Lukes, Jennifer R

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2004 - 20107
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  • Publication
    Acoustic phonon scattering from particles embedded in an anisotropic medium: A molecular dynamics study
    (2008-03-05) Zuckerman, Neil; Lukes, Jennifer R
    Acoustic phonon scattering from isolated nanometer-scale impurity particles embedded in anisotropic media is investigated using molecular dynamics simulation. The spectral-directional dependence of the scattering, for both longitudinal and transverse modes, is found through calculation of scattering cross sections and three dimensional scattering phase functions for inclusions of varying sizes, shapes, and stiffnesses and for waves of different wave numbers. The technique enables direct observation of the effects of mode conversion, lattice mismatch strain, elastic anisotropy, and atomistic granularity on acoustic phonon scattering from nanoparticles. The results will be useful for the design of nanoparticle-based thermal insulating materials, for example, quantum dot superlattices for thermoelectric energy conversion.
  • Publication
    Molecular Dynamics Simulation of Thermal Conduction in Nanoporous Thin Films
    (2004-10-01) Lukes, Jennifer R; Tien, C. L.
    Molecular dynamics simulations of thermal conduction in nanoporous thin films are performed. Thermal conductivity displays an inverse temperature dependence for films with small pores and a much less pronounced dependence for larger pores. Increasing porosity reduces thermal conductivity, while pore shape has little effect except in the most anisotropic cases. The pores separate the film into local regions with distinctly different temperature profiles and thermal conductivities, and the effective film thermal conductivity is lowest when the pores are positioned in the center of the film. Such tunability by pore placement highlights new possibilities for engineering nanoscale thermal transport.
  • Publication
    Molecular Dynamics Study of the Lattice Thermal Conductivity of Kr/Ar Superlattice Nanowires
    (2004-06-15) Chen, Yunfei; Li, Deyu; Yang, Juekuan; Wu, Yonghua; Lukes, Jennifer R; Majumdar, Arun
    The nonequilibrium molecular dynamics (NEMD) method has been used to calculate the lattice thermal conductivities of Ar and Kr/Ar nanostructures in order to study the effects of interface scattering, boundary scattering, and elastic strain on lattice thermal conductivity. Results show that interface scattering poses significant resistance to phonon transport in superlattices and superlattice nanowires. The thermal conductivity of the Kr/Ar superlattice nanowire is only about 1/3 of that for pure Ar nanowires with the same cross sectional area and total length due to the additional interfacial thermal resistance. It is found that nanowire boundary scattering provides significant resistance to phonon transport. As the cross sectional area increases, the nanowire boundary scattering decreases, which leads to increased nanowire thermal conductivity. The ratio of the interfacial thermal resistance to the total effective thermal resistance increases from 30% for the superlattice nanowire to 42% for the superlattice film. Period length is another important factor affecting the effective thermal conductivity of the nanostructures. Increasing the period length will lead to increased acoustic mismatch between the adjacent layers, and hence increased interfacial thermal resistance. However, if the total length of the superlattice nanowire is fixed, reducing the period length will lead to decreased effective thermal conductivity due to the increased number of interfaces. Finally, it is found that the interfacial thermal resistance decreases as the reference temperature increases, which might be due to the inelastic interface scattering.
