Browsing by Subject "Thermal conductivity"
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Item Basal plane thermal conductivity of thin germanane layers(2014-05) Coloyan, Gabriella Marie Gregson; Shi, Li, Ph. D.The thermal conductivity of thin Germanane (GeH) layers was measured using suspended micro-devices with integrated heaters and thermometers. The thermal contact resistance of the GeH samples suspended on the measurement devices was determined from the measured thermal resistance values of samples with different suspended lengths. The room-temperature thermal conductivity of the GeH samples was observed to be 0.6-1.0 Wm⁻¹K⁻¹. This low thermal conductivity is attributed to phonon scattering by defects and grain boundaries in the layered materials, including scattering caused by gangling bonds associated with missing Hydrogen atoms between adjacent layers.Item Characterization of Thermal Properties of Depleted Uranium Metal Microspheres(2012-07-16) Humrickhouse, Carissa JoyNuclear fuel comes in many forms; oxide fuel is the most commonly used in current reactor systems while metal fuel is a promising fuel type for future reactors due to neutronic performance and increased thermal conductivity. As a key heat transfer parameter, thermal conductivity describes the heat transport properties of a material based upon the density, specific heat, and thermal diffusivity. A material?s ability to transport thermal energy through its structure is a measurable property known as thermal diffusivity; the units for thermal diffusivity are given in area per unit time (e.g., m2/s). Current measurement methods for thermal diffusivity include LASER (or light) Flash Analysis and the hot-wire method. This study examines an approach that combines these previous two methods to characterize the diffusivity of a packed bed of microspheres of depleted uranium (DU) metal, which have a nominal diameter of 250 micrometers. The new apparatus is designated as the Crucible Heater Test Assembly (CHTA), and it induces a radial transient across a packed sample of microspheres then monitors the temperature profile using an array of thermocouples located at different distances from the source of the thermal transient. From the thermocouple data and an accurate time log, the thermal diffusivity of the sample may be calculated. Results indicate that DU microspheres have very low thermal conductivity, relative to solid uranium metal, and rapidly form an oxidation layer. At 500?C, the thermal conductivity of the DU microspheres was 0.431 ? 13% W/m-K compared to approximately 32 W/m-K for solid uranium metal. Characterization of the developed apparatus revealed a method that may be useful for measuring the thermal diffusivity of powders and liquids.Item Energy Carrier Transport In Surface-Modified Carbon Nanotubes(2012-11-30) Ryu, YeontackCarbon nanotubes are made into films or bulks, their surface or junction morphology in the networks can be modified to obtain desired electrical transport properties by various surface modification methods. The methods include incorporation of organic molecules or inorganic nanoparticles, debundling of nanotubes by dispersing agents, and microwave irradiation. Because carbon nanotubes have unique carrier transport characteristics along a sheet of graphite in a cylindrical shape, the properties can be dramatically changed by the modification. This is ideal for developing high-performance materials for thermoelectric and photovoltaic energy conversion applications. In this research, decoration of various organic/inorganic nanomaterials on carbon nanotubes was employed to enhance their electrical conductivity, to improve thermoelectric power factor by modulating their electrical conductance and thermopower, or to obtain n-type converted carbon nanotube. The electrical conductivity of double-wall nanotubes (DWNTs) decorated with tetrafluoro-tetracyanoquinodimethane (F4TCNQ) was increased up to 5.9 ? 10^5 S/m. The sheet resistances were measured to be 42 ?/sq at 75% of transmittance for HNO3/SOCl2-treated DWNT films, making their electrical conductivities 200~300% better than those of the pristine DWNT films. A series of experiments at different ion concentrations and reaction time periods were systematically performed in order to find optimum nanomaterial formation conditions and corresponding electronic transport changes for better thermoelectric power factor. For example, the thermoelectric power factors were improved by ~180% with F4TCNQ on DWNTs, ~200% with Cu on SWNTs, and ~140% with Fe on single-walled nanotubes (SWNTs). Also SWNTs was converted from p-type to n-type with a large thermopower (58 ?V/K) by using polyethyleneimine (PEI) without vacuum or controlled environment. This transport behavior is believed to be from charge interactions resulted from the difference between the work functions/reduction