Thermal Transport Coefficients Research Articles

Microscopic Model to Quantify the Difference of Energy-Transfer Rates between Bonded and Nonbonded Monomers in Polymers

Thermal transport properties often dictate the usefulness of materials in a variety of applications. In this context, polymers are an important material class because they provide different pathways of energy transport due to the distinct microscopic interactions, i.e., via stiff, covalently bonded backbone interactions or via soft, nonbonded interactions, such as van der Waals (vdW) forces and/or hydrogen bonds (H-bond). Therefore, the precise control of the delicate balance between bonded and nonbonded energy transfer rates provides a possible strategy to tailor the thermal conductivity of a material. In this work, we devise a simple analytical model that decouples the microscopic bonded, Gb, and nonbonded, Gnb, contributions to the heat transport in polymeric materials. This model considers the diffusion of energy along the macromolecular backbone, involving multiple transfers before it can hop off to a neighboring chain molecule. We show how these individual microscopic components can be combined to obtain a diffusive contribution to the macroscopic thermal transport coefficient, κ. The ability of the model to describe thermal transport is validated by molecular simulations of one universal polymer model and three, chemically specific, all-atom polymer models. These results suggest strategies for tailoring κ of polymeric materials by macromolecular engineering of molecular architecture and conformations.

Self-Doping for Synergistically Tuning the Electronic and Thermal Transport Coefficients in n-Type Half-Heuslers.

Ternary intermetallic half-Heusler (HH) compounds (XYZ) with 18 valence electron count, namely, ZrCoSb, ZrNiSn, and ZrPdSn, have revealed promising thermoelectric properties. Exemplarily, it has been experimentally observed that a slight change in the content of Y site atoms (by ∼3-12.5% i.e., m = 0.03 and 0.125 in ZrY1+mZ) leads to a drastic decrease in lattice thermal conductivity κL by more than 65-80% in many of these compounds. The present work aims at exploring the possibility of maximizing the electronic transport scenario after achieving the low κL limit in these compounds. By taking into account the full anharmonicity of the lattice dynamics, Boltzmann transport calculations are performed under the framework of density functional theory. Our results show that these excess atoms present in the vacant lattice site induce scattering either by acting as a rattling mode or by hybridizing with the acoustic modes of the host depending upon their mass and bonding chemistry, respectively. Furthermore, the introduction of these scattering centers may lead either to the formation of a defect midgap state in the electronic band structure (detrimental for electronic transport) or to light doping of the host compound. The latter is found to be particularly conducive for attaining synergy in both thermal and electronic transport.

Thermo-Magneto-Electric Transport through a Torsion Dislocation in a Type I Weyl Semimetal

Herein, we study electronic and thermoelectric transport in a type I Weyl semimetal nanojunction, with a torsional dislocation defect, in the presence of an external magnetic field parallel to the dislocation axis. The defect is modeled in a cylindrical geometry, as a combination of a gauge field accounting for torsional strain and a delta-potential barrier for the lattice mismatch effect. In the Landauer formalism, we find that due to the combination of strain and magnetic field, the electric current exhibits chiral valley-polarization, and the conductance displays the signature of Landau levels. We also compute the thermal transport coefficients, where a high thermopower and a large figure of merit are predicted for the junction.

QEHeat: An open-source energy flux calculator for the computation of heat-transport coefficients from first principles

We give a detailed presentation of the theory and numerical implementation of an expression for the adiabatic energy flux in extended systems, derived from density-functional theory. This expression can be used to estimate the heat conductivity from equilibrium ab initio molecular dynamics, using the Green-Kubo linear response theory of transport coefficients. Our expression is implemented in an open-source component of the Quantum ESPRESSO suite of computer codes for quantum mechanical materials modeling, which is being made publicly available. Program summaryProgram title:QEHeatCPC Library link to program files:https://doi.org/10.17632/c6wxkvy4z3.1Licensing provisions: GPLv3Programming language: FortranNature of problem: The computation of thermal transport coefficients via equilibrium molecular dynamics and the Green-Kubo theory of linear response requires the definition of a heat-flux describing the instantaneous flow of energy. When considering predictive first-principles methods, a definition of the heat-flux compatible with density-functional theory is required [1]. The evaluation of such a heat flux requires an extension of state-of-the-art atomic simulation codes.Solution method: This work describes in detail the numerical implementation of the adiabatic energy current derived in Refs. [1,2] and makes it available to the users of the Quantum ESPRESSO suite of computer codes [3]. Used in conjunction with the cp.x code, to perform Car-Parrinello ab initio molecular dynamics, and the SPORTRAN post-processing tool for data analysis [4–6], the program allows to estimate heat transport coefficients in extended systems entirely from first principles. The new code provides as well to developers a modular and easily extendable framework to evaluate time derivatives of electronic properties (e.g. electronic densities or potentials) via a finite difference approach.

