Thermal Transport Coefficients Research Articles

Direct optimization of neoclassical ion transport in stellarator reactors

Abstract We directly optimize stellarator neoclassical ion transport while holding neoclassical electron transport at a moderate level, creating a scenario favorable for impurity expulsion and retaining good ion confinement. Traditional neoclassical stellarator optimization has focused on minimizing ϵ eff , the geometric factor that characterizes the amount of radial transport due to particles in the 1 / ν regime. Under expected reactor-relevant conditions, core electrons will be in the 1 / ν regime and core fuel ions will be in the ν regime. Traditional optimizations thus minimize electron transport and rely on the radial electric field (Er ) that develops to confine the ions. This often results in an inward-pointing Er that drives high-Z impurities into the core, which may be troublesome in future reactors. In this work, we increase the ratio of the thermal transport coefficients L 11 e / L 11 i , which previous research has shown can create an outward-pointing Er . This effect is very beneficial for impurity expulsion. We obtain self-consistent density, temperature, and Er profiles at reactor-relevant conditions for an optimized equilibrium. This equilibrium is expected to enjoy significantly improved impurity transport properties.

Thermal Energy Transport through Nonbonded Native Contacts in Protein.

Within the protein interior, where we observe various types of interactions, nonuniform flow of thermal energy occurs along the polypeptide chain and through nonbonded native contacts, leading to inhomogeneous transport efficiencies from one site to another. The folded native protein serves not merely as thermal transfer medium but, more importantly, as sophisticated molecular nanomachines in cells. Therefore, we are particularly interested in what sort of "communication" is mediated through native contacts in the folded proteins and how such features are quantitatively depicted in terms of local transport coefficients of heat currents. To address the issue, we introduced a concept of inter-residue thermal conductivity and characterized the nonuniform thermal transport properties of a small globular protein, HP36, using equilibrium molecular dynamics simulation and the Green-Kubo formula. We observed that the thermal transport of the protein was dominated by that along the polypeptide chain, while the local thermal conductivity of nonbonded native contacts decreased in the order of H-bonding > π-stacking > electrostatic > hydrophobic contacts. Furthermore, we applied machine learning techniques to analyze the molecular mechanism of protein thermal transport. As a result, the contact distance, variance in contact distance, and H-bonding occurrence probability during MD simulations are found to be the top three important determinants for predicting local thermal transport coefficients.

Real-space calculation of thermal Hall effects in strained and disordered kagome ferromagnets

Ferromagnetic insulators may exhibit magnon Hall effects when subjected to a temperature gradient due to the Dzyaloshinsky–Moriya interaction. In this study, we investigate the magnon thermal Hall conductivity κxy of kagome ferromagnets in real space using the kernel polynomial method. We first establish the formalism in real space within the framework of linear response theory, which enables efficient numerical calculations of thermal transport properties under various imperfections. The validity and accuracy of the real-space approach are confirmed by comparing the calculations with those obtained in momentum space. This approach is particularly advantageous for computing the thermal transport coefficients of disordered lattices. We consider two types of disorder in kagome ferromagnets and observe that both types significantly influence κxy across the entire temperature range. This is in contrast to the effects of strains, where strains primarily impact the maximum values of κxy .

Heat Generation and Diffusion in an Assembly of Magnetic Nanoparticles: Application to Magnetic Hyperthermia

We investigate the thermal generation and transport properties of an assembly of magnetic nanoparticles embedded in a solid or fluid matrix, subjected to an AC magnetic field. For this purpose, we first build the heat equation for the assembly using the effective thermal transport coefficients obtained within the effective medium approach. In the present calculation, the SAR is obtained from the (linear) dynamic response of the assembly to the AC magnetic field. We numerically solve the extended heat equation and, as a preliminary study, we obtain the space-time profile of the temperature and total power absorbed by the system.

Roles of non-axisymmetric perturbations in free drift vertical displacement events on EAST

The safe operation of most tokamaks, especially the large ones, relies on the feedback control of vertical displacement events (VDEs). However, most of these feedback control systems are based on axisymmetric VDE models. In this study, we use NIMROD simulations to study the role of non-axisymmetric perturbations in free drift vertical displacement events on EAST. The high-n modes in the non-axisymmetric VDE grow first, which drives the formation of high-n magnetic island chains. Subsequently, the magnetic island chains grow and overlap with each other, leading to the destruction of the magnetic flux surface, which induces a minor disruption and accelerates the start of the following major disruption. The magnetic island and the stochastic magnetic field allow the toroidally non-axisymmetric poloidal plasma current to jet towards the hoop force direction, forming finger-like and filamentary structures. Such a plasma current non-axisymmetry strongly depends on the anisotropy in the thermal transport coefficients.