potentials of nanotubes and nanomaterials. In addition, different dispersing agents were utilized with DWNT and SWNTs to see a debundling effect in a film network. The highest electrical conductivity of ~1.72?10^6 S/m was obtained from DWNT film which was fabricated with a nanotube solution dispersed by chlorosulfonic acid. Debundling of nanotubes in the film network has been demonstrated to be a critical parameter in order to get such high electrical property. In the last experiment, Au nanoparticle decoration on carbon nanotube bundle was performed and a measurement of themophysical properties has done before and after modifying carbon nanotube surface. Carbon nanotube bundle, herein, was bridged on microdevice to enable the measurement work. This study demonstrates a first step toward a breakthrough in order to extract the potential of carbon nanotubes regarding electron transport properties.Item Experimental investigation of thermal transport in graphene and hexagonal boron nitride(2012-12) Jo, Insun; Yao, Zhen, Ph. D.; Shi, Li, Ph. D.Two-dimensional graphene, a single layer of graphite, has emerged as an excellent candidate for future electronic material due to its unique electronic structure and remarkably high carrier mobility. Even higher carrier mobility has been demonstrated in graphene devices using hexagonal boron nitride as an underlying dielectric support instead of silicon oxide. Interestingly, both graphene and boron nitride exhibit superior thermal properties, therefore may potentially offer a solution to the increasingly severe heat dissipation problem in nanoelectronics caused by increased power density. In this thesis, we focus on the investigation of the thermal properties of graphene and hexagonal boron nitride. First, scanning thermal microscopy based on a sub-micrometer thermocouple at the apex of a microfabricated tip was employed to image the temperature profiles in electrically biased graphene devices with ~ 100 nm scale spatial resolution. Non-uniform temperature distribution in the devices was observed, and the "hot spot" locations were correlated with the charge concentrations in the channel, which could be controlled by both gate and drain-source biases. Hybrid contact and lift mode scanning has enabled us to obtain the quantitative temperature profiles, which were compared with the profiles obtained from Raman-based thermometry. The temperature rise in the channel provided an important insight into the heat dissipation mechanism in Joule-heated graphene devices. Next, thermal conductivity of suspended single and few-layer graphene was measured using a micro-bridge device with built-in resistance thermometers. Polymer-assisted transfer technique was developed to suspend graphene layers on the pre-fabricated device. The room temperature thermal conductivity values of 1-7 layer graphene were measured to be lower than that of bulk graphite, and the value appeared to increase with increasing sample thickness. These observations can be explained by the impact of the phonon scattering by polymer residue remaining on the sample surfaces. Lastly, thermal conductivity of few-layer hexagonal boron nitride sample was measured by using the same device and technique used for suspended graphene. Measurements on samples with different suspended lengths but similar thickness allowed us to extract the intrinsic thermal conductivity of the samples as well as the contribution of contact thermal resistance to the overall thermal measurement. The room temperature thermal conductivity of 11 layer sample approaches the basal-plane value reported in the bulk sample. Lower thermal conductivity was measured in a 5 layer sample than an 11 layer sample, which again supports the polymer effect on the thermal transport in few-layer hexagonal boron nitride.Item Experimental investigations of thermal transport in carbon nanotubes, graphene and nanoscale point contacts(2011-05) Pettes, Michael Thompson, 1978-; Shi, Li, Ph. D.As silicon-based transistor technology continues to scale ever downward, anticipation of the fundamental limitations of ultimately-scaled devices has driven research into alternative device technologies as well as new materials for interconnects and packaging. Additionally, as power dissipation becomes an increasingly important challenge in highly miniaturized devices, both the implementation and verification of high mobility, high thermal conductivity materials, such as low dimensional carbon nanomaterials, and the experimental investigation of heat transfer in the nanoscale regime are requisite to continued progress. This work furthers the current understanding of structure-property relationships in low dimensional carbon nanomaterials, specifically carbon nanotubes (CNTs) and graphene, through use of combined thermal conductance and transmission electron microscopy (TEM) measurements on the same