Browsing by Author "Mavrokefalos, Anastassios"
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Item An Experimental Investigation of Electrowetting Modulated Nucleate Boiling(2014-12) Sur, Aritra; Liu, Dong; Mavrokefalos, Anastassios; Ruchhoeft, Paul; Witte, Larry C.; Dunbar, Bonnie J.Electrowetting (EW) has drawn significant research interests in droplet-based microfluidics, and most applications focus on electronic displays, lab-on-a-chip devices and electro-optical switches, etc. In this work, we report a novel application of EW in enhancing nucleate boiling heat transfer. The working approach capitalizes on the complimentary roles of hydrophobicity and hydrophilicity in the fundamental processes of nucleate boiling, and takes advantage of the ability of EW to dynamically alter the surface wettability at different thermal loads. For instance, at low-to-moderate heat fluxes, the boiling surface remains hydrophobic so that onset of nucleate boiling (ONB) commences spontaneously and excellent boiling heat transfer can be obtained. When the bubble growth and merger intensify at high fluxes, EW will be activated to change the surface to hydrophilic, thereby delaying the critical heat flux (CHF). In this work, we demonstrate the creation of such tunable adaptive boiling surfaces, and examine the effects of both direct current (DC) EW and alternating current (AC) EW on the overall boiling heat transfer characteristics. A synchronized high-speed optical imaging and infrared thermography approach is taken to obtain simultaneous measurements of the bubble dynamics and temperature distribution on the boiling surface. Boiling curves are constructed and the boiling heat transfer coefficients are computed for an EW-modulated surface, and the comparison with those for hydrophilic and hydrophobic surfaces shows clearly the efficacy of using EW to drastically improve the boiling heat transfer performance. Some insights are also offered on the boiling heat transfer mechanisms under the influence of EW.Item Atomistic Modeling of Nanostructures via Molecular Dynamics and Time-Scaling Methods(2016-08) Hammami, Farah; Kulkarni, Yashashree; Sharma, Pradeep; Ardebili, Haleh; Willam, Kaspar J.; Mavrokefalos, AnastassiosNanostructures are emerging as novel materials with revolutionary application in electronics, nuclear reactors, structures, aerospace, and energy. Nanocrystalline structures owe their outstanding mechanical properties to their nanoscale grain size and high density of crystalline interfaces called grain boundaries. Recently, nanotwinned structures, containing special grain boundaries called twin boundaries, have become quite attractive as optimal motifs for strength, ductility, and grain stability in metals. This dissertation presents our atomistic study of the role of these grain boundaries and twin boundaries in governing the mechanical response of nanostructures by way of different atomistic simulation methods. Nanopillar compression is first used to investigate the interplay between size effects associated with the twin spacing and the finite size of nanopillars by molecular dynamics. Simulations reveal that there exists an optimal aspect ratio for which the yield strength of twinned nanopillars is higher than even single crystal nanopillars. In addition, it is observed that twin boundaries facilitate dislocation-starvation as defects glide along twin boundaries and are annihilated at the free surface. Approaching experimentally-relevant strain rates has been a long-standing bottleneck for molecular dynamics. In this study, shearing of a nanopillar with a grain boundary is used as a paradigmatic problem to investigate the rate dependence of grain boundary sliding in nanostructures. A combination of time-scaling approaches is used including the recently developed autonomous basin climbing method, the nudged elastic band method, and kinetic Monte Carlo, to access strain rates ranging from 0.5s-1 to 107s-1. Although grain boundary sliding is the primary mechanism observed in all simulations, at lower strain rate, sliding initiates at significantly lower stress and occurs on the time-scale of seconds which is beyond the reach of conventional molecular dynamics. Finally the time scaling approach is used to investigate the diffusion of radiation-induced point defects through nanotwinned metals. The simulations reveal that dumbbell interstitials can cross coherent twin boundaries in three low energy barrier steps which can occur even at room temperature. Furthermore, the method shows that Frenkel pairs have greater probability to recombine in the vicinity of coherent twin boundaries which is consistent with observations reported by other computational studies.Item Atomistic Study of Fracture and Deformation Mechanisms in Nanotwinned FCC Metals(2014-08) Sinha, Tanushree; Kulkarni, Yashashree; Sharma, Pradeep; Nakshatrala, Kalyana