ICMCTF2011 Session TS3: Surface Engineering for Thermal Transport, Storage, and Harvesting

Wednesday, May 4, 2011 1:30 PM in Room Royal Palm 1-3
Wednesday Afternoon

Time Period WeA Sessions | Abstract Timeline | Topic TS3 Sessions | Time Periods | Topics | ICMCTF2011 Schedule

Start Invited? Item
1:30 PM Invited TS3-1 New Approaches with Organic and Inorganic Films for Thermal Energy Conversion
Max Shtein, Kevin Pipe (University of Michigan); Yansha Jin, Huarui Sun (Michigan State University); Abhishek Yadav (University of Michigan)
Efficient solid-state thermal energy conversion via the thermoelectric effect requires a combination of large Seebeck coefficient, low thermal conductivity, and high electrical conductivity. An additional avenue for performance improvement involves device engineering to enable, for example, substantial increase in the number of thermocouple junction density, flexibility, and cost-effectiveness. In this talk I will discuss strategies for controlling thermal and electrical conductivity in hybrid organic-inorganic materials via interface engineering, and scalable fabrication methods. Time-permitting, I will describe some novel architectures for cost-effective and versatile, textile-integrated thermoelectric generators.
2:10 PM TS3-3 Effects of Ni Diffusion Barrier on CNT Growth on Metal Foils for Thermal Interface Applications
Olalekan Adewuyi, Anuradha Bulusu, Samuel Graham, Baratunde Cola (Georgia Institute of Technology)

Thermal interface materials (TIMs) based on carbon nanotube (CNT) arrays are attractive for thermal management of high-power microelectronic devices. However, growth of CNTs using chemical vapor deposition (CVD) requires temperatures that are usually too high for direct growth on devices. A promising approach to circumvent this integration challenge is to grow CNTs directly on thin, flexible metal foils to produce CNT-based TIMs that can be inserted between the device and heat sink. We simultaneously grew CNT arrays on both sides of thin Al and Cu foils using a trilayer catalyst (30 nm Ti/10 nm Al/3 nm Fe), and thermal CVD with H2 and C2H2 as process gases. The G/D ratios of CNT arrays produced with this catalyst stack were measured with Raman spectroscopy to be 0.9925 on average. Poor adhesion and inconsistent sample-to-sample CNT coverage were qualitatively observed. X-tray photoelectron spectroscopy (XPS) analyses of the catalyzed metallic foils annealed at the CNT growth temperature showed significant amounts of Cu and Al at the surface. This suggests that atoms of the metal foils diffuse through the catalyst stack during CNT growth, thereby changing the structure and chemistry of the catalyst. The catalyst stack was modified by adding 20 to 100 nm of Ni film directly on the thin metal foil substrates to serve as a diffusion barrier before depositing the trilayer catalyst. CNT coverage and sample-to-sample consistency improved significantly with the inclusion of Ni barrier. When the barrier was at least 50 nm thick XPS analysis revealed no trace of Cu or Al atoms near the surface of the trilayer catalyst stack. Use of the Ni barrier increased G/D ratios by 8 percent and lead to significant qualitative improvements to the CNT-substrate adhesion (i.e., the CNTs were more difficult to scratch from the foil substrate)as well as reduced thermal interface resistance in comparison to CNT-foil TIMs fabricated without the Ni barrier.

2:30 PM TS3-4 Carbon Nanotube-Coated Foils as Low-Resistance Thermal Interface Materials
Stephen Hodson (Purdue University & Birck Nanotechnology Center); Anuradha Bulusu (Georgia Institute of Technology); Joseph Wasniewski, David Altman (Raytheon Integrated Defense Systems); Baratunde Cola, Samuel Graham (Georgia Institute of Technology); Xianfan Xu (Purdue University & Birck Nanotechnology Center); Anurag Gupta (Raytheon Integrated Defense Systems); Timothy Fisher (Purdue University & Air Force Research Laboratory)

