Thermal Transport Behavior Research Articles

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.

Electrical and Thermal Transport Behavior in Zn-Doped BiCuSeO Oxyselenides

Oxide-based thermoelectric materials such as BiCuSeO have been researched for several years to optimize their thermoelectric performance. In this article, we present a series of Zn-doped BiCuSeO ceramics produced by fine processing. The results indicate that Zn doping can effectively increase the power factor through Cu vacancies along with a suppressed thermal conductivity through increased phonon scattering at ZnSe impurity phase. Consequently, a higher ZT value of 0.65 at 873 K is obtained for 10% Zn doping, being about three times larger than that of the pure sample.

Interfacial thermal conductance of a silicene/graphene bilayer heterostructure and the effect of hydrogenation.

van der Waals heterostructures, obtained by stacking layers of isolated two-dimensional atomic crystals like graphene (GE) and silicene (SE), are one of emerging nanomaterials for the development of future multifunctional devices. Thermal transport behaviors at the interface of these heterostructures play a pivotal role in determining their thermal properties and functional performance. Using molecular dynamics simulations, the interfacial thermal conductance G of an SE/GE bilayer heterostructure is studied. Simulations show that G of a pristine SE/GE bilayer at room temperature is 11.74 MW/m(2)K when heat transfers from GE to SE, and is 9.52 MW/m(2)K for a reverse heat transfer, showing apparent thermal rectification effects. In addition, G increases monotonically with both the temperature and the interface coupling strength. Furthermore, hydrogenation of GE is efficient in enhancing G if an optimum hydrogenation pattern is adopted. By changing the hydrogen coverage f, G can be controllably manipulated and maximized up to five times larger than that of pristine SE/GE. This study is helpful for understanding the interface thermal transport behaviors of novel van der Waals heterostructures and provides guidance for the design and control of their thermal properties.

Thermal transport properties of defective graphene: A molecular dynamics investigation

In this work the thermal transport properties of graphene nanoribbons with randomly distributed vacancy defects are investigated by the reverse non-equilibrium molecular dynamics method. We find that the thermal conductivity of the graphene nanoribbons decreases as the defect coverage increases and is saturated in a high defect ratio range. Further analysis reveals a strong mismatch in the phonon spectrum between the unsaturated carbon atoms in 2-fold coordination around the defects and the saturated carbon atoms in 3-fold coordination, which induces high interfacial thermal resistance in defective graphene and suppresses the thermal conductivity. The defects induce a complicated bonding transform from sp2 to hybrid sp—sp2 network and trigger vibration mode density redistribution, by which the phonon spectrum conversion and strong phonon scattering at defect sites are explained. These results shed new light on the understanding of the thermal transport behavior of graphene-based nanomaterials with new structural configurations and pave the way for future designs of thermal management phononic devices.

From kinetic to collective behavior in thermal transport on semiconductors and semiconductor nanostructures

We present a model which deepens into the role that normal scattering has on the thermal conductivity in semiconductor bulk, micro, and nanoscale samples. Thermal conductivity as a function of the temperature undergoes a smooth transition from a kinetic to a collective regime that depends on the importance of normal scattering events. We demonstrate that in this transition, the key point to fit experimental data is changing the way to perform the average on the scattering rates. We apply the model to bulk Si with different isotopic compositions obtaining an accurate fit. Then we calculate the thermal conductivity of Si thin films and nanowires by only introducing the effective size as additional parameter. The model provides a better prediction of the thermal conductivity behavior valid for all temperatures and sizes above 30 nm with a single expression. Avoiding the introduction of confinement or quantum effects, the model permits to establish the limit of classical theories in the study of the thermal conductivity in nanoscopic systems.

Atomistic study of the influence of lattice defects on the thermal conductivity of silicon

Lattice defects such as vacancies, voids and dislocations are inevitably present in any material of technological interest. In this work, non-equilibrium molecular dynamics simulations are conducted to investigate how the monatomic vacancies and nanovoids influence the lattice thermal conductivity of silicon. The results show a clear non-linear decrease of the thermal conductivity with increasing defect volume fraction. Furthermore, it is found that for a given volume fraction of defects, a random distribution shows a lower lattice thermal conductivity. To develop a fundamental understanding of these observations, the spectral energy densities for all phonon branches obtained from 2D Fourier transformations of the atomic trajectories are analyzed. This yields the mean phonon group velocities and relaxation times, which are the main physical quantities contributing to the lattice thermal conductivity. Our analysis reveals that the phonon relaxation time is the most important parameter for describing the degrading of the thermal transport behavior in the defected structures.

