Calculation Of Thermal Conductivity Research Articles

Thermal conductivity calculation based on Green–Kubo formula using ANN potential for β-Ag2Se

For calculating thermal conductivity (TC) of the nonsuperionic β-Ag2Se, we used the computational framework that combined molecular dynamics simulations using interatomic potentials based on artificial neural networks (ANN potential) and the Green–Kubo formula. The ANN potential trained on first-principles molecular dynamics (FPMD) data. Despite the framework being essentially based on one developed in our previous study in which an empirical interatomic potential was employed for the superionic alpha phase to obtain the training data, it worked well in the current study based on the FPMD simulations for the beta phase. The estimated value was consistent with the extremely low experimental lattice TC of ∼0.3 Wm−1K−1. This result demonstrates the effectiveness of our framework for estimating TC.

Electronic structure and thermal conductance of the MASnI3/Bi2Te3 interface: a first-principles study

To develop high-performance thermoelectric devices that can be created using printing technology, the interface of a composite material composed of MASnI3 and Bi2Te3, which individually show excellent thermoelectric performance, was studied based on first-principles calculations. The structural stability, electronic state, and interfacial thermal conductance of the interface between Bi2Te3 and MASnI3 were evaluated. Among the interface structure models, we found stable interface structures and revealed their specific electronic states. Around the Fermi energy, the interface structures with TeII and Bi terminations exhibited interface levels attributed to the overlapping electron densities for Bi2Te3 and MASnI3 at the interface. Calculation of the interfacial thermal conductance using the diffuse mismatch model suggested that construction of the interface between Bi2Te3 and MASnI3 could reduce the thermal conductivity. The obtained value was similar to the experimental value for the inorganic/organic interface.

Maps of deep heat flow and geothermal resources in Ukraine http://gpimo.nas.gov.ua/en/node/1176

A technique for processing the material used to map the deep heat flow (HF) on the territory of Ukraine and constructing a map of geothermal resources on its basis is considered. We are talking about the deep (corrected) HF, since its error (determined by the differences in values at close points) is noticeably smaller than in the observed one, and only its values can be used to build thermal models of the earth's crust and upper mantle and solve the indicated applied problem. Due to the unique study of Ukraine in this parameter, which has been going on for almost 60 years, there are still significant fragments of the territory where the HF has not been determined. This is due to the lack of boreholes in such areas suitable for temperature measurements in the required depth interval with the required accuracy. Of course, any means of processing the experimental material in the territories adjacent to such white spots cannot completely replace a full-fledged study of HF within them. Nevertheless, the existing methods of geostatistics allow, by processing the available network of results, to ensure the construction of a map of the entire study area. In our case, the kriging technique was applied. The method works best inside a wrapper defined by peripheral data. In areas with a stable «wavelength» of HF anomalies, it is possible to fill in the gaps of the map without any problems. Naturally, with a noticeable smoothing of field variations. Most of the processed fragments of the map belonged to this type. A new edition of the description of the distribution of Ukraine's geoenergy resources is considered. We used a map of the deep heat flow processed by the kriging method and the construction of maps by region with the calculation of thermal conductivity at depths of up to 6 km at their contacts. This allowed to fill in the gaps in the network of experimental data and clarify the position and intensity of parameter anomalies in the southern part of the Carpathians, the north-eastern slope of the shield, in the Donbass and improve the overall assessment of geoenergy resources (up to 1.1 trillion tons of reference fuel). Geothermal energy from the deepest possible extraction interval (5.5—6.0 km) is about 26 times larger than the reserves of all fossil fuels in Ukraine.

Optimal reinforcement for improving the thermal performance of a permafrost tunnel based on coupled heat-mass transfer

Over 80% of cold-region tunnels (CRTs) suffer from heavy periodic freeze–thaw actions, and therefore, their normal safe operations cannot be guaranteed. To mitigate freeze–thaw damage to CRTs, thermal insulation layers (TILs) were used to prevent external heat from penetrating the surrounding rock of CRTs. TILs were often laid between two linings of CRTs with a uniform thickness along the whole tunnel based on a single thermal conduction calculation or engineering analogy method, which was not only uneconomical but also unsafe. Thus, freeze–thaw damage still occurred at the entrance of the CRTs. To solve this engineering problem, we took the Fenghuoshan Tunnel in permafrost regions as an example and proposed a coupled heat-mass transfer model based on mass, energy and momentum conservation principles. The numerical model was indicated to be accurate by comparing the computed and monitored temperatures. The subsequent numerical results show that the heat and mass exchanges between the air inside the tunnel and tunnel wall make the air velocities vary with seasonal alternation, and therefore, temperatures of the surrounding permafrost exhibit evident differences on the temporal and spatial scales, according to which the most unfavorable position and time point are selected to design an optimal reinforcement for improving the thermal performance. Based on a series of heat-mass transfer simulations with different thicknesses of aerogel felt, an optimal changeable thickness of aerogel felt along the whole tunnel is obtained to improve the thermal performance of the Fenghuoshan Tunnel. If the permafrost tunnel is reinforced by this approach, the highest temperatures of the surrounding permafrost near the tunnel are less than approximately −0.44 °C, implying that the optimal reinforcement is effective and can control the freeze–thaw damage of the Fenghuoshan Tunnel. This study provides theoretical support for designing a safe and economical reinforcement on permafrost tunnels and understanding the coupled heat-mass transfer processes in tunnels.