Intrinsic thermoelectric figure of merit of bulk compositional SiGe alloys: A first-principles study

A systematic study based on state-of-the-art first-principles calculations has been carried out to determine intrinsic thermoelectric properties in $n$- and $p$-type SiGe alloys. Both electronic and thermal transport coefficients have been evaluated using the Boltzmann transport equation, applied on the electronic and phonon band structure, respectively. The Seebeck coefficient, electrical conductivity, and thermal conductivity have been analyzed focusing on the effect of carrier concentration, temperature, and alloy composition. The resulting figure of merit is highest for heavy doping and at elevated temperatures ($>$1000 K) in the SiGe alloy with Ge content from 50% to 80%.

Thermoelectric and thermal properties of the weakly disordered non-Fermi liquid phase of Luttinger semimetals

We compute the thermoelectric and thermal transport coefficients in the weakly disordered non-Fermi liquid phase of the Luttinger semimetals at zero doping, where the decay rate associated with the (strong) Coulomb interactions is much larger than the electron-impurity scattering rate. To this end, we implement the Mori-Zwanzig memory matrix method, that does not rely on the existence of long-lived quasiparticles in the system. We find that the thermal conductivity at zero electric field scales as κ¯∼T−n (with 0≲n≲1) at low temperatures, whereas the thermoelectric coefficient has the temperature dependence given by α∼Tp (with 1/2≲p≲3/2). These unconventional properties turn out to be key signatures of this long sought-after non-Fermi liquid state in the Luttinger semimetals, which is expected to emerge in strongly correlated spin-orbit coupled materials like the pyrochlore iridates. Finally, our results indicate that these materials might be good candidates for achieving high figure-of-merit for thermoelectric applications.

Study of engine-oil based CNT nanofluid flow on a rotating cylinder with viscous dissipation

An axisymmetric viscous carbon nanotube (CNT) nanofluid flow exterior to a rotating circular cylinder of radius a 0 along with viscous dissipation and magnetohydrodynamic (MHD) is considered here. The two-phased Buongiorno’s model is employed to observe the thermal energy transport in nanofluid with engine-oil as a background fluid and two distinct nanoparticles, i.e., single-walled (SWCNTs) and multi-walled (MWCNTs). The flow is driven because of the torsional motion and stretching of the cylinder along the z-direction, which constitutes the axial pressure gradient. It is seen that in the presence of a magnetic field, the axially dependent swirling motion of the cylinder creates a light transverse flow in the meridional plane, which is the principal cause for the wall jet phenomenon on the axial axis. The function computing the numerical solution is bvp5c, which is available in Matlab®. It is noted that for both SWCNTs/MWCNTs cases, thermal and mass transport coefficients are decreasing function of Prandtl and Schmidt numbers, respectively. However, they are augmented due to the presence of strong Lorentz force. The values for wall stress parameters (WSPs) are determined as a function of a magnetic parameter for 0.01 ≤ Re ≤ 102 (where Re is Reynolds number). It is also perceived that the axial WSP decreases due to an increase in values of Re, but an increase in numerical values of azimuthal WSP and Nusselt number is noticed.

Total-transmission and total-reflection of individual phonons in phononic crystal nanostructures

The control of thermal waves by the phononic crystal exhibits peculiar behaviors different from the particle picture of phonons and thus has attracted increasing interest. However, the wave nature of phonons is only indirectly reflected in most studies via the macroscopic thermal transport coefficient, such as thermal conductivity. In this work, we investigate directly the coherent interference effect in a graphene superlattice structure at the microscopic phonon mode level via wave-packet simulations. The constructive interference and destructive interference between the reflected phonons give rise to valleys and peaks in the transmission coefficient, respectively, leading to the periodic oscillation of the transmission function with the variation of the superlattice period length. More importantly, both total-transmission and total-reflection of individual phonons have been clearly demonstrated. The physical conditions for realizing the phonon interference have been proposed, which are quantitatively in good agreement with independent wave-packet simulations. Our study provides direct evidence for the coherent phonon interference effect, which might be helpful for the regulation of phonon transport based on its wave nature.

