Thermal Transport Behavior Research Articles

Functionalization mediates heat transport in graphene nanoflakes.

The high thermal conductivity of graphene and few-layer graphene undergoes severe degradations through contact with the substrate. Here we show experimentally that the thermal management of a micro heater is substantially improved by introducing alternative heat-escaping channels into a graphene-based film bonded to functionalized graphene oxide through amino-silane molecules. Using a resistance temperature probe for in situ monitoring we demonstrate that the hotspot temperature was lowered by ∼28 °C for a chip operating at 1,300 W cm−2. Thermal resistance probed by pulsed photothermal reflectance measurements demonstrated an improved thermal coupling due to functionalization on the graphene–graphene oxide interface. Three functionalization molecules manifest distinct interfacial thermal transport behaviour, corroborating our atomistic calculations in unveiling the role of molecular chain length and functional groups. Molecular dynamics simulations reveal that the functionalization constrains the cross-plane phonon scattering, which in turn enhances in-plane heat conduction of the bonded graphene film by recovering the long flexural phonon lifetime.

Novel Strategies for the Mass Production of Corrosion-Resistant Nanowires and Their Bottom-up Assembly into Isotropic/Anisotropic Energy Conversion Devices

Over the last two decades, nanomaterials have been demonstrated to be useful in renewable energy generation. In particular, nanowires, a subset of the broad class of nanomaterials, have been employed not only for efficiently converting solar energy into electricity (using dye-sensitized solar cells), but also efficiently converting heat energy into electricity (using thermoelectrics). In addition, they have been utilized as components in the design and fabrication of optoelectronics, sensors, and photocatalysts for producing hydrogen fuel from water. Enhanced charge transfer resulting from the single-crystalline nature of nanowires, coupled with the control of their electrical, thermal and electronic properties through size variations, allowed for accomplishing this task. Most of these demonstrations were made using devices either based on single nanowires or small-scale nanowire arrays and mats. From these studies, however, it is not yet clear whether novel properties exhibited by individual nanowires and small-scale nanowire mats and arrays are extendable to devices utilizing large-scale nanowire assemblies. More specifically, these studies do not indicate whether it is possible to assemble these single-crystalline nanowires in an interface-engineered manner into assemblies with controlled porosity so as to ensure that the resulting assemblies also exhibit the superior properties observed in individual nanowires. These studies also fail to account for the environmental impact of nanowires. They do not account for the air-, moisture- and acid-assisted degradation of the nanowires. This data is essential for two reasons: the enhanced surface areas of nanowires make them more vulnerable to air- and moisture-assisted degradation, compared to bulk materials; and the operation of devices employing these nanowires in ambient air necessitates that the nanowire components of these devices be resistant to acid-assisted degradation, as rain water is acidic in nature. Therefore, at this juncture, it is imperative to ask the following questions. What quantities of nanowires need to be produced for mass producing the renewable energy devices needed for mankind? What should be the set of properties nanowires need to exhibit for their use in renewable energy device fabrication for mankind? Finally, what strategies should be employed for industrial scale production and assembly of nanowires into energy conversion devices? Answering the first two questions is possible using a photocatalysts useful for spilling water into hydrogen fuel with the aid of sunlight as an illustrative example. Considering a hydrogen production rate of 10 mM/hour per gram of nanowire photocatalyst, 1.65 x 108 Kg of nanowires are required to generate energy from hydrogen at a rate equal to 1% of the current mankind’s energy consumption rate (estimated to be 13 terawatts as of 2007). To put this in perspective, the scale of production of nanowires should be on a level similar to that accomplished by the steel industry (≈2x1012 kg of steel was produced in the world in 2005). Therefore, photoelectrochemical production of hydrogen from water requires the production of the photocatalysts on million metric ton scales. Secondly, the nanowires produced should exhibit the following characteristics: They should be (a) stable in water for extended periods of time (corrosion resistant), (b) not degrade and release their constituents into water, (c) resistant to fouling for extended periods of time, (d) made from inexpensive raw materials, (e) bandgap engineerable, (f) easily removable from water following disinfection, without the need for specialized membranes and filters (i.e., they should also be amenable to removal by filtration from water through conventional water treatment facilities used by municipalities, counties and states across US). Mass producing such corrosion-resistant nanowires is ideally possible by developing strategies that rely only on direct reactions of the raw materials at low/moderate temperatures, without reliance on expensive catalysts/templates. Assembly strategies developed for consolidating nanowires should ensure control over the properties of the interfaces between them after assembly. In an ideal case, interfaces (or bridges) between the nanowires should have the same chemical composition and phase as the nanowires themselves. In addition, assembly strategies should offer the possibility of consolidating nanowires either in a randomly oriented fashion or aligned fashion. Assemblies of aligned nanowires aid in utilizing any direction dependent properties of nanostructured materials. This cannot be accomplished using hot pressing or spark plasma sintering. In this talk, strategies developed for the mass production of nanowires by direction reaction of component elements, their assembly via welding and imparting stabilities to them using non-conformal/conformal decoration of their surfaces with organic/inorganic molecules will be discussed. In addition, shear assisted consolidation of nanowires into aligned and welded nanowire assemblies will be discussed. Finally, the electrical and thermal transport behavior through these nanowire assemblies will be discussed.

