Nanostructured composites containing aligned carbon nanotubes (CNTs) are very promising as interface materials for electronic systems and thermoelectric power generators. We report the first data for the thermal conductivity of densified, aligned multiwall CNT nanocomposite films for a range of CNT volume fractions. A 1 vol % CNT composite more than doubles the thermal conductivity of the base polymer. Denser arrays (17 vol % CNTs) enhance the thermal conductivity by as much as a factor of 18 and there is a nonlinear trend with CNT volume fraction. This article discusses the impact of CNT density on thermal conduction considering boundary resistances, increased defect concentrations, and the possibility of suppressed phonon modes in the CNTs.

Thermal Conduction in Aligned Carbon Nanotube–Polymer Nanocomposites with High Packing Density

The extremely high thermal conductivities of carbon nanotubes have motivated a wealth of research. Progress includes innovative conduction metrology based on microfabricated platforms and scanning thermal probes as well as simulations exploring phonon dispersion and scattering using both transport theory and molecular dynamics. This article highlights these advancements as part of a detailed review of heat conduction research on both individual carbon nanotubes and nanostructured films consisting of arrays of nanotubes or disordered nanotube mats. Nanotube length, diameter, and chirality strongly influence the thermal conductivities of individual nanotubes and the transition from primarily diffusive to ballistic heat transport with decreasing temperature. A key experimental challenge, for both individual nanotubes and aligned films, is the separation of intrinsic and contact resistances. Molecular dynamics simulations have studied the impacts of specific types of imperfections on the nanotube conductance and its variation with length and chirality. While the properties of aligned films fall short of predictions based on individual nanotube data, improvements in surface engagement and postfabrication nanotube quality are promising for a variety of applications including mechanically compliant thermal contacts.

http://nanoheat.stanford.edu/sites/default/files/publications/RevModPhys%20Marconnet%20et%20al..pdf

Thermal conduction phenomena in carbon nanotubes and related nanostructured materials

Thermally conductive functionalized multilayer graphene sheets (fMGs) are efficiently aligned in large-scale by a vacuum filtration method at room temperature, as evidenced by SEM images and polarized Raman spectroscopy. A remarkably strong anisotropy in properties of aligned fMGs is observed. High electrical (∼386 S cm−1) and thermal conductivity (∼112 W m−1 K−1 at 25 °C) and ultralow coefficient of thermal expansion (∼−0.71 ppm K−1) in the in-plane direction of A-fMGs are obtained without any reduction process. Aligned fMGs are vertically assembled between contacted silicon/silicon surfaces with pure indium as a metallic medium. Thus-constructed three-dimensional vertically aligned fMG thermal interfacial material (VA-fMG TIM) architecture has significantly higher equivalent thermal conductivity (75.5 W m−1 K−1) and lower contact thermal resistance (5.1 mm2 K W−1), compared with their counterpart from A-fMGs that are recumbent between silicon surfaces. This finding provides a throughout approach for a g...

A three-dimensional vertically aligned functionalized multilayer graphene architecture: an approach for graphene-based thermal interfacial materials.

This article presents a general formulation of an atomistic Green's function (AGF) method. The atomistic Green's function approach combines atomic-scale fidelity with asymptotic treatment of large-scale (bulk) features, such that the method is particularly well suited to address an emerging class of multiscale transport problems. A detailed mathematical derivation of the phonon transmission function is provided in terms of Green's functions and, using the transmission function, the heat flux integral is written in Landauer form. Within this theoretical framework, the required inputs to calculate heat flux are equilibrium atomic locations and an appropriate interatomic potential. Relevant algorithmic and implementation details are discussed. Several examples including a homogeneous atomic chain and two heterogeneous atomic chains are included to illustrate the applications of this methodology.

The Atomistic Green's Function Method: An Efficient Simulation Approach for Nanoscale Phonon Transport

This work describes an experimental study of thermal conductance across multiwalled carbon nanotube (CNT) array interfaces, one sided (Si-CNT-Ag) and two sided (Si-CNT-CNT-Cu), using a photoacoustic technique (PA). Well-anchored, dense, and vertically oriented multiwalled CNT arrays have been directly synthesized on Si wafers and pure Cu sheets using plasma-enhanced chemical vapor deposition. With the PA technique, the small interface resistances of the highly conductive CNT interfaces can be measured with accuracy and precision. In addition, the PA technique can resolve the one-sided CNT interface component resistances (Si-CNT and CNT-Ag) and the two-sided CNT interface component resistances (Si-CNT, CNT-CNT, and CNT-Cu) and can estimate the thermal diffusivity of the CNT layers. The thermal contact resistances of the one- and two-sided CNT interfaces measured using the PA technique are 15.8±0.9 and 4.0±0.4mm2K∕W, respectively, at moderate pressure. These results compare favorably with those obtained using a steady state, one-dimensional reference bar method; however, the uncertainty range is much narrower. The one-sided CNT thermal interface resistance is dominated by the resistance between the free CNT array tips and their opposing substrate (CNT-Ag), which is measured to be 14.0±0.9mm2K∕W. The two-sided CNT thermal interface resistance is dominated by the resistance between the free tips of the mating CNT arrays (CNT-CNT), which is estimated to be 2.1±0.4mm2K∕W.

