ICMCTF2010 Session TS4: Surface Engineering for Thermal Transport, Storage, and Harvesting

Monday, April 26, 2010 1:30 PM in Room Sunrise

Monday Afternoon

Start	Invited?	Item
1:30 PM	Invited	TS4-1 Thermoelectric Energy Conversion in Multilayers and Embedded Nanoparticle Materials Ali Shakouri (University of California at Santa Cruz) In this talk we will review the trade offs between electrical conductivity, Seebeck coefficient and thermal conductivity. We describe how these parameters are related to the bandstructure and the transport of various phonon modes. Theoretical calculations show that superlattices, embedded nanoparticles and hot electron filtering can improve the thermoelectric energy conversion. Novel metal-semiconductor nanocomposites are developed to modify the transport of both electrons and phonons. Theory and experiment are compared for a series of samples based on rare-earth nanoparticles in III-V semiconductor matrix as well in nitride metal/semiconductor multilayer films. Potential to reach energy conversion efficiencies exceeding 15-20% is discussed. We also describe how similar principles can be used to make micro refrigerators on a chip and remove hot spots in integrated circuits. In this case the three dimensional heat and current spreading plays an important role.
2:10 PM		TS4-3 Thermal Conductivity in Anisotropic Thin Film Materials Christopher Muratore (Air Force Research Laboratory); Jianjun Hu (UDRI/Air Force Research Laboratory); Jamie Gengler (Spectral Energies/Air Force Research Laboratory); Ryan McLaren (University of Illinois Urbana-Champaign); Shawn Putnam (Air Force Research Labs/Universal Technology Corp.); William King, David Cahill (University of Illinois Urbana-Champaign); Andrey Voevodin (Air Force Research Laboratory) Materials with hexagonal crystal structures, such as graphite, molybdenum disulfide and boron nitride are composed of atomic lamellae. Crystals comprised of these materials possess markedly different physical properties along these atomic planes compared to those measured across atomic layers due to the nature of the intra- versus inter planar chemical bonding. For example, in MoS₂, each layer consists of two planes of sulfur atoms and an intermediate plane of molybdenum atoms, all covalently bonded. The layers themselves, however, are held together by weaker Van der Waals bonds. Thus, the shear strength, surface energy and electrical conductivity all vary by orders of magnitude, depending on crystal orientation. Such crystallographically anisotropic compounds also offer an opportunity to study fundamentals of thermal conductivity in crystals, as well as applications for directing or spreading heat within a compositionally homogeneous material. To this end, plasma-based deposition methods to control the c-axis orientation of MoS₂ and BN films with thicknesses of 0.1 – 1 mm were identified. The orientation of MoS₂ films was dictated primarily by deposition rate during pulsed magnetron sputtering of a MoS₂ target. The intensity of (002) peaks increased with deposition rate and also ion energy. Boron nitride films were oriented by mixing hydrogen with the reactive nitrogen gas while dc sputtering a pure boron target. Increasing the hydrogen flow promoted (002) orientation of BN films, presumably by increasing the etch rate of (100) crystals during competitive growth of both orientations. Amorphous films and films with mixed orientations were also examined. Laser thermoreflectance-based techniques were used to measure the through-thickness thermal conductivity of the films. The thermal conductivity was dependent on the orientation and thickness of the films for both materials. In initial experiments, thermal conductivity was as low as 0.2 W m^-1K^-1 through (002) oriented MoS₂, and as high as 1.4 along the 100 planes. For 100 nm thick boron nitride films, thermal conductivity went from 0.7 for the 002 orientation to over 5 W m^-1K^-1 for the 001 films. The measured values of thermal conductivity were compared to values predicted by molecular dynamics models and models in the literature presented by Slack and others to gain insight with respect to mechanisms of phonon transport in anisotropic crystalline thin film materials.
