Download or read book Thermal Resistance of Graphene Based Device written by Roisul Hasan Galib and published by . This book was released on 2020 with total page 74 pages. Available in PDF, EPUB and Kindle. Book excerpt: Thermal transport in low dimensional materials play a critical role in the functionality and reliability of modern electronics. In 2D material based device, interface between 2D materials and substrates often limit the heat flow through the device. This thesis discusses the experimental measurements and theoretical modeling of thermal resistances at 2D material based device. First, we measure thermal conductivity and thermal resistance of bulk substrate by three-omega method. Next, we model the interfacial thermal resistance between the 2D material and substrates with the aid of phonon mismatch modelling. Finally, we quantify the total thermal resistance of a graphene based device by series resistance model. Our analysis shows majority of the resistance comes from the interfaces, and material's intrinsic resistance becomes less significant at nanoscale. We find that the thermal resistance at the interface of graphene and substrate contributes to more than 50% of the total resistance. We attribute this high resistance at interface to weak Van der Waals interactions at the interface and dissimilar phonon vibrational properties of the materials. Our results suggest that increasing bond strength at the interface is an effective way to reduce the overall thermal resistance of the device. We compare our results with commonly used materials and interfaces, demonstrating the role of interface as potential application for heat guide or block in 2D material based device. This study will provide guide into the energy-efficient design and thermal management of 2D material devices.

Download or read book Electronic and Thermal Properties of Graphene written by Kyong Yop Rhee and published by Mdpi AG. This book was released on 2020-07 with total page 322 pages. Available in PDF, EPUB and Kindle. Book excerpt: This Special Issue includes recent research articles and extensive reviews on graphene-based next-generation electronics, bringing together perspectives from different branches of science and engineering. The papers presented in this volume cover experimental, computational and theoretical aspects of the electrical and thermal properties of graphene and its applications in batteries, electrodes, sensors and ferromagnetism. In addition, this Special Issue covers many important state-of-the-art technologies and methodologies regarding the synthesis, fabrication, characterization and applications of graphene-based nanocomposites.

Download or read book Polymer Nanocomposites written by Xingyi Huang and published by Springer. This book was released on 2016-05-06 with total page 354 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book focuses on the fundamental principles and recent progress in the field of electrical and thermal properties of polymer nanocomposites. The physical and chemical natures determining the electrical and thermal properties of polymer nanocomposites are discussed in detail. The authors describe the range of traditional and emerging polymer nanocomposites from nanoparticle and polymer composites to novel nanostructure based polymer nanocomposites. They include novel properties and potential applications, such as high-k, low-k, high thermal conductivity, antistatic, high voltage insulation, electric stress control, and thermal energy conversion among others.

Download or read book Polymer Nanocomposites written by Aravind Dasari and published by Springer. This book was released on 2016-06-28 with total page 311 pages. Available in PDF, EPUB and Kindle. Book excerpt: This highlights ongoing research efforts on different aspects of polymer nanocomposites and explores their potentials to exhibit multi-functional properties. In this context, it addresses both fundamental and advanced concepts, while delineating the parameters and mechanisms responsible for these potentials. Aspects considered include embrittlement/toughness; wear/scratch behaviour; thermal stability and flame retardancy; barrier, electrical and thermal conductivity; and optical and magnetic properties. Further, the book was written as a coherent unit rather than a collection of chapters on different topics. As such, the results, analyses and discussions presented herein provide a guide for the development of a new class of multi-functional nanocomposites. Offering an invaluable resource for materials researchers and postgraduate students in the polymer composites field, they will also greatly benefit materials

Download or read book Experimental and Theoretical Investigations of Thermal Transport in Graphene written by Mir Mohammad Sadeghi and published by . This book was released on 2015 with total page 246 pages. Available in PDF, EPUB and Kindle. Book excerpt: Graphene has been actively investigated because its unique structural, electronic, and thermal properties are desirable for a number of technological applications ranging from electronic to energy devices. The thermal transport properties of graphene can influence the device performances. Because of the high surface to volume ratio and confinement of phonons and electrons, the thermal transport properties of graphene can differ considerably from those in graphite. Developing a better understanding of thermal transport in graphene is necessary for rational design of graphene-based functional devices and materials. It is known that the thermal conductivity of single-layer graphene is considerably suppressed when it is in contact with an amorphous material compared to when it is suspended. However, the effects of substrate interaction in phonon transport in both single and multi-layer graphene still remains elusive. This work presents sensitive in-plane thermal transport measurements of few-layer and multi-layer graphene samples on amorphous silicon dioxide with the use of suspended micro-thermometer devices. It is shown that full recovery to the thermal conductivity of graphite has yet to occur even after the thickness of the supported multi-layer graphene sample is increased to 34 layers, which is considerably thicker than previously thought. This surprising finding is explained by the long intrinsic scattering mean free paths of phonons in graphite along both the basal-plane and cross-plane directions, as well as partially diffuse scattering of phonons by the graphene-amorphous support interface, which is treated by an interface scattering model developed for highly anisotropic materials. In addition, an experimental method is introduced to investigate electronic thermal transport in graphene and other layered materials through the measurement of longitudinal and transverse thermal and electrical conductivities and Seebeck coefficient under applied electric and magnetic fields. Moreover, this work includes an investigation of quantitative scanning thermal microscopy measurements of electrically biased graphene supported on a flexible polyimide substrate. Based on a triple scan technique and another zero heat flux measurement method, the temperature rise in flexible devices is found to be higher by more than one order of magnitude, and shows much more significant lateral heat spreading than graphene devices fabricated on silicon.

