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1-7 of 7
Timothy S. English
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Journal Articles
Journal:
Journal of Heat Transfer
Article Type: Research-Article
J. Heat Transfer. September 2013, 135(9): 091103.
Paper No: HT-12-1408
Published Online: July 26, 2013
Abstract
Accurate thermal conductivity values are essential for the successful modeling, design, and thermal management of microelectromechanical systems (MEMS) and devices. However, the experimental technique best suited to measure the thermal conductivity of these systems, as well as the thermal conductivity itself, varies with the device materials, fabrication processes, geometry, and operating conditions. In this study, the thermal conductivities of boron doped single-crystal silicon microbridges fabricated using silicon-on-insulator (SOI) wafers are measured over the temperature range from 80 to 350 K. The microbridges are 4.6 mm long, 125 μm tall, and either 50 or 85 μm wide. Measurements on the 85 μm wide microbridges are made using both steady-state electrical resistance thermometry (SSERT) and optical time-domain thermoreflectance (TDTR). A thermal conductivity of 77 Wm−1 K−1 is measured for both microbridge widths at room temperature, where the results of both experimental techniques agree. However, increasing discrepancies between the thermal conductivities measured by each technique are found with decreasing temperatures below 300 K. The reduction in thermal conductivity measured by TDTR is primarily attributed to a ballistic thermal resistance contributed by phonons with mean free paths larger than the TDTR pump beam diameter. Boltzmann transport equation (BTE) modeling under the relaxation time approximation (RTA) is used to investigate the discrepancies and emphasizes the role of different interaction volumes in explaining the underprediction of TDTR measurements.
Proceedings Papers
Timothy S. English, Thomas W. Kenny, Justin L. Smoyer, Christopher H. Baker, Nam Q. Le, John C. Duda, Pamela M. Norris, Patrick E. Hopkins
Proc. ASME. HT2012, Volume 1: Heat Transfer in Energy Systems; Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Transport Phenomena in Materials Processing and Manufacturing; Heat and Mass Transfer in Biotechnology; Environmental Heat Transfer; Visualization of Heat Transfer; Education and Future Directions in Heat Transfer, 617-624, July 8–12, 2012
Paper No: HT2012-58459
Abstract
This paper investigates anharmonic phonon dispersion relations measured directly from molecular dynamics simulations at finite temperatures and pressure. The measured dynamical matrix and resulting anharmonic dispersion relations do not require an a-priori analytical expression regarding the strength of anharmonic processes. Therefore, no assumptions concerning the degree of anharmonicity are made beyond specifying an interatomic potential. We calculate phonon properties pertinent to thermal transport in graphene. Specifically, we demonstrate the calculation of phonon dispersion relations and group velocities over the entire Brillouin Zone, as well as the branch-dependent contribution to specific heat capacity and ballistic thermal conductance. We highlight the capabilities of this technique to lend fundamental insight into the anharmonic characteristics of phonon-mediated transport. Finally, we discuss how anharmonic phonon dispersion relations may be used to evaluate the differences in phonon properties between various interatomic potentials commonly used in the simulation of phonon-mediated thermal transport.
Proceedings Papers
Proc. ASME. MNHMT2012, ASME 2012 Third International Conference on Micro/Nanoscale Heat and Mass Transfer, 713-722, March 3–6, 2012
Paper No: MNHMT2012-75179
Abstract
Accurate thermal conductivity values are essential to the modeling, design, and thermal management of microelectromechanical systems (MEMS) and devices. However, the experimental technique best suited to measure thermal conductivity, as well as thermal conductivity itself, varies with the device materials, fabrication conditions, geometry, and operating conditions. In this study, the thermal conductivity of boron doped single-crystal silicon-on-insulator (SOI) microbridges is measured over the temperature range from 77 to 350 K. The microbridges are 4.6 mm long, 125 μm tall, and two widths, 50 or 85 μm. Measurements on the 85 μm wide microbridges are made using both steady-state electrical resistance thermometry and optical time-domain thermoreflectance. A thermal conductivity of ∼ 77 W/mK is measured for both microbridge widths at room temperature, where both experimental techniques agree. However, a discrepancy at lower temperatures is attributed to differences in the interaction volumes and in turn, material properties, probed by each technique. This finding is qualitatively explained through Boltzmann transport equation modeling under the relaxation time approximation.
