AVS2010 Session SE-TuA: Surface Engineering for Thermal Management

Tuesday, October 19, 2010 2:00 PM in Room Cimmaron

Tuesday Afternoon

Start	Invited?	Item
2:00 PM	Invited	SE-TuA-1 Nanometer-Scale and Interfacial Thermal Transport and Thermal Properties Characterization William P. King (University of Illinois at Urbana-Champaign) This talk discusses recent work on measurements of nanometer-scale and interfacial heat transfer as well as measurements of nanometer-scale thermophysical properties of solid materials. The research combines atomic force microscope (AFM)- based measurements, nanometer-scale thermal processing, and nanometer-scale infrared spectroscopy. In the first research thrust, an AFM cantilever probe can be used to measure thermomechanical expansions with spatial resolution smaller than 10 nm and out-of-plane displacements as small as 3 pm. Such displacements correspond to about 10 mK temperature changes. We use this technique to measure temperature distributions in graphene and carbon nanotube devices. It is possible to make a quantitative measurement of temperature rise in carbon nanoelectronic devices that are one atom thick. In the second research thrust, we use AFM cantilever probes with integrated heaters. When the AFM tip is in contact with a solid substrate, the tip-substrate contact is an ultrasmall hotspot with a diameter as small as 1 nm. This tip can be used to measure nanometer-scale temperature-dependent mechanical, chemical, and electronic properties of surfaces.
2:40 PM		SE-TuA-3 Determination of Thermal Accommodation Coefficients from Heat Transfer Measurements Between Parallel Plates Wayne Trott, John Torczynski, Michael Gallis, Daniel Rader, Jaime Castañeda (Sandia National Laboratories) Thermal accommodation coefficients have been derived for a variety of gas-surface combinations using an experimental apparatus developed to measure the pressure dependence of the conductive heat flux between parallel plates at unequal temperature separated by a gas-filled gap. The heat flux is inferred from temperature-difference measurements across the plates in a configuration where the plate temperatures are set with two carefully controlled thermal baths. Temperature-controlled shrouds provide for environmental isolation of the opposing test plates. Since the measured temperature differences in these experiments are very small (typically 0.3^o C or less over the entire pressure range), high-precision thermistors are used to acquire the requisite temperature data. High-precision components have also been utilized on the other control and measurement subsystems in this apparatus, including system pressure, gas flow rate, plate alignment, and plate positions. The apparatus also includes the capability for in situ plasma cleaning of the installed test plates. Measured heat-flux results are used in a formula based on Direct Simulation Monte Carlo (DSMC) code calculations to determine the thermal accommodation coefficients. Thermal accommodation coefficients have been determined for three different gases (argon, nitrogen, helium) in contact with various surfaces. Materials include metals and alloys such as aluminum, gold, platinum, and 304 stainless steel. A number of materials important to fabrication of Micro Electro Mechanical Systems (MEMS) devices have also been examined. For most surfaces, coefficient values are near 0.95, 0.85, and 0.45 for argon, nitrogen, and helium, respectively. Only slight differences in accommodation as a function of surface roughness have been seen. Surface contamination appears to have a more significant effect: argon plasma treatment has been observed to reduce thermal accommodation by as much as 0.10 for helium. Mixtures of argon and helium have also been examined, and the results have been compared to DSMC simulations incorporating thermal-accommodation values from single-species experiments. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
3:20 PM		BREAK
4:40 PM		SE-TuA-9 Thermal Conductance of Pt/VO₂/Pt Heterointerfaces Dong-Wook Oh, David Cahill (University of Illinois at Urbana-Champaign) The metal-insulator-transition (MIT) of VO₂ at ≈68°C allows us to systematically explore heat transport at interfaces between metals and correlated-electron systems. We use time domain thermoreflectance (TDTR) to measure interfacial thermal conductance of Pt/VO₂/Pt structures. Pt/VO₂/Pt layers are deposited by reactive dc sputtering on sapphire substrates. High throughput measurements of dependence of the conductance on the VO₂ thickness h are enabled by creating a lateral thickness gradient, 0<h<30 nm, across the width of the sample. The thermal conductance of Pt/VO₂/Pt structures with h<10 nm was found to be large (~800 MW m^-2K^-1), implying that the VO₂ layer is not planar. The thermal conductance of thicker VO₂, h>10 nm, was ~200 MW m^-2K^-1. We have not yet been able to resolve a difference in the thermal conductance between the metal and insulator phases of VO₂; the upper limit on the change in conductance at the metal-to-insulator transition is ~40 MW m^-2K^-1. This conductance implies that the specific electrical resistance of an interface between metallic VO₂ and Pt is >2´10^-8 W cm².
