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

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.

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.

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 1400^oC. 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).

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.

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.

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.