ICMCTF2009 Session TS4: Surface Engineering for Thermal Interfaces, Heat Flow Control and Thermal Energy Management

Wednesday, April 29, 2009 8:00 AM in Room Sunset

Wednesday Morning

Start	Invited?	Item
8:00 AM	Invited	TS4-1 Thermal Conductivity Control with Thin Films D.G. Cahill (University of Illinois Urbana-Champaign) The control and management of high heat fluxes is an increasingly critical issue for a wide range of technologies. Thin film materials have a vital role to play in thermal management at both ends of the spectrum: for example, improved thermal barriers are needed to raise the operating temperatures of heat engines and high thermal conductivity coatings are needed for heat spreading and thermal interfaces. In this talk, I will discuss the basic science and current limits to the thermal conductivity of thin films and the thermal conductance of interfaces. Our experiments are enabled by recent advances in time-domain thermoreflectance (TDTR); although the equipment required for TDTR is still relatively complex, a measurement of the thermal conductivity of thin films is relatively simple and requires a level of experimental effort that is similar to using ellipsometry to measure optical constants. Carbon nanotubes may have the highest thermal conductivity of any material but the superlative thermal properties of nanotubes have not found immediate application in thermal interfaces because of difficulties in making good thermal contact with nanotubes, i.e., the thermal conductance of interfaces with nanotubes is very small. Solids that combine order and disorder in the random stacking of two-dimensional crystalline sheets, so-called “disordered layered crystals” show the lowest thermal conductivity ever observed in a fully dense solid, only a factor of 2 larger than air.
8:40 AM		TS4-3 Thermal Conductivity Anisotropy in Molybdenum Disulfide Thin Films R. McLaren, D.G. Cahill, W. King (University of Illinois Urbana-Champaign); C. Muratore (Air Force Research Lab/UTC, Inc.); J. Hu, A.A. Voevodin (Air Force Research Laboratory) Hexagonal molybdenum disulfide (MoS₂) is composed of atomic lamellae. Each layer consists of two planes of sulfur atoms and an intermediate plane of molybdenum atoms, bound with covalent bonds. The layers themselves are held together by weaker van der Waal bonds. Such crystallographically anisotropic compounds offer an opportunity to study intrinsic anisotropic thermal conductivity, which may be of interest for heat spreading or thermal isolation within a compositionally homogeneous material. To this end, MoS₂ films with thicknesses of 0.3 – 1 µm were grown by pulsed magnetron sputtering of an MoS₂ target in an argon background. The deposition rate was modulated with the duty factor of the pulsed power applied to the sputter target to control the orientation of the atomic lamellae. Conditions were selected to grow the MoS₂ films with the c-axis of the hexagonal crystals oriented perpendicular (002) or parallel (100) to the substrate, as determined by X-ray diffraction and transmission electron microscopy of film cross-sections. Amorphous films were also examined. Laser thermoreflectance was used to measure the out-of plane thermal conductivity of the films. The thermal conductivity for all of the samples was in the range 0.3-1 W/m-K. The films with (002) orientation of the MoS₂ sheets had out-of-plane thermal conductivity of 0.3-0.6 W/m-K, while films with (100) orientation had thermal conductivity 0.65-1 W/m-K. The thermal conductivity was dependent on the orientation of the films and on film thickness, with thinner films having higher thermal conductivity. Discussion of conductivity changes based on crystal anisotropy, and orientation dependence on thickness will be presented.