5:00 PM		SE-TuA-10 Synthesis and Thermoelectric Properties of RuO₂ Nanorods Denis Music, Felix H.-U. Basse (RWTH Aachen University, Germany); Jamie J. Gengler, Andrey A. Voevodin (Air Force Research Laboratory); Jochen M. Schneider (RWTH Aachen University, Germany) We have explored the effect of the O/Ru ratio on the morphology and the Seebeck coefficient of RuO₂ nanorods (space group P4₂/mnm) synthesized by reactive sputtering. At an O/Ru ratio of 1.69, a faceted surface is observed, while nanorod formation occurs at O/Ru ratios of 2.03 and 2.24. Using classical molecular dynamics with the potential parameters derived in this work, we show that volatile species enable nanorod formation. Based on ab initio calculations, two effects of the nanorod formation on the Seebeck coefficient are observed: (i) increase due to additional states in the vicinity of the Fermi level and (ii) decrease due to oxygen point defects (volatile species). These two competing effects give rise to a moderate increase of the Seebeck coefficient upon nanorod formation.
5:20 PM		SE-TuA-11 Thermal Conductivity Measurements of RuO_x Thin Films Jamie J. Gengler, Andrey A. Voevodin (Air Force Research Laboratory); Denis Music, Felix H.-U. Basse, Jochen M. Schneider (RWTH Aachen University, Germany) Thermal conductivity trends in RuO_x thin films of varying stoichiometry were characterized with a time-domain thermoreflectance (TDTR) technique. At an O/Ru ratio of x = 1.69, a faceted film surface is observed with a measured thermal conductivity value of 28.8 ± 0.8 W m^-1 K^-1. With an O/Ru ratio of x = 2.24, nanorod formation occurs. These films were grown by a reactive magnetron sputtering technique with nonrotating substrates oriented 20⁰ normal to the Ru target. Such material synthesis conditions resulted in a gradient sample structure at the onset of nanorod formation. As a result, the RuO_2.24 samples exhibited gradually changing surface roughness (rms of 12 nm – 200 nm) and thermal conductivity values (22 W m^-1 K^-1 – 5 W m^-1 K^-1), respectively. The thermal conductivity of the thin film samples studied here are all well below that of single crystal RuO₂ with tetragonal rutile structure (50 W m^-1 K^-1 [1]). The samples also have an inverse relationship of thermal conductivity with Seebeck coefficient [2], which is desirable for improving the figure of merit for thermoelectric performance. References [1] Ferizovic D, Hussey LK, Huang Y-S, Munoz M. Determination of the room temperature thermal conductivity of RuO₂ by the photothermal deflection technique. Appl Phys Lett 2009;94:131913. [2] Music D, Basse F H-U, Habdorf R, Schneider JM. Synthesis and thermoelectric properties of RuO₂ nanorods. J Appl Phys, submitted for review.
5:40 PM		SE-TuA-12 Heat Transport at Water Interfaces in the Proximity of Micro- and Nano-Structured Surfaces Shawn A. Putnam, John G. Jones (Wright Patterson Air Force Base) Breakthroughs in many of today’s advanced technologies depend on the ability to reliably dissipate enormous amounts of thermal energy (heat) from very small areas. The most demanding applications are managed with nucleate boiling-based cooling schemes (e.g. spray cooling, heat pipes, thermosyphons, flow boiling, and jet impingement ), where the cooling effectiveness is dictated by both the cooling configuration and the coolant itself. Surface features can also play a crucial role in boiling/cooling processes because they can, for example, i) increase the total wettable surface area, 2) control the bubble nucleation dynamics at the surface (e.g. vapor bubble release rate and size), and iii) change the effective surface energy (i.e., the intrinsic driving mechanism for wetting the hot surface with a coolant). Here we present our studies on thermal transport at liquid interfaces, focusing on 1) our recent experimental data and corresponding numerical simulations of water microdroplets evaporating, wetting, and bouncing on micro- and nano-structured surfaces and 2) our time-domain thermoreflectance (TDTR) experiments for the interfacial thermal conductance (G) of evaporating water microdroplets on aluminum thin-films as a function of surface temperature.