Heat and Fluid Flow Analysis and ANN-Based Prediction of A Novel Spring Corrugated Tape

A circular tube fitted with novel corrugated spring tape inserts has been investigated. Air was used as the working fluid. A thorough literature review has been done and this geometry has not been studied previously, neither experimentally nor theoretically. A novel experimental investigation of this enhanced geometry can, therefore, be treated as a new substantial contribution in the open literature. Three different spring ratio and depth ratio has been used in this study. Increase in thermal energy transport coefficient is noticed with increase in depth ratio. Corrugated spring tape shows promising results towards heat transfer enhancement. This geometry performs significantly better (60% to 75% increase in heat duty at constant pumping power and 20% to 31% reduction in pumping power at constant heat duty) than simple spring tape. This paper also presented a statistical analysis of the heat transfer and fluid flow by developing an artificial neural network (ANN)-based machine learning (ML) model. The model is evaluated to have an accuracy of 98.00% on unknown test data. These models will help the researchers working in heat transfer enhancement-based experiments to understand and predict the output. As a result, the time and cost of the experiments will reduce. The results of this investigation can be used in designing heat exchangers.

Transport Properties of Superfluid Phonons in Neutron Stars

We review the effective field theory associated with the superfluid phonons that we use for the study of transport properties in the core of superfluid neutrons stars in their low temperature regime. We then discuss the shear and bulk viscosities together with the thermal conductivity coming from the collisions of superfluid phonons in neutron stars. With regard to shear, bulk, and thermal transport coefficients, the phonon collisional processes are obtained in terms of the equation of state and the superfluid gap. We compare the shear coefficient due to the interaction among superfluid phonons with other dominant processes in neutron stars, such as electron collisions. We also analyze the possible consequences for the r-mode instability in neutron stars. As for the bulk viscosities, we determine that phonon collisions contribute decisively to the bulk viscosities inside neutron stars. For the thermal conductivity resulting from phonon collisions, we find that it is temperature independent well below the transition temperature. We also obtain that the thermal conductivity due to superfluid phonons dominates over the one resulting from electron-muon interactions once phonons are in the hydrodynamic regime. As the phonons couple to the Z electroweak gauge boson, we estimate the associated neutrino emissivity. We also briefly comment on how the superfluid phonon interactions are modified in the presence of a gravitational field or in a moving background.

Tuning thermal transport in highly cross-linked polymers by bond-induced void engineering

Tuning the heat flow is fundamentally important for the design of advanced functional materials. Here, polymers are of particular importance because they provide different pathways for the energy transfer. More specifically, the heat flow between two covalently bonded monomers is over 100 times faster than between the two non-bonded monomers interacting via van der Waals (vdW) forces. Therefore, the delicate balance between these two contributions often provide a guiding tool for the tunability in thermal transport coefficient k of the polymeric materials. Traditionally most studies have investigated k in the linear polymeric materials, the recent interests have also been directed towards the highly cross-linked polymers (HCP). In this work, using the generic molecular dynamics simulations we investigate the factors effecting k of HCP. We emphasize on the importance of the cross-linking bond types and its influence on the network microstructure with a goal to provide a guiding principle for the tunability in k. While these simulation results are discussed in the context of the available experimental data, we also make predictions.

Heat transfer and fluid flow analysis using nanofluids in diamond-shaped cavities with novel obstacles

This work computationally explores the two-phase flow of nanofluids and their thermal energy transport coefficients in 3D diamond-shaped cavities with square-shaped barriers having reducing dimensions. Materials with two emissivity values, and 0.9, have been considered to investigate the effect of the radiation thermal energy transport coefficient while the hot side is maintained at 400 or 500 K. Two values of the Rayleigh number, Ra = 106 and 108, are used for the study. Cu nanoparticles (NPs) with an average size of 25 nm have been used at a concentration of 0.01–0.05% in the base fluid. The temperature gradients and thermal energy transport coefficient characteristics are enhanced by raising the volume concentration of nanoparticles, but the streamlines do not alter substantially. By increasing Ra, the thermal energy transport coefficient rate is further augmented. It is also found that increasing the Ra and volume concentration of NPs results in enhanced heat transfer inside a cavity, while a change in the emissivity coefficient has no significant impact on the thermal and flow characteristics of the nanofluid. For each case, there is an optimum NP volume fraction for each model that leads to the highest Nusselt number.

