Tuning Thermal and Thermoelectric Transport in Polycrystalline Films

In today’s ever-shrinking, fast paced, Internet of Things (IoT) generation, the need for thermal management has become one of the most important and key requirements to ensure the longevity of the high-power density devices. Thermal transport in material has been well documented and studied for various single-crystalline material systems. However, their applications are limited to high-performance system, which makes them commercially challenging due to the high-cost associated with processing of these materials. In real world applications though, polycrystalline material are preferred due to their cost benefits, hence understanding and manipulating thermal transport in these material systems are paramount. In the past decades, researchers have tried to understand the thermal transport using both experimental and theoretical approaches. Thermal transport in materials, often needs to be tuned for specific design requirements or applications. For example, flexible substrate that have high thermal conductivity are required for high-power LED operation whereas materials with suppressed thermal conductivity are required for heat generation using solid-state thermoelectric devices. Polycrystalline materials are highly desirable for such applications and since their thermoelectric properties are sensitive to the material microstructure, careful studies are needed to understand the causal effect of the microstructure-thermoelectric property relationship. The ambit of this dissertation is to further the current understanding of the microstructural effect on the thermal transport and thermoelectric transport in various polycrystalline materials. This objective was accomplished by performing thermal, structural and electrical characterization on polycrystalline thin films and novel flexible substrates. The first study reported in this thesis, is the tuning of thermal transport in flexible polycrystalline yttria-stabilized zirconia (YSZ) flexible substrates. Thermal transport of these substrates was enhanced by increasing the grain size without deteriorating their flexibility. We reported thermal conductivity values of 4.16 Wm-1K-1, four times higher than their state-of-the-art polymeric counterparts, while keeping the bending radius intact. Further, we demonstrated the application of these thin YSZ flexible substrate in Flip-chip light emitting diodes, which showed enhancement in device operation due to the reduction in the hot-spot formation during the operation of the LEDs in comparison to the polymeric substrate The second work reported here is on tuning thermal transport in aluminum nitride thin films, where the effect of controlling the c-axis alignment (mosaicity) to the cross-plane thermal transport was experimentally and theoretically investigated. We show that enhancement in the grain interface quality due to improvement of the c-axis alignment resulted in significant increase of the thermal conductivity from 3.5 to 6 Wm-1K-1. Essentially, providing a tuning knob to control the thermal transport of the polycrystalline film. Lastly, we investigated the thermo-electric properties of a new class of flexible single-crystal-like biaxially-textured with low angle grain boundaries GaAs films. Coupled with microstructural and thermoelectric analysis we observed enhanced power factor values of 1300 μW/mK2, the highest value for the non-toxic thin film inorganic flexible films. And three-fold enhancement in the figure of merit compared to the bulk GaAs.