individual nanomaterials suspended between two micro-resistance thermometers. Through the development of a method to measure thermal contact resistance, the intrinsic thermal conductivity, [kappa], of multi-walled (MW) CNTs is found to correlate with TEM observed defect density, linking phonon-defect scattering to the low [kappa] in these chemical vapor deposition (CVD) synthesized nanomaterials. For single- (S) and double- (D) walled (W) CNTs, the [kappa] is found to be limited by thermal contact resistance for the as-grown samples but still four times higher than that for bulk Si. Additionally, through the use of a combined thermal transport-TEM study, the [kappa] of bi-layer graphene is correlated with both crystal structure and surface conditions. Theoretical modeling of the [kappa] temperature dependence allows for the determination that phonon scattering mechanisms in suspended bi-layer graphene with a thin polymeric coating are similar to those for the case of graphene supported on SiO₂. Furthermore, a method is developed to investigate heat transfer through a nanoscale point contact formed between a sharp silicon tip and a silicon substrate in an ultra high vacuum (UHV) atomic force microscope (AFM). A contact mechanics model of the interface, combined with a heat transport model considering solid-solid conduction and near-field thermal radiation leads to the conclusion that the thermal resistance of the nanoscale point contact is dominated by solid-solid conduction.Item Fabrication and characterization of open celled micro and nano foams(2013-08) Srinivas Sundarram, Sriharsha, 1985-; Li, Wei, doctor of mechanical engineeringOpen celled micro and nano foams fabricated from polymers and metals have attracted tremendous attention in the recent past because of their applications in numerous areas such as catalyst carriers, filtration media, ion exchange membranes and tissue engineering scaffolds. In this study open celled polymer micro- and nano foams with controllable pore size and porosity were fabricated via solid state foaming of immiscible blends. The polymer foams were used as templates for fabricating nickel foams using an ethanol based electroless plating process. Thermal conductivity of micro- and nano foams was studied as a function of pore size and porosity using finite element and molecular dynamics based models. The effect of pore size and porosity on performance of phase change material infiltrated metal foams for thermal management was investigated via numerical models. Open celled micro foams were fabricated via solid state foaming of ethylene acrylic acid (EAA) and polystyrene (PS) co-continuous blends. Blending temperature was the main parameters affecting the formation of co-continuous structure. Gas saturation and foaming studies were performed to determine ideal processing conditions for the blend. The results indicated that saturation pressure and foaming temperature were major process parameters determining the porosity of the foamed samples. Open celled polymer templates were obtained by selective extraction of PS phase using dichloromethane (DCM). Foaming resulted in faster extraction of PS and also in a higher porosity. Open celled nano foams were fabricated via solid state foaming of polyetherimide (PEI) and polyethersulfone (PES). The effect of process parameters namely saturation pressure and temperature, desorption time, and foaming temperature and time on porosity and pore size was studied. A high gas concentration and foaming temperature were required to obtain nano pore-sized foams. Throughout the cross section there existed regions with varying pore size and porosity and solid skins at the surface regions of the foam. A solvent surface dissolution process using dimethylformamide (DMF) was employed to access the internal porous structure. Micro- and nano cellular nickel foams were fabricated from EAA and PES templates via electroless plating. The structure of the nickel foams was an inverse of the polymer templates. Ethanol based electroless plating solutions were used to ensure infiltration into the porous structure because of the small pore sizes. Finite element and molecular dynamics based models were developed to predict thermal conductivity of polymer foams as a function of pore size and porosity. Pore sizes ranging from 1 nm to 1 mm were studied. Models were partially validated using experimental data. The results showed that pore size has significant effect on thermal conductivity even for microcellular and conventional foams. When the pore size is reduced to the nanometer scale, the thermal conductivity of the nano foam dramatically reduces and the value could be lower than that of air for certain porosity levels. The extremely low thermal conductivity of polymer nanofoams is possibly due to increased phonon-phonon scattering in the solid phases of the polymer matrix in