Thermal Transport in Single-Layer MoS2and Black Phosphorus Transistors

Ballistic thermal conductance in single-layer MoS2 and black phosphorus (BP) is calculated based on the phonon dispersion. In single-layer MoS2, the ballistic thermal conductance is almost isotropic of a value of $10.7\times 10^{8}$ W/ $\text{m}^{2}\text{K}$ ; while in single-layer BP, the in-plane ballistic thermal conductance is anisotropic with a value of $12.8\times 10^{8}$ W/ $\text{m}^{2}\text{K}$ in the zigzag direction and $7.1\times 10^{8}$ W/ $\text{m}^{2}\text{K}$ in the armchair direction. Thermal transport behaviors at both steady state and transient state in double-gated MOSFET-like transistors with single-layer MoS2 and BP used as the channel material are studied. The conventional Fourier’s law with contact temperatures as the boundary conditions and modified boundary conditions that can capture quasi-ballistic phonon transport effects are used. The simulation results show that Fourier’s law with contact temperatures as the boundary conditions underestimate the peak temperature rise by 64% in single-layer MoS2 transistors and 56% in single-layer BP transistors when the channel length is comparable to the phonon mean free path. And the strong anisotropic thermal properties of single-layer BP lead to higher temperature rise in the armchair direction than that in the zigzag direction.

Molecular dynamics study of thermal transport across grain boundaries in silicon carbide nanorod

The thermal transport behaviors of Σ3, Σ9 and Σ11 grain boundaries (GBs) in silicon carbide nanorod are investigated by using nonequilibrium molecular dynamics (NEMD) simulation. Temperature changes suddenly at the boundaries if a constant heat flux is assumed. The thermal conductance of these GBs is found several times larger than that of interfaces of other materials previously reported. Furthermore the interfacial thermal resistance increases with elevated temperature above 500 K. Our results give theoretical guidance to understand the underlying thermal transport mechanism in silicon carbide, and may be helpful to design silicon carbide materials for high temperature applications.

Design of crystal-like aperiodic solids with selective disorder-phonon coupling.

Functional materials design normally focuses on structurally ordered systems because disorder is considered detrimental to many functional properties. Here we challenge this paradigm by showing that particular types of strongly correlated disorder can give rise to useful characteristics that are inaccessible to ordered states. A judicious combination of low-symmetry building unit and high-symmetry topological template leads to aperiodic ‘procrystalline' solids that harbour this type of disorder. We identify key classes of procrystalline states together with their characteristic diffraction behaviour, and establish mappings onto known and target materials. The strongly correlated disorder found in these systems is associated with specific sets of modulation periodicities distributed throughout the Brillouin zone. Lattice dynamical calculations reveal selective disorder-driven phonon broadening that resembles the poorly understood ‘waterfall' effect observed in relaxor ferroelectrics. This property of procrystalline solids suggests a mechanism by which strongly correlated topological disorder might allow independently optimized thermal and electronic transport behaviour, such as required for high-performance thermoelectrics.