A metadata schema for lattice thermal conductivity from first-principles calculations

Materials genome engineering databases represent fundamental infrastructures for data-driven materials design, in which the data resources should satisfy the FAIR (Findable, Accessible, Interoperable and Reusable) principles. However, a variety of challenges, such as data standardization, veracity and longevity, still impede the progress of data-driven materials science, including both high-throughput experiments and simulations. In this work, we propose a metadata schema for lattice thermal conductivity from first-principles calculations. The calculation workflow for lattice thermal conductivity includes structural optimization and the calculation of interatomic force constants and lattice thermal conductivity. The data generated during the calculation process corresponds to the virtual sample information, virtual source data and processed data, respectively, as specified in the General rule for materials genome engineering data of the Chinese Society for Testing and Materials. Following this general rule, the metadata structure and schema for each action are systematically defined and all metadata elements can be collected completely. Although this metadata schema is specific to lattice thermal conductivity calculations, it provides general rules and insights for other computational materials data in materials genome engineering.

Thermal conductivity of ethylene-vinyl acetate copolymers with different vinyl acetate contents dependent on temperature and crystallinity

In this work, the thermal conductivity of a series of ethylene-vinyl acetate copolymers (EVAs) with different vinyl acetate (VAc) contents at different temperatures are measured by the transient hot line method. It is found that the thermal conductivity of a series of EVA samples with different VAc contents decreased smoothly with increasing temperature at temperatures from -20 to 80 °C. Meanwhile, the crystallinity and melting point of a series of EVA samples are determined by differential scanning calorimetry (DSC), and an empirical formula between the thermal conductivity of EVA and temperature and crystallinity is established by using a data-fitting method. The theoretical calculation of thermal conductivity obtained from this empirical formula is compared with the measured results, and the error is within 5%. Therefore, this empirical formula is for guidance for predicting the thermal conductivity of EVA samples with different VAc contents at different temperatures in the future.

Pivotal role of magnetic ordering and strain in lattice thermal conductivity of chromium-trihalide monolayers

Understanding the coupling between spin and phonons is critical for controlling the lattice thermal conductivity (κ l ) in magnetic materials, as we demonstrate here for CrX3 (X = Br and I) monolayers. We show that these compounds exhibit large spin-phonon coupling (SPC), dominated by out-of-plane vibrations of Cr atoms, resulting in significantly different phonon dispersions in ferromagnetic (FM) and paramagnetic (PM) phases. Lattice thermal conductivity calculations provide additional evidence for strong SPC, where particularly large κ l is found for the FM phase. Most strikingly, PM and FM phases exhibit radically different behavior with tensile strain, where κ l increases with strain for the PM phase, and strongly decreases for the FM phase—as we explain through analysis of phonon lifetimes and scattering rates. Taken all together, we uncover the high significance of SPC on the phonon transport in CrX3 monolayers, a result extendable to other 2D magnetic materials, that will be useful in further design of thermal spin devices.

Revisiting phonon transport in perovskite SrTiO3 : Anharmonic phonon renormalization and four-phonon scattering

We investigate the role of anharmonic phonon renormalization and four-phonon scattering in the thermal transport of cubic ${\mathrm{SrTiO}}_{3}$ from 200 to 800 K by constructing an accurate first-principles machine-learning potential with active learning. The anharmonic phonon dispersion relations show obvious low-frequency phonon hardening as temperature increases, implying potential importance of fourth order lattice anharmonicity in cubic ${\mathrm{SrTiO}}_{3}$. Under the perturbation theory and Boltzmann transport equation scheme, we find that phonon frequency renormalization can make a significant improvement in the calculations of thermal conductivity and its temperature dependence of cubic ${\mathrm{SrTiO}}_{3}$ compared to experimental data. Four-phonon scattering is usually important in strongly anharmonic materials with ultralow thermal conductivity and simple crystals featured by large acoustic-optical phonon band gaps. Nonetheless, we find that four-phonon scattering can make a considerable contribution to suppressing phonon transport in cubic ${\mathrm{SrTiO}}_{3}$ which has a moderate thermal conductivity. This study deepens the understanding of anharmonic phonon renormalization and four-phonon scattering in oxide perovskites.

