Patrick K. Schelling

Phonon wave-packet dynamics at semiconductor interfaces by molecular-dynamics simulation

We directly observe phonon wave packets of well-defined frequency and polarization scattering at a coherent semiconductor interface using molecular-dynamics simulations. We find that in the low-frequency limit the transmission coefficients of both longitudinal and transverse acoustic phonons agree well with those predicted by the continuum-level based acoustic mismatch model. However, the transmission coefficients rapidly decrease close to the cutoff frequency, a result that can be understood within a simple one-dimensional discrete atomic-chain model. We also find that the transmission coefficient for transverse acoustic phonons depends strongly on the relative orientation of the polarization and the Si-Si bonds in the diamond lattice structure.

Kapitza conductance and phonon scattering at grain boundaries by simulation

We use a nonequilibrium molecular-dynamics method to compute the Kapitza resistance of three twist grain boundaries in silicon, which we find to increase significantly with increasing grain boundary energy, i.e., with increasing structural disorder at the grain boundary. The origin of this Kapitza resistance is analyzed directly by studying the scattering of packets of lattice vibrations of well-defined polarization and frequency from the grain boundaries. We find that scattering depends strongly on the wavelength of the incident wave packet. In the case of a high-energy grain boundary, the scattering approaches the prediction of the diffuse mismatch theory at high frequencies, i.e., as the wavelength becomes comparable to the lattice parameter of the bulk crystal. We discuss the implications of our results in terms of developing a general model of scattering probabilities that can be applied to mesoscale models of heat transport in polycrystalline systems.

Thermal transport and grain boundary conductance in ultrananocrystalline diamond thin films

Although diamond has the highest known room temperature thermal conductivity, k∼2200W∕mK, highly sp3 amorphous carbon films have k<15W∕mK. We carry out an integrated experimental and simulation study of thermal transport in ultrananocrystalline diamond (UNCD) films. The experiments show that UNCD films with a grain size of 3–5nm have thermal conductivities as high as k=12W∕mK at room temperature, comparable with that of the most conductive amorphous diamond films. This value corresponds to a grain boundary (Kapitza) conductance greater than 3000MW∕m2K, which is ten times larger than that previously seen in any material. Our simulations of both UNCD and individual diamond grain boundaries yield values for the grain boundary conductance consistent with the experimentally obtained value, leading us to conclude that thermal transport in UNCD is controlled by the intrinsic properties of the grain boundaries.

Managing heat for electronics

Increasing power densities and decreasing transistor dimensions are hallmarks of modern computer chips. Both trends are increasing the thermal management challenge within the chip and surrounding packaging, as well as accelerating research progress on high-conductivity materials. This article reviews recent materials advances with a focus on novel composite substrates and interface materials, including those composites leveraging the high conductivities of carbon nanotubes. Furthermore, attention is given to the special properties of one-dimensional structures that are likely to be of increasing importance in future applications.

Optimum pyrochlore compositions for low thermal conductivity

The thermal conductivities of 40 pyrochlores with the composition A2B2O7 (A = La, Pr, Nd, Sm, Eu, Gd, Y, Er or Lu; B = Ti, Mo, Sn, Zr or Pb) are predicted by molecular dynamics simulations. The trends in the behaviour can be fully understood in terms of the differences in the density and the speed of sound in the materials. Increased structural disorder, arising from O diffusion in most of the Pb-containing systems, leads to a further reduction in the thermal conductivity. We suggest strategies for lowering the thermal conductivity even further.

Towards more accurate molecular dynamics calculation of thermal conductivity: Case study of GaN bulk crystals

Significant differences exist among literature for thermal conductivity of various systems computed using molecular dynamics simulation. In some cases, unphysical results, for example, negative thermal conductivity, have been found. Using GaN as an example case and the direct nonequilibrium method, extensive molecular dynamics simulations and Monte Carlo analysis of the results have been carried out to quantify the uncertainty level of the molecular dynamics methods and to identify the conditions that can yield sufficiently accurate calculations of thermal conductivity. We found that the errors of the calculations are mainly due to the statistical thermal fluctuations. Extrapolating results to the limit of an infinite-size system tend to magnify the errors and occasionally lead to unphysical results. The error in bulk estimates can be reduced by performing longer time averages using properly selected systems over a range of sample lengths. If the errors in the conductivity estimates associated with each of the sample lengths are kept below a certain threshold, the likelihood of obtaining unphysical bulk values becomes insignificant. Using a Monte Carlo approach developed here, we have determined the probability distributions for the bulk thermal conductivities obtained using the direct method. We also have observed a nonlinear effect that can become a source of significant errors. For the extremely accurate results presented here, we predict a [0001] GaN thermal conductivity of

Density functional theory study of water adsorption at reduced and stoichiometric ceria (111) surfaces

185\text{ }\text{W}/\text{K}\text{ }\text{m}

Scattering of g -process longitudinal optical phonons at hotspots in silicon

at 300 K,

Multiscale simulation of phonon transport in superlattices

102\text{ }\text{W}/\text{K}\text{ }\text{m}

Thermal resistivity of Si–Ge alloys by molecular-dynamics simulation

at 500 K, and