Thermal Conductivity Of Bulk Research Articles

Effect of four-phonon scattering on anisotropic thermal transport in bulk hexagonal boron nitride by machine learning interatomic potential

The highly anisotropic thermal conductivity in layered materials is crucial for a broad range of applications such as thermal management of electronic devices, thermal insulation, and thermoelectrics. Understanding of anisotropic thermal transport in layered materials largely depends on atomistic simulations based on density functional theory (DFT) or empirical potentials, which however suffer either low computational efficiency or accuracy. Recently, machine learning interatomic potentials (MLIPs) are emerging as a powerful tool to bridge the gap. Despite the recent progress in developing MLIPs, little attention has been paid to constructing a potential that can accurately predict the thermal properties of layered materials, which is more challenging compared with the case of isotropic materials because of the highly anisotropic bonding and weak van der Waals interactions in layered materials. Here, we introduce a MLIP within the Gaussian approximation potential (GAP) framework for bulk hexagonal boron nitride (h-BN) with a typical layered structure. We demonstrate that the GAP can well predict the highly anisotropic phonon transport properties and thermal conductivity of bulk h-BN with DFT-level accuracy at 3 orders of magnitude reduced cost. Particularly, we investigate the effect of four-phonon scattering on the thermal conductivity of bulk h-BN using the GAP. The suppression of thermal conductivity by four-phonon scattering is negligible below 300 K due to the orders of magnitude weaker four-phonon scattering than three-phonon scattering, which is mainly attributed to the breaking of the reflection symmetry selection rule and the splitting of the flexural acoustic branch in bulk h-BN. As temperature increases, four-phonon scattering results in a larger reduction in thermal conductivity, which increases up to 21.64% (in-plane) and 35.71% (cross-plane) at 1000 K. Atomistic simulations based on MLIPs are expected to be able to greatly promote the understanding of phonon transport and the prediction of thermophysical properties in layered materials.

Thermal conductivity of bulk and porous ThO2: Atomistic and experimental study

Thorium dioxide (ThO2) is proposed to play a vital role in the world's future energy needs and is considered a better and safer alternative to the currently used nuclear fuel, uranium dioxide (UO2). Thermo-physical properties of ThO2 are superior to UO2, but the fundamental physics governing the heat transport in ThO2 is still ambiguous, and the available data for the thermal conductivity (k) of ThO2 was scattered. In this article, a systematic investigation regarding the lattice thermal conductivity (kL) of the bulk and porous ThO2 is carried out theoretically and validated with experiments. The phonon transport calculations were done using two different methods; ab-initio calculations combined with the Boltzmann transport equation (BTE) and the equilibrium molecular dynamics (EMD) simulations using Green Kubo (GK) approach. An extensive examination of the phonon mode contribution, available three-phonon scattering phase space modes, Grüneisen parameter, and mean free path (MFP) distributions were analyzed to understand the underlying physics in the thermal transport of ThO2. The effect of porosity on the kL by measurements and molecular dynamics (MD) simulations was explored. The measurements were performed on specimens with different porosity, that were prepared by spark plasma sintering (SPS) using the laser flash (LFA) technique. The results obtained demonstrated that the kL values predicted by both the BTE and the EMD simulations were in excellent agreement with our experimental measurements. Moreover, the model to simulate the 95% theoretical density (TD) using MD simulations also captured the decrease in thermal conductivity with porosity and agreed well with the measured results for 95% TD dense sintered pellets.

Atomistic and experimental study on thermal conductivity of bulk and porous cerium dioxide

Cerium dioxide (CeO2) is a surrogate material for traditional nuclear fuels and an essential material for a wide variety of industrial applications both in its bulk and nanometer length scale. Despite this fact, the underlying physics of thermal conductivity (kL), a crucial design parameter in industrial applications, has not received enough attention. In this article, a systematic investigation of the phonon transport properties was performed using ab initio calculations unified with the Boltzmann transport equation. An extensive examination of the phonon mode contribution, available three-phonon scattering phase space, mode Grüneisen parameter and mean free path (MFP) distributions were also conducted. To further augment theoretical predictions of the kL, measurements were made on specimens prepared by spark plasma sintering using the laser flash technique. Since the sample porosity plays a vital role in the value of measured kL, the effect of porosity on kL by molecular dynamics (MD) simulations were investigated. Finally, we also determined the nanostructuring effect on the thermal properties of CeO2. Since CeO2 films find application in various industries, the dependence of thickness on the in-plane and cross-plane kL for an infinite CeO2 thin film was also reported.