Babu; White, Kenneth W.; Mavrokefalos, AnastassiosNanotwinned metals have opened up exciting avenues for the design of high-strength, high-ductility materials owing to the extraordinary properties of twin boundaries. This dissertation presents insights into the deformation mechanisms governing the high temperature response and fracture behavior of nanotwinned face-centered-cubic (fcc) metals using molecular dynamics simulations. The aim of our atomistic modeling is to elucidate the role of coherent twin boundaries (CTB) in the interaction with dislocations (thus mediating strength and hardening) and in inhibiting crack propagation (thus contributing to toughness). Our simulations reveal an intriguing transition in the behavior of CTBs at higher temperatures as the deformation mechanism changes from shear-coupled normal motion to deformation twinning, an occurrence that has not been reported before in fcc metals. This anomalous response of twin boundaries at high temperatures is studied for different fcc metals and analyzed based on the energetics of the competing mechanisms. Our simulations of pre-existing cracks along CTBs reveal that CTBs in nanotwinned structures exhibit alternating intrinsic brittleness and intrinsic ductility. This is a startling consequence of the directional anisotropy of an atomically sharp crack along a twin boundary that favors cleavage in one direction and dislocation emission from the crack tip in the opposite direction owing to the effect of the crystallographic orientations in the adjoining twins. These results shed light on the previously held notion that twin boundaries are inherently brittle, and can also explain the brittle versus ductile behavior of CTBs reported in recent literature. We also investigate the effect of twin boundary spacing, and sample thickness on the crack-propagation in twinned nanopillars. The simulations show that CTBs serve as effective barriers for dislocation motion and restrict the plasticity in the vicinity of the crack tip. We finally extend our study of crack propagation to polycrystalline nanotwinned structures. We observe multiple mechanisms such as dislocation-twin interactions, twin migration, and dislocation nucleation from grain boundaries that govern the ductile response of a pre-existing crack. The findings reported in this dissertation demonstrate remarkable properties of twin boundaries and open further avenues for the design of novel nanotwinned structures for next-generation structural applications.Item Characterization of Temperature-Dependent Mechanical Properties of Half-Heusler Thermoelectric Material and Microstructural Evolution of Reactively-Brazed Half-Heusler/Incusil Aba/Copper Interfaces(2018-12) Gahlawat, Sonika; White, Kenneth W.; Sun, Li; Ryou, Jae-Hyun; Mavrokefalos, Anastassios; Ren, ZhifengThermoelectric materials, which have the capability of converting heat into electricity and vice-versa, provide a possible solution for harvesting waste heat from conventional energy conversion processes. Development of economically viable thermoelectric devices for large-scale applications warrants high figure-of-merit, a quantity that characterizes the performance of thermoelectric materials, as well as mechanical robustness and chemical stability of the device and its components. This work determines the hardness and elastic properties of commonly-used thermoelectric materials through nanoindentation and atomic force microscopy. Based on these results, p-type thermoelectric half-Heusler, Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2, was selected for a comprehensive mechanical characterization according to ASTM standards. This study probed its flexure and fracture behavior at ambient and elevated temperatures chosen accoding to the operational temperature range half-Heusler-based thermoelectric devices and reported the temperature-dependent bending strength and SENB plane strain fracture toughness of the material. Microstructural characterization of fractured surfaces allowed for establishment of fracture mechanism and microstructure-strength correlation for half-Heusler. Additionally, the work investigated elastic anisotropy of the micron-sized grain p-type half-Heusler microstructure through nanoindentation combined with electron backscatter diffraction, offering the first experimental reporting of the elasticity tensor for p-type half-Heusler. It also reported other elastic properties computed from the elasticity tensor and the results were validated with those obtained for a simpler TiCoSb half-Heusler from first principle calculations as reported in the literature. Finally, given that the thermoelectric modules fail primarily at the thermoelectric leg/electrode interfaces near the hot side, as reported in numerous studies, the last part of this work assessed ceramic thermoelectric/braze interfaces in reactively-brazed half-Heusler/Incusil