Exact solution of the Boltzmann equation for low-temperature transport coefficients in metals. I. Scattering by phonons, antiferromagnons, and helimagnons

We present a technique for an exact solution of the linearized Boltzmann equation for the electrical and thermal transport coefficients in metals in the low-temperature limit. This renders unnecessary an uncontrolled approximation that has been used in all previous solutions of the integral equations for the transport coefficients. Applications include electron-phonon scattering in nonmagnetic metals, as well as the magnon contribution to the electrical and thermal conductivities, and to the thermopower, in metallic ferromagnets, antiferromagnets, and helimagnets. In this paper, the first of a pair, we set up the technique and apply it to the scattering of electrons by phonons, antiferromagnons, and helimagnons. We show that the Bloch $T^5$ law for the electrical resistivity, the $T^2$ law for the thermal resistivity, and the $T$ law for the thermopower due to phonon and antiferromagnon scattering are exact, and determine the prefactors exactly. The corresponding exact results for helimagnons are $T^{5/2}$, $T^{1/2}$, and $T$, respectively. In a second paper we will consider the scattering by ferromagnons.

Exact solution of the Boltzmann equation for low-temperature transport coefficients in metals. II. Scattering by ferromagnons

In a previous paper (Paper I) we developed a technique for exactly solving the linearized Boltzmann equation for the electrical and thermal transport coefficients in metals in the low-temperature limit. Here we adapt this technique to determine the magnon contribution to the electrical and thermal conductivities, and to the thermopower, in metallic ferromagnets. For the electrical resistivity $\rho$ at asymptotically low temperatures we find $\rho \propto \exp{(-T_{\rm min}/T)}$, with $T_{\rm min}$ an energy scale that results from the exchange gap and a temperature independent prefactor of the exponential. The corresponding result for the heat conductivity is $\sigma_h \propto T^3\,\exp{(T_{\rm min}/T)}$, and thermopower is $S \propto T$. All of these results are exact, including the prefactors.

Using High Multipolar Orders to Reconstruct the Sound Velocity in Piezoelectrics from Lattice Dynamics.

Information over the phonon band structure is crucial to predicting many thermodynamic properties of materials, such as thermal transport coefficients. Highly accurate phonon dispersion curves can be, in principle, calculated in the framework of density-functional perturbation theory. However, well-established techniques can run into trouble (or even catastrophically fail) in the case of piezoelectric materials, where the acoustic branches hardly reproduce the physically correct sound velocity. Here we identify the culprit in the higher-order multipolar interactions between atoms and demonstrate an effective procedure that fixes the aforementioned issue. Our strategy drastically improves the predictive power of perturbative lattice-dynamical calculations in piezoelectric crystals and is directly implementable for high-throughput generation of materials databases.

Potential thermoelectric candidate monolayer silicon diphosphide (SiP2) from a first-principles calculation

Recently layered silicon diphosphide (SiP2) single crystal has been successfully synthesized in the experiment. Based on the unique geometric structure and potential excellent physical properties, the thermoelectric transport performance of monolayer SiP2 is investigated by using Boltzmann transport theory combined with the first-principles calculations. Both thermal and electronic transport coefficients of monolayer SiP2 present obvious anisotropic behavior, in which the thermal conductivity and electrical conductivity in the a direction are less than those in the b direction. At 300 K, the phonon thermal conductivity in the a and b direction are 2.22 and 16.23 W/mK, respectively. However, the Seebeck coefficient of monolayer SiP2 is almost isotropic. Its values at room temperature could approach 3.02 mV/K and 3.15 mV/K in the a and b direction, respectively. By using the electron relaxation time calculated from the deformation potential theory, the thermoelectric figure of merit of 0.90 and 0.74 along the b- and a-axis at 300 K are obtained, respectively. The results presented in this work indicate that monolayer SiP2 processes fascinating thermoelectric performance, and foreshadow the great potential in future thermoelectric applications.

