Engineering Nanocomposites and Interfaces for Conduction and Radiation Thermal Management

Xiangyu Li, Purdue University

Abstract

The rapid advance in nanoscale fabrication of nanoparticles and thin films has urged a new perspective in the field of heat transfer. In one of the applications, composite materials, fillers are added in polymer matrix to enhance certain properties. Thermal interface materials are such composites to increase thermal conductivity of polymer for better heat transfer between interfaces. Nanoparticles introduces other mechanisms rarely seen in large fillers, such as aggregation and sintering effect. Such nanocomopsites can be utilized as thermal interface materials for better thermal transport between macro-interfaces. Though thermal interface materials have been studied for decades, aggregation and size effect have never been of great attention as micro-particles diffuse slowly, stay isolated and get separated easily. Thus, aggregation does not show much impact on the overall thermal conductivity. Most of the theoretical models simply ignore aggregation to assume isolated or uniform particle dispersion. However, with size shrinking down to nanometers, nanoparticles diffuse much faster and form clusters more frequently. Along with sintering effect during curing process, clusters with continuous filler phase are common, resulting in a further increase for thermal conductivity. In this work, we focus on the aggregation and size effect on thermal conductivity of metal-polymer nanocomposite. We have fabricated nickel-epoxy nanocomposites and observed higher thermal conductivity than effective medium theory predicts. Contrary to classical models indicate, smaller particles are also found to show higher thermal conductivity with the same particle concentration. A two-level EMA model is developed to account for the aggregation effect and to explain the size-dependent enhancement of the thermal conductivity by introducing local concentration in aggregation structures. An aggregation simulation during the curing process is followed to illustrate both qualitatively and quantitatively that the size-dependent enhancement of thermal conductivity is caused by the aggregation effect due to higher diffusing speed of the smaller particles. The results help a better understanding on the impact of aggregation, provide guidance in nanocomposite designing and can also apply for other areas such as composite aging process. Metal-nonmetal interfacial thermal resistance that exist in the nanocomposites is further studied in details using thin-film sandwich structures, as quantifying interfaces in nanocomposite is challenging especially with aggregation effect. Heat generation in modern microelectronic device surges as the number of transistors skyrockets along with the prediction of the Moore’s Law. What is worse, with a much higher density of interfaces, especially metal-dielectric interfaces, their contribution has dominated the overall thermal resistance and greatly impeded the thermal management, resulting in one of the largest challenges for enhancing performance nowadays. In this work, in order to decrease gold-alumina interfacial thermal resistance, we inserted an intermediate metal layer nickel between gold and alumina and observed a 70% reduction in total interfacial thermal resistance. Though one more interface is introduced with the inserted nickel layer, the higher electron-phonon coupling factor and the lattice constant of nickel reduce the total thermal resistance. The two temperature model (TTM) is applied to explain the reduction of interfacial resistance, and the results show that the nickel layer functions as a bridge that reduces the phonon mismatch between gold and aluminum oxide. Moreover, nickel has strong electron-phonon coupling, which reduces the thermal resistance caused by the weak electron-phonon coupling in gold. On the other hand, the thermal conductivity of nanoparticles and thin films changes as the scale goes down to the mean free path of phonons. This size effect opens up more opportunities for thermal conductivity engineering for applications such as thermoelectric energy harvesting. Superlattices or multi-layer structures with minimal lattice mismatch can achieve higher thermal conductivity than expected due to coherent phonons and coupled interfaces. However, such multi-layer structures are rarely seen in actual devices, where the coupling effect of two adjacent thermal interfaces is yet to be well understood. In this work, sandwich structures of aluminum oxide, nickel, and aluminum oxide films are fabricated with atomic layer deposition to study thermal interfacial resistance between metal and dielectric material and interfacial coupling effect across a thin metal layer. Thermal resistances of thin nickel layer and two interfaces are measured with the 3ω method. Experimental results show interfacial thermal resistance between nickel and aluminum oxide as 6.8 × 10−3mm2K/W at 300K, with weak dependence on metal thickness and temperature. Two-temperature model and detailed diffuse mismatch model have been used to estimate interfacial resistance theoretically, and the results agree reasonably well with experiments. Estimations from the two temperature model indicate that in the overall thermal interfacial resistance, the phonon-phonon interfacial resistance dominates over the resistance due to electron-phonon coupling effect and inside the metal layer. Also, the phonon-phonon interfacial resistance does not vary as the metal layer thickness decreases below electronphonon cooling length indicating the two adjacent interfaces are not thermally coupled.

Degree

Ph.D.

Advisors

Ruan, Purdue University.

Subject Area

Engineering|Condensed matter physics|Materials science|Nanotechnology|Physics|Polymer chemistry|Thermodynamics

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