Apparent Temperature Jump and Thermal Transport in Channels With Streamwise Rib and Cavity Featured Superhydrophobic Walls at Constant Heat Flux

This paper presents an analytical investigation of constant property, steady, fully developed, laminar thermal transport in a parallel-plate channel comprised of metal superhydrophobic (SH) walls. The superhydrophobic walls considered here exhibit microribs and cavities aligned in the streamwise direction. The cavities are assumed to be nonwetting and contain air, such that the Cassie–Baxter state is the interfacial state considered. The scenario considered is that of constant heat flux through the rib surfaces with negligible thermal transport through the air cavity interface. Closed form solutions for the local Nusselt number and local wall temperature are presented and are in the form of infinite series expansions. The analysis show the relative size of the cavity regions compared to the total rib and cavity width (cavity fraction) exercises significant influence on the aggregate thermal transport behavior. Further, the relative size of the rib and cavity module width compared to the channel hydraulic diameter (relative module width) also influences the Nusselt number. The spatially varying Nusselt number and wall temperature are presented as a function of the cavity fraction and the relative module width over the ranges 0–0.99 and 0.01–1.0, respectively. From these results, the rib/cavity module averaged Nusselt number was determined as a function of the governing parameters. The results reveal that increases in either the cavity fraction or relative module width lead to decreases in the average Nusselt number and results are presented over a wide range of conditions from which the average Nusselt number can be determined for heat transfer analysis. Further, analogous to the hydrodynamic slip length, a temperature jump length describing the apparent temperature jump at the wall is determined in terms of the cavity fraction. Remarkably, it is nearly identical to the hydrodynamic slip length for the scenario considered here and allows straightforward determination of the average Nusselt number for any cavity fraction and relative rib/cavity module width.

The Influence of Carbon Nanotube Aspect Ratio on Thermal Conductivity Enhancement in Nanotube–Polymer Composites

We report and model a linear increase in the thermal conductivity (κ) of polymer composites incorporated with relatively low length/diameter aspect ratio multiwalled carbon nanotubes (CNTs). There was no evidence of percolation-like behavior in the κ, at/close to the theoretically predicted threshold, which was attributed due to the interfacial resistance between the CNT and the polymer matrix. Concomitantly, the widely postulated high thermal conductivity of CNTs does not contribute to the net thermal conductivity of the composites. Through estimating the interfacial resistance and the thermal conductivity of the constituent CNTs, we conclude that our experimental and modeling approaches can be used to study thermal transport behavior in nanotube–polymer composites.

Isoflux Nusselt Number and Slip Length Formulae for Superhydrophobic Microchannels

We analytically and numerically consider the hydrodynamic and thermal transport behavior of fully developed laminar flow through a superhydrophobic (SH) parallel-plate channel. Hydrodynamic slip length, thermal slip length and heat flux are prescribed at each surface. We first develop a general expression for the Nusselt number valid for asymmetric velocity profiles. Next, we demonstrate that, in the limit of Stokes flow near the surface and an adiabatic and shear-free liquid–gas interface, both thermal and hydrodynamic slip lengths can be found by redefining existing solutions for conduction spreading resistances. Expressions for the thermal slip length for pillar and ridge surface topographies are determined. Comparison of fundamental half-space solutions for the Laplace and Stokes equations facilitate the development of expressions for hydrodynamic slip length over pillar-structured surfaces based on existing solutions for the conduction spreading resistance from an isothermal source. Numerical validation is performed and an analysis of the idealized thermal transport behavior suggests conditions under which superhydrophobic microchannels may enhance heat transfer.

Trap state controlled bipolar resistive switching effect and electronic transport in LaAlO3/Nb:SrTiO3 heterostructures

We studied the resistive switching (RS) effect in LaAlO3/Nb:SrTiO3 heterostructures at different temperatures with AC impedance technique in addition to the conventional I–V measurements. It was demonstrated that the bipolar RS effect originates from LaAlO3/Nb:SrTiO3 interface and the resistance and capacitance states are controlled by the filling status of traps. A model based on the variation of trap state was proposed to explain the RS effect and the thermal history dependent electronic transport behavior. This work demonstrates the key role of trap state in the RS effect and electronic transport.