9:00 AM		TS4-4 Fabrication and Characterisation of Nanoscale Heating Sources (‘NanoHeaters’) for Thermal Nanomanufacturing C. Rebholz, K. Fadenberger, I.E. Gunduz, M. Kokonou, C.C. Doumanidis (University of Cyprus); T. Ando (Northeastern University); J. Chen, Z Gu (University of Massachusetts Lowell); P. Wong (Tufts University) Nanoheaters consist of nanoscale heterostructures of reactive multi-material systems, which upon controlled external ignition release instantaneous, locally singular exothermic heat to a substrate or surrounding material. This presentation overviews progress in fabrication of nanoheater systems based on the exothermic reaction between aluminum and nickel. Multi-layer nanoheater reaction thermodynamics and formation of nickel aluminides have been studied by differential scanning calorimetry tests, contributing to 3D simulations of the temperature and concentration distributions, validated by high-speed and infrared pyrometry experiments on a special fixture. Results on the self propagating reaction of Ni-Al thin film multilayers with various bilayer thicknesses (in the range of 20-100 nm) indicate a two-stage reaction with two different characteristic temperatures. Alternative ignitable bulk nanoheater fabrication was investigated through preheated ultrasonic consolidati on or mechanical alloying of bi-metallic micropowders, and also in the form of Ni-Al nanorods, deposited by electron gun evaporation in nanoporous anodized aluminium oxide membranes. Applications of nanoheaters in rapid thermal processing of semiconductors, self-heated materials, MEMS actuators, thermal nanobatteries, etc. are explored, along with nanoheater fabrication and operation safety.
9:20 AM		TS4-6 Processing and Characterization of Carbon Nanotubes on Diamond/Si Substrates for Thermal Management Applications C. Varanasi (University of Dayton Research Institute); J. Petry (Air Force Research Laboratory); J. Burke, L. Brunke (University of Dayton Research Institute); J. Bulmer, K. Yost (Air Force Research Laboratory); W. Lanter (Innovative Scientific Solutions, Inc.); J. Scofield, P. Barnes (Air Force Research Laboratory) Carbon nanotubes (CNTs) with a high thermal conductivity (~3000 W/m-K) are very attractive for their potential use in a variety of thermal management applications. Diamonds films deposited by chemical vapor deposition with a thermal conductivity >1800 W/m-K are useful to act as heat sinks in high power devices. A structure consisting of both CNTs and diamonds is explored in the present study for thermal management. Diamond films were initially grown on Si wafers at 1100°C in a 5KW microwave plasma enhanced chemical vapor deposition (PECVD) reactor using CH₄ as the carbon precursor. Diamond films on the Si as well as free standing diamond films obtained after etching off the Si substrate were used to subsequently grow CNTs. Substrates were decorated with nanoparticles of various metals (Ni, Ni-Mo, Ni-W-Fe) as catalysts for the CNT nucleation and growth. Both magnetron sputtering and pulsed laser ablation were used to deposit the catalyst nanoparticles. CNTs were then gr own on the catalyst-decorated diamond films using thermal chemical vapor deposition technique. Ni- W- Fe was found to be the most suitable to grow a high density of CNTs on the diamond. Processing parameters to deposit catalyst particles as well as the CNT growth parameters such as pressure, temperature, gas flow rate, etc. were optimized to obtain the high density. Processing details and characterization of the CNT/Diamond nanostructures, including microstructural and Raman spectroscopy data, will be discussed.
9:40 AM	Invited	TS4-7 Strain, Electronic Structure, Phonons and Thermal Properties of ScN:ZrN Superlattices: A First-Principles Study U.V. Waghmare (J Nehru Centre for Advanced Scientific Research, India) We use first-principles density functional theory-based simulations to determine electronic structure and vibrational spectra of ScN:ZrN superlattices, aiming at (a) understanding the role of interfaces and epitaxial strain in controlling their thermal and thermoelectric properties, and (b) developing models that can be employed in determination such properties of superlattices with periodicity of longer length-scales. We first obtain electronic structure and vibrational spectrum of ScN and ZrN crystals with rocksalt structure, and use them in construction of lattice dynamical models and statistical thermodynamic analysis to estimate their specific heat and thermal conductivity. Through comparison of the vibrational and electronic structure of bulk ScN and ZrN with that of their superlattices, we develop a strategy of modeling the interface between them. We use the resultant model in predicting properties of ZrN:ScN super-lattices of different periodicities and epitaxial strain. *Work done in collaboration with Bivas Saha, Jagaran Acharya and Timothy Sands.