Download or read book Graphene written by Hongwei Zhu and published by Academic Press. This book was released on 2017-09-01 with total page 400 pages. Available in PDF, EPUB and Kindle. Book excerpt: Graphene: Fabrication, Characterizations, Properties and Applications presents a comprehensive review of the current status of graphene, especially focused on synthesis, fundamental properties and future applications, aiming to giving a comprehensive reference for scientists, researchers and graduate students from various sectors. Graphene, a single atomic layer of carbon hexagons, has stimulated a lot of research interest owing to its unique structure and fascinating properties. The book is devoted to understanding graphene fundamentally yet comprehensively through a wide range of issues in the areas of materials science, chemistry, physics, electronics and biology. The book is an important resource of comprehensive knowledge pertinent to graphene and to related expanding areas. This valuable book will attract scientists, researchers and graduate students in physics and chemistry because it aims at providing all common knowledge of these communities including essential aspects of material synthesis and characterization, fundamental physical properties and detailed chapters focused on the most promising applications. Presents a comprehensive and up-to-date review of current research of graphene, especially focused on synthesis, fundamental properties and future applications Includes not only fundamental knowledge of graphene materials, but also an overview of special properties for different potential applications of graphene in the fields of solar cells, photodetectors, energy storage, composites, environmental materials and bio-materials Emphasizes graphene-based applications that are quickly emerging as potential building blocks for nanotechnological commercial applications

Download or read book 2D Materials written by Phaedon Avouris and published by Cambridge University Press. This book was released on 2017-06-29 with total page 521 pages. Available in PDF, EPUB and Kindle. Book excerpt: Learn about the most recent advances in 2D materials with this comprehensive and accessible text. Providing all the necessary materials science and physics background, leading experts discuss the fundamental properties of a wide range of 2D materials, and their potential applications in electronic, optoelectronic and photonic devices. Several important classes of materials are covered, from more established ones such as graphene, hexagonal boron nitride, and transition metal dichalcogenides, to new and emerging materials such as black phosphorus, silicene, and germanene. Readers will gain an in-depth understanding of the electronic structure and optical, thermal, mechanical, vibrational, spin and plasmonic properties of each material, as well as the different techniques that can be used for their synthesis. Presenting a unified perspective on 2D materials, this is an excellent resource for graduate students, researchers and practitioners working in nanotechnology, nanoelectronics, nanophotonics, condensed matter physics, and chemistry.

Download or read book Impact of Thermal Conductivity on Energy Technologies written by Aamir Shahzad and published by BoD – Books on Demand. This book was released on 2018-09-05 with total page 210 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book is intended to provide a deep understanding on the advanced treatments of thermal properties of materials through experimental, theoretical, and computational techniques. This area of interest is being taught in most universities and institutions at the graduate and postgraduate levels. Moreover, the increasing modern technical and social interest in energy has made the study of thermal properties more significant and exciting in the recent years. This book shares with the international community a sense of global motivation and collaboration on the subject of thermal conductivity and its wide spread applications in modern technologies. This book presents new results from leading laboratories and researchers on topics including materials, thermal insulation, modeling, steady and transient measurements, and thermal expansion. The materials of interest range from nanometers to meters, bringing together ideas and results from across the research field.

Download or read book Graphene Based Polymer Nanocomposites in Electronics written by Kishor Kumar Sadasivuni and published by Springer. This book was released on 2015-01-19 with total page 383 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book covers graphene reinforced polymers, which are useful in electronic applications, including electrically conductive thermoplastics composites, thermosets and elastomers. It systematically introduces the reader to fundamental aspects and leads over to actual applications, such as sensor fabrication, electromagnetic interference shielding, optoelectronics, superconductivity, or memory chips. The book also describes dielectric and thermal behaviour of graphene polymer composites - properties which are essential to consider for the fabrication and production of these new electronic materials. The contributions in this book critically discuss the actual questions in the development and applications of graphene polymer composites. It will thus appeal to chemists, physicists, materials scientists as well as nano technologists, who are interested in the properties of graphene polymer composites.

