Ballistic Phonon Penetration Depth in Amorphous Silicon Dioxide.

Thermal transport in amorphous silicon dioxide (a-SiO2) is traditionally treated as random walks of vibrations owing to its greatly disordered structure, which results in a mean free path (MFP) approximately the same as the interatomic distance. However, this picture has been debated constantly and in view of the ubiquitous existence of thin a-SiO2 layers in nanoelectronic devices, it is imperative to better understand this issue for precise thermal management of electronic devices. Different from the commonly used cross-plane measurement approaches, here we report on a study that explores the in-plane thermal conductivity of double silicon nanoribbons with a layer of a-SiO2 sandwiched in-between. Through comparing the thermal conductivity of the double ribbon samples with that of corresponding single ribbons, we show that thermal phonons can ballistically penetrate through a-SiO2 of up to 5 nm thick even at room temperature. Comprehensive examination of double ribbon samples with various oxide layer thicknesses and van der Waals bonding strengths allows for extraction of the average ballistic phonon penetration depth in a-SiO2. With solid experimental data demonstrating ballistic phonon transport through a-SiO2, this work should provide important insight into thermal management of electronic devices.

Tunable Mechanochemistry of Lithium Battery Electrodes.

The interplay between mechanical strains and battery electrochemistry, or the tunable mechanochemistry of batteries, remains an emerging research area with limited experimental progress. In this report, we demonstrate how elastic strains applied to vanadium pentoxide (V2O5), a widely studied cathode material for Li-ion batteries, can modulate the kinetics and energetics of lithium-ion intercalation. We utilize atomic layer deposition to coat V2O5 materials onto the surface of a shapememory superelastic NiTi alloy, which allows electrochemical assessment at a fixed and measurable level of elastic strain imposed on the V2O5, with strain state assessed through Raman spectroscopy and X-ray diffraction. Our results indicate modulation of electrochemical intercalation potentials by ∼40 mV and an increase of the diffusion coefficient of lithium ions by up to 2.5-times with elastic prestrains of <2% imposed on the V2O5. These results are supported by density functional theory calculations and demonstrate how mechanics of nanomaterials can be used as a precise tool to strain engineer the electrochemical energy storage performance of battery materials.

Thermal measurements using ultrasonic acoustical pyrometry.

Reflections from geometric discontinuities can be used with ultrasonic energy to predict the temperature of an interface where classical temperature measurement techniques are impractical because of physical access limitations or harsh environmental conditions. Additionally, these same ultrasonic measurements can be used with inversion methods commonly applied to ill-posed heat transfer problems to increase the accuracy of the measurement of surface temperature or heat flux at the surface of interest. Both methods for determining surface temperature are presented, along with a comparison of results both from a verification example and using data gathered in a field test of the methods. The results obtained with these two methods are shown to be in good agreement with an empirical relationship used in the design of large caliber guns.

Probing and controlling photothermal heat generation in plasmonic nanostructures.

In the emerging field of thermoplasmonics, Joule heating associated with optically resonant plasmonic structures is exploited to generate nanoscale thermal hotspots. In the present study, new methods for designing and thermally probing thermoplasmonic structures are reported. A general design rationale, based on Babinet's principle, is developed for understanding how the complementary version of ideal electromagnetic antennae can yield efficient nanoscale heat sources with maximized current density. Using this methodology, we show that the diabolo antenna is more suitable for heat generation compared with its more well-known complementary structure, the bow-tie antenna. We also demonstrate that highly localized and enhanced thermal hot spots can be realized by incorporating the diabolo antenna into a plasmonic lens. Using a newly developed thermal microscopy method based on the temperature-dependent photoluminescence lifetime of thin-film thermographic phosphors, we experimentally characterize the thermal response of various antenna and superstructure designs. Data from FDTD simulations and the experimental temperature measurements confirm the validity of the design rationale. The thermal microscopy technique, with its robust sensing method, could overcome some of the drawbacks of current micro/nanoscale temperature measurement schemes.

Transient measurements using thermographic phosphors.

The decay of thermographic phosphors has been used to measure temperature in a wide variety of applications. Because measurements of a single temperature are obtained from intensity decay in time, the use of phosphors is predicated on the fact that the temperature does not change during the decay. This may not be valid in some engineering applications. A new model for phosphor data reduction designed to recover transient effects is presented. A heated wire experiment is used to determine the efficacy of the approach. Results for a particular microsecond phosphor indicate that transients can be resolved, but not to a great deal of accuracy. Nevertheless, the steady model predicted temperatures that were 10 degrees C off compared to the transient model during high heating.