Thermal conductivity of bulk and nanowire of cubic-SiC from ab initio calculations

In this work, we predict the structural, elastic, thermal and thermodynamic properties of alpha-silicon carbide (SiC) using the plane-wave pseudopotential approach to density functional theory (DFT) in the local density approximation (LDA). The lattice thermal conductivity of SiC is calculated using the Slack model and Boltzmann transport equation (BTE). We also provide the thermal conductivity of SiC nanowires and the dependence of the thermal conductivity on the diameter and their orientations. The ground state structural and elastic properties show excellent agreement with the experiments. The calculated phonon dispersion curve shows good agreement with the experimental values measured at room temperature. The thermodynamic properties are studied using quasi-harmonic approximation (QHA), and the predicted properties agree well with the experiment. This study demonstrates the need for including the normal phonon scattering and boundary scattering to evaluate the thermal conductivities at low temperature and is evident from the fact that the thermal conductivity at low temperature predicted by the Slack model is higher compared to the value predicted by the BTE. Our BTE calculation for the bulk SiC agrees well with the known experimental results, and we also provide predictions for the case of SiC in the form of nanowires.

Measurement Techniques for Thermal Conductivity and Interfacial Thermal Conductance of Bulk and Thin Film Materials

Thermal conductivity and interfacial thermal conductance play crucial roles in the design of engineering systems where temperature and thermal stress are of concerns. To date, a variety of measurement techniques are available for both bulk and thin film solid-state materials with a broad temperature range. For thermal characterization of bulk material, the steady-state method, transient hot-wire method, laser flash diffusivity method, and transient plane source (TPS) method are most used. For thin film measurement, the 3ω method and the transient thermoreflectance technique including both time-domain and frequency-domain analysis are widely employed. This work reviews several most commonly used measurement techniques. In general, it is a very challenging task to determine thermal conductivity and interfacial thermal conductance with less than 5% error. Selecting a specific measurement technique to characterize thermal properties needs to be based on: (1) knowledge on the sample whose thermophysical properties are to be determined, including the sample geometry and size, and the material preparation method; (2) understanding of fundamentals and procedures of the testing technique, for example, some techniques are limited to samples with specific geometries and some are limited to a specific range of thermophysical properties; and (3) understanding of the potential error sources which might affect the final results, for example, the convection and radiation heat losses.

Thermal conductivity of bulk and monolayer MoS2

We show that the lattice contribution to the thermal conductivity of MoS2 strongly dominates the carrier contribution in a broad temperature range from 300 to 800 K. Since theoretical insight into the lattice contribution is largely missing, though it would be essential for materials design, we solve the Boltzmann transport equation for the phonons self-consistently in order to evaluate the phonon lifetimes. In addition, the length scale for transition between diffusive and ballistic transport is determined. The low out-of-plane thermal conductivity of bulk MoS2 ( at 300 K) is useful for thermoelectric applications. On the other hand, the thermal conductivity of monolayer MoS2 ( at 300 K) is comparable to that of Si.

Low thermal conductivity of Lu4Si2O7N2: Theoretical and experimental studies

Thermal conductivity of bulk and dense oxynitride Lu4Si2O7N2 was measured at various temperatures. Results show that Lu4Si2O7N2 possesses low thermal conductivity and has potential applications as thermal insulation material. Detailed analysis shows that the experimental thermal diffusivity is significantly influenced by defects at low temperature and infrared radiation at high temperature. Therefore, we combined Debye model and Slack's equation to predict the temperature dependent intrinsic lattice thermal conductivity of Lu4Si2O7N2. All parameters indispensable in the calculation (equilibrium crystal structure, second order elastic constants, elastic modulus, Debye temperature, and Grüneisen constant) were predicted with the help of first-principles calculation methods. Finally, factors that influence the thermal conductivity of Lu4Si2O7N2 were discussed and possible explanations for the difference between experimental and theoretical thermal conductivity were illustrated.