Download or read book Natural Fiber Reinforced Composites written by Senthilkumar Krishnasamy and published by John Wiley & Sons. This book was released on 2022-04-18 with total page 356 pages. Available in PDF, EPUB and Kindle. Book excerpt: Natural Fiber-Reinforced Composites In-depth overview of thermal analysis of natural fiber-reinforced composites In Natural Fiber-Reinforced Composites: Thermal Properties and Applications, a team of distinguished researchers has delivered a comprehensive overview of the thermal properties of natural fiber-reinforced polymer composites. The book brings together information currently dispersed throughout the scientific literature and offers viable and environmentally friendly alternatives to conventional composites. The book highlights the thermal analysis of natural fiber-reinforced composites with techniques such as Thermogravimetric Analysis, Dynamic Mechanical Analysis, Thermomechanical Analysis, Differential Scanning Calorimetry, etc. This book provides: A thorough review of the thermal characterization of natural fiber-based hybrid composites Detailed investigation of the thermal properties of polymer composites reinforced with various natural fibers such as flax fiber, pineapple leaf fiber, sisal, sugar palm, grass fiber and cane fiber Discussions on the thermal properties of hybrid natural fiber-reinforced composites with various thermosetting and thermoplastic polymers Influence of nanofillers on the thermal stability and thermal decomposition characteristics of the natural fiber-based hybrid composites Natural Fiber-Reinforced Composites: Thermal Properties and Applications is a must-read for materials scientists, polymer chemists, and professionals working in the industry. This book is ideal for readers seeking to make an informed decision regarding materials selection for applications involving thermal insulation and elevated temperature. The suitability of natural fiber-reinforced composites in the automotive, mechanical, and civil engineering sectors is highlig

Download or read book Thermal Transport in Low dimensional Materials written by Prabhakar Marepalli and published by . This book was released on 2015 with total page 360 pages. Available in PDF, EPUB and Kindle. Book excerpt: Recent 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.

Download or read book Experimental Investigation of Thermal Transport in Graphene and Hexagonal Boron Nitride written by Insun Jo and published by . This book was released on 2012 with total page 270 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.

Download or read book Graphene and Its Derivatives written by Ishaq Ahmad and published by BoD – Books on Demand. This book was released on 2019-11-27 with total page 102 pages. Available in PDF, EPUB and Kindle. Book excerpt: Graphene and its derivatives are potential nanomaterials currently being widely investigated for diverse applications due to its exceptional mechanical, electrical, physical, and chemical properties. Examples of the applications include drug delivery, shape memory polymers, gene delivery, biosensor, tissue engineering, flexible electronic devices, antibacterial composites, photovoltaic devices, and physical sensors. Its excellent properties can be used to develop smart nanomaterials with enhanced properties for various advanced applications. There is no doubt that graphene-based nanomaterials are helping to develop next generation technologies with enhancing properties to change people's lifestyles. This book provides an overview of recent research and development of synthesis of graphene and its applications.

Download or read book Graphene based Energy Devices written by A. Rashid bin Mohd Yusoff and published by John Wiley & Sons. This book was released on 2015-02-17 with total page 464 pages. Available in PDF, EPUB and Kindle. Book excerpt: This first book dedicated to the topic provides an up-to-date account of the many opportunities graphene offers for robust, workable energy generation and storage devices. Following a brief overview of the fundamentals of graphene, including the main synthesis techniques, characterization methods and properties, the first part goes on to deal with graphene for energy storage applications, such as lithium-ion batteries, supercapacitors and hydrogen storage. The second part is concerned with graphene-based energy-generation devices, in particular conventional as well as microbial and enzymatic fuel cells, with chapters on graphene photovoltaics rounding off the book. Throughout, device architectures are not only discussed on a laboratory scale, but also ways for upscaling to an industrial level, including manufacturing processes and quality control. By bridging academic research and industrial development this is invaluable reading for materials scientists, physical chemists, electrochemists, solid state physicists, and those working in the electrotechnical industry.

Download or read book Electronic and Thermal Properties of Graphene written by Kyong Yop Rhee and published by . This book was released on 2020 with total page 324 pages. Available in PDF, EPUB and Kindle. Book excerpt: This Special Issue includes recent research articles and extensive reviews on graphene-based next-generation electronics, bringing together perspectives from different branches of science and engineering. The papers presented in this volume cover experimental, computational and theoretical aspects of the electrical and thermal properties of graphene and its applications in batteries, electrodes, sensors and ferromagnetism. In addition, this Special Issue covers many important state-of-the-art technologies and methodologies regarding the synthesis, fabrication, characterization and applications of graphene-based nanocomposites.