addition to low thermal conductivity of gas trapped in nano sized pores. Finite element based models were also developed to study the effect of pore size and porosity on performance of phase change material infiltrated metal foams for thermal management applications. The results showed that foams with smaller pore sizes can delay the temperature rise of the heat source for an extended period of time by rapidly dissipating heat in the phase change material. The lower temperatures resulting from the use of a smaller pore size metal foam could significantly increase the lifetime of IC chips.Item Four-probe measurements of anisotropic in-plane thermal conductivities of thin black phosphorus(2016-08) Smith, Brandon Paul; Shi, Li, Ph. D.; Akinwande, DejiPhosphorene, a two-dimensional material exfoliated from black phosphorus (BP), is a promising p-type, high-mobility semiconductor. Phosphorene and BP display intrinsic in-plane anisotropic transport properties due to its puckered honeycomb lattice with distinct armchair and zigzag crystallographic orientations. The anisotropic thermal transport properties of BP and phosphorene influence the performance and reliability of functional devices made from these materials, and remain to be better understood. Here, we report the anisotropic in-plane thermal conductivities of suspended multi-layer BP samples, which are measured by a four-probe thermal transport measurement method. The measurement device consists of four microfabricated, suspended Pd/SiNx lines that act as resistive heaters and thermometers. The BP flake is suspended across the microstructure in contact with all four lines. This four-probe thermal transport measurement is equipped with the unique ability to isolate the intrinsic thermal resistance from the contact thermal resistance, which can be a major source of error in thermal conductivity measurements of nanostructures. Four BP samples were measured with thicknesses ranging from 39.2 nm to 274 nm and a peak thermal conductivity of 142 W m-1 K-1 at 80 K for a 55.6 nm thick zigzag oriented flake. The measurement results exhibit more pronounced temperature dependence with a higher peak thermal conductivity together with a weaker thickness dependence than prior reports. The results suggest the important role of defects in thermal transport in thin BP flakes, which can degrade upon exposure to air and water.Item Four-probe thermal measurement of a carbon nanotube sheet(2015-08) Ou, Eric; Shi, Li, Ph. D.As advances are made in top-down nanofabrication and bottom-up syntheses of nanostructures, the characteristic length scales encountered in these structures are on the order of the mean free path of the heat carriers or smaller. Therefore, the thermal transport properties of these nanostructures can be different from the bulk counterparts. A number of experimental techniques have been developed for characterizing the size-dependent thermal transport properties of nanostructures. However, it is difficult to eliminate contact thermal resistance, an important error source, from the measurement results. Recently, a four-probe thermal measurement technique has been developed to measure the intrinsic thermal conductance of a suspended sample as well as isolate the values of contact resistance between the sample and measurement device. Here, the fabrication process of the four-probe measurement device is described. In addition, numerical heat conduction simulation is used to verify the analytical model of the measurement method. This method is further used to measure the thermal conductance of a carbon nanotube sheet.Item Interface and Size Effects on TiN-based Nanostructured Thin Films(2012-07-16) Kim, IckchanTitanium nitride coatings have been widely applied and studied as high temperature diffusion barrier for silicon devices in microelectronics, wear resistant coatings in turbine blade materials, and materials for future high temperature nuclear reactors. In order to enhance the material property, superlattices is one of artificially engineered protective coatings, such as AlN/TiN and TaN/TiN multilayered films. Epitaxial cubic multilayer films, TaN/TiN and AlN/TiN nanolayers were grown on Si(001) by Pulsed Laser Deposition (PLD) with various nanolayer thicknesses and number of interfaces. Microstructural studies include X-ray diffraction (XRD), transmission electron microscopy (TEM), and high resolution TEM with ion-irradiation experiments. Electrical, mechanical and thermal property studies were conducted for the interface and size effects on the nanolayers by using nanoindentation and Transient Thermo-Reflectance (TTR) methods. The microstructural and hardness study on TaN/TiN films with ion irradiation (12 keV and 50 keV He ) suggest no obvious microstructural or mechanical behavior change due to ion irradiation. In addition, titanium nitride that serves as effective