ICMCTF2012 Session TS1-1: Surface Engineering for Thermal Transport, Storage and Harvesting

To investigate mechanisms of thermal conductivity in amorphous materials, we developed a technique for microfabricating freestanding ultrathin films with built-in strain actuation and instrumentation for thermal conductivity measurement. Using a combination of infrared thermal micrography, 3-omega analysis, and multi-physics simulations, we measured the thermal conductivity of amorphous silicon nitride and silicon oxide films under a range of tensile strains. The thermal conductivity of the silicon nitride film showed a remarkable decrease with tensile strain, dropping down by an order of magnitude at approximately 2% strain when compared to the unstrained film. Silicon oxide showed no change in thermal conductivity up to 1% tensile strain. The theoretical analysis was performed using classical molecular dynamics and lattice dynamics simulations, showing that amorphous silicon nitride has unusual vibrational properties resembling those of amorphous silicon and other chemically uniform glasses. The unusual relationship between strain and thermal conductivity in amorphous silicon nitride suggests that an additional mechanism such as long-range unharmonic coupling between oscillators plays an important role in heat conduction, as the conductivity agrees with harmonic theory predictions at large values of tensile strain (>2%). This work is supported by AFOSR Low Density Materials Program,Task #2306CR7P.

Improved understanding of thermal energy transport at nanometer scales has enabled a broad range of technological advances in recent years. Today, new materials can be designed at the atomic level and are projected to improve the efficiency of information processing, heat transfer, and energy conversion, among other applications relevant to aerospace vehicles and systems. For the transfer of energy by phonons, approaches based on atomistic Green’s functions have been recently developed and offer the possibility of including atomic-scale detail at material interfaces, while mesoscopic scales can be modeled with the particle-based Boltzmann transport equation. This review will summarize a framework for the inclusion of such high-fidelity atomistic modeling within multi-scale modeling tools that are needed to understand complex interfacial transport processes and scaling principles in thermal interface materials enhanced with carbon nanotubes (CNTs). Experimental validation and refinement on model components is essential to this work, and includes highly localized techniques such as transient thermoreflectance techniqes, as well as traditional 1D reference bar approaches to assess overall performance. As an example of recent work, we briefly describe critical results related to contact resistance measurements with CNT arrays and other graphene-based structures. The talk will conclude with enumeration of important questions related to heterojunction bonding and materials processing for scaled-up manufacturing.

Liquid to vapor phase change technology utilizing latent heat of vaporization, which can have heat transfer rates orders of magnitude higher than single phase liquid cooling, is necessary for advanced aircraft due to the onboard heat generated by high power (electrical and chemical) components. The wettability of engineered surfaces used in these cooling systems, in addition to other parameters such as surface roughness, is strongly correlated to the performance of the heat exchanger systems that use these materials. In addition to optimizing performance using passive means (e.g., surface texturing), it would also be advantageous to control heat transfer rates using applied stimuli (e.g., light or sound waves), which could result in weight savings and/or energy optimization. Oxide photocatalysts have been investigated to influence boiling performance on surfaces¹, but time scales for switching between wettability states are on the order of tens of minutes to hours, much longer than for practical use to control boiling processes. In previous work, synthesis of [Ru(bpy)₂(pox)]Cl₂, an organometallic complex which undergoes reversible photo-isomerization that changes the inherent water affinity of the molecule, was achieved². In the current research, new ruthenium-centered organometallic complexes with functionalized bipyridine (bpy) ligands are synthesized. The functionalization allows the Ru complexes to be covalently tethered to metallic substrates. Surface chemistry is investigated with XPS, indicating a character of the bonding linkage, which is then correlated with contact angle measurements of the surface energy modification with and without UV-VIS light irradiation. Water boiling heat transfer studies during UV-VIS light irradiation are conducted on these samples and correlated with the reversible switching of surface wetting.

¹Takata, et al., International Journal of Chemistry Research, 2003.