ABA/copper joints. Electron backscatter diffraction and energy dispersive spectroscopy allowed for microstructural characterization of these ceramic/braze interfaces processed under varying brazing times with the primary goal assessing the chemical stability of these ceramic/braze interfaces and identifying optimal brazing parameters for development of robust interfaces for thermoelectric modules.Item Developing Magnetic Tweezers for Magnetic Material Manipulation(2013-08) Liu, Yun; Sun, Li; Ardebili, Haleh; Mavrokefalos, AnastassiosSingle-molecule research has stimulated the development of a wide range of technologies that are capable of manipulating very small structures and materials. Among current available methods, the magnetic tweezers show high efficiency and cost effectiveness in generating strong and direction adjustable interactions with magnetic materials. Since our research group has extensive experience in synthesizing magnetic nanostructures, it is of great interest to develop magnetic force-based nanomaterials manipulation techniques. In this thesis, we describe the design and construct of tip based electromagnetic tweezers. We focused on the investigation and quantification of interactions between a soft magnetic tip and a superparamagnetic bead suspended in liquid. We studied the effects of tip taper length and current in the solenoid surrounding the tip on resulted forces. An axisymmetric 2D model using COMSOL Multiphysics has also been developed to analyze the magnetic field and force acting on the superparamagnetic beads based on experimental design.Item Electrowetting Enhancement of Critical Heat Flux(2017-05) Lu, Yi; Liu, Dong; Ruchhoeft, Paul; Mavrokefalos, Anastassios; Ghasemi, Hadi; Yu, CunjiangCritical heat flux (CHF) represents the upper limit of nucleate boiling heat transfer, beyond which boiling transitions to the inefficient film boiling regime. Potentially catastrophic burnout conditions may ensue and endanger the safe and reliable operation of the boiling device. Thus, it is highly desirable to augment CHF in order to boost the thermal performance of various energy-intensive applications that rely on boiling to transport a large amount of thermal energy. In this work, CHF enhancement was explored both theoretically and experimentally by capitalizing on the ability of electrowetting (EW) to modulate the liquid-vapor interfacial stabilities and the liquid-vapor-solid three-phase contact line motion. To do so, a Leidenfrost drop (i.e., a liquid drop hovering over a highly superheated solid surface) was first employed as a model system, due to its simplicity and close connection to film boiling and CHF, to investigate the effect of the electric field on the dynamics of the vapor film that separates the drop from the hot surface. It was found that the electrostatic attraction force alone cannot destabilize the vapor film, instead, it is the accelerated vapor flow that changes the critical wavelength of the Kelvin-Helmholtz instability, thus causing the film to collapse. The results show that, without the need for any complicated surface micro/nanostructures, the Leidenfrost point (LFP) temperature of water can be increased from 200°C to 380°C with a moderate voltage of 56 V a frequency of 50 Hz. Subsequently, to better understand the impact of EW on the liquid-vapor interfacial behaviors, the dynamics of EW-induced motion of both liquid droplets and vapor bubbles was studied. Computational fluid dynamics models were developed by using the Volume of Fluid (VOF)-Continuous Surface Force (CSF) method to scrutinize the response of a droplet when subject to EW actuating signals. In particular, a dynamic contact angle model based on the molecular kinetic theory was implemented as the boundary condition at the moving contact line, which considers the effects of both the contact line friction and the pinning force. The droplet shape evolution and the interfacial resonance oscillation were investigated in detail. On the bubble aspect, the nucleation, growth and departure of vapor bubbles on a hydrophilic surface, a hydrophobic surface both with and without the influence of EW, were compared, which revealed the significant effect of the EW force on the contact line and, therefore, on the bubble dynamics. Lastly, to demonstrate the EW enhancement of CHF, a synchronized high-speed optical imaging and infrared (IR) thermographic technique was used to characterize boiling heat transfer at the CHF conditions. Simultaneous measurements of the bubble dynamics and the wall temperature and heat flux distributions on the boiling surface were acquired. The results showed that CHF can be enhanced by 133% by the use of EW. Additionally, by considering the force balance at the contact line of a nucleate bubble, a theoretical model was developed to delineate the threshold conditions for CHF to occur, which show very good agreement