10:20 AM		TS4-9 GaN-on-Diamond Wafers for HEMT, the Diamond Side of the Story C. Engdahl, E. Francis (Crystallume) HEMT devices fabricated on GaN-on-diamond wafers have demonstrated greatly improved thermal management properties, and have the potential to increase temperature of operation, power and RF performance. GaN-on-Diamond up to 2” in diameter have been fabricated as a thin (~ 100 µm), both freestanding and bonded on silicon wafers. GaN-on-diamond wafers present unique challenges for device fabrication. First, since diamond wafers are extremely difficult to thin post-process, the initial thickness of the wafer must equal the final desired thickness. Second, the bow of freestanding diamond wafers could be several hundred microns, which can preclude the use of standard photolithography techniques. Third any attempt to improve and adjust any physical properties of diamond should not impact the thermal conductivity of the material negatively. This paper presents the latest progress in hot filament assisted CVD diamond hardware and process necessary for integration of diamond with GaN. In this investigation, we study the successful demonstration of GaN on diamond and review and compare the results with that of GaN on silicon. The effects of process conditions on the characteristics such as stress and thermal conductivity are described. The as-deposited diamond films were characterized by stress and morphology. The GaN on diamond stacks were evaluated optically and electrically. A steady-state technique was used to measure the thermal conductivity of the deposited and free standing diamond and the GaN/Diamond layers.
10:40 AM		TS4-11 Modeling of Anisotropic Thermal Transport Behavior in Molybdenum Disulphide (MoS₂) V. Varshney (Air Force Research Laboraotry/RXBT); S. Patnaik (Air Force Research Laboratory); A. Roy (Air Force Research Laboratories/RXBT); A.A. Voevodin (Air Force Research Lab/University of Dayton); B. Farmer (Air Force Research Laboratory) In this study, we present an investigation of the anisotropic nature of heat transport in Molybdenum Disulphide (MoS₂) crystallites using atomistic molecular dynamics simulations. The motivation for this work originates from recent findings of variation in the in-plane (parallel to the substrate) thermal conductivity of MoS₂ coating depending on the relative orientations of basal planes of MoS₂ crystallites at the surface (ratio of parallel and perpendicular orientation of basal planes). The objective of this modeling work is to provide deeper insight into the thermal transport behavior of MoS₂ . The anisotropic thermal conductivity along the basal plane and normal to the basal plane will be shown followed by a systematic study of thermal conductivity estimates of crystallites with different concentration of parallel and perpendicular basal planes. A model parameter set from literature, based on empirical force-fields, has been developed to reproduce crystal structure o f MoS₂.The parameter set will be further verified and refined to reproduce experimentally known thermal properties of MoS₂ such as CTE, heat capacity etc. Equilibrium molecular dynamics simulations based on Green-Kubo formalism to estimate thermal conductivities of MoS₂ and its dependence on possible orthogonal orientations of basal planes will be presented. This will provide further insight into interfacial thermal resistance at the grain boundaries of the crystallites.
11:00 AM		TS4-13 Models of Thermal Condutivity of Multilayer Coatings for Cutting Applications L. Braginsky, V. Shklover (ETH Zürich, Switzerland); A. Gusarov (ENISE, France) The applicability of two earlier elaborated by us models of thermal conductivity to the multilayer coatings for cutting applications is discussed. First is the hopping model, which has been elaborated to consider the phonon hopping between the grains in the nanocrystalline and grained coatings. The model uses the structure parameters of the coating: the grain size, the size of the necks between the grains, coordination number, and cohesion. Transparency of the grains for the phonon hopping is the only adjustable parameter used in the model. Good agreement with experimental data for nanocrystalline metal oxide coatings, where the phonon mean free path is larger or comparable to the grain size has been shown. The second model was suggested for the porous coatings, where the phonon mean free path is much smaller than the pore size. The model permits us also to estimate the thermal conductivity of the coating using its optical or SEM image. Two-dimensional image of the coating can be used to consider the three-dimensional problem of the heat transport. Interesting results were obtained for the use of the model, combining both models. This approach is particularly important for modeling thermal conductivity of the coatings where the defects of two or more different scales are present. Temperature deviations in the multilayer coatings due to roughness of the layers or local thermal gradients will be considered. Being large, such deviations can destroy the coating owing to the temperature stress. We discuss also some other aspects related to thermal conductivity of multilayer coatings, which can be used to ensure anisotropic heat spreading in the cutting instruments.