diffusion barrier to prevent the inter-diffusion between the nuclear fuel and the cladding material was studied in order to enhance the lifetime of the fuels and the reliability of the fuel claddings. The TiN has good adhesion with the stainless steel and higher hardness than that of bulk TiN on the stainless steel. Thermal conductivity test demonstrates that thin TiN film has compatible thermal conductivity as the MA957 and HT-9 bars. The size effect on electrical resistivity is dominant in both of the epitaxial cubic and the polycrystalline TiN thin films in the thickness ranged from ~60 nm down to ~35nm. In the TaN/TiN multilayer, the grain scattering effect on resistivity is dominant rather than interface influence on the resistivity with comparing epitaxial cubic phase and polycrystalline phase. The microstructure and hardness studies of the AlN/TiN multilayer films with He implantation present that the suppression of amorphization in AlN layers and the reduction of radiation-induced softening were achieved in all nanolayer films. Radiation tolerance was found to be size dependent and the layer thickness leading to the highest radiation tolerance was around 10 nm. In addition, the embedded epitaxial cubic AlN with cladding TiN nanolayers showed higher effective thermal conductivity than that of AlN single layer as well as the embedded polycrystalline AlN in the thickness ranged from 10 nm down to 2 nm. It confirms a suppressed size effect, which reduces the amount of decrease in through-plane thermal conductivity.Item Molecular Dynamic Simulation of Thermo-Mechanical Properties of Ultra-Thin Poly(methyl methacrylate) Films(2011-08-08) Silva Hernandez, Carlos Ardenis A.The thermal conductivity of PMMA films with thicknesses from 5 to 50 nanometers and layered over a treated silicon substrate is explored numerically by the application of the reverse non-equilibrium molecular dynamics (NEMD) technique and the development of a coarse-grained model for PMMA, which allows for the simulation time of hundreds of nanoseconds required for the study of large polymer systems. The results showed a constant average thermal conductivity of 0.135 W/m_K for films thickness ranging from 15 to 50 nm, while films under 15 nm in thickness showed a reduction of 30% in their conductivity. It was also observed that polymer samples with a degree of polymerization equal to 25% of the entanglement length had 50% less thermal conductivity than films made of longer chains. The temperature profiles through the films thickness were as predicted by the Fourier equation of heat transfer. The relative agreement between the thermal conductivity from experiments (0.212 W/m_K for bulk PMMA) and the results from this investigation shows that with the proper interpretation of results, the coarse-grained NEMD is a useful technique to study transport coefficients in systems at larger nano scales.Item A study of bond-length fluctuations in transition metal oxides(2004) Yan, Jiaqiang; Goodenough, John B.Bond-length fluctuations with different origins have been investigated by thermal conductivity measurement performed on La1.60-xNd0.40SrxCuO4, RCoO3, and RVO3 single crystals grown by floating zone method. Thermal conductivity has been proved to be a sensitive probe to bond-length fluctuations in stronglycorrelated transition-metal oxides. Superconductivity in cuprates occurs at a crossover from localized to itinerant electronic behavior. The segregation of localized spins and delocalized holes into hole-poor and hole-rich regions in La2-xSrxCuO4 induces bond-length fluctuations via a strong electron-lattice coupling. This bond-length fluctuation suppresses in-plane thermal conductivity due to charge fluctuations in this quasi- 2D system. In the La1.60-xNd0.40SrxCuO4 system, the low-temperature orthorhombic (LTO) phase transforms into a low-temperature-tetragonal (LTT) phase with decreasing temperature. The hole-rich regions order into static stripes in the LTT phase of La2-x-yNdySrxCuO4; this charge order revives the phonon contribution to the thermal conductivity. The phonon thermal conductivity in the normal state of LTT phase and the LTO phase of some underdoped compositions of LSCO calls for reconsideration of the role of bond-length fluctuations on superconducting pairing in different structures. Suppression of the phonon thermal conductivity in the Mott-Hubbard insulator RCoO3 is interpreted to be caused by the spin-state transition from the low-spin t6 e 0 ground state to a higher spin-state, either intermediate-spin t5 e 1 or high-spin t4 e 2 , with increasing temperature. RVO3 offers us a unique chance to study the bond-length fluctuations caused by strong spinorbital-lattice coupling. An unusually strong orbital-lattice and spin-lattice coupling has been clearly demonstrated.Item Thermal and thermoelectric transport in organic and inorganic