Length Dependence Of Thermal Conductivity Research Articles

Semiballistic thermal conduction in polycrystalline SiGe nanowires

Ballistic heat conduction remains a controversial nanoscale phenomenon because of its occurrence and strength depending on the material, alloy composition, and temperature. Here, we discuss the impact of ballistic thermal conduction and compare the results with theoretical predictions. We experimentally investigate ballistic thermal transport in SiGe polycrystalline nanowires by measuring the length dependence of thermal conductivity for different alloy compositions and temperatures. At room temperature, our experiments show that ballistic effects are negligible in nanowires made of pure polycrystalline Si but become stronger as the Ge composition increases. As we decreased the temperature, we observed that ballistic contribution becomes even stronger.

Thermal conductivity and scattering models for graphene: From intrinsic scattering of pristine graphene to strong extrinsic scattering of functionalized graphene

The present study used various scattering models in a Callaway framework to elucidate the relationship between the scattering mechanisms and thermal conductivity of graphene. As well as intrinsic scattering, the scattering caused by external conditions was constructed using a combination of boundary, imperfection, and interface scattering. The effect of each type of scattering on the length-dependent thermal conductivity of graphene, which is a subject of fundamental importance, was evaluated. The change in specularity shifted the logarithmic length-dependent regime from the micrometer scale for a diffuse boundary to the submicrometer scale for high specularity. Imperfection scattering had little effect on the intrinsically determined length dependence of thermal conductivity. Substrate scattering, that is, interface scattering by van der Waals interaction, caused large decreases of both the thermal conductivity and length scale. A rough model for functionalized graphene was constructed by considering both impurity and interface scattering. The spring constant in interface scattering with functionalizing bond was nearly 40 times larger than that for substrate scattering, in combination with an imperfection source, explained the length-independent low thermal conductivity of functionalized graphene. This model was extended to nonsubstitutionally functionalized graphene, for which imperfection scattering strength was proportional to the mass of the adatom.

Ballistic thermal transport in silicon nanowires

We have experimentally investigated the impact of dimensions and temperature on the thermal conductivity of silicon nanowires fabricated using a top-down approach. Both the width and temperature dependences of thermal conductivity agree with those in the existing literature. The length dependence of thermal conductivity exhibits a transition from semi-ballistic thermal phonon transport at 4 K to fully diffusive transport at room temperature. We additionally calculated the phonon dispersion in these structures in the framework of the theory of elasticity and showed that the thermal conductance increases with width. This agrees with our experimental observations and supports the pertinence of using the modified phonon dispersion at low temperatures.

Understanding the chain length dependence of thermal conductivity of liquid alcohols at 298 K on the basis of molecular-scale energy transfer

Many of the specifics of microscopic energy transfer in liquids are not sufficiently understood in spite of their importance in controlling heat transport in practical devices. The present study aims to clarify the details of microscopic energy transfer in associated liquids with the aid of non-equilibrium molecular dynamics (NEMD) simulations on the liquids of linear alcohols from ranging ethanol to decanol at normal conditions. The NEMD simulations reasonably reproduce the experimentally observed chain length dependence of thermal conductivity. The microscopic energy transfer corresponding to each type of interatomic interactions are analyzed in relation to the liquid structures and molecular morphologies. Our analysis shows that the heat transfer by the Coulomb interaction has only a limited contribution to the alcohol thermal conductivity. Molecular morphology also has little effect. Rather, the chain length dependence of the thermal conductivity is determined by the result of the competition between the intermolecular and intramolecular energy transfers. The new insights obtained in the present study are a step towards a molecular theory of the thermal conductivity of liquids.

Thermal conduction in Si and SiGe phononic crystals explained by phonon mean free path spectrum

Thermal phonon transport in single-crystalline Si, amorphous SiGe, and poly-SiGe nanostructures was investigated experimentally at room temperature. The characteristic length dependence of thermal conductivity was compared across these three materials by changing the shortest distance between the circular holes of phononic crystals formed in the membranes. The dependences clearly differ for these materials, and these differences can be explained by the thermal phonon mean free path spectra of the three materials. Nanostructuring has a larger impact on thermal conductivity reduction when the characteristic length is comparable to that in the region where the thermal phonon mean free path spectrum is dense. The results suggest that thermal phonon mean free path spectra can be estimated qualitatively by thermal conductivity measurements with characteristic length sweeps.