Thermal Conductivity of Bottle-Brush Polymers.

Using molecular dynamics (MD) simulations of a generic model, we investigated heat propagation in bottle-brush polymers (BBP). An architecture is referred to as a BBP when a linear (backbone) polymer is grafted with the side chains of different length Ns and grafting density ρg, which control the bending stiffness of a backbone. Investigating κ-behavior in BBP is of particular interest due to two competing mechanics: increased backbone stiffness, via Ns and ρg, increases the thermal transport coefficient κ, while the presence of side chains provides additional pathways for heat leakage. We show how a delicate competition between these two effects controls κ. These results reveal that going from a weakly grafting (ρg < 1) to a highly grafting (ρg ≥ 1) regime, κ changes non-monotonically that is independent of Ns. The effect of side chain mass on κ and heat flow in the BBP melts is also discussed.

Crystal Thermal Transport in Altermagnetic RuO_{2}.

We demonstrate the emergence of a pronounced thermal transport in the recently discovered class of magnetic materials-altermagnets. From symmetry arguments and first-principles calculations performed for the showcase altermagnet, RuO_{2}, we uncover that crystal Nernst and crystal thermal Hall effects in this material are very large and strongly anisotropic with respect to the Néel vector. We find the large crystal thermal transport to originate from three sources of Berry's curvature in momentum space: the Weyl fermions due to crossings between well-separated bands, the strong spin-flip pseudonodal surfaces, and the weak spin-flip ladder transitions, defined by transitions among very weakly spin-split states of similar dispersion crossing the Fermi surface. Moreover, we reveal that the anomalous thermal and electrical transport coefficients in RuO_{2} are linked by an extended Wiedemann-Franz law in a temperature range much wider than expected for conventional magnets. Our results suggest that altermagnets may assume a leading role in realizing concepts in spin caloritronics not achievable with ferromagnets or antiferromagnets.

Thermoelectric transport in Weyl semimetals under a uniform concentration of torsional dislocations

In this article, we present an effective continuum model for a Weyl semimetal, to calculate its thermal and thermoelectric transport coefficients under the presence of a uniform concentration of torsional...

Thermal transport through carbon nanotubes based nanofluid flow over a rotating cylinder with statistical analysis for heat transfer rate

This study investigates the impact of magnetohydrodynamic effects and thermal radiation on axisymmetric flow with carbon nanotubes over a rotating cylinder. The thermal energy behavior of the nanofluid is analyzed using the Xue model, with water as the base fluid and two different types of nanoparticles: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The governing flow problem is transformed through appropriate transformations, resulting in a system of ordinary differential equations. Numerical solutions are obtained using the bvp4c function in MATLAB. The results indicate that the thermal transport coefficient increases with the radiation number in both SWCNTs and MWCNTs cases. Further, the presence of nanoparticles in the nanofluid leads to an increase in the thermal transport coefficient due to the volume fraction of the nanoparticles. Moreover, the study reveals that water-based MWCNTs exhibit a higher heat transfer rate than water-based SWCNTs. These findings contribute to understanding thermal energy characteristics in nanofluids and have implications for applications such as heat exchangers, electronic cooling systems, and the thermal management of advanced materials.

Unsteady MHD hybrid nanofluid flow over a convectively heated linear stretching cylinder with velocity slip: A comparative study

This paper provides a comparative numerical analysis of unsteady hybrid nanofluid ([Formula: see text]/H2O) and nanofluid (TiO2/H2O) flows through a permeable linear stretching cylinder placed in a porous medium has been presented in the presence of oblique Lorentz force. The flow regulating boundary layer equations has been governed due to the sway of the suction, viscous-Ohmic dissipation, heat source, thermal radiation and first-order velocity slip with imposing convective boundary conditions on the surface. The self-similar transformation has been employed to convert the governing coupled nonlinear PDEs into a set of ODEs and finally solved them numerically by fifth-order Runge–Kutta method with shooting technique. Finally, a comparative study has been made on the thickness of the momentum and thermal boundary layers due to the effect of governing parameters for simple and hybrid nanofluids. Further, a numerical observation on local skin friction and thermal-transport coefficients is presented with the help of tables. The result indicates that the progressiveness of the velocity profile reduces for the unsteadiness parameter, Hartree pressure gradient, velocity slip parameter, inclination parameter, porous medium parameter, magnetic parameter and suction parameter while improving for velocity ratio parameter and temperature profiles increases with Biot number, Eckert number, heat generation parameter and thermal radiation parameter, but decreases for Hartree pressure gradient and velocity ratio parameter. This work has found various applications in high-temperature and cooling processes, paints, conductive coatings, medicines, space technology, biosensors, conductive coatings and so on.