Journal Articles
Journal:
Journal of Heat Transfer
Article Type: Technical Briefs
J. Heat Transfer. January 2012, 134(1): 014501.
Published Online: October 28, 2011
Abstract
Many random substitutional solid solutions (alloys) will display a tendency to atomically order given the appropriate kinetic and thermodynamic conditions. Such order–disorder transitions will result in major crystallographic reconfigurations, where the atomic basis, symmetry, and periodicity of the alloy change dramatically. Consequently, phonon behavior in these alloys will vary greatly depending on the type and degree of ordering achieved. To investigate these phenomena, the role of the order–disorder transition on phononic transport properties of Lennard–Jones type binary alloys is explored via nonequilibrium molecular dynamics simulations. Particular attention is paid to regimes in which the alloy is only partially ordered. It is shown that by varying the degree of ordering, the thermal conductivity of a binary alloy of fixed composition can be tuned across an order of magnitude at 10% of the melt temperature, and by a factor of three at 40% of the melt temperature.
Proceedings Papers
Proc. ASME. AJTEC2011, ASME/JSME 2011 8th Thermal Engineering Joint Conference, T30009, March 13–17, 2011
Paper No: AJTEC2011-44160
Abstract
Many random substitutional solid solutions (alloys) will display a tendency to chemically order given the appropriate kinetic and thermodynamic conditions. Such order-disorder transitions will result in major crystallographic reconfigurations, where the atomic basis, symmetry, and periodicity of the alloy change dramatically. Consequently, phonon behavior in these alloys will vary greatly depending on the type and degree of ordering achieved. To investigate these phenomena, the role of the order-disorder transition on phononic transport properties of Lennard-Jones type binary alloys is explored via non-equilibrium molecular dynamics simulations. Particular attention is paid to regimes in which the alloy is only partially ordered. It is shown that, through exploitation of the long-range order parameter, thermal conductivity of binary alloys can be effectively tuned across half an order of magnitude at low-to-moderate temperatures.
Proceedings Papers
Timothy S. English, Justin L. Smoyer, John C. Duda, Pamela M. Norris, Thomas E. Beecham, Patrick E. Hopkins
Proc. ASME. AJTEC2011, ASME/JSME 2011 8th Thermal Engineering Joint Conference, T30036, March 13–17, 2011
Paper No: AJTEC2011-44657
Abstract
This work develops a new model for calculating the thermal conductivity of polycrystalline silicon using an effective medium approach which discretizes the contribution to thermal conductivity into that of the grain and grain boundary regions. While the Boltzmann transport equation under the relaxation time approximation is used to model the grain thermal conductivity, a lower limit thermal conductivity model for disordered layers is applied in order to more accurately treat phonon scattering in the grain boundary regions, which simultaneously removes the need for fitting parameters frequently used in the traditional formation of grain boundary scattering times. The contributions of the grain and grain boundary regions are then combined using an effective medium approach to compute the total thermal conductivity. The model is compared to experimental data from literature for both undoped and doped polycrystalline silicon films. In both cases, the new model captures the correct temperature dependent trend and demonstrates good agreement with experimental thermal conductivity data from 20 to 300K.
Proceedings Papers
Proc. ASME. IHTC14, 2010 14th International Heat Transfer Conference, Volume 6, 443-448, August 8–13, 2010
Paper No: IHTC14-22953
Abstract
Thermal transport at the interface between Lennard-Jones crystals is explored via non-equilibrium molecular dynamics simulations. The vibrational properties of each crystal are varied by changing the atomic mass of the crystal. By applying a constant thermal flux across the two-crystal composite system, a steady-state temperature gradient is established and thermal boundary conductance at the interface between the crystals is calculated via Fourier’s law. With the material properties of the two crystals fixed, thermal boundary conductance can be affected by an intermediate layer inserted between the two crystals. It is found that when the interstitial layer atomic mass is between those values of the crystals comprising the interface, interfacial transport is enhanced. This layer helps bridge the gap between the different vibrational spectra of the two materials, thus enhancing thermal transport, which is maximized when the interstitial layer atomic mass approaches the average mass of the two fixed crystals. The degree of enhancement depends on the vibrational mismatch between the interstitial layer and the crystals comprising the interface, and we report an increase in thermal boundary conductance of up to 50%.