nanostructures(2012-08) Weathers, Annie C.; Shi, Li, Ph. D.; Tutuc, EmanuelThermal transport in nanowires and nanotubes has attached much attention due to their use in various functional devices and their use as a model system for low dimensional transport phenomena. The precise control of the crystal structure, defects, characteristic size, and electronic properties of nanowires has allowed for fundamental studies of phonon and electron transport in a variety of nanoscale systems. The thermal conductivity in nanostructured materials can vary greatly compared to bulk values owing to classical and quantum size effects. In this work, two model systems for investigating fundamental phonon transport were investigated for potential use in thermoelectric and thermal management applications. The thermoelectric properties of twin defect indium arsenide nanowires and the thermal conductivity of polythiophene nanofibers with improved polymer chain crystallinity were measured with a microfabricated measurement device. The effects of twin planes on reducing the mean free path of phonons in indium arsenide and the effects of improved chain alignment in increasing the thermal conductivity in polymer fibers is discussed.Item Thermal conductivity measurements of polyamide powder(2011-12) Yuan, Mengqi; Bourell, David Lee; Beaman, JosephAn important component in understanding the laser sintering process is knowledge of the thermal properties of the processed material. Thermal conductivity measurements of pure polyamide 12 and polyamide11 with multi-wall carbon nanotubes were conducted based on transient plane source technology using a Hot Disk® TPS500 conductivity measurement device. Polyamide powder was packed to three different densities in nitrogen at room temperature. Thermal diffusivity and conductivity were measured from 40°C to 170°C for both fresh powder and previously heated (“recycled”) powder. The fresh powder tests revealed that thermal conductivity increased linearly with temperature whereas for previously heated powder, more constant and higher thermal conductivity was observed as it formed a powder cake. Tests were also performed on fully dense polyamide 12 to establish a baseline. Polyamide 12 powder had a room-temperature thermal conductivity of approximately 0.1 W/mK which increased with temperature, whereas the bulk laser sintered polyamide 12 room-temperature value was 0.26 W/mK and generally decreased with increasing temperature.Item Thermal conductivity of silicon nanostructures containing impurities(2012-05) Gibbons, Michael; Estreicher, Stefan K.; Peralta, Luis G.; Holtz, Mark; Jiang, HongxingIn recent years the thermal conductivity of materials has become an important area of research. High thermal conductivity is useful for heat removal in electronic devices, and low thermal conductivity results in a larger thermoelectric figure of merit. The question is how to control the thermal conductivity. Many physical properties of semiconductors are successfully controlled by adding impurity atoms. The electrical conductivity can be substantially increased using dopants, the mechanical strength of Si can be improved by introducing O and N, optical properties are tailored using rare-earth elements such as Er. But is it possible to control the thermal conductivity with impurities as well? The impact of 'impurity scattering' (whatever this really means) on the thermal conductivity of materials is well known, but only qualitatively. However, it is never described at the atomic level. A better understanding of the physics behind the impact of impurities on the thermal conductivity could lead to devices which are tuned to the the desired thermal conductivity through the use of impurities. This dissertation introduces a first-principles method for calculating the thermal conductivity of semiconductor nanostructures containing impurities. The method is used to study the effects of various impurities on the thermal conductivity of silicon nanowires. The effects of impurity type, isotope, and concentration are studied. Surprisingly, the isotopic mass of the impurity appears to have a substantial impact on the thermal conductivity. Attempts to correlate this effect with the vibrational lifetime of impurity-related (localized) vibrational modes are discussed.Item Thermal transport in low-dimensional materials(2015-12) Marepalli, Prabhakar; Murthy, Jayathi; Shi, Li; Akinwande, Deji; Wang, Yaguo; Singh, DhruvRecent years have witnessed a paradigm shift in the world of electronics. Researchers have not only continued to postpone the long dreaded end-of-Moore’s-law, but have also opened up a new world of possibilities with electronics. The future of electronics is widely anticipated to be dominated by wearable and implantable devices, the realization of which will be made possible by the discovery of new materials. Graphene and