Rapid prediction of phonon structure and properties using the atomistic line graph neural network (ALIGNN)

The phonon density of states (DOS) summarizes the lattice vibrational modes supported by a structure and gives access to rich information about the material's stability, thermodynamic constants, and thermal transport coefficients. Here, we present an atomistic line graph neural network (ALIGNN) model for the prediction of the phonon density of states and the derived thermal and thermodynamic properties. The model is trained on a database of over 14 000 phonon spectra included in the joint automated repository for various integrated simulations: density functional theory (jarvis-dft) database. The model predictions are shown to capture the spectral features of the phonon density of states, effectively categorize dynamical stability, and lead to accurate predictions of DOS-derived thermal and thermodynamic properties, including heat-capacity ${C}_{\mathrm{V}}$, vibrational entropy ${S}_{\mathrm{vib}}$, and the isotopic phonon-scattering rate ${\ensuremath{\tau}}_{\mathrm{i}}^{\ensuremath{-}1}$. A comparison of room temperature thermodynamic property predictions reveals that the DOS-mediated ALIGNN model provides superior predictions when compared to a direct deep-learning prediction of these material properties as well as predictions based on analytic simplifications of the phonon DOS, including the Debye or Born--von Karman models. Finally, the ALIGNN model is used to predict the phonon spectra and properties for about 40 000 additional materials listed in the jarvis-dft database, which are validated as far as possible against other open-sourced high-throughput DFT phonon databases.

Natural response of thermoelectric materials and devices

The research aims to introduce the theory of the natural response of electrical circuits to studying thermoelectricity and the characterization of thermoelectric devices and materials. New equations were found to calculate the thermal resistance of the contacts Rcold and Rhot, the thermal conductance K0, and the figure of merit ZT. Furthermore, it permits determining characteristic time constants τ0, τ1, τ2, and relaxation times and calculating the thermoelectric capacitances related to the device, material, and thermal contacts. Also, the characteristic angular frequencies are predicted ω0, ω1and, ω2. The described theory satisfies Newton’s law of cooling, Luttinger’s thermal transport coefficients theory, and the first and second-order electric circuit’s behavior. Additionally, it makes available the prediction of the transport coefficients and the characterization in situ. Like Harman’s method, these parameters can be measured simultaneously on the same device or sample.

Forced Response of Thermoelectric Materials and Devices

The theory of the forced response of electric circuits applied to the study of the thermoelectricity here described allows the characterization of thermoelectric devices and materials determining the resistance of the thermal contacts, and the thermoelectric resistance. The successive computer analysis yields values for the Seebeck coefficient, electrical resistance, thermal conductance, and the figure of merit. The forced response of the thermoelectric materials and devices satisfies the Luttingers thermal transport coefficients theory, and the first-order electric circuitss behavior. Also, permits to find the thermoelectric time constant, and predicting the thermoelectric angular frequency which is necessary to determinate the complex impedance graphically through the Nyquist plot, due to that the thermoelectric time constant is inversely proportional to the thermoelectric angular frequency, as well as it makes accessible the prediction of the impedance spectroscopy measurements beyond the restrictive case of adiabatic boundary conditions regularly dicult to achieve experimentally, and therefore the characterization in situ. Like Harmans method, these parameters can be measured simultaneously on the same device or sample, and no requires neither reference nor standard material for comparison.

Comparison of all atom and united atom models for thermal transport calculations of amorphous polyethylene

Polymer simulations routinely employ models with different molecular resolutions. United atom (UA) models are one such example, where groups of certain atoms in a molecule are clustered into superatoms. Although their computational simplicity makes them particularly attractive for studying a wide range of polymer properties, the missing degrees of freedom in UA models can impact certain properties that are intimately linked to localized vibrations, such as the heat capacity and the thermal transport coefficient κ. In contrast, the numerically exhausting all atom (AA) models produce results that better match experimental data. In this work, we systematically investigate and compare κ obtained from an AA and a UA models for an amorphous polyethylene system. The results indicate that the UA description may not be a suitable model for evaluating thermal transport, since it underestimates κ in comparison to an AA description and the experimental value. The coarse-graining leads to the softer interactions and its presence is highlighted in a weaker mechanical response from the UA model, thus also underestimates κ. We further consolidate our findings by extracting the bonded and the nonbonded contributions to κ within the framework of the single chain energy transfer model.