Simple model for effective thermal conductivity of bulk nanostructured materials

The ability to design and predict effective thermal transport properties in bulk nanostructured materials is becoming increasingly important as the demand for higher performance and higher efficiency electronics and energy conversion devices grows. A tremendous effort has been focused on understanding and improving our ability to predict thermal transport at an individual interface, resulting in new models that account for phonon populations and electron–phonon coupling. Despite the success of this work, there has been no effort to produce a simple analytic model to predict the effective thermal conductivity of bulk materials that include nanoscale features.This paper focuses on applying knowledge of thermal interface resistances to the development of a simple analytic model that can be used for the efficient prediction of effective thermal transport behavior to aid in the design of thermoelectric materials and microelectronic devices. To this end, molecular dynamics simulations are performed to generate an initial model based on the effective thermal conductivity of in-plane and cross-plane superlattices and embedded nanoparticle and nanowire arrays. The model is then validated and generalized by comparing to existing computational and experimental data. Results shows that the effective thermal conductivity calculated from the analytic model agrees well with that of various systems of different materials and geometries, and provides the ability to predict effective system thermal conductivity of these material systems with variable interface area for optimization of thermal transport properties.

Unusual thermal transport behavior in self-assembled fullerene nanorods

Ultralow thermal conductivity (less than 0.06 W m−1 K−1) is observed for self-assembled C60 nanorods at room temperature.

Characteristics of acoustic phonon transport and thermal conductance in multi-terminal graphene junctions

By using non-equilibrium Greens function method, we investigate the transmission rate of acoustic phonon and thermal conductance through a parallel multi-terminal graphene junctions, the relationship between the thermal-transport property in each terminal and the number of quantum terminals, the relationship between the thermal-transport property in each terminal and the relative position of quantum terminals in quantum structure, and also study the thermaltransport property in each terminal and the rough degree of edge structure. The results show that when the graphene chains (dimer lines) across the ribbon width are fixed, the increase of the number of the parallel multi-terminal graphene junctions can reduce the transmission rate of the phonons and the thermal conductance of each output terminal as well. This is because the increase of the number of the graphene junctions can lead to the decrease of the transverse dimension of the each output terminal, which enlarges the strength of the phonon scattering and results in the reduction of the phonon transmission. Owing to long distance scattering, the transmission rate of the phonons of the furthest distant output terminal is the smallest, and also the thermal conductance of the furthest output terminal is the smallest. On the contrary, the strength of the phonon scattering is the weakest for the closest output terminal. So the transmission rate of the phonons is the biggest, which induces the thermal conductance to be the biggest. The thermal conductance of the middle-output terminal depends sensitively on the structural parameters of each terminal. This is because mainly the relative position between the middle-output terminal and the phonon-input terminal is related closely to the structural parameters of each terminal, which can influence the strength of the phonon scattering and the transmission rate of the phonons. However, the thermal conductances in the top and bottom output terminals are just sensitively dependent on the structural parameters of the respective output terminal. This is because the relative position between the top (or bottom) output terminal and the phonon-input terminal is only related to the structural parameters of the respective output terminal. The rough edge structure can reduce obviously the transmission rate of the phonons, and the thermal conductance of the closest output terminal as well. The rough edge structure can modulate slightly the transmission rate of the phonons and the thermal conductance of the other output terminal. The total thermal conductance is related closely to the number of total graphene chains, the number of the multi-terminal graphene junctions, and the rough degree of edge structure. These results shed new light on the understanding of the thermal transport behaviors of multi-terminal junction quantum devices based on graphene-based nanomaterials in practical application.