Molecular dynamics approach to study the effects of MgCl2 salt atomic ratio on the phase transition phenomenon in the phase change materials

In general, phase change materials reduce energy consumption in different structures by increasing the thermal conductivity or decreasing the phase change time. On the other hand, hydrated salts are the most important group of phase change materials with high phase change temperatures and are extensively studied in latent thermal energy storage systems. Today, these atomic structures are used in different usages such as thermal transfer compounds, commercial and industrial applications, for cooling and heating on a small scale, etc. In the current computational study, we utilized Molecular Dynamics procedure to explain the thermal and phase transition (from liquid to solid-state) behavior of water liquid in the presence of MgCl2 salt as salts hydrate compound. Computationally, physical parameters like temperature, potential/total energy, phase transition time, thermal conductivity, and velocity/density profile of H2O-MgCl2 nanofluid were reported for time=15 ns. Molecular Dynamics outputs indicated that the phenomenon of phase transition in simulated nanofluid happens in a shorter time using a 4% atomic ratio salt structure. This atomic behavior of simulated structure causes thermal behavior improvement is defined nanofluid. Numerically, by MgCl2 salt ratio optimizing, H2O-MgCl2 nanofluid thermal conductivity converged to 0.67 W/m.k. Technically, thermal conductivity calculation in current research is done by using the Green-Kubo method.

Study on pore size distribution and thermal conductivity of aramid nanofiber aerogels based on fractal theory

Aramid nanofiber aerogels (ANFAs) have been considered as ideal thermal insulation materials due to their low bulk density, hierarchical porous structure, high thermal insulation, and thermal stability. Thermal conductivity of ANFAs is highly determined by their complicated porous structure. Fractal geometry theory can effectively simulate pore size distribution, but it is rarely used in the study of the thermal conductivity of aerogels. In this work, the Sierpinski carpet model and two modified Sierpinski carpet models were established to study the pore size distribution of ANFAs, and then the equivalent resistance method was used to transform the models into series and parallel fractal resistance networks, which were applied for the calculation of the effective thermal conductivity of ANFAs. Results show that the calculated results are closer to the experimental data, with a minimum error of 0.78%, indicating the thermal conductivity prediction model is effective and reliable.

Inspecting the electronic structure and thermoelectric power factor of novel p-type half-Heuslers

In line for semiconducting electronic properties, we systematically scrutinize the likely to be grown half-Heusler compounds XTaZ (X = Pd, Pt and Z = Al, Ga, In) for their stability and thermoelectric properties. The energetically favored F-43m configuration of XTaZ alloys at equilibrium lattice constant is a promising non-magnetic semiconductor reflected from its total valence electron count (NV = 18) and electronic structure calculations. Alongside mechanical stability, the dynamic stability is guaranteed from lattice vibrations and the phonon studies. The energy gaps of these stable Ta-based materials with Z = Ga are estimated to reach as high as 0.46 eV when X = Pd and 0.95 eV when X = Pt; however, this feature is reduced when Z = Al/In and X = Pd/Pt, respectively. Lattice thermal conductivity calculations are achieved to predict the smallest room temperature value of KL = 33.6 W/K (PdTaGa) and 38.0 W/mK (for PtAlGa) among the proposed group of Heusler structures. In the end, we investigated the plausible thermoelectric performance of XTaZ alloys, which announces a comparable difference for the n-type and p-type doping regions. Among the six alloys, PtTaAl, PtTaGa and PtTaIn are predicted to be the most efficient materials where the power factor (PF) elevates up to ~ 90.5, 106.7, 106.5 mW/(K2m), respectively at 900 K; however the lower values are recorded for PdTaAl (~ 66.5), PdTaGa (~ 76.5) and PdTaIn (~ 73.4) alloys. While this reading unlocks avenues for additional assessment of this new class of Half Heuslers, the project approach used here is largely appropriate for possible collection of understandings to realize novel stable materials with potential high temperature applications.