hexagonal boron nitride (hBN) are two such materials that have shown promising properties to make these devices possible. It has been shown that an energy bandgap can be opened in graphene by patterning it as a narrow ribbon, by applying an electric displacement field to a bilayer configuration, and by other means. The possibility of tuning the bandgap makes graphene an ideal channel material for future electronics. Similarly, hexagonal boron nitride (hBN) and its ribbon configurations have been shown to be excellent dielectric materials. In addition, the similarities in the atomic configurations of graphene and hBN allow them to conform extremely well to each other, achieving atomically smooth interfaces. Graphene devices on hBN substrates have been shown to have mobilities an order of magnitude larger than graphene devices fabricated on silicon dioxide. In addition to their outstanding electrical properties, graphene and hBN have been shown to have excellent thermal properties compared to their traditional counterparts (silicon and silicon dioxide, respectively). More specifically, these materials have been shown to have size dependent thermal properties which may be used to tune device performance. In this thesis, we study the thermal transport of three important classes of materials – graphene nanoribbons, hBN nanoribbons and graphene-hBN heterostructures using the phonon Boltzmann transport equation in a linearized framework. An exact solution of the Boltzmann transport equation is obtained ensuring that normal and umklapp phonon scattering processes are appropriately treated. In the first part of the thesis, we present a computational technique called method of automatic code differentiation to calculate sensitivities in nanoscale thermal transport simulations. Key phonon parameters like force constants, group velocities, the Gruneisen parameter, etc., which can be expressed as sensitivities or derivatives, are computed using this technique. The derivatives computed using this technique are exact and can be generalized to any order with minimal effort. This technique can be unintrusively integrated with existing first-principles simulation codes to obtain the sensitivities of parameters computed therein to chosen inputs. The next focus is to investigate the thermal properties of three main classes of materials – graphene nanoribbons, hBN nanoribbons,and graphene-hBN heterostructures. For nanoribbons, we consider ribbons of varying widths to investigate the transition of key thermal properties with width. The lattice structure of the ribbon structures considered is fully resolved. An efficient parallelization technique is developed to handle the large number of atoms in a unit cell. The thermal conductivity is obtained by an iterative solution of the linearized Boltzmann transport equation. For graphene and hBN ribbons, we find that the thermal conductivity increases with the ribbon width following a power-law trend. The rate of increase of thermal conductivity with width for hBN ribbons is found to be slower compared to graphene. Flexural phonons are found to contribute to the majority of heat conduction in both the materials. Frequency- and polarization-resolved transport is analyzed for ribbon of all widths. The thermal conductivity of single- and few-layer hexagonal boron nitride is also computed and compared with measured data. It is found that the thermal conductivity of hBN based nanostructures (single-layer, few-layer and ribbons) is around 6-8 times smaller than that for the corresponding graphene-based nanostructure. The effect of strain in both these materials is investigated. We find that the thermal conductivity of single-layer hBN is very sensitive to strain whereas graphene shows relatively less sensitivity for the same strains. Finally, thermal transport in graphene-hBN heterostructures is simulated. Two different structures are considered – single-layer graphene on an hBN substrate, and bilayer graphene on an hBN substrate. Substrates of different thickness are considered. Due to the weak interlayer coupling in these heterostructures, it is found that the phonon dispersion remains largely unchanged from the dispersions of the individual layers. The only difference in dispersion is noticed for flexural phonons, which are the only modes affected by interlayer coupling. The addition of an hBN layer underneath the graphene/bilayer graphene layer is found to drastically reduce the thermal conductivity of the heterostructures. This reduction is due to breakdown of the selection rule for flexural phonons which results in increased scattering channels for these phonons. The thermal conductivity gradually decreases, saturating to a bulk value with an increase in the number of hBN layers. The results presented in this thesis are expected to help guide the design of graphene/hBN based flexible electronics.