熱伝導性フィラー含有シリコーン樹脂の熱伝導挙動の可視化

熱伝導性フィラー含有シリコーン樹脂の熱伝導挙動の可視化

Electronic and thermal transport study of sinusoidally corrugated nanowires aiming to improve thermoelectric efficiency

Improvement of thermoelectric efficiency has been very challenging in the solid-state industry due to the interplay among transport coefficients which measure the efficiency. In this work, we modulate the geometry of nanowires to interrupt thermal transport with causing only a minimal impact on electronic transport properties, thereby maximizing the thermoelectric power generation. As it is essential to scrutinize comprehensively both electronic and thermal transport behaviors for nano-scale thermoelectric devices, we investigate the Seebeck coefficient, the electrical conductance, and the thermal conductivity of sinusoidally corrugated silicon nanowires and eventually look into an enhancement of the thermoelectric figure-of-merit from the modulated nanowires over typical straight nanowires. A loss in the electronic transport coefficient is calculated with the recursive Green function along with the Landauer formalism, and the thermal transport is simulated with the molecular dynamics. In contrast to a small influence on the thermopower and the electrical conductance of the geometry-modulated nanowires, a large reduction of the thermal conductivity yields an enhancement of the efficiency by 10% to 35% from the typical nanowires. We find that this approach can be easily extended to various structures and materials as we consider the geometrical modulation as a sole source of perturbation to the system.

A dynamic tester to evaluate the thermal and moisture behaviour of the surface of textiles

The thermal and moisture behaviour of the microclimate of textiles is crucial in determining the physiological comfort of apparel, but it has not been investigated sufficiently due to the lack of particular evaluation techniques. Based on sensing, temperature controlling and wireless communicating technology, a specially designed tester has been developed in this study to evaluate the thermal and moisture behaviour of the surface of textiles in moving status. A temperature acquisition system and a temperature controllable hotplate have been established to test temperature and simulate the heat of human body, respectively. Relative humidity of the surface of fabric in the dynamic process has been successfully tested through sensing. Meanwhile, wireless communication technology was applied to transport the acquired data of temperature and humidity to computer for further processing. Continuous power supply was achieved by intensive contact between an elastic copper plate and copper ring on the rotating shaft. This tester provides the platform to evaluate the thermal and moisture behaviour of textiles. It enables users to conduct a dynamic analysis on the temperature and humidity together with the thermal and moisture transport behaviour of the surface of fabric in moving condition. Development of this tester opens the door of investigation on the micro-climate of textiles in real time service, and eventually benefits the understanding of the sensation comfort and wellbeing of apparel wearers.

A dynamic tester to evaluate the thermal and moisture behaviour of the surface of textiles

The thermal and moisture behaviour of the microclimate of textiles is crucial in determining the physiological comfort of apparel, but it has not been investigated sufficiently due to the lack of particular evaluation techniques. Based on sensing, temperature controlling and wireless communicating technology, a specially designed tester has been developed in this study to evaluate the thermal and moisture behaviour of the surface of textiles in moving status. A temperature acquisition system and a temperature controllable hotplate have been established to test temperature and simulate the heat of human body, respectively. Relative humidity of the surface of fabric in the dynamic process has been successfully tested through sensing. Meanwhile, wireless communication technology was applied to transport the acquired data of temperature and humidity to computer for further processing. Continuous power supply was achieved by intensive contact between an elastic copper plate and copper ring on the rotating shaft. This tester provides the platform to evaluate the thermal and moisture behaviour of textiles. It enables users to conduct a dynamic analysis on the temperature and humidity together with the thermal and moisture transport behaviour of the surface of fabric in moving condition. Development of this tester opens the door of investigation on the micro-climate of textiles in real time service, and eventually benefits the understanding of the sensation comfort and wellbeing of apparel wearers.

Networked nanoconstrictions: An effective route to tuning the thermal transport properties of graphene

Tuning the thermal transport properties of graphene is under intense investigation to achieve novel material functionalities. Here we propose a strategy of networked nanoconstrictions to maintain the ultrahigh thermal conductivity like in design of graphene-based integrated circuits, or to reduce to the minimum for thermoelectrics of energy conversion. By using molecular dynamics simulations, we study the thermal transport behavior in the 18.2-nm-long graphene sheet and firstly report the characteristics of the thermal resistance arising from single-nanoconstriction, inversely proportional to the constriction width and independent of geometry shapes, which agrees well with the derived two-dimensional ballistic resistance model. After the nanoconstrictions are networked, the results elucidate a parallel relationship between ballistic resistances in parallel systems, and especially, a complicated superimposed effect of arrangement mode on ballistic resistances in series systems governed by the phonon localization and corresponding change of phonon transmission angle. Such anomalous phenomenon causes a decrease or further increase in the total ballistic resistance, e.g., tuning the thermal transport property of graphene as much as or more than 96% with specific nanoconstriction networks. We believe this feasible and versatile route will effectively expand potential applications of two-dimensional graphene and also pave the way for three-dimensional materials in the future.