2:30 PM		TS4-4 Time Domain Thermoreflectance and 3-Omega Comparison Studies of Polymer-Metallic-Ceramic Nanolaminate Coatings Adam Waite (Air Force Research Labs/Universal Technology Corp.); Jamie Gengler (Spectral Energies, LLC); John Jones (Air Force Research Labs); Christopher Muratore, Andrey Voevodin (Air Force Research Laboratory) Multilayered polymer-metal-ceramic nanolaminate coatings were grown by room temperature plasma enhanced chemical vapor deposition (PECVD) and magnetron sputtering processes in a dual chamber PVD-CVD system to examine optical coatings with tailored, through-thickness thermal conductivity. Highly cross-linked fluoropolymer films were grown by PECVD from an octafluorocyclobutane gas precursor. High refractive index ceramic layers were deposited by pulsed DC magnetron sputtering of a TiO₂ target. Thin (5-50 nm) silver interlayers with thicknesses on the order of phonon mean free paths were also integrated into the nanolaminate stack. The thickness and position of the layers with high and low refractive index layers could by adjusted to develop optical coatings with desired functionality for different wavelengths of incident light, while metal layers were integrated to distribute heat and eliminate decomposition of the polymer films during heating by incident light. The through-thickness thermal conductivity of the films with and without the integrated silver layers was compared by time domain thermoreflectance (TDTR) and 3-Omega techniques. The 3-Omega analysis provided the bulk thermal conductivity of the nanolaminate stack which was compared to the constructed thermal transport model from the TDTR analysis of each film material and their respective interfaces.
2:50 PM		TS4-5 Effects of Combined Macro- and Nano-Scale Surface Structuring on Pool Boiling Chad Hunter (Air Force Research Laboratory); Shawn Putnam, Adam Waite (Air Force Research Labs/Universal Technology Corp.); Christopher Muratore, John Jones, Lois Gschwender (Air Force Research Laboratory) In cooling technologies that utilize the latent heat of vaporization (e.g. pool boiling, heat pipes, spray cooling, etc.), knowledge of the critical heat flux (CHF) is “critical”, as it dictates the maximum safe operating temperature for the actively cooled system. Interactions between the liquid coolant and the heated surface largely dictate the magnitude of the CHF, however a combination of macro- and nano-scale surface engineering has only recently been applied to control boiling phenomena. For example, at superheats beyond the critical heat flux, a continuous vapor layer forms between the liquid coolant and the solid heater substrate that impedes heat transfer. The existence of this thermally-resistive vapor layer is well known and engineering systems are designed to operate below this CHF threshold, however a clear understanding of how multi-scale manipulation of surfaces can increase the CHF is only beginning to emerge. In this work we present results on how combined macro- and nano-scale surface structuring influences the onset of the Leidenfrost point and the magnitude of the CHF. The basic mechanisms controlling formation of the resistive vapor layer are linked to surface wetting by the liquid coolant, solid-liquid contact area, spacing between features and other physical characteristics of the surface. Examples demonstrating this effect include superheats (ΔT) greater than 100˚C for unstable film boiling of water on macro-scale patterned MoS₂-Si thin films and macro-scale patterned PTFE on Cu and Si thin-film substrates of different surface energies and surface roughness. We also present CHF data for patterned Cu and Si nanowhisker substrates of different aspect ratios and packing densities
3:10 PM		TS4-6 Chromium-Containing Amorphous Hydrogenated Carbon Thin Films as Solar Selective Absorber Coatings: Effects of Hydrogen Content and Metal-Concentration Hsin-Yen Cheng, Jyh-Ming Ting (National Cheng Kung University, Taiwan) Various chromium-containing amorphous hydrogenated carbon (a-C:H/Cr) thin films were prepared and investigated for use as solar absorber coatings. The films were deposited on oxygen-free copper substrates using a reactive co-sputter deposition technique. The obtained films were characterized using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), secondary ion mass spectrometer (SIMS), X-ray absorption near edge spectroscopy (XANES), and valence-band photoemission spectroscopy (PES). The hydrogen content was measured by elastic recoil detection (ERD). The optical performance was examined using UV-Vis-NIR spectrophotometry and Fourier transform infrared spectrometry. Effects of hydrogen content and metal-concentration on the optical properties were studied. We show that a-C:H/Cr films having appropriate hydrogen contents and metal-concentrations exhibit high absorption greater than 96 % from 0.3 to 2.5 μm at room temperature and low emittance less than 1 % from 2.5 to 10 μm at 100℃.