with the experimental measurements.Item Enhancing Thermoelectric Performance in Single-Crystal-Like Semiconducting Films by Tuning the Carrier Scattering Mechanism(2017) Singh, Shivkant; Dutta, Pavel; Rathi, Monika; Yao, Yao; Gao, Ying; Sun, Sicong; Khatiwada, Devendra; Selvamanickam, Venkat; Mavrokefalos, AnastassiosItem Evaluation of Implementing Thermoelectric Materials as a Power Generator(2016-05) Baseri, Ehsan; Masson, Philippe J.; Mavrokefalos, Anastassios; Tekin, EylemThe challenge of finding new energy sources or improving the efficiency of machines has become an important priority to scientists. A huge portion of provided energy becomes waste in the form of thermal energy. This fact sets the stage to look at recovering this wasted heat by Direct Heat-to-Electricity Conversion. Thermoelectric materials provide the opportunity to convert heat to electricity by thermoelectric generators (TEGs), which work under the Seebeck Effect. In this study, we focused on two less developed applications of TEGs. First, the feasibility of exciting High Temperature Superconductor coils by means of a TEG has been investigated. In this study, a physical model of the system in Simscape™/MATLAB has been prepared. Second, we focused on energy recovery from the waste heat source in cylindrical shaped equipment. An analytical approach has been served to offer the relation between geometrical variables or thermoelectric properties, and the maximum output power of Tubular Thermoelectric Generators (TTEGs).Item Experimental and Theoretical Investigation of Thermal, Thermoelectrical, Optical and Structural Properties in Nanostructures(2016-08) Brahmi, Hatem; Mavrokefalos, Anastassios; White, Kenneth W.; Varghese, Oomman K.; Masson, Philippe J.; Ardebili, HalehNanomaterials keep showing huge potential for diver electronic, computing, sensing, biomedical and energy conversion applications. Due to their size confinement effect, nanostructures materials (thin film, nanotubes, nanowires, two dimensional thick material and quantum dots) have very different physical properties compared to their bulk counterparts. Thus, they are promising candidates for more efficient solid state thermal, electronic, optical, biomedical devices. However it is very challenging to investigate their properties and detect their real potential. This dissertation presents thermophysical, thermoelectric and optical characterization of nanotubes, 2D material and thin films. Three different nanostructures have been experimentally and theoretically investigated: Titania nanotubes TiO2, atomically thin molybdenum diselenide MoSe2 and nickel silicide NiSi ultrathin film. The thermal and thermoelectrical measurements for both TiO2 nanotubes and MoSe2 monolayer were performed using suspended micro fabricated devices that allows the simultaneous measurements of Seebeck coefficient S, electrical conductivity σ and thermal conductivity κ on the same nanostructure that eliminated the parasitic effects of thermal contact and electrical contact resistances from the measured properties. Also the microdevice presents a through-substrate hole under the suspended membranes that makes the structural characterization on the same sample possible using Transmission Electron Microscope (TEM) analysis TEM. The assembly of the samples were performed using different techniques: First, the TiO2 nanotubes were assembled either by drop casting method or by use of a micromanipulator under an optical microscope the nanotube on the microdevice. Second, the PMMA transfer method that uses polymethyl methacrylate polymer as a transfer carrier for the monolayer MoSe2. In the second technique polymerization process was developed to anchor the sample to the device. For the TiO2 sample the measurements are done on both intrinsic, as synthesized, and doped samples and for both amorphous and polycrystalline anatase phase. The thermal conductivity of single nanotubes was found to be up six times lower than their bulk counterpart and very close to that of the amorphous phase. The thermoelectric properties were measured just for the doped samples and the results show semiconducting behavior which are sensitive to the doping concentration. Furthermore at high doping concentrations TiO2 nanotubes present an n-type behavior at high temperature, that switches to p-type behavior at very low temperature. The same characterization method was used to measure the thermal conductivity of atomically thick (0.7 nm) monolayer MoSe2. The measurements are done on intrinsic and tungsten doped samples. The results (30 W/mK and 17 W/mK for intrinsic and tungsten doped samples respectively) reveal nanostructure behavior for the thermal conductivity and they are in agreement with the reported values from first principle calculations. Also, the results show more than 50% decrease in the thermal conductivity due to the phonon scattering by