Justin L. Smoyer

Extension of the diffuse mismatch model for thermal boundary conductance between isotropic and anisotropic materials

This model is an extension of the diffuse mismatch model (DMM), tailored to accurately predict thermal boundary conductance (hBD) at interfaces where one material comprising the interface is characterized by high elastic anisotropy. Temperature-dependent specific heat is calculated with this vibrational model and compared to published values. Modifications to the DMM that incorporate the vibrational model are presented with predictions of hBD at a metal-graphite interface. This model slightly underestimates experimental data, as expected, as the large acoustic mismatch between metals and graphite suggests inelastic scattering, something the DMM does not take into account.

Role of dispersion on phononic thermal boundary conductance

The diffuse mismatch model (DMM) is one of the most widely implemented models for predicting thermal boundary conductance at interfaces where phonons dominate interfacial thermal transport. In the original presentation of the DMM, the materials comprising the interface were described as Debye solids. Such a treatment, while accurate in the low temperature regime for which the model was originally intended, is less accurate at higher temperatures. Here, the DMM is reformulated such that, in place of Debye dispersion, the materials on either side of the interface are described by an isotropic dispersion obtained from exact phonon dispersion diagrams in the [100] crystallographic direction. This reformulated model is applied to three interfaces of interest: Cr–Si, Cu–Ge, and Ge–Si. It is found that Debye dispersion leads to substantially higher predictions of thermal boundary conductance. Additionally, it is shown that optical phonons play a significant role in interfacial thermal transport, a notion not pre...

On the Assumption of Detailed Balance in Prediction of Diffusive Transmission Probability During Interfacial Transport

Models intended to predict interfacial transport often rely on the principle of detailed balance when formulating the interfacial carrier transmission probability. However, assumptions invoked significantly impact predictions. Here, we present six derivations of the transmission probability, each subject to a different set of preliminary assumptions regarding the type of scattering at the interface. Application of each case to phonon flux and thermal boundary conductance allows for a final quantitative comparison. Depending on the preliminary assumptions, predictions for thermal boundary conductance span over two orders of magnitude, demonstrating the need for transparency when assessing the accuracy of any predictive model.

Prediction and measurement of thermal transport across interfaces between isotropic solids and graphitic materials.

Due to the high intrinsic thermal conductivity of carbon allotropes, there have been many attempts to incorporate such structures into existing thermal abatement technologies. In particular, carbon nanotubes (CNTs) and graphitic materials (i.e., graphite and graphene flakes or stacks) have garnered much interest due to the combination of both their thermal and mechanical properties. However, the introduction of these carbon-based nanostructures into thermal abatement technologies greatly increases the number of interfaces per unit length within the resulting composite systems. Consequently, thermal transport in these systems is governed as much by the interfaces between the constituent materials as it is by the materials themselves. This paper reports the behavior of phononic thermal transport across interfaces between isotropic thin films and graphite substrates. Elastic and inelastic diffusive transport models are formulated to aid in the prediction of conductance at a metal-graphite interface. The temperature dependence of the thermal conductance at Au-graphite interfaces is measured via transient thermoreflectance from 78 to 400 K. It is found that different substrate surface preparations prior to thin film deposition have a significant effect on the conductance of the interface between film and substrate.

Effects of intra- and interband transitions on electron-phonon coupling and electron heat capacity after short-pulsed laser heating.

This work considers the effects of intra- and direct interband transitions on electron heat capacity and the electron-phonon coupling factor in metals. In the event of an interband transition, the population of the electron bands around the Fermi level will change, affecting the electron density of states and subsequently the thermophysical properties. In the event of photon-induced interband transitions, this repopulation can occur even at relatively low temperatures. This introduces a photon energy-dependent population term into density of states calculations when examining the effects of direct interband transitions on electron heat capacity and the electron-phonon coupling factor in temperature regimes where traditionally only intraband transitions are considered. Example calculations are shown for copper and the two-temperature model is solved using the interband-dependent values of electron heat capacity and electron-phonon coupling factor to examine the effect of these transitions on the transient electron temperature after short-pulsed laser heating.

Effects of subconduction band excitations on thermal conductance at metal-metal interfaces

Increased power densities combined with the decreased length scales of nanosystems give rise to large thermal excitations that can drastically affect the electron population near the Fermi surface. In light of such conditions, a model is developed for electron thermal boundary conductance (eTBC) that accounts for significant changes in the electron and hole populations around the Fermi level that occur at heightened temperatures. By including the contribution of subconduction band electrons to transport and evaluating the transmission coefficient based upon the total number of available states, an extension of eTBC predictions to high temperatures is made possible.

Ultrafast thermoelectric properties of gold under conditions of strong electron-phonon nonequilibrium

The electronic scattering rates in metals after ultrashort pulsed laser heating can be drastically different than those predicted from free electron theory. The large electron temperature achieved after ultrashort pulsed absorption and subsequent thermalization can lead to excitation of subconduction band thermal excitations of electron orbitals far below the Fermi energy. In the case of noble metals, which all have a characteristic flat d-band several electron volts well below the Fermi energy, the onset of d-band excitations has been shown to increase electron-phonon scattering rates by an order of magnitude. In this paper, we investigate the effects of these large electronic thermal excitations on the ultrafast thermoelectric transport properties of gold, a characteristic noble metal. Under conditions of strong electron-phonon nonequilibrium (relatively high electron temperatures and relatively low lattice temperatures, Te⪢TL), we find that the Wiedemann–Franz law breaks down and the Seebeck coefficien...

Modeling Grain Boundary Scattering and Thermal Conductivity of Polysilicon Using an Effective Medium Approach

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.Copyright

Brief Historical Perspective in Thermal Management and the Shift Toward Management at the Nanoscale

ABSTRACT Since the early days of computing, excess heat has been a major road block in the design and development of faster, more efficient, and more compact electronic devices. Coupled with improvements in thermal management has been a reduction in the size of major electronic components, primarily transistors. This has expanded the field of thermal management down into the nanoscale, where the “rules” of thermal transport become more complicated. This paper presents a brief perspective on the historical challenges in thermal management and outlines the major length scale regimes where thermal management is being developed. In particular, the expansion of thermal management into the nanoscale is presented due to the consequence that as the feature size of most nano-devices continues to diminish, the impact of thermal transport across solid-solid interfaces on device performance, reliability, and lifetime becomes increasingly important.

Anharmonic Phonon Dispersion Relations, Group Velocities, and Branch-Dependent Specific Heat Capacities Measured Directly From Molecular Dynamics Simulations at Finite Temperatures

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.Copyright