/pdf/photoacoustic-characterization-of-carbon-nanotube-array-10mkiopurp.pdf

Photoacoustic characterization of carbon nanotube array thermal interfaces

In this work, a photothermal experiment is designed and conducted to characterize the thermal transport in carbon nanotubes (CNTs) along the axial direction exclusively. We characterize the thermal contact resistance between CNTs and the substrate. The measured value demonstrates a sound thermal contact between them. Our experimental result reveals a low thermal conductivity of the CNTs along their axial direction. The unique structure of the CNTs characterized in our work indicates that the thermal transport in the axial direction is across the carbon atomic layers. This explains why the measured thermal conductivity is much lower than theoretical predictions, in which the thermal conductivity is along the atomic layer direction. In addition, our transmission electron microscope observation of the CNTs reveals poor structural qualities, which can strongly enhance phonon scattering and reduce the thermal conductivity.

/pdf/noncontact-thermal-characterization-of-multiwall-carbon-7901ji3xqe.pdf

Noncontact thermal characterization of multiwall carbon nanotubes

In this work, thermal transport in nanocrystalline materials is studied using large-scale equilibrium molecular dynamics simulation. Nanocrystalline materials with different grain sizes are studied to explore how and to what extent the size of nanograins affects the thermal conductivity and specific heat. Substantial thermal conductivity reduction is observed and the reduction is stronger for nanocrystalline materials with smaller grains. On the other hand, the specific heat of nanocrystalline materials shows little change with the grain size. Based on the calculated thermal conductivity, the thermal resistance at grain boundaries is calculated and found to be in the order of 10−9m2K∕W. The simulation results are compared with the thermal transport in freestanding nanograins based on molecular dynamics simulation. Further discussions are provided to explain the fundamental physics behind the observed thermal phenomena in this work.

/pdf/thermal-transport-in-nanocrystalline-materials-2r7yd99ykr.pdf

Thermal transport in nanocrystalline materials

In this work, the thermal conductivity of nanofilms, nanowires, and nanoparticles are studied using molecular dynamics simulation. It is found that their thermal conductivity depends significantly on the characteristic size until it reaches a large value. Comparison with results of the lattice Boltzmann method reflects strong effects of surface structure, especially when the film thickness is comparable to the mean free path of phonons. Study of the phonon thermal transport in nanowires and nanoparticles reveals much stronger boundary-scattering effect on thermal transport than in nanofilms, which is attributed to the more confined phonon movements in these two- and one-dimensional nanomaterials.

/pdf/equilibrium-molecular-dynamics-study-of-phonon-thermal-4thryjxcrx.pdf

Equilibrium molecular dynamics study of phonon thermal transport in nanomaterials

In this work, a novel photothermal experiment is designed and conducted to characterize the thermal transport in carbon nanotubes (CNTs) along the axial direction exclusively. For the first time, we characterize the thermal contact resistance between CNTs and the substrate. The measured value demonstrates a sound thermal contact between the CNTs and the substrate. Our experimental result reveals a low thermal conductivity of the CNTs along their axial direction. The unique structure of the CNTs characterized in our work indicates that the thermal transport in the axial direction is across the carbon atomic layers. This explains why the measured thermal conductivity is much lower than theoretical predictions, in which the thermal conductivity is along the atomic layer direction. In addition, our TEM observation of the CNTs reveals poor structural qualities, which can strongly enhance phonon scattering and reduce the thermal conductivity.

Non-Contact Thermal Characterization of Individual Multi-wall Carbon Nanotubes

In this work, thermal transport in nanocrystalline materials is studied using large-scale equilibrium molecular dynamics (MD) simulation. Nanocrystalline materials with different grain sizes are studied to explore how and to what extent the size of nanograins affects the thermal conductivity and specific heat. Substantial thermal conductivity reduction is observed and the reduction is stronger for nanocrystalline materials with smaller grains. On the other hand, the specific heat of nanocrystalline materials shows little change with the grain size. The simulation results are compared with the thermal transport in individual nanograins based on MD simulation. Further discussions are provided to explain the fundamental physics behind the observed thermal phenomena in this work.Copyright © 2005 by ASME

Zhanrong Zhong

Papers

Noncontact thermal characterization of multiwall carbon nanotubes

Thermal transport in nanocrystalline materials

Equilibrium molecular dynamics study of phonon thermal transport in nanomaterials

Non-Contact Thermal Characterization of Individual Multi-wall Carbon Nanotubes

Thermal Transport in Nanocrystalline Materials