tungsten atoms. Furthermore, optical and electrical properties of nickel silicide NiSi thin films are investigated over a broad range of wavelengths from 300 nm to 1000 nm. Those ultrathin films (thickness of 3 to 5 nm) show wafer scale uniformity, optical transparency of 60-90% and the resistivity is 1.29×10-01 to 9.93×10-02 μΩcm.Item Investigation of Dielectrophoresis-directed Fluidic Assembly(2015-08) He, Guoliang; Liu, Dong; Pan, Tsorng-Whay; Franchek, Matthew A.; Mavrokefalos, Anastassios; Yu, CunjiangDielectrophoresis (DEP)-based fluidic self-assembly of nanoscale building blocks, such as nanoparticles and nanowires, is a promising alternative to the current micro/nanofabrication techniques to manufacture functional micro/nanodevices. While individual particles can be manipulated with reasonable precision, it remains a grand challenge to scale up the assembly process to reproducibly assemble a large number of particles. This is partially due to the lack of a quantitative understanding of the complex fluid-particle dynamics when numerous nanostructures are interacting both electrically and hydrodynamically. In this work, both experiment and numerical study were conducted to explore the electrohydrodynamic effects during the assembly of multiple nanostructures driven by DEP. Direct numerical simulations were conducted that combine the Maxwell Stress Tensor (MST) approach and the Distributed Lagrange Multiplier/Fictitious Domain (DLM/FD) method to solve the conjugate fluid-particle interaction problem. The MST approach was used to compute the DEP forces and torques exerted on the particles, which yields rigorous solutions even for highly non-uniform electric field and for particles of irregular shapes. The DLM/FD method was then employed to simulate the hydrodynamic equations of the particle-fluid system involving multiple particles. The motion of the individual particles and the subsequent aggregation of adjacent particles under three major driving mechanisms for directed self-assembly, namely, DEP, traveling-wave DEP and electrorotation, were studied in details. In addition, microfluidic DEP devices were fabricated and self-assembly experiments were carried out for polystyrene microparticles suspended in colloidal solutions. The observed particle motion and the assembly patterns were compared to the numerical simulation results. The good agreement suggests the comprehensive numerical framework developed in this work can be used as a powerful tool for the fundamental study of colloidal hydrodynamics with coupled electrokinetic effects. With further advancement, this work will help to push forward the development of more effective and robust fluidic assembly techniques, and lay the foundation towards large-scale parallel manufacturing of functional nanostructures for various engineering applications.Item Lifetime Predictions of Actuation Fatigue for Shape Memory Alloy Notched Members(2018-12) Mehta, RutvikShape Memory Alloy (SMA)-based solid state actuators are an attractive alternative to conventional actuators when a small volume and/or large force and stroke are required. These alloys have the unique characteristic of being able to accommodate large recoverable strains through repeated martensitic-austenitic phase transformation. Insufficient understanding of the SMA "actuation" fatigue properties and lack of theoretical models for accurate prediction of fatigue life are the main limiters for their wider acceptance in engineering applications. The efficiency of the Smith-Watson-Topper model combined with the field intensity approach in estimating fatigue life for loaded notched SMA members undergoing thermal cycling is demonstrated. The field intensity approach adopted, which characterizes damage over a critical region where failure mechanisms are highly active rather than at a single point, is more reasonable from the point of view of fatigue failure mechanisms and more comprehensive from the point of view of explaining fatigue phenomena.Item Thermal and Thermoelectric Transport Measurements of Silicon Nanomembranes, Transition Metal Dichalcogenides, and Carbon Nanotube Networks(2017-12) Yarali, Milad; Mavrokefalos, Anastassios; Liu, Dong; Kulkarni, Yashashree; Ryou, Jae-Hyun; Bao, JimingDeep understanding and manipulating of energy transport characteristics in nanoscale systems is of fundamental importance in realizing high-performance solid-state devices. As dimension of these devices progressively shrink, the size effect of nanostructures, their interfacial scattering and interactions with the surrounding environment are increasingly dominating the electron and phonon transport properties. The objective of the work presented in this dissertation is to further the current understanding of interplay between structure and thermal and thermoelectric properties in nanofilm materials through experimental investigations. This