Because of their extraordinarily high thermal conductivities and mechanical conformability, carbon nanotubes (CNTs) offer a compelling alternative to traditional thermal interface materials in electronics packages. We note that the conformability feature is particularly advantageous in addressing CTE mismatch under extreme thermal conditions encountered in advanced electronics applications. Prior results for dry CNT array thermal interface materials compare very favorably with state-of-the-art thermal greases and other non-bonded materials. In this work, we report the thermal interface behavior of CNT-coated Cu foils and the commensurate effects of a wide range of synthesis parameters. The results indicate that reasonably low resistance is possible for unbonded form, but when bonded with metals by either diffusion bonding or solder reflow, the performance improves markedly to levels below 10 mm2K/W as measured by both photoacoustic and steady reference bar techniques. General observations from parametric variations include the effect of CNT array length, in which short CNTs produce superior performance.

2:50 PM TS3-5 Crystalline Thin Film Materials with Anisotropic Thermal Conductivity
Chris Muratore (Air Force Research Laboratory); Vikas Varshney (Air Force Research Laboratory/UTC); Jamie Gengler (Air Force Research Laboratory/Spectral Energies); Jianjun Hu, John Bultman (Air Force Research Laboratory/UDRI); Timothy Smith, Andrey Voevodin (Air Force Research Laboratory)
Transition metal dichalcogenide (TMD) crystals are characterized by their distinct layered atomic structures, with strong covalent bonds comprising each layer, but weak van der Waals forces holding the layers together. The relationship between chemical bonding in a material and its thermal conductivity (k) is well-known, however the thermal properties of TMD thin films with such highly anisotropic chemical bonds have only recently been investigated with remarkable results, such as ultra-low kz. Materials with very low thermal conductivity in the z-axis, but higher kx and ky have potential as next-generation thermal barrier or heat spreading materials. Molecular dynamics (MD) simulations predicted kx=ky=4kz for perfect TMD crystals (MoS2 in this case). Experiments to determine kx,y and kz were conducted by developing a process to grow crystalline TMD thin film materials with strong (002) (basal planes parallel to surface) or (100) (perpendicular basal planes) crystal orientations. Initially, no correlation between structure and thermal conductivity was apparent, as water intercalation and reactivity to ambient air resulted in a thermal “short-circuit” across basal planes, such that the time between deposition and k measurement had a stronger impact on thermal conductivity than film orientation. Experiments to measure intrinsic thermal conductivity of MoS2 revealed values approximately one order of magnitude lower than those predicted using MD simulations, however, measurement of kx=ky=4kz was consistent with simulation results. Simulations to evaluate the dependence of thermal conductivity on grain size was evaluated, which correlated well to measured values. Comparison of measured k values for MoS2, WS2 and WSe2 are discussed in the context of the Slack Law, which accounts for intrinsic physical properties of the crystal, but not film microstructure. Alternatives to TMDs, with less environmental sensitivity, will also be illustrated.
3:10 PM TS3-6 Thermal Conductivity of Si-B-C-N Thin Films
Jamie Gengler (Air Force Research Laboratory/Spectral Energies); Jianjun Hu (Air Force Research Laboratory/UDRI); John Jones, Andrey Voevodin (Air Force Research Laboratory); Petr Steidl, Jaroslav Vlcek (University of West Bohemia, Czech Republic)

Thin films of amorphous silicon, boron, carbon, and nitrogen (Si-B-C-N) were recently shown to have an exceptionally high thermal oxidation resistance and are potential candidates for high temperature protective coatings [1]. Such applications would also require a low thermal conductivity through the coating thickness. Thermal transport was investigated in this study for ceramic films with different Si-B-C-N composition, where the microstructure varied from amorphous to nanocrystalline in order to investigate the effect of morphology on thermal barrier properties. Thermal conductivity trends of several ceramic thin films were characterized with a time-domain thermoreflectance (TDTR) technique. Samples containing two different Si-B-C-N chemical compositions were created by reactive magnetron sputtering and then subjected to annealing at temperatures up to 1400oC. The thermal conductivity of the samples prepared via a 50% Ar / 50% N2 gas mixture remained constant near 1.3 W m-1 K-1, while samples prepared via a 75% Ar / 25% N2 gas mixture exhibited an increase in thermal conductivity of 2.2 W m-1 K-1 (or higher). X-ray diffraction data demonstrated that the former samples were unstructured, while the latter samples formed silicon nitride (Si3N4) crystals. The experiments reveal which chemical composition remains stable in the amorphous state at high temperatures, thereby retaining lower thermal transport properties. These material aspects are ideal for thermal barrier applications such as non-oxide ceramic coatings for cutting tools.