Download or read book Electronic Thermal Conductivity Measurements in Graphene written by Serap Yiğen and published by . This book was released on 2015 with total page 122 pages. Available in PDF, EPUB and Kindle. Book excerpt: The electronic thermal conductivity in graphene describes how energy is transported by the charge carriers in graphene, and how these carriers lose their energy via diffusion and interactions with phonons and impurities. Understanding these interactions can shed light on electron-phonon scatterings, thermal relaxation processes, and the electron cooling mechanisms in graphene. We developed a method to experimentally isolate the electronic thermal conductivity in suspended graphene transistors by adapting a Joule self-heating method. We extracted the electronic thermal conductivity, $K_{e}$, as a function of electron temperature and charge carrier density.​ We fabricated two-point suspended graphene transistors using micro-fabrication methods. We used the electrical contacts as source and drain to apply a bias voltage and a back-gate electrode to tune the carrier density. We adapted a Joule self-heating method in which we used graphene as its own heater and thermometer. To do so, we prepared thermometry (calibration) curves by measuring low-bias resistance of the graphene devices versus temperature. As we increased the bias voltage, we could measure and control the temperature of electrons. We solved a one-dimensional heat diffusion equation and extracted the electronic thermal conductivity. We studied our samples at low bias voltages and intermediate temperatures where the electron and lattice temperatures are decoupled. This minimized the energy transfer between phonons and charge carriers. Since the suspended devices isolate the graphene crystals from the substrate, there were no interactions with the substrate phonons and no heat leakage to the substrate. Therefore, the heat was diffused only by the charge carriers.​ We extracted the electronic thermal conductivity in intrinsic monolayer graphene over a temperature range of 20 to 300 K. We found that $K_{e}$ has a strong temperature dependence, ranging from 0.5 to 11 W/m.K. We compared our data with a model of diffusing charged quasiparticles which have the same mean free path and velocity as graphene's charge carriers. Data from three different devices are in very good agreement with the model, supporting that the heat is carried by diffusing Dirac quasiparticles. ​ We doped our devices using the back-gate electrode, and extracted $K_{e}$ in doped graphene over a temperature range of 50 to 160 K. We found that $K_{e}$ is proportional to the charge conductivity times the temperature, and thus the Wiedemann-Franz Law is obeyed in suspended graphene. The Lorenz coefficient is estimated to be 1.1 to 1.7 $ imes$ 10${̂-8}$ W $Omega$ K${̂-2}$. We observed a strong thermal transistor effect in our devices as the charge carrier density is changed from $pprox$ 0.5 to 1.8 $ imes$ 10${̂11}$ cm${̂-2}$, showing that $K_{e}$ can be tuned by more than a factor of 2 by applying a few volts of gate voltage. ​ The methods presented here could be extended to bilayer graphene devices and other two-dimensional materials to isolate $K_{e}$ to study electron-electron and electron-phonon interactions. The ability to control $K_{e}$ could be useful for energy harvesting in nano and opto-electronic devices.

Download or read book Optothermal Raman Studies of Thermal Properties of Graphene Based Films written by Hoda Malekpour and published by . This book was released on 2017 with total page 118 pages. Available in PDF, EPUB and Kindle. Book excerpt: Efficient thermal management is becoming a critical issue for development of the next generation of electronics. As the size of electronic devices shrinks, the dissipated power density increases, demanding a better heat removal. The discovery of graphene's unique electrical and thermal properties stimulated interest of electronic industry to development of graphene based technologies. In this dissertation, I report the results of my investigation of thermal properties of graphene derivatives and their applications in thermal management. The dissertation consists of three parts. In the first part, I investigated thermal conductivity of graphene laminate films deposited on thermally insulating polyethylene terephthalate substrates. Graphene laminate is made of chemically derived graphene and few layer graphene flakes packed in overlapping structure. Two types of graphene laminate were studied: as deposited and compressed. The thermal conductivity of the laminate was found to be in the range from 40 W/mK to 90 W/mK at room temperature. It was established that the average size and the alignment of graphene flakes are parameters dominating the heat conduction. In the second part of this dissertation, I investigated thermal conductivity of chemically reduced freestanding graphene oxide films. It was found that the in-plane thermal conductivity of graphene oxide can be increased significantly using chemical reduction and temperature treatment. Finally, I studied the effect of defects on thermal conductivity of suspended graphene. The knowledge of the thermal conductivity dependence on the concentration of defects can shed light on the strength of the phonon - point defect scattering in two-dimensional materials. The defects were introduced to graphene in a controllable way using the low-energy electron beam irradiation. It was determined that as the defect density increases the thermal conductivity decreases down to about 400 W/mK, and then reveal saturation type behavior. The thermal conductivity dependence on the defect density was analyzed using the Boltzmann transport equation and molecular dynamics simulations. The obtained results are important for understanding phonon transport in two-dimensional systems and for practical applications of graphene in thermal management.