objective is accomplished by utilizing a micro-fabricated device to perform coupled electrical-thermal-structural characterizations on the same individual nanofilm suspended between two resistance thermometers. First, an approach was developed to manage thermal conductivity (κ) of Si thin-film based nanoarchitectures through the formation of radial and planar Si/SiOx hybrid nanomembrane superlattices (HNMSLs). For the 24 nm thick one-winding tube at room temperature κ = 7.64 W m-1 K-1 which is 20 times smaller than the value of bulk single-crystalline silicon. Interestingly, a continuous reduction in κ with increasing number of windings was observed. Meanwhile, the planar Si/SiOx HNMSL shows κ = 5.3 W m-1 K-1, being the smallest in-plane thermal conductivity among all the reported values for Si-based superlattices. Next, the effect of metal doping and intrinsic structural defects on κ of monolayer undoped MoS2 and MoSe2, and doped Mo0.82W0.18Se2 grown by chemical vapor deposition were investigated. The results show the grain boundaries and vacancies are responsible for over 2 times reduction in the room temperature κ in our samples compared to their exfoliated counterpart, while κ remains intact upon isoelectronic substitution of W for Mo atoms. Also, boundary scattering dominates over defects and phonon-phonon scattering at low temperatures. Lastly, the effect of physisorbed vs chemisorbed oxygen on transport properties of aligned single walled carbon nanotubes (SWCNT) nanofilm was investigated. The physisorbed oxygen molecules on the SWCNTs surface make them initially p-type with metallic behavior. Vacuum annealing leads to desorb these molecules resulting in transition to n-type with semiconducting behavior while κ remains intact. On the other hand, SWCNTs with chemisorbed oxygen molecules exhibit purely p-type metallic behavior with lower κ.Item Thermal and Thermoelectric Transport Measurements of Silicon Nanomembranes, Transition Metal Dichalcogenides, and Carbon Nanotube Networks(2017-12) Yarali, Milad; Mavrokefalos, Anastassios; Liu, Dong; Kulkarni, Yashashree; Ryou, Jae-Hyun; Bao, JimingDeep understanding and manipulating of energy transport characteristics in nanoscale systems is of fundamental importance in realizing high-performance solid-state devices. As dimension of these devices progressively shrink, the size effect of nanostructures, their interfacial scattering and interactions with the surrounding environment are increasingly dominating the electron and phonon transport properties. The objective of the work presented in this dissertation is to further the current understanding of interplay between structure and thermal and thermoelectric properties in nanofilm materials through experimental investigations. This objective is accomplished by utilizing a micro-fabricated device to perform coupled electrical-thermal-structural characterizations on the same individual nanofilm suspended between two resistance thermometers. First, an approach was developed to manage thermal conductivity (κ) of Si thin-film based nanoarchitectures through the formation of radial and planar Si/SiOx hybrid nanomembrane superlattices (HNMSLs). For the 24 nm thick one-winding tube at room temperature κ = 7.64 W m-1 K-1 which is 20 times smaller than the value of bulk single-crystalline silicon. Interestingly, a continuous reduction in κ with increasing number of windings was observed. Meanwhile, the planar Si/SiOx HNMSL shows κ = 5.3 W m-1 K-1, being the smallest in-plane thermal conductivity among all the reported values for Si-based superlattices. Next, the effect of metal doping and intrinsic structural defects on κ of monolayer undoped MoS2 and MoSe2, and doped Mo0.82W0.18Se2 grown by chemical vapor deposition were investigated. The results show the grain boundaries and vacancies are responsible for over 2 times reduction in the room temperature κ in our samples compared to their exfoliated counterpart, while κ remains intact upon isoelectronic substitution of W for Mo atoms. Also, boundary scattering dominates over defects and phonon-phonon scattering at low temperatures. Lastly, the effect of physisorbed vs chemisorbed oxygen on transport properties of aligned single walled carbon nanotubes (SWCNT) nanofilm was investigated. The physisorbed oxygen molecules on the SWCNTs surface make them initially p-type with metallic behavior. Vacuum annealing leads to desorb these molecules resulting in transition to n-type with semiconducting behavior while κ remains intact. On the other hand, SWCNTs with chemisorbed oxygen molecules exhibit purely p-type metallic behavior with lower κ.Item Tuning Thermal and Thermoelectric Transport in Polycrystalline Films(2018-05) Singh, Shivkant D.; Mavrokefalos, Anastassios; Kulkarni, Yashashree; Ryou, Jae-Hyun; Selvamanickam, Venkat; White, Kenneth W.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.