[1] J. Vlček, S. Hřeben, J. Kalaš, J. Čapek, P. Zeman, R. Čerstvý, V. Peřina, and Y. Setsuhara. Magnetron sputtered Si-B-C-N films with high oxidation resistance and thermal stability in air at temperatures above 1500°C, J. Vac. Sci. Technol. A, 26, 1101 (2008).

3:30 PM TS3-8 Thermal Properties of Diamond/Ag Composites Fabricated by Salt Bath Coating
Mu Tse Lee, Jiun-Shian Liu, Chih-Yu Chung, Su-Jien Lin (National Tsing Hua University, Taiwan)
Thermal management in microelectronic technology has become an important issue due to the increase of device power and integration levels. Diamond and silver were selected for the fabrication of composites with high thermal conductivity and low coefficient of thermal expansion (CTE). However, the low thermal conductivity may be caused from the weak bonding between diamonds and silver in the consolidated composite. Improvements in bonding strength and thermal properties of the composites were achieved. A Cr film with a thickness of 150nm was formed on the surface of diamond particles using salt bath coating with additions of chromium to increase the interfacial bonding in diamond/Ag composites. These Diamond/Ag composites have potential applications for the high integration electronic devices.
3:50 PM TS3-9 Homogeneous Solution of Ca(BH4)2 as a Thermal Energy Storage Material
Placidus Amama, Jonathan Spowart, Andrey Voevodin (Air Force Research Laboratory); Timothy Fisher (Purdue University & Air Force Research Laboratory); Patrick Shamberger (Air Force Research Laboratory)

The development of new and efficient thermal energy storage (TES) materials remains a major challenge in addressing needs in a variety of areas from intermittent solar energy harvesting to thermal management of transient, high-flux heat loads. Calcium borohydride (Ca(BH4)2) is a potential TES material because of its high thermal storage capacity (2.0 MJ/kg) [1]. However, the high decompostion temperatures at atmospheric pressure (> 300°C ) in the solid state and slow kinetics represent significant challenges in its use for TES. To date, research efforts aimed at addressing these challenges have focused on engineering the solid-state reaction of hydrides, and the results, though somewhat promising for fuel cell applications, still do not meet the temperature and rate requirements for TES. To circumvent the complex processes associated with solid-state reactions and further to reduce the desorption temperature for fast, on-demand H2 release, homogeneous dehydrogenation of Ca(BH4)2 in various aprotic polar solvents is explored in this work. The modification of Ca(BH4)2 in solution with traditional catalysts is also studied. Preliminary analysis suggests that decomposition near room temperature is possible with enhanced kinetic rates.


[1] Siegel et al., Phys. Rev. B 76 (2007) 134102.

4:10 PM TS3-10 Infrared Study of Ta2O5 and HfO2 Thin Films on Si Substrates
Trevor Bright, Zhuomin Zhang (Georgia Institute of Technology); Chris Muratore, Andrey Voevodin (Air Force Research Laboratory)

Traditional dielectric materials used in the electronics and photonics industries are intended for use at moderate to low temperatures and do not have suitable properties for high-temperature applications (up to 1000 K). This requires us to experiment with new materials that can be used as high/low refractive index materials at elevated temperatures to engineer surface micro/nanostructures such as photonic crystals. The thermal radiative properties of these materials, such as Ta2O5 and HfO2, have not been well documented and the available data are very limited. We have deposited thin films of tungsten, hafnium oxide, and other films over a range of thicknesses using DC and pulsed DC magnetron sputtering processes on lightly doped, infrared transparent silicon substrates. For oxide films, deposition was conducted in a reactive sputtering process with pure targets of hafnium and tantalum. Post annealing allows the oxide film to crystallize as observed by x-ray diffraction. A Fourier-transform infrared spectrometer (FTIR) was used to measure the transmittance and reflectance (for incidence from either the film side or the substrate side) at room temperature, in the wavelength region from 1 to 20 mm. The FTIR system was purged with dry nitrogen to minimize measurement errors due to water absorption. FTIR measurements were tested against samples with known optical properties to minimize experimental uncertainty. The transmittance and reflectance spectra of the film-substrate composite were analyzed to determine the infrared optical constants of Ta2O5 and HfO2 films. The results will be presented and compared with data available in the literature.