Length dependence of thermal conductivity by approach-to-equilibrium molecular dynamics

The length dependence of thermal conductivity over more than two orders of magnitude has been systematically studied for a range of materials, interatomic potentials, and temperatures using the atomistic approach-to-equilibrium molecular dynamics (AEMD) method. By comparing the values of conductivity obtained for a given supercell length and maximum phonon mean free path (MFP), we find that such values are strongly correlated, demonstrating that the AEMD calculation with a supercell of finite length actually probes the thermal conductivity corresponding to a maximum phonon MFP. As a consequence, the less pronounced length dependence usually observed for poorer thermal conductors, such as amorphous silica, is physically justified by their shorter average phonon MFP. Finally, we compare different analytical extrapolations of the conductivity to infinite length and demonstrate that the frequently used Matthiessen rule is not applicable in AEMD. An alternative extrapolation more suitable for transient-time, finite-supercell simulations is derived. This approximation scheme can also be used to classify the quality of different interatomic potential models with respect to their capability of predicting the experimental thermal conductivity.

Molecular dynamics simulations of thermal conductivity of carbon nanotubes: Resolving the effects of computational parameters

Predicting thermal conductivity, k, of carbon nanotubes (CNTs) has been the focus of many molecular dynamics (MD) simulation studies reported in the literature. The values of k obtained in these studies exhibit a large, up to an order of magnitude, variability that is commonly attributed to the variations in the computational setups adopted in different studies. The sensitivity of the computational results to the choice of individual parameters of the simulation setups, however, has not been systematically investigated and is often overlooked when the predicted values of k are compared across the literature. Here we present the results of several series of simulations specifically designed to evaluate the effects of common computational parameters of non-equilibrium MD (NEMD), such as the type of boundary conditions, size and location of heat bath regions, definition of the CNT length, and the choice of interatomic potential, on the computational predictions. The length dependence of thermal conductivity is found to exhibit a gradual transition from a strong increase of k with CNT length for nanotubes that are shorter than ∼200nm to a much weaker dependence for longer CNTs, reflecting the transition from ballistic to diffusive-ballistic heat transport regimes. The effect of increasing length of thermal bath regions is found to be nearly indistinguishable from the effect of increasing length of the unperturbed region between the bath regions, suggesting that the value of k is defined by the total length of the CNT (including the length of the heat bath regions) in NEMD simulations employing uni-directional heat flux. The choice of interatomic potential is shown to be responsible for an up to fourfold variability in predictions of k for otherwise identical simulation conditions. Overall, the results of this study help elucidate the cause of quantitative discrepancies across published data and provide recommendations on the choice of simulation setups that may improve the consistency of the computational predictions.

Enhanced anisotropic heat conduction in multi-walled carbon nanotubes

Anisotropy of heat conduction in multi-walled carbon nanotubes (MWNTs) is investigated by measuring heat flows in a pristine MWNT and in a MWNT with defects. The in- and out-of-shell thermal conductivities of each MWNT graphite shell are determined, and differences of more than four orders of magnitude are obtained because of the inter-shell gaps. This enhanced anisotropy reduces the conductance by 74% compared with that of the pristine MWNT because of the presence of outer shell defects, which comprise only 2.8% volume ratio. Furthermore, the anisotropy-assisted length dependence of thermal conductivity is demonstrated, even though there is no ballistic phonon transport.

Phonon surface scattering controlled length dependence of thermal conductivity of silicon nanowires

We present a kinetic model to investigate the anomalous thermal conductivity in silicon nanowires (SiNWs) by focusing on the mechanism of phonon-boundary scattering. Our theoretical model takes into account the anharmonic phonon-phonon scattering and the angle-dependent phonon scattering from the SiNWs surface. For SiNWs with diameter of 27.2 nm, it is found that in the case of specular reflection at lateral boundaries, the thermal conductivity increases as the length increases, even when the length is up to 10 μm, which is considerably longer than the phonon mean free path (MFP). Thus the phonon-phonon scattering alone is not sufficient for obtaining a normal diffusion in nanowires. However, in the case of purely diffuse reflection at lateral boundaries, the phonons diffuse normally and the thermal conductivity converges to a constant when the length of the nanowire is greater than 100 nm. Our model demonstrates that for observing the length dependence of thermal conductivity experimentally, nanowires with smooth and non-contaminated surfaces, and measuring at low temperature, are preferred.

