The percolating conductance of a new class of nanocomposite thin-film transistors, with channels composed of isotropic ensembles of nanotubes or nanowires, is analyzed as a function of wire/tube density and channel length. The conductance exponents are validated against analytical results for short channel transistors, and against available experimental data for longer channel devices. Our plots of conductance exponents as a function of tube-to-tube coupling strength provide a unified framework to interpret future experiments and should help design better nanocomposite transistors.
An evaporating meniscus in a microchannel is investigated through an augmented Young-Laplace model and the kinetic theory-based expression for mass transport across a liquid-vapor interface. The complete expression for mass transport is employed without any approximations and boundary conditions for the film profile are developed. The thin-film and the intrinsic-meniscus regions are distinguished based on the disjoining pressure variation along the meniscus. While heat transfer in the thin-film region is found to be relatively insensitive to channels larger than a few micrometers in radius, that in the intrinsic meniscus is quite sensitive to channel size. The role of evaporation suppression due to capillary pressure in both regions is discussed. Compared to the relatively small contribution to overall heat transfer from the thin-film region, the micro region (defined here as extending from the non-evaporating region to a location where the film is 1 m thick) is found to account for more than 50% of the total heat transfer.
Thermal challenges in next-generation electronic systems, as identified through panel presentations and ensuing discussions at the workshop, Thermal Challenges in Next Generation Electronic Systems, held in Santa Fe, NM, January 7-10, 2007, are summarized in this paper. Diverse topics are covered, including electrothermal and multiphysics codesign of electronics, new and nanostructured materials, high heat flux thermal management, site-specific thermal management, thermal design of next-generation data centers, thermal challenges for military, automotive, and harsh environment electronic systems, progress and challenges in software tools, and advances in measurement and characterization. Barriers to further progress in each area that require the attention of the research community are identified.
Using the linearized Boltzmann transport equation and perturbation theory, we analyze the reduction in the intrinsic thermal conductivity of few-layer graphene sheets accounting for all possible three-phonon scattering events. Even with weak coupling between layers, a significant reduction in the thermal conductivity of the out-of-plane acoustic modes is apparent.The main effect of this weak coupling is to open many new three-phonon scattering channels that are otherwise absent in graphene. However, reflection symmetry is only weakly broken with the addition of multiple layers, and ZA phonons still dominate thermal conductivity. We also find that reduction in thermal conductivity is mainly caused by lower contributions of the higher-order overtones of the fundamental out-of-plane acoustic mode. The results compare remarkably well over the entire temperature range with measurements of graphene and graphite.
A numerical model is developed for the evaporating liquid meniscus in wick microstructures under saturated vapor conditions. Four different wick geometries representing common wicks used in heat pipes, viz., wire mesh, rectangular grooves, sintered wicks and vertical microwires, are modeled and compared for evaporative performance. The solid-liquid combination considered is copper-water. Steady evaporation is modeled and the liquid-vapor interface shape is assumed to be static during evaporation. Liquid-vapor interface shapes in different geometries are obtained by solving the Young-Laplace equation using Surface Evolver. Mass, momentum and energy equations are solved numerically in the liquid domain, with the vapor assumed to be saturated. Evaporation at the interface is modeled by using heat and mass transfer rates obtained from kinetic theory. Thermocapillary convection due to non-isothermal conditions at the interface is modeled for all geometries and its role in heat transfer enhancement from the interface is quantified for both low and high superheats. More than 80% of the evaporation heat transfer is noted to occur from the thin-film region of the liquid meniscus. The very small Capillary and Weber numbers resulting from the small fluid velocities near the interface for low superheats validate the assumption of a static liquid meniscus shape during evaporation. Solid-liquid contact angle, wick porosity, solid-vapor superheat and liquid level in the wick pore are varied to study their effects on evaporation from the liquid meniscus.
Raman spectroscopy has been widely used to measure thermal conductivity (κ) of 2D materials such as graphene. This method is based on a well-accepted assumption that different phonon polarizations are in near thermal equilibrium. However, in this work we show that in laser irradiated single layer graphene, different phonon polarizations are in strong non-equilibrium, using predictive simulations based on first principles density functional perturbation theory (DFPT) and a multi-temperature model. We first calculate the electron cooling rate due to phonon scattering as a function of the electron and phonon temperatures, and the results clearly illustrate that optical phonons dominate the hot electron relaxation process. We then use these results in conjunction with the phonon scattering rates computed using perturbation theory to develop a multi-temperature model, and resolve the spatial temperature distributions of the energy carriers in graphene under steady state laser irradiation. Our results show that electrons, optical phonons, and acoustic phonons are in strong non-equilibrium, with the ZA phonons showing the largest non-equilibrium to other phonon modes, mainly due to their weak coupling to other carriers in suspended graphene. Since ZA phonons are the main heat carriers in graphene, we estimate that neglecting this non-equilibrium leads to underestimation of thermal conductivity in experiments at room temperature by a factor of 1.35 to 2.6, depending on experimental conditions and assumptions used. Underestimation is also expected in Raman measurements of other 2D materials when the opticalacoustic phonon coupling is weak.
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