Interfacial thermal conductance of partially unzipped carbon nanotubes: Linear scaling and exponential decay

The development of integrated circuits urgently demands improved understanding of interfacial thermal transport at the nanoscale. We report a systematical nonequilibrium Green's function study on thermal transport in a class of experimentally available carbon-based nanostructures, partially unzipped carbon nanotubes (PUCNTs), where unusual interfacial thermal transport behaviors are revealed. We find that thermal conductance of PUCNTs shows a perfect linear dependence on the width of the unzipped part and an exponential decay (to a finite value) with the length of the unzipped part. The exponential length dependence observed at interfaces is a pure quantum phenomenon without classical counterpart. The improved understanding of nanoscale thermal transport can find applications in designing high-performance thermal management and/or thermoelectric nanodevices.

Hybridization, Inter-Ion Correlation, and Surface States in the Kondo InsulatorSmB6

As an exemplary Kondo insulator, SmB6 has been studied for several decades; however, direct evidence for the development of the Kondo coherent state and the evolution of the electronic structure in the material has not been obtained due to the rather complicated electronic and thermal transport behavior. Recently, these open questions attracted increasing attention as the emergence of a time-reversal invariant topological surface state in the Kondo insulator has been suggested. Here, we use point-contact spectroscopy to reveal the temperature dependence of the electronic states in SmB6. We demonstrate that SmB6 is a model Kondo insulator: below 100 K, the conductance spectra reflect the Kondo hybridization of Sm ions, but below ~ 30 K, signatures of inter-ion correlation effects clearly emerge. Moreover, we find evidence that the low-temperature insulating state of this exemplary Kondo lattice compound harbors conduction states on the surface, in support of predictions of nontrivial topology in Kondo insulators.

Three dimensional microstructure and image-based simulation of a fly ash/al syntactic foam using X-ray micro-computed tomography

An aluminum matrix syntactic foam, incorporated with hollow-structured fly ash particles, was fabricated by pressure infiltration technique. X-ray micro-computed tomography was used to characterize its heterogeneous microstructure three dimensionally (3D). The quantification of some microstructure features, such as content and size distribution of hollow fly ash particles, was acquired in 3D. The tomographic data were exploited as a rapid method to generate a microstructurally accurate and robust 3D meshed model. The thermal transport behavior has been modeled using a commercial finite-element code to conduct steady state analyses. Simulation of the thermal conductivity showed good correlation with experimental result.

Molecular dynamics simulations for thermal transport behavior of InAs nanotubes: A role of symmetry

Molecular dynamics simulations for the indium arsenide (InAs) nanowires and nanotubes are performed to understand the improved (decreased) thermal transport behavior in nanostructured systems. The InAs nanotubes have a significantly reduced heat flow as compared to the nanowire analogue due to the increased surface area to volume effect. A 53% reduction in thermal conductivity was observed with a 17% reduction in the nanowire cross sectional area. Local heat current analysis shows that the interior atoms being removed in nanotubes have the largest local heat current contribution, thereby punching a hole in the middle leading to the largest reduction in thermal conductivity of the material. We then find that the broken symmetry can lower the thermal conductivity even further. As a result, a sweet spot for the lowest thermal conductivity for nanotubes is found at the nearest displacement of the hole from the center due to the two opposing factors: highest local heat currents for the innermost atoms and the effect of broken symmetry. We expect that this new physical mechanism of heat transport in InAs nanotubes can be generalized to other thermoelectric materials such as silicon nanowires and nanotubes to reduce a lattice thermal conductivity even further.

A review on modeling of the thermal conductivity of polymeric nanocomposites

AbstractThis review reports recent advances in the field of thermal conductivity of polymeric nanocomposites. Thermally conductive polymeric nanocomposites can be used for replacing metal parts in several applications, such as liquid cooling and ventilation garment, power electronics, electric motors and generators, heat exchangers, etc., because the polymers have some privileges such as light weight, corrosion resistance, lower manufacturing cost and ease of processing. In this study, the thermal conductivity measurement and modeling of polymeric nanocomposites are discussed in general, and detailed examples are also drawn from the scientific literature. Many theoretical models are available to predict the thermal conductivity of nanocomposites. The simplest of these are mixture rules such as series, parallel, and geometric models. However, the series model typically over predicts the thermal conductivity, whereas the parallel model tends to under predict the thermal conductivity of the nanocomposites. Other models such as the Hamilton-Crosser model and the Lewis-Nielsen model are based on particle size, geometry, and the manner of particle packing in the matrix. Also, there are various effective medium approaches (EMA) like the Maxwell-Garnett (MG) approximation to analyze the thermal transport behaviour in heterogeneous media such as thermal conductivity of some composite structures.