Development of a generalized thermal resistance model for the calculation of effective thermal conductivities in periodic open cellular structures (POCS)

Periodic open cellular structures (POCS) are a group of porous media with promising attributes for the use in heat transfer enhancement. They consist of a periodic arrangement of unit cells that offer a vast design freedom and therefore high adaptability to the needs of a specific application. In order to use POCS efficiently, a fundamental understanding of the relation between their geometry and thermal transport properties is essential.This contribution focuses on the investigation and modeling of the effective thermal conductivity. It is based on a homogenization approach that averages the heat conduction properties over an entire unit cell, while neglecting the influence of the fluid. To this end, the investigation provides detailed insight and an important component for the subsequent model development on the overall heat transfer capabilities of POCS, including convection and conjugate heat transfer. A numerical simulation setup is developed, which allows the determination of this quantity in isotropic as well as anisotropic POCS with varying unit cell type, porosity, tapering and strut cross-sectional shape. The results indicate the impact of both struts as well as nodes, on the heat transfer and form the foundation for a new generalized thermal resistance network model. In contrast to similar approaches, the impact of multidimensional heat transfer within the nodes is considered enabling an accurate prediction of the effective thermal conductivity of all POCS investigated in this work. Due to its modularity, it is easily transferable to other unknown geometries, thus contributing to the toolbox for the efficient design of POCS.

Effect of regulating compressive strains on thermal transport of silicon-based amorphous silica thin films and interfacial thermal resistance

To investigate the modulation of compressive strains on the thermal transport of silica thin films, a test method is proposed. A more accurate and anti-interference thermal conductivity test system was built, and the thermal-induced compressive strain was regulated by the external tensile stress setup. The microstructure and composition of sample were analyzed, and regulation law of compressive strains on the thermal conductivity of silica thin films was obtained. The effect of compressive strains on the thermal conduction of amorphous silica and interfacial thermal resistance was analyzed based on molecular dynamics simulation. The thermal conductivity calculation result is consistent with the change trend of the test result. The interfacial thermal resistance is small and the strain effect can be ignored.

Preference Parameters for the Calculation of Thermal Conductivity by Multiparticle Collision Dynamics.

Calculation of the thermal conductivity of nanofluids by molecular dynamics (MD) is very common. Regrettably, general MD can only be employed to simulate small systems due to the huge computation workload. Instead, the computation workload can be considerably reduced due to the coarse-grained fluid when multiparticle collision dynamics (MPCD) is employed. Hence, such a method can be utilized to simulate a larger system. However, the selection of relevant parameters of MPCD noticeably influences the calculation results. To this end, parameterization investigations for various bin sizes, number densities, time-steps, rotation angles and temperatures are carried out, and the influence of these parameters on the calculation of thermal conductivity are analyzed. Finally, the calculations of thermal conductivity for liquid argon, water and Cu-water nanofluid are performed, and the errors compared to the theoretical values are 3.4%, 1.5% and 1.2%, respectively. This proves that the method proposed in the present work for calculating the thermal conductivity of nanofluids is applicable.

Atomistic calculations of thermal conductivity in films made from graphene sheets for electron emitter applications

Simulations for the thermal conductivity of carbon fibers (CFs) are carried out based on a molecular dynamics (MD) approach. Our calculations are geared toward providing a general trend for thermal conductivity and its dependence on sample length and to probe the suitability of this material for high power applications from the thermal management standpoint. Since abundant data are not available for carbon fibers, calculations are first carried out in silicon to validate the predictions. The MD results yield a bulk thermal conductivity of 148 W m−1 K−1 for silicon in good agreement with published reports. However, for CF, a much lower bulk value of ∼14 W m−1 K−1 is predicted. This suggests that thermal management would likely be an issue for this material and that CF emitters of lengths below 2 µm should be avoided. The predicted increases in thermal conductivity with temperature may help alleviate the issue to a minor degree. Carbon nanotubes would likely be a better alternative in this context of field emitter arrays.

Drastic reduction of thermal conductivity in hexagonal AX (A = Ga, In & Tl, X = S, Se & Te) monolayers due to alternative atomic configuration