Thermal conductivity of bulk nanostructured lead telluride

Thermal conductivity of lead telluride with embedded nanoinclusions was studied using Monte Carlo simulations with intrinsic phonon transport properties obtained from first-principles-based lattice dynamics. The nanoinclusion/matrix interfaces were set to completely reflect phonons to model the maximum interface-phonon-scattering scenario. The simulations with the geometrical cross section and volume fraction of the nanoinclusions matched to those of the experiment show that the experiment has already reached the theoretical limit of thermal conductivity. The frequency-dependent analysis further identifies that the thermal conductivity reduction is dominantly attributed to scattering of low frequency phonons and demonstrates mutual adaptability of nanostructuring and local disordering.

Thermal conductivity of bulk and nanowire InAs, AlN, and BeO polymorphs from first principles

We compute the thermal conductivity of the alternative zincblende (ZB) and wurtzite (WZ) phases of InAs, AlN, and BeO. The bulk thermal conductivity of the ZB phase of BeO is predicted to be even higher than that of its WZ phase (the highest amongst all ceramics used in electronic technology). Our calculations agree well with the available experimental measurements for bulk ZB InAs, WZ AlN, WZ BeO, and WZ and ZB InAs nanowires, and we provide predictions for the remaining cases. The predicted good thermal conductor ZB BeO might have interesting applications in improved heat sinks for high performance semiconductor electronics.

Thermal conductivity of bulk and thin-film silicon: A Landauer approach

The question of what fraction of the total heat flow is transported by phonons with different mean-free-paths is addressed using a Landauer approach with a full dispersion description of phonons to evaluate the thermal conductivities of bulk and thin film silicon. For bulk Si, the results reproduce those of a recent molecular dynamic treatment showing that about 50% of the heat conduction is carried by phonons with a mean-free-path greater than about 1 μm. For the in-plane thermal conductivity of thin Si films, we find that about 50% of the heat is carried by phonons with mean-free-paths shorter than in the bulk. When the film thickness is smaller than ∼0.2 μm, 50% of the heat is carried by phonons with mean-free-paths longer than the film thickness. The cross-plane thermal conductivity of thin-films, where quasi-ballistic phonon transport becomes important, is also examined. For ballistic transport, the results reduce to the well-known Casimir limit [H. B. G. Casimir, Physica 5, 495–500 (1938)]. These results shed light on phonon transport in bulk and thin-film silicon and demonstrate that the Landauer approach provides a relatively simple but accurate technique to treat phonon transport from the ballistic to diffusive regimes.

Predicting Thermal Transport in Bi2Te3: From Bulk to Nanostructures

ABSTRACTTwo-body interatomic potentials in the Morse potential form have been developed for bismuth telluride, and the potentials are used in molecular dynamics (MD) simulations to predict the thermal conductivity of Bi2Te3 bulk, nanowires and few-quintuple thin films. The density functional theory with local density approximations is first used to calculate the total energies for many artificially distorted Bi2Te3 configurations to produce the energy surface. Then by fitting to this energy surface and other experimental data, the Morse potential form is parameterized. Molecular dynamics simulations are then performed to predict the thermal conductivity of bulk Bi2Te3 at different temperatures, and the results agree with experimental data well. We also predicted the thermal conductivity of Bi2Te3 nanowires with diameter ranging from 3 to 30 nm with both smooth (SMNW) and rough (STNW) surfaces. It is found that when the nanowire diameter decreases to the molecular scale (below 10 nm, or the so called "quantum wire"), the thermal conductivity shows significant reduction as compared to bulk value. We find the dimensional crossover behavior of thermal transport in few quintuple layer (QL) thin films at room temperature, and we attribute it to the interplay between phonon Umklapp scattering and boundary scattering. Also, nanoporous films show significantly reduced thermal conductivity compared to perfect thin films, indicating that they can be very promising thermoelectric materials.

Thermal conductivity of silicon bulk and nanowires: Effects of isotopic composition, phonon confinement, and surface roughness