Tremendous Thermal Conductivity Reduction in Anti-Dotted Graphene Nanoribbons

Using non-equilibrium molecular dynamics method, we have investigated the antidot effect on thermal conductivity of graphene nanoribbons (GNRs). It is found that the thermal conductivity of GNRs is significantly reduced by introducing periodic antidots, especially by square antidots. The length dependence of thermal conductivity is further explored. Different from that of perfect GNRs, the thermal conductivity of anti-dotted GNRs barely changes with length increasing, while it is mainly determined by the antidot concentration, indicating a dramatic reduction of thermal conductivity can be obtained for longer GNRs. We predict that the thermal conductivity of perfect armchair GNRs is nearly 15 times larger than that with square antidots in the experimental length. Our results suggest an effective way to reduce the thermal conductivity of GNRs, which provides a promising way to the GNR's application in thermoelectrics.

Thermal conductivity of graphene nanoribbons

We have investigated the thermal conductivity of graphene nanoribbons (GNRs) with different edge shapes as a function of length, width, and strain using nonequilibrium molecular dynamics method. The initial GNR for the functional variations has dimensions of 2×11 nm2. Strong length dependence of thermal conductivity is obtained, indicating high thermal conductivities of GNRs, which is consistent with the experimental results for graphene. A tensile/compressive uniaxial strain can remarkably decrease the thermal conductivity of GNR.

Molecular Dynamics of Diffusive-Ballistic Heat Conduction in Single-Walled Carbon Nanotubes

The diffusive-ballistic heat conduction of finite-length single-walled carbon nanotubes has been studied by means of nonequilibrium molecular dynamics simulations. The length dependence of thermal conductivity is quantified for a range of nanotube lengths up to 1.6 µm at room temperature. A gradual transition from nearly pure ballistic to diffusive-ballistic heat conduction was identified from the thermal conductivity profile. In the diffusive-ballistic regime, the profile exhibits power-law length dependence and does not converge even for a tube length of 1.6 µm. Furthermore, the diameter dependence of thermal conductivity suggests considerable suppression of the diffusion effect as the diameter decreases.

Experimental and theoretical study of the length-dependent thermal conductivity of individual single-walled carbon nanotubes

We report measurements on the length-dependent thermal conductivity of individual single-walled carbon nanotubes (SWNTs) on a substrate using a four-pad 3ω method with the consideration of heat loss between the sample and substrate. An increase in thermal conductivity with length (0.5—7μm) was observed at room temperature. The length-dependence of thermal conductivity was theoretically predicted by the modified WV model with consideration of second order-3-phonon process. The predicted phonon mean free path is about 175nm. Both the measurement and the prediction show the scale effect of the length on the thermal conductivity of SWNTs.

Length Dependence of Thermal Conductivity of Single-WalledCarbon Nanotubes

Dependence of the thermal conductivity on the length of two armchairsingle-walled carbon nanotubes (SWNTs) is studied by the nonequilibriummolecular dynamics (MD) method with Brenner II potential. The thermalconductivities are calculated for (5, 5) and (7, 7) SWNTs with lengthsranging from 22 to 155 nm. The results show that the thermalconductivity of SWNTs is sensitive to the length and it does notconverge to a finite value when the tube length increases up to 155 nm,however it obeys a power law relation.

Molecular Dynamics Simulations of Diffusive-Ballistic Heat Conduction in Carbon Nanotubes

AbstractHeat conduction of finite-length single-walled carbon nanotubes (SWNTs) has been studied by means of non-equilibrium molecular dynamics (MD) simulations. The length-dependence of the thermal conductivity was quantified for a range of nanotube-lengths at room temperature. The length dependence of thermal conductivity exhibits a gradual transition from nearly pure ballistic heat conduction to diffusive-ballistic heat conduction. The results show that the thermal conductivity profile does not converge even beyond a micrometer nanotube-length. Furthermore, the diameter dependence suggests that the phonon diffusion is reduced with the diameter.

Thermal conductivity of nanotubes revisited: Effects of chirality, isotope impurity, tube length, and temperature

We study the dependence of the thermal conductivity of single-walled nanotubes on chirality, isotope impurity, tube length, and temperature by nonequilibrium molecular-dynamics method with accurate potentials. It is found that, contrary to electronic conductivity, the thermal conductivity is insensitive to the chirality. The isotope impurity, however, can reduce the thermal conductivity up to 60% and change the temperature dependence behavior. We also found that the tube length dependence of thermal conductivity is different for nanotubes of different radii at different temperatures.