β phase transition manifests itself by anomalies in the resistivity, thermopower, and specific heat at 250 K as well as by a reduction in the thermal conductivity as Zn4Sb3 changes phase from the ordered …
Direct Experimental Evidence For Atomic Tunneling Of Europium In Crystalline Eu8Ga16Ge30, Raphaël P. Hermann, Veerle Keppens, Pierre Bonville, George S. Nolas, Fernande Grandjean, Gary J. Long, Hans M. Christen, Bryan C. Chakoumakos, Brian C. Sales, David Mandrus
Direct Experimental Evidence For Atomic Tunneling Of Europium In Crystalline Eu8Ga16Ge30, Raphaël P. Hermann, Veerle Keppens, Pierre Bonville, George S. Nolas, Fernande Grandjean, Gary J. Long, Hans M. Christen, Bryan C. Chakoumakos, Brian C. Sales, David Mandrus
Faculty Publications and Other Works -- Materials Science & Engineering
Mössbauer-effect and microwave absorption experimental evidence unambiguously demonstrates the presence of slow, ∼450 MHz, tunneling of magnetic europium between four equivalent sites in Eu8Ga16Ge30, a stoichiometric clathrate. Remarkably, six of the eight europium atoms, or 11% of the constituents in this solid, tunnel between these four sites separated by 0.55 Å. The off centering of the atoms or ions in crystalline clathrates appears to be a promising route for producing Rabi oscillators in solid-state materials.
Magnetic And Electronic Properties Of Eu4Sr4Ga16Ge30, G.T. Woods, J. Martin, M. Beekman, Raphaël P. Hermann, Fernande Grandjean, Veerle Keppens, O. Leupold, Gary J. Long, G.S. Nolas
Magnetic And Electronic Properties Of Eu4Sr4Ga16Ge30, G.T. Woods, J. Martin, M. Beekman, Raphaël P. Hermann, Fernande Grandjean, Veerle Keppens, O. Leupold, Gary J. Long, G.S. Nolas
Faculty Publications and Other Works -- Materials Science & Engineering
Magnetization, static and ac magnetic susceptibility, nuclear forward scattering, and electrical resistivity measurements have been performed on polycrystalline Eu4Sr4Ga16Ge30, a type I clathrate that has divalent strontium and europium ions encapsulated within a Ga-Ge framework. These data are compared with those of type I clathrates Eu8Ga16Ge30 and Eu6Sr2Ga16Ge30. The ferromagnetic ordering of these Eu-containing clathrates is substantially altered by the incorporation of strontium, as compared to Eu8Ga16Ge30. Ferromagnetism, accompanied by a relatively large …
Structure Formation Under Steady-State Isothermal Planar Elongational Flow Of N-Eicosane: A Comparison Between Simulation And Experiment, David J. Keffer, T. C. Ionescu, C. Baig, B. J. Edwards, A. Habenschuss
Structure Formation Under Steady-State Isothermal Planar Elongational Flow Of N-Eicosane: A Comparison Between Simulation And Experiment, David J. Keffer, T. C. Ionescu, C. Baig, B. J. Edwards, A. Habenschuss
Faculty Publications and Other Works -- Chemical and Biomolecular Engineering
We use nonequilibrium molecular dynamics simulations to investigate the structural properties of an oriented melt of n-eicosane under steady-state planar elongational flow. The flow-induced structure was evaluated using the structure factor s(k) taken as the Fourier transform of the total pair correlation function g(r). We found that the equilibrium liquid structure factor is in excellent agreement with the one determined via x-ray diffraction. Moreover, a new x-ray diffraction experiment has been performed on a crystalline n-eicosane sample. The resulting intramolecular contribution to the structure factor was found to be in very good agreement with the simulated one at …
Rheological And Structural Studies Of Linear Polyethylene Melts Under Planar Elongational Flow Using Nonequilibrium Molecular Dynamics Simulations, David J. Keffer, C. Baig, B. J. Edwards, H. D. Cochran, V. A. Harmandaris
Rheological And Structural Studies Of Linear Polyethylene Melts Under Planar Elongational Flow Using Nonequilibrium Molecular Dynamics Simulations, David J. Keffer, C. Baig, B. J. Edwards, H. D. Cochran, V. A. Harmandaris
Faculty Publications and Other Works -- Chemical and Biomolecular Engineering
We present various rheological and structural properties of three polyethylene liquids, C50H102, C78H158, and C128H258, using nonequilibrium molecular dynamics simulations of planar elongational flow. All three melts display tension-thinning behavior of both elongational viscosities, η1 and η2. This tension thinning appears to follow the power law with respect to the elongation rate, i.e., η ∼ Eb, where the exponent b is shown to be approximately −0.4 for η1 and η2. More specifically, b of η1 is shown to be …
A Validation Of The P-Sllod Equations Of Motion For Homogeneous Steady-State Flows, David J. Keffer, B. J. Edwards, C. Baig
A Validation Of The P-Sllod Equations Of Motion For Homogeneous Steady-State Flows, David J. Keffer, B. J. Edwards, C. Baig
Faculty Publications and Other Works -- Chemical and Biomolecular Engineering
A validation of the p-SLLOD equations of motion for nonequilibrium molecular dynamics simulation under homogeneous steady-state flow is presented. We demonstrate that these equations generate the correct center-of-mass trajectory of the system, are completely compatible with (and derivable from) Hamiltonian dynamics, satisfy an appropriate energy balance, and require no fictitious external force to generate the required homogeneous flow. It is also shown that no rigorous derivation of the SLLOD equations exists to date.
A Comparison Of Simple Rheological Models And Simulation Data Of N-Hexadecane Under Shear And Elongational Flows, David J. Keffer, C. Baig, B. Jiang, B. J. Edwards, H. D. Cochran
A Comparison Of Simple Rheological Models And Simulation Data Of N-Hexadecane Under Shear And Elongational Flows, David J. Keffer, C. Baig, B. Jiang, B. J. Edwards, H. D. Cochran
Faculty Publications and Other Works -- Chemical and Biomolecular Engineering
The microscopic origins of five rheological models are investigated by comparing their predictions for the conformation tensor and stress tensor with the same tensors obtained via nonequilibrium molecular dynamics simulations for n-hexadecane. Steady-state simulations were performed under both planar Couette and planar elongational flows, and the results of each are compared with rheological model predictions in the same flows, without any fitting parameters where possible. The use of the conformation tensor for comparisons between theory and experiment/simulation, rather than just the stress tensor, allows additional information to be obtained regarding the physical basis of each model examined herein. The character …