Recent advances in aerospace and defense technologies have lead to an increasing need to develop novel materials with exotic physical properties for use in a variety of applications involving extreme, high-temperature, high mechanical stress, and oxidizing environments. Transition metal nitrides (TMN), oxides, and oxynitrides are well known to have remarkable range of unique physical properties including high hardness and mechanical strength, high melting temperatures (>>2000 °C), and tunable optical-electronic properties. One method to further enhance the physical properties of many binary transition metal nitrides is to alloy them with a second thermodynamically immiscible nitride to form metastable compounds with enhanced physical properties. Many of these properties are accompanied by the formation of nanoscale compositional modulations during film growth as well as post annealing experiments, however, very little has been reported on the ability to control this nanostructure, and as a result, the effects of these nanostructures on fundamental physical properties is relatively unknown.

Nanostructuring methods using these kinetically-limited growth techniques involving high-flux low-energy ion bombardment during film growth, lead to a unique independent control of both electric and thermal transport. The right materials system, combined with said growth techniques, would allow for the realization of hard, chemically inert, environmentally-friendly, and refractory thermal-to-electrical energy conversion thin films materials to tackle a variety of demanding defense applications.

I have used Hf_1-xAl_xN as a model system to study the nanostructures of interest. I begin by reporting on the effects of nanostructure on the optical, electronic, thermal transport and elastic constant properties of Hf_1-xAl_xN single crystal layers grown on MgO(001) by reactive unbalanced magnetron cosputtering using ellipsometry, temperature dependent hall effect, picosecond probe thermoreflectance and acoustic transport measurements, respectively. I will also present thermal transport studies of nanocrystallione SrTiO₃ (a promising oxide thermoelectric) and SrTiO₃/TiO₂ systems, as a first step towards understanding thermal and electron transport in nanostructured oxide thin films.

The development of novel and efficient thermal energy storage (TES) materials is a major challenge in addressing needs in a variety of applications, from intermittent solar energy harvesting to thermal management of transient, high-flux heat loads. Lithium alanate (LiAlH₄) is a potential TES material that possesses extraordinarily high inherent thermal energy density, and the possibility of a system that is compact and lightweight. However, the high desorption temperature at atmospheric pressure and slow kinetics represent significant challenges to the use of this material for TES. In order to address these challenges, the present work focuses on the modification of LiAlH₄ via high-energy ball milling with Ti-based catalysts, resulting in nanoscale features. The doping of LiAlH₄ with Ti-based catalysts has resulted in improved thermal energy storage properties. Quasi in situ X-ray photoelectron (XPS) study of the dehydrogenation reaction of LiAlH₄ has provided new insights into the role of the catalysts.

Light concentration is important for the development of advanced PV system using high efficiency solar cells. High efficiency multi-junction cells under high concentration operations have been investigated for terrestrial application. Also, low concentration operations with multi-junction cells have been investigated for space satellite. It is considered that the temperature of solar cells considerably rises under light concentrating operations.

It is very important for concentration photovoltaic (CPV) modules to decrease their temperature. A heat release from aluminum chassis of CPV module should be enhanced. In this study, a heat radiation layer was coated on the aluminum chassis of CPV module, and the effect of the layer on cell temperature was evaluated and discussed.

A CPV module consisted of 25 pairs of Fresnel lens (160 mm x 160 mm) and triple-junction solar cell (7 mm x 7mm), and an aluminum chassis was used. A heat radiation layer (PELNOX LTD., PELCOOL(R)) was coated on the aluminum chassis of CPV module. The triple-junction solar cells were arrayed on the aluminum chassis. In order to detect the cell temperature, a Pt100 was embedded just below the triple-junction solar cell. A CPV module was fabricated by connecting 25 lens-cell pairs in series.

The cell temperature of CPV module with heat release coating was about 15K lower than that without coating. The effect of the high radiation layer was remarkable, and the output powers of the CPV module with and without coating were 170.7 W and 162.4 W, respectively.

This study was carried out in Japan. Since the effect of the high radiation layer becomes remarkable, as the temperature of the heat source is higher, it can be expected that the high radiation coating for the CPV module works more effectively in more higher temperature area.