  • Publication
    Velocity and Stress Autocorrelation Decay in Isothermal Dissipative Particle Dynamics
    (2010-02-19) Chaudhri, Anuj; Lukes, Jennifer R
    The velocity and stress autocorrelation decay in a dissipative particle dynamics ideal fluid model is analyzed in this paper. The autocorrelation functions are calculated at three different friction parameters and three different time steps using the well-known Groot/Warren algorithm and newer algorithms including selfconsistent leap-frog, self-consistent velocity Verlet and Shardlow first and second order integrators. At low friction values, the velocity autocorrelation function decays exponentially at short times, shows slower-than exponential decay at intermediate times, and approaches zero at long times for all five integrators. As friction value increases, the deviation from exponential behavior occurs earlier and is more pronounced. At small time steps, all the integrators give identical decay profiles. As time step increases, there are qualitative and quantitative differences between the integrators. The stress correlation behavior is markedly different for the algorithms. The self-consistent velocity Verlet and the Shardlow algorithms show very similar stress autocorrelation decay with change in friction parameter, whereas the Groot/Warren and leap-frog schemes show variations at higher friction factors. Diffusion coefficients and shear viscosities are calculated using Green-Kubo integration of the velocity and stress autocorrelation functions. The diffusion coefficients match well-known theoretical results at low friction limits. Although the stress autocorrelation function is different for each integrator, fluctuates rapidly, and gives poor statistics for most of the cases, the calculated shear viscosities still fall within range of theoretical predictions and nonequilibrium studies.
  • Publication
    Minimum superlattice thermal conductivity from molecular dynamics
    (2005-11-01) Chen, Yunfei; Li, Deyu; Lukes, Jennifer R; Ni, Zhonghua; Chen, Minhua
    The dependence of superlattice thermal conductivity on period length is investigated by molecular dynamics simulation. For perfectly lattice matched superlattices, a minimum is observed when the period length is of the order of the effective phonon mean free path. As temperature decreases and interatomic potential strength increases, the position of the minimum shifts to larger period lengths. The depth of the minimum is strongly enhanced as mass and interatomic potential ratios of the constituent materials increase. The simulation results are consistent with phonon transmission coefficient calculations, which indicate increased stop bandwidth and thus strongly enhanced Bragg scattering for the same conditions under which strong reductions in thermal conductivity are found. When nonideal interfaces are created by introducing a 4% lattice mismatch, the minimum disappears and thermal conductivity increases monotonically with period length. This result may explain why minimum thermal conductivity has not been observed in a large number of experimental studies.
  • Publication
    Interfacial Thermal Resistance Between Carbon Nanotubes: Molecular Dynamics Simulations and Analytical Thermal Modeling
    (2006-09-01) Zhong, Hongliang; Lukes, Jennifer R
    Interfacial thermal transport between offset parallel (10,10) single-wall carbon nanotubes is investigated by molecular dynamics simulation and analytical thermal modeling as a function of nanotube spacing, overlap, and length. A four order of magnitude reduction in interfacial thermal resistance is found as the nanotubes are brought into intimate contact. A reduction is also found for longer nanotubes and for nanotubes with increased overlap area. Thermal resistance between a nanotube and a reservoir at its boundary increases with decreasing reservoir temperature. Additionally, length-dependent Young's moduli and damping coefficients are calculated based on observed nanotube deflections.
  • Publication
    Thermal expansion and impurity effects on lattice thermal conductivity of solid argon
    (2004-02-22) Chen, Yunfei; Lukes, Jennifer R; Li, Deyu; Yang, Juekuan; Wu, Yonghua
    Thermal expansion and impurity effects on the lattice thermal conductivity of solid argon have been investigated with equilibrium molecular dynamics simulation. Thermal conductivity is simulated over the temperature range of 20 – 80 K. Thermal expansion effects, which strongly reduce thermal conductivity, are incorporated into the simulations using experimentally measured lattice constants of solid argon at different temperatures. It is found that the experimentally measured deviations from a T-1 high-temperature dependence in thermal conductivity can be quantitatively attributed to thermal expansion effects. Phonon scattering on defects also contributes to the deviations. Comparison of simulation results on argon lattices with vacancy and impurity defects to those predicted from the theoretical models of Klemens and Ashegi et al. demonstrates that phonon scattering on impurities due to lattice strain is stronger than that due to differences in mass between the defect and the surrounding matrix. In addition, the results indicate the utility of molecular dynamics simulation for determining parameters in theoretical impurity scattering models under a wide range of conditions. It is also confirmed from the simulation results that thermal conductivity is not sensitive to the impurity concentration at high temperatures.