Several two-dimensional chalcogenide materials have been in the limelight in the recent past for their promising thermoelectric properties. It is well established that the thermoelectric performance of materials improves on reducing the physical dimensionality of the system. Two-dimensional hexagonal chalcogen (S, Se and Te) bearing compounds of Ga, In and Tl have already been studied extensively in literature. But in those phases, the group-13 non-chalcogen atoms occupy the two inner planes while the chalcogens occupy the two outer planes of the unit cell. In this work, we have proposed the alternate arrangement in which the chalcogen atoms occupy the two inner planes while the group-13 atoms occupy the two outer planes of the unit cell. Unprecedentedly, this alternate arrangement shows much lower thermal conductivity that leads to superior thermoelectric performance. In this work we have studied in details the thermoelectric properties of hexagonal AX (A = Ga, In & Tl, X = S, Se & Te) monolayers and compare the results having both the atomic arrangements. The very low lattice thermal conductivity of this new arrangement is due to the outermost valence s-orbital lone pair of the chalcogens which leads to enhanced anharmonicity. We have explained these results from the anti-crossing of the phonon branches as well. The electronic, dynamical, thermodynamical and elastic properties have also been studied. We think that these results should have significant impact on the synthesis of high-performance thermoelectric materials based on chalcogenides of gallium, indium and thallium. • The position of the group 13 and chalcogen atoms in AX (A = Ga, In & Tl, X = S, Se & Te) have been flipped. • Ab initio calculations of thermal conductivity, electronic properties and mechanical properties have been determined. • The new configuration yields dynamically stable structures too with much lower elastic constants and Debye temperatures. • The high anharmonic scattering and weak bonds lead to ultralow thermal conductivity due to the lattice vibrations. • The ultralow thermal conductivity lead to extremely high values of thermoelectric figure of merit.

Intrinsic nanostructure induced ultralow thermal conductivity yields enhanced thermoelectric performance in Zintl phase Eu2ZnSb2

The Zintl thermoelectric phase Eu2ZnSb2 has a remarkable combination of high mobility and low thermal conductivity that leads to good thermoelectric performance. The key feature of this compound is a crystal structure that has a Zn-site with a 50% occupancy. Here we use comparison of experimental thermal conductivity measurements and first principles thermal conductivity calculations to characterize the thermal conductivity reduction. We find that partial ordering, characterized by local order, but Zn-site disorder on longer scales, leads to an intrinsic nanostructuring induced reduction in thermal conductivity, while retaining electron mobility. This provides a direction for identifying Zintl compounds with ultralow lattice thermal conductivity and good electrical conductivity.

FourPhonon: An extension module to ShengBTE for computing four-phonon scattering rates and thermal conductivity

FourPhonon is a computational package that can calculate four-phonon scattering rates in crystals. It is built within ShengBTE framework, which is a well-recognized lattice thermal conductivity solver based on Boltzmann transport equation. An adaptive energy broadening scheme is implemented for the calculation of four-phonon scattering rates. In analogy with thirdorder.py in ShengBTE, we also provide a separate python script, Fourthorder.py, to calculate fourth-order interatomic force constants. The extension module preserves all the nice features of the well-recognized lattice thermal conductivity solver ShengBTE, including good parallelism and straightforward workflow. In this paper, we discuss the general theory, program design, and example calculations on Si, BAs and LiCoO2. Program summaryProgram Title:FourPhononCPC Library link to program files:https://doi.org/10.17632/jjcwt5436h.1Developer's repository link:https://github.com/FourPhononLicensing provisions: GNU General Public License version 3Programming language: Fortran 90, MPINature of problem: Calculation of lattice thermal conductivity and related quantities, determination of both three-phonon and four-phonon scattering ratesSolution method: Four-phonon scattering rates at RTA level, adaptive broadening schemeAdditional comments including restrictions and unusual features: For a productive run, one needs to use High Performance Computing facilities and it takes several hours to several days to finish calculations

Computational Revolutions in Lattice Thermal Conductivity

Understanding and controlling the phonon, the dominant heat carrier of semiconductor materials, is essential to developing a wide variety of applications. This article studies the theoretical and computational approach of the calculation of lattice thermal conductivity of semiconducting materials. Despite having different methods to calculate the lattice thermal conductivity, first-principle estimates predict more accurately in most applications. This motivates to present the descriptive explanation on first-principle calculation with the combination of lattice dynamics and Boltzmann transport equation. Finally, we summarized an overview of the recent achievements and opportunities.

Ultrahigh thermal conductivity in three-dimensional covalent organic frameworks

Covalent organic frameworks (COFs) are a new, growing class of crystalline polymers with exceptional characteristics such as low density, mechanical strength, and flexibility. Taking three-dimensional (3D) COF-300 derivatives as an example, we apply molecular dynamics simulations to show that ultrahigh thermal conductivity (>15 W/(mK)) is achieved at a pore size of d = 0.63 nm. This is the first observation of such an outstanding intrinsic thermal conductivity of a 3D polymer in all directions. Further temperature-dependent thermal conductivity calculations and X-ray diffraction (XRD) patterns highlight the importance of maintaining microscopic order for a large thermal conductivity. We also demonstrate that the tunability and anisotropy in COFs provide sufficient room to engineer and obtain desired thermal conductivity. These findings provide a fundamental understanding of structure-thermal conductivity relation in COFs and shine a light on high-thermal-conductivity COFs for thermal management applications.