Diverse lattice dynamics in ternary Cu-Sb-Se compounds.

Searching and designing materials with extremely low lattice thermal conductivity (LTC) has attracted considerable attention in material sciences. Here we systematically demonstrate the diverse lattice dynamics of the ternary Cu-Sb-Se compounds due to the different chemical-bond environments. For Cu3SbSe4 and CuSbSe2, the chemical bond strength is nearly equally distributed in crystalline bulk, and all the atoms are constrained to be around their equilibrium positions. Their thermal transport behaviors are well interpreted by the perturbative phonon-phonon interactions. While for Cu3SbSe3 with obvious chemical-bond hierarchy, one type of atoms is weakly bonded with surrounding atoms, which leads the structure to the part-crystalline state. The part-crystalline state makes a great contribution to the reduction of thermal conductivity that can only be effectively described by including a rattling-like scattering process in addition to the perturbative method. Current results may inspire new approaches to designing materials with low lattice thermal conductivities for high-performance thermoelectric conversion and thermal barrier coatings.

Size- and structure-dependence of thermal and mechanical behaviors of single-crystalline and polytypic superlattice ZnS nanowires

Molecular dynamics (MD) simulations are carried out to study the thermal and mechanical behaviors of single-crystalline wurtzite (WZ), zinc-blende (ZB), and polytypic superlattice ZnS nanowires containing alternating WZ and ZB regions with thicknesses between 1.85 nm and 29.62 nm under tensile loading. The wires analyzed have diameters between 1.77 nm and 5.05 nm. The Green-Kubo method is used to calculate the thermal conductivity of the wires at different deformed states. A non-equilibrium MD approach is used to analyze the thermal transport behavior at the interfaces between different structural regions in the superlattice nanowires (SLNWs). The Young's modulus and thermal conductivity of ZB nanowires are approximately 2%–12% and 23%–35% lower than those of WZ nanowires, respectively. The lower initial residual compressive stress due to higher irregularity of surface atoms causes the Young's modulus of ZB nanowires to be lower. The dependence of the thermal conductivity on structure comes from differences in phonon group velocities associated with the different wires. The thermal conductivity of polytypic superlattice nanowires is up to 55% lower than that of single-crystalline nanowires, primarily because of phonon scattering at the interfaces and the resulting lower effective phonon mean free paths for each structural region. As the periodic lengths (1.85–29.62 nm) and specimen lengths (14.81–59.24 nm) of SLNWs decrease, these effects become more pronounced, causing the thermal conductivity to further decrease by up to 30%.

Thermal transport behaviors of suspended graphene sheets with different sizes

We experimentally investigated the thermal transport behaviors of suspended single-layer graphene (SLG) and graphene sheets with different sizes. The length of graphene sheet suspended on a groove of SiO2/Si substrate is equal to the groove width (508 μm). Laser cutting technology is used to tailor SLG to obtain graphene sheets with different widths. A one-dimensional steady-state method is applied to measure the thermal conductivity, k, of graphene sheet and SLG at different conditions. k of the graphene sheets with different widths is much higher than that of SLG and increases with the decrease of graphene sheet width, and the increase in k appears significant when the width decreases greatly. We proposed that the Umklapp scattering may become dominant in phonon transport with the increasing of graphene sheet width, resulting in the thermal conductivity decrease. Another anomalous behavior is that k increases firstly and then decreases with an increase in temperature, and has a peak value at around 50 °C. The competition effects of boundary scattering, point defect scattering, grain-boundary phonon scattering, and Umklapp scattering in phonon transport may contribute to this phenomenon. A maximum k value of 2450.55 W m−1 K−1 is obtained under the condition of 50 °C when the SLG is cut for the fourth times. The electrical conductivity of graphene sheets with different widths at room temperature is in the range of 0.7584–2.7002 × 106 Ω−1 m−1. The graphene sheet with small width exhibits better thermal and electronic properties that make them attractive for the fabrication of nanoscale electronics and promising for the application in thermal management.

