Timothy S. English

Thermal Conduction in Vertically Aligned Copper Nanowire Arrays and Composites

The ability to efficiently and reliably transfer heat between sources and sinks is often a bottleneck in the thermal management of modern energy conversion technologies ranging from microelectronics to thermoelectric power generation. These interfaces contribute parasitic thermal resistances that reduce device performance and are subjected to thermomechanical stresses that degrade device lifetime. Dense arrays of vertically aligned metal nanowires (NWs) offer the unique combination of thermal conductance from the constituent metal and mechanical compliance from the high aspect ratio geometry to increase interfacial heat transfer and device reliability. In the present work, we synthesize copper NW arrays directly onto substrates via templated electrodeposition and extend this technique through the use of a sacrificial overplating layer to achieve improved uniformity. Furthermore, we infiltrate the array with an organic phase change material and demonstrate the preservation of thermal properties. We use the 3ω method to measure the axial thermal conductivity of freestanding copper NW arrays to be as high as 70 W m(-1) K(-1), which is more than an order of magnitude larger than most commercial interface materials and enhanced-conductivity nanocomposites reported in the literature. These arrays are highly anisotropic, and the lateral thermal conductivity is found to be only 1-2 W m(-1) K(-1). We use these measured properties to elucidate the governing array-scale transport mechanisms, which include the effects of morphology and energy carrier scattering from size effects and grain boundaries.

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.

Reducing thermal conductivity of binary alloys below the alloy limit via chemical ordering

Substitutional solid solutions that exist in both ordered and disordered states will exhibit markedly different physical properties depending on their exact crystallographic configuration. Many random substitutional solid solutions (alloys) will display a tendency to 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, the dominant scattering mechanism in ordered alloys will be different than that in disordered alloys. In this study, we present a hypothesis that ordered alloys can exhibit lower thermal conductivities than their disordered counterparts at elevated temperatures. To validate this hypothesis, we investigate the phononic transport properties of disordered and ordered AB Lennard-Jones alloys via non-equilibrium molecular dynamics and harmonic lattice dynamics calculations. It is shown that the thermal conductivity of an ordered alloy is the same as the thermal conductivity of the disordered alloy at ≈0.6T(melt) and lower than that of the disordered alloy above 0.8T(melt).

Bidirectionally tuning Kapitza conductance through the inclusion of substitutional impurities

We investigate the influence of substitutional impurities on Kapitza conductance at coherent interfaces via non-equilibrium molecular dynamics simulations. The reference interface is comprised of two mass-mismatched Lennard-Jones solids with atomic masses of 40 and 120 amu. Substitutional impurity atoms with varying characteristics, e.g., mass or bond, are arranged about the interface in Gaussian distributions. When the masses of impurities fall outside the atomic masses of the reference materials, substitutional impurities impede interfacial thermal transport; on the other hand, when the impurity masses fall within this range, impurities enhance transport. Local phonon density of states calculations indicate that this observed enhancement can be attributed to a spatial grading of vibrational properties near the interface. Finally, for the range of parameters investigated, we find that the mass of the impurity atoms plays a dominant role as compared to the impurity bond characteristics.

Mean Free Path Effects on the Experimentally Measured Thermal Conductivity of Single-Crystal Silicon Microbridges

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 lm tall, and either 50 or 85 lm wide. Measurements on the 85 lm 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. [DOI: 10.1115/1.4024357]

Strategies for tuning phonon transport in multilayered structures using a mismatch-based particle model

The performance of many micro- and nanoscale devices depends on the ability to control interfacial thermal transport, which is predominantly mediated by phonons in semiconductor systems. The phonon transmissivity at an interface is therefore a quantity of interest. In this work, an empirical model, termed the thermal mismatch model, is developed to predict transmissivity at ideal interfaces between semiconductor materials, producing an excellent agreement with molecular dynamics simulations of wave packets. To investigate propagation through multilayered structures, this thermal mismatch model is then incorporated into a simulation scheme that represents wave packets as particles, showing a good agreement with a similar scheme that used molecular dynamics simulations as input [P. K. Schelling and S. R. Phillpot, J. Appl. Phys. 93, 5377 (2003)]. With these techniques validated for both single interfaces and superlattices, they are further used to identify ways to tune the transmissivity of multilayered str...

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...

Controlling Thermal Conductivity of Alloys via Atomic Ordering

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.

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

Enhancing user presence in a chest tube simulator by Joining virtual reality with a mannequin

Tube thoracostomy (chest tube insertion) is a common clinical procedure that all medical and nursing students must learn. In order to lower complication rates and improve patient safety, virtual reality (VR) training has been employed in medical education. Despite the capabilities of VR and haptic interfaces, most VR simulators offer a lesser sense of presence than mannequin-based models. To improve the users sense of presence in a VR simulator, this work combines VR with a mannequin to join a central point of physical interaction with a virtual 3-D environment. The human chest mannequin is integrated with two force feedback devices (SensAble OMNI), three haptic interfaces (pen, Kelly clamp, and finger constriction device), and a 3-D monitor. These elements are arranged to provide a greater degree of immersion as well as a more realistic user experience. By calibrating the angles of the mannequin and the 3-D monitor, the system aims to align the users physical and visual sensory perceptions. Shifting the simulator - along the VR to mannequin continuum of interaction - to a hybrid system may provide users with a greater degree of presence which can enhance learning and reduce procedural complications.