We present a rigorous analysis of the thermal conductivity of bulk silicon (Si) and Si nanowires (Si NWs) which takes into account the exact physical nature of the various acoustic and optical phonon mechanisms. Following the Callaway solution for the Boltzmann equation, where resistive and nonresistive phonon mechanisms are discriminated, we derived formalism for the lattice thermal conductivity that takes into account the phonon incidence angles. The phonon scattering processes are represented by frequency-dependent relaxation time. In addition to the commonly considered acoustic three-phonon processes, a detailed analysis of the role of the optical phonon decay into acoustic phonons is performed. This optical phonon decay mechanism is considered to act as acoustic phonon generation rate partially counteracting the acoustic phonon scattering rates. We have derived the analytical expression describing this physical mechanism which should be included in the general formalism as a correction to the resistive phonon-point-defects and phonon-boundary scattering expressions. The phonon-boundary scattering mechanism is taken as a function of the phonon frequency, incidence angles, and surface roughness. The importance of all the mechanisms we have involved in the model is demonstrated clearly with reference to reported data regarding the isotopic composition effect in bulk Si and Si NW samples. Namely, our model accounts for previously unexplained experimental results regarding (i) the isotope composition effect on the thermal conductivity of bulk silicon reported by Ruf et al. [Solid State Commun. 115, 243 (2000)], (ii) the size effect on κ(T) of individual Si NWs reported by Li et al. [Appl. Phys. Lett. 83, 2934 (2003)], and (iii) the dramatic decrease in the thermal conductivity for rough Si NWs reported by Hochbaum et al. [Nature (London) 451, 163 (2008)].

Thermal Conductivity of Bulk ${\rm MgB}_{2}$ Produced by Infiltration of Different Boron Powders

The thermal conductivity (TC) of superconducting bulk materials is a key property in determining its stability at the cryogenic conditions and the superconducting behavior in applications as the current leads and the resistive current limiters. The thermal conductivity of bulk MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> materials obtained by the reactive liquid Mg infiltration technique has been measured, in the temperature range from 4.2 K to 300 K, on samples having different grain size and derived from boron powders of different crystallinity and different granulometry. A wide spread of the TC values has been detected for the different samples and a typical maximum of TC at temperatures around T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> (critical temperature) has been detected only in the materials derived by very pure and macro-crystalline boron. On the contrary, the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> materials derived from less pure commercial so called "amorphous" boron, in which finer crystalline grains are detectable, show very low values of the thermal conductivity without any maxima in the lower temperatures range. The different behavior can be phenomenologically explained in terms of dependence of the electronic thermal conductivity on the lattice defects and impurities in the samples, with a larger amount of these defects in the microcrystalline samples. An interesting contribution is suggested to be due to the lately described Mg <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">25</sub> structure.

Thermal conductivity of high Tc YBaCuO bulk prepared by melt growth technique

A YBa2Cu3O7 sample with Tc of about 80 K and Jc of about 3000 Acm-2 at 50 K was prepared by a melt powdering melt growth technique. Thermal conductivity lambda of the sample was about 10-2 Wcm-1K-1 at 15 K. Addition of about 10 wt.%Ag increased a value of lambda about two times as large. Temperature dependence of lambda suggested the sample includes many defects. The sample is a hopeful material for superconducting current leads.

Thermal conductivity of thick anodic oxide coatings on aluminum

Thermal conductivity data for thick anodized aluminum coatings prepared in sulfuric acid solutions are found using a transient heat-flow method. For the coatings investigated, thermal conductivities ranged from 0.5 to 1.0 W/m K, more than an order of magnitude lower than the thermal conductivity of bulk, polycrystalline aluminum oxide (about 30 W/m K), which is frequently used to estimate the thermal conductivity of these coatings.

Thermal properties of small Pb spheres at low temperatures

The specific heat and thermal conductivity of 0.10-mm- and 0.34-mmdiameter spheres of Pb are reported (2–30 K). The former spheres are 99.99% Pb, the latter spheres are Pb + 5% Sb. Both types of spheres undergo a superconducting transition at 7 K, and the changes in the specific heats at the transition correlate well with the electronic coefficient determined for bulk Pb. The 5% Sb addition increases the specific heat by as much as 30%, due to an enhancement of the localized, non-Debye excitations present in pure Pb. The Debye temperatures of the spheres are 102–103 K. The thermal conductivities of packed columns of the spheres are due to lattice rather than electron transport and are about 103 times smaller than the thermal conductivity of bulk superconducting Pb. The 0.34-mm-diameter spheres have double the thermal conductivity of the 0.10-mm-diameter spheres, in contrast to the predictions of the elastic theory of Chan and Tien. An oxide layer may be the cause of the additional thermal resistance of the smaller spheres. T 3 boundary scattering occurs below 3 K for both sphere sizes and is consistent with specific heat and elastic-constant data for bulk Pb.