Transport and thermodynamic properties under anharmonic motion in type-I Ba8Ga16Sn30clathrate

Anharmonic guest atom oscillation has direct connection to the thermal transport and thermoelectric behavior of type-I Ba${}_{8}$Ga${}_{16}$Sn${}_{30}$ clathrates. This behavior can be observed through several physical properties, with for example the heat capacity providing a measure of the overall excitation level structure. Localized anharmonic excitations also influence the low-temperature resistivity, as we show in this paper. By combining heat capacity, transport measurements, and our previous NMR relaxation results, we address the distribution of local oscillators in this material, as well as the shape of the confining potential and the excitation energies for Ba(2) ions in the cages. We also compare to the soft-potential model and other models used for similar systems. The results show good agreement between the previously deduced anharmonic rattler potential and experimental data.

Simultaneous solutions of coupled thermal airflow problem for natural ventilation in buildings

Natural and hybrid ventilation can be sustainable building ventilation strategies, where airflow is driven naturally by thermal buoyancy and/or wind forces other than pure mechanical means. The simulation and design of these systems need to consider the combined impact of thermal and airflow transport behaviors. The numerical solution of such combined thermal airflow problems often employs a segregate and iterative approach. Either the air temperatures in the thermal problem or the air pressures in the airflow problem are solved separately with the other parameter known from a previous iteration. The newly solved parameters are then substituted successively into the other calculation. For highly coupled thermal airflow problems, the segregate method can cause solution fluctuation or even divergence when relaxation factors are not carefully selected to avoid abrupt changes of air parameters in the successive substitution procedure. This article investigated two nonsegregate methods to solve thermal and airflow problems simultaneously. In the fully simultaneous method, air temperatures and pressures for all rooms of a building are solved simultaneously using a single Jacobian matrix. In the semi-simultaneous method, a Jacobian matrix for the air temperature and pressure of one room is solved when air temperatures and pressures of other rooms are kept as constants. The same procedure is then repeated for each room of a building. In both cases, relaxation factors are not required. The simultaneous solution methods are demonstrated for a two-zone building with thermal buoyancy-driven flows and validated for an experimental study of combined wind and buoyancy forces in a light well. It was shown that the simultaneous solvers provide stable solutions without using any relaxation in both cases. The predicted results also agree reasonably well with the experimental data.

Graphene-nanotube 3D networks: intriguing thermal and mechanical properties

Carbon-based nanomaterials have drawn strong interest for potential applications due to their extraordinary stability and unique mechanical, electrical and thermal properties. For the minimization of microelectronics/micromechanics circuits, bridging the low dimensional microscopic structure and mesoscopic modeling is indispensable. Graphene and carbon nanotubes are suggested as ideal ‘building blocks’ for the bottom-up strategy, and recently the integration of both materials has stimulated research interests. In this work we investigated the thermal and mechanical performance in the pillared-graphene – constructed by combining graphene sheets and carbon nanotubes to create a three-dimensional nano network. Reverse non-equilibrium molecular dynamics simulations were carried out to analyze the thermal transport behavior. The obtained thermal conductivities are found to be possibly isotropic in two specific directions or highly anisotropic for certain structure configurations. In the mechanical performance analysis, tensile deformations are loaded along graphene plane and along tube axis. The elongation responses and stress-strain relations are observed to be nearly linear, and the calculated strength, fracture strain and Young's moduli are lower than the pristine graphene or carbon nanotubes. The alterations in the thermal and mechanical performances are ascribed to the bond conversion on the junctions.

Prediction and Measurement of Thermal Transport Across Interfaces Between Isotropic Solids and Graphitic Materials

Due to the high intrinsic thermal conductivity of carbon allotropes, there have been many attempts to incorporate such structures into existing thermal abatement technologies. In particular, carbon nanotubes (CNTs) and graphitic materials (i.e., graphite and graphene flakes or stacks) have garnered much interest due to the combination of both their thermal and mechanical properties. However, the introduction of these carbon-based nanostructures into thermal abatement technologies greatly increases the number of interfaces per unit length within the resulting composite systems. Consequently, thermal transport in these systems is governed as much by the interfaces between the constituent materials as it is by the materials themselves. This paper reports the behavior of phononic thermal transport across interfaces between isotropic thin films and graphite substrates. Elastic and inelastic diffusive transport models are formulated to aid in the prediction of conductance at a metal-graphite interface. The temperature dependence of the thermal conductance at Au-graphite interfaces is measured via transient thermoreflectance from 78 to 400 K. It is found that different substrate surface preparations prior to thin film deposition have a significant effect on the conductance of the interface between film and substrate.