Understanding unusual thermal transport behavior in soft materials under mechanical strain – A molecular dynamics study

Experiments have shown a dependence of the thermal conductivity of soft polymer materials on shear stress, which is common to several applications, such as film processing, fiber spinning, blow molding, and vacuum forming. Experiments reveal that the conductivity initially decreases with shear, but then increases as additional shear rate is applied. Based on molecular principles, we hypothesize that when molecules are initially placed under tension and extended, they disentangle, which reduces the number of points of interaction and diminishes the heat flux. Further molecular stretching increases this flux because the molecules are now better axially aligned. Molecular dynamics simulations confirm this competition and reproduce the inflection in the flux-strain relationship, which has not been previously explained.

Sub-amorphous thermal conductivity in ultrathin crystalline silicon nanotubes.

Thermal transport behavior in nanostructures has become increasingly important for understanding and designing next generation electronic and energy devices. This has fueled vibrant research targeting both the causes and ability to induce extraordinary reductions of thermal conductivity in crystalline materials, which has predominantly been achieved by understanding that the phonon mean free path (MFP) is limited by the characteristic size of crystalline nanostructures, known as the boundary scattering or Casimir limit. Herein, by using a highly sensitive measurement system, we show that crystalline Si (c-Si) nanotubes (NTs) with shell thickness as thin as ∼5 nm exhibit a low thermal conductivity of ∼1.1 W m(-1) K(-1). Importantly, this value is lower than the apparent boundary scattering limit and is even about 30% lower than the measured value for amorphous Si (a-Si) NTs with similar geometries. This finding diverges from the prevailing general notion that amorphous materials represent the lower limit of thermal transport but can be explained by the strong elastic softening effect observed in the c-Si NTs, measured as a 6-fold reduction in Young's modulus compared to bulk Si and nearly half that of the a-Si NTs. These results illustrate the potent prospect of employing the elastic softening effect to engineer lower than amorphous, or subamorphous, thermal conductivity in ultrathin crystalline nanostructures.

First principles study of thermal conductivity cross-over in nanostructured zinc-chalcogenides

Systematic first principles studies of zinc-chalcogenides have been performed to understand their thermal transport behaviour. We have applied the Boltzmann transport equation in the relaxation time approximation to calculate the thermal conductivity of ZnS, ZnSe, and ZnTe. We find a thermal conductivity cross-over between ZnS and ZnSe at nanostructure sizes around 0.1–0.2 μm and explain this in terms of the different contributions of phonon modes in these materials. We study the effect of nanostructuring using both the diffusive boundary scattering and confined mean free path limit and discuss the variations in the results. Furthermore, we show the strong influence of isotope scattering on the thermal conductivity. The calculated thermal conductivity is found to be strongly dependent on the volume and we explain the observed differences between local density and generalized gradient approximation calculations. We compare further calculated thermal properties, such as the thermal expansion coefficient, to experiment to validate our approach.

Thermal transport behavior of polycrystalline graphene: A molecular dynamics study

The thermal transport behavior of polycrystalline graphene is studied using molecular dynamics simulations, with focus on the effects of grain size, tensile strain, and temperature on the thermal conductivity. All the simulation samples have the same overall dimensions of 30 × 30 nm with average grain sizes ranging from 2.5 to 12.5 nm. It is found that polycrystalline graphene exhibits a significant reduction in thermal conductivity compared to single-crystalline graphene, and the smaller the grain size is, the more the thermal conductivity drops. The thermal conductivity of polycrystalline graphene with average grain size of 2.5 nm is only about 20% of single-crystalline graphene. However, the thermal conductivity of polycrystalline graphene is less sensitive to both the applied strain and temperature than that of single-crystalline graphene. The underlying mechanisms for the differences in thermal behavior are examined and discussed. These findings are important for the thermal management of graphene-based devices.