4:30 PM TS3-11 Photonically Enhanced Flow Boiling from Nanostructured Surfaces
Chad Hunter (Air Force Research Laboratory); S.K. Arun (Purdue Univeristy); S.A. Putnam (Universal Technology Corporation); Nicholas Glavin (Air Force Research Laboratory); Timothy Fisher (Purdue University & Air Force Research Laboratory)
Devices that generate high heat fluxes, such as advanced electronic components, usually require multi-phase cooling because of the extremely high heat transfer coefficients that can be achieved. By operating devices in the so called ‘nucleate boiling’ regime, large rates of heat transfer can be achieved while maintaining acceptably low device temperatures. The addition of forced convection through flow boiling further enhances heat transfer beyond that of pool boiling. Surface characteristics play a critical role in the transition from nucleate boiling to critical heat flux (CHF), in which a stable vapor film forms at the device interface and results in an uncontrolled increase in substrate temperature. Modifying the boiling surface characteristics through surface roughness, micro/nano patterning and other means has been shown to enhance wetted surface area and nucleation site density, thereby allowing control over the CHF and increase of the boiling heat transfer coefficient. The aim of the present work is to assess new means of heat transfer enhancement by altering surface characteristics such as surface energy and wettability through light-surface interactions during flow boiling. Such photolytic and photocatalytic processes can be potentially controlled by wavelength and intensity of the irradiation emitted from a lamp source or laser diode, and the surface wetting enhancing coating on nanostructures. In this study, conformal TiO2 surfaces on CNT-coated copper substrates are exposed to parametric variations in UV (395 nm) intensity and mesoscopic spatial pattern during flow boiling in ultra-pure water. Measurements of system parameters such as CHF, and heat transfer coefficient are performed, and the effects of different surface and light characteristics are quantified.
4:50 PM TS3-12 Enhanced Surfaces in Conjunction with Single and Two-Phase Flows for Power Electronics Cooling Applications
Sreekant Narumanchi, Patrick McCluskey, Gilbert Moreno, Charles King (National Renewable Energy Laboratory)

Within the Department of Energy (DOE) Advanced Power Electronics and Electrical Machines (APEEM) program, cooling methodologies are being developed to enable high heat flux dissipation while maintaining low die operating temperatures, and to enable the program goals of weight, volume and cost. Existing hybrid vehicle power electronics cooling systems rely on single-phase, channel flow cooling configurations which provide relatively low heat transfer capability. Consequently, these cooling systems tend to be bulky and add significant mass and volume to the system.

In this study, we investigate efficient cooling schemes such as single-phase jet impingement and two-phase boiling heat transfer in combination with enhanced surfaces, with the overall goal of helping decrease the size and weight of automotive power electronics (PE) cooling systems. With a single-phase liquid (water) free jet impingement configuration, a copper microporous coating (3M) yielded a heat transfer enhancement of about 130% as compared to free jet impingement on a plain surface, while with a submerged jet configuration, the MicroCool finned surface (Wolverine Tube, Inc.) yielded a heat transfer enhancement of about 100% as compared to impingement on a plain surface. In two-phase flow configurations, the copper microporous coating (3M) increased nucleate boiling heat transfer rates by 100 to 500% and increased the critical heat flux (CHF) by 7 to 20% in the pool boiling and spray impingement boiling configurations using HFE7100 dielectric coolant.

The implications of these heat transfer performance enhancements in the context of power electronics package cooling will also be discussed.

Time Period WeA Sessions | Abstract Timeline | Topic TS3 Sessions | Time Periods | Topics | ICMCTF2011 Schedule