Heat transfer and exergy analysis of solar air heater tube with helical corrugation and perforated circular disc inserts

The effects of perforated circular disc swirl generator on heat transfer (HT) and flow fields in a solar air heater helical corrugated tube have been investigated experimentally. Thermal energy transport coefficient at different values of the corrugation angle (θ), the corrugation pitch ratio (y), the perforation ratio (k), and the perforation disc pitch ratio (s) is studied for Reynolds numbers (Re) ranging from 10,000 to 52,000. Isothermal pressure drop tests and heat transfer experiments under a uniform heat flux conditions have been carried out. The results indicate that in the presence of a perforated circular disc inside the helically corrugated tube, heat transfer is augmented by around 50–60%. Entropy generation in the form of irreversibility is reported. Exergy analysis, in terms of exergy efficiency, is presented. The corrugated tube with swirl generator inserts augments the thermal energy transport coefficient mostly, which is accompanied by a minimum pressure penalty. The combined geometry augments more thermal energy transport coefficient than those of acting alone. A predictive Nusselt number and friction factor correlation are also developed. A large data set has been created for thermal energy transport coefficient and thermal–hydraulic performance, which is beneficial for the design of solar thermal air heaters and heat exchangers.

Thermal transport in a weakly magnetized hot QCD medium

The thermal transport coefficients in a weakly magnetized quark-gluon plasma have been investigated within the ambit of a quasiparticle model to encode the effects of the realistic equation of state. The presence of a weak magnetic field leads to the Hall-type conductivity associated with thermal transport in the medium. An effective covariant kinetic theory has been employed to quantify the thermal dissipation while incorporating the mean field contributions in the medium. The interplay of thermal transport and electric charge transport in the weakly magnetized medium has been explored in terms of the Wiedemann-Franz law. Strong violation of the Wiedemann-Franz law has been observed in temperature regimes near to the transition temperature. The behaviour of thermal conductivity in the strong magnetic field limit has also been studied. It is observed that both the magnetic field and equation of state have a significant impact on the thermal dissipation in the medium.

Anomalous transverse response of Co2MnGa and universality of the room-temperature αijA/σijA ratio across topological magnets

The off-diagonal (electric, thermal and thermoelectric) transport coefficients of a solid can acquire an anomalous component due to the non-trivial topology of the Bloch waves. We present a study of the anomalous Hall (AHE), Nernst (ANE) and thermal Hall effects (ATHE) in the Heusler Weyl ferromagnet Co$_2$MnGa. The Anomalous Wiedemann-Franz law, linking electric and thermal responses, was found to be valid over the whole temperature window. This indicates that the AHE has an intrinsic origin and the Berry spectrum is smooth in the immediate vicinity of the Fermi level. From the ANE data, we extract the magnitude and temperature dependence of $\alpha^A_{ij}$ and put under scrutiny the $\alpha^A_{ij}/\sigma^A_{ij}$ ratio, which approaches k$_B$/e at room temperature. We show that in various topological magnets the room-temperature magnitude of this ratio is a sizeable fraction of k$_B$/e and argue that the two anomalous transverse coefficients depend on universal constants, the Berry curvature averaged over a window set by either the Fermi wavelength (for Hall) or the de Broglie thermal length (for Nernst). Since the ratio of the latter two is close to unity at room temperature, such a universal scaling finds a natural explanation in the intrinsic picture of anomalous transverse coefficients.

Gauge Fixing for Heat-Transport Simulations.

Thermal and other transport coefficients were recently shown to be largely independent of the microscopic representation of the energy (current) densities or, more generally, of the relevant conserved densities/currents. In this Article, we show how this gauge invariance, which is intimately related to the intrinsic indeterminacy of the energy of individual atoms in interacting systems, can be exploited to optimize the statistical properties of the current time series from which the transport coefficients are evaluated. To this end, we introduce and exploit a variational principle that relies on the metric properties of the conserved currents, treated as elements of an abstract linear space. Different metrics would result in different variational principles. In particular, we show that a recently proposed data-analysis technique based on the theory of transport in multicomponent systems can be recovered by a suitable choice of this metric.