3d‐Transition metal doped two-dimensional SnTe: Modulation of thermoelectric properties

Newly explored hexagonal phase of two-dimensional (2D) SnTe has been shown to be thermodynamically as well as dynamically stable and it exhibits high thermoelectric performance. In this study, we mainly investigate the effect in terms of electrical and thermal transport properties due to 3d-transition metal (TM) doping in 2D-SnTe. Based on first-principles calculations and Boltzmann transport theory, our study encompasses that both V and Mn doped SnTe exhibit moderate intrinsic carrier mobility because of lower elastic constant and higher carrier effective mass. Furthermore, the incorporation of V and Mn in the atomic layer of SnTe slightly enhances the Seebeck coefficient than pristine. While calculating thermal transport coefficient, V and Mn doping show a significant reduction in lattice thermal conductivity than pristine 2D SnTe due to enhanced phonon-phonon scattering strength, and as a result, we achieve very high zT ~ 2.24 at 900 K for Mn-doped SnTe. The replacement of Sn by cost-effective and environmentally-friendly 3d-TM with enhanced thermoelectric performance by 5%, would be beneficial during the energy crisis.

Mechanical, optical and thermoelectric properties of Janus BiTeCl monolayer

We report mechanical, optical and thermoelectric properties of recently fabricated Janus BiTeCl monolayer using density functional and semi-classical Boltzmann transport theory. Janus BiTeCl monolayer exhibits a direct bandgap, high carrier mobility (∼103 cm2V−1s−1) and high optical absorption in the UV–visible region. The mechanical behavior of the Janus BiTeCl monolayer is nearly isotropic having an ideal tensile strength ∼15 GPa. The higher value of the Gruneisen parameter (γ), a low value of phonon group velocity (vg), and very little phonon scattering time (τp) lead to low lattice thermal conductivity (1.46 W/mK) of Janus BiTeCl monolayer. The combined effect of thermal conductivity and electronic transport coefficients of Janus BiTeCl monolayer results in the figure of merit (ZT) in the range of 0.43–0.75 at 300–500 K. Our results suggest Janus BiTeCl monolayer be a potential candidate for optoelectronic and moderate temperature thermoelectric applications.

Dynamics of electron internal transport barrier formation at the H–L transition on EAST

The dynamics of electron-internal transport barrier (e-ITB) formation is studied in the hybrid plasma of neutral beam injection and lower hybrid wave heating on EAST. The e-ITB is observed to be repetitively formed at the H–L transition, where the relaxation process of core radiation triggers an increase in neutron yield and core electron temperature. A reversed q-profile is beneficial for generating the barrier, identified by the appearance of reversed shear Alfvén eigenmodes (RSAEs). The dimensionless parameter of the normalized Larmor radius is estimated to visualize the dynamic behavior of the e-ITB. It is found that the e-ITB forms once the electron exceeds 0.014, and the RSAEs are accompanied by the e-ITB formation. The barrier foot moves inwards until the L–H transition occurs, where the H-mode pedestal or type-I edge localized mode (ELM) strongly influences the e-ITB intensity. The results of thermal transport modeling show a significant reduction in the core thermal transport coefficient in the electron channel while maintaining a nearly unchanged ion transport level as the e-ITB evolves.

Kinetic Landau-fluid closures of non-Maxwellian distributions

New kinetic Landau-fluid closures, based on the cutoff Maxwellian distribution, are derived. A special static case is considered (the frequency ω=0). In the strongly collisional regime, our model reduces to Braginskii's heat flux model, and the transport is local. In the weak collisional regime, our model indicates that the heat flux is non-local and recovers the Hammett–Perkins model while the value of the cutoff velocity approaches to infinity. We compare the thermal transport coefficient χ of Maxwellian, cutoff Maxwellian and super-Gaussian distribution. The results show that the reduction of the high-speed tail particles leads to the corresponding reduction of the thermal transport coefficient χ across the entire range of collisionality, more reduction of the free streaming transport toward the weak collisional regime. In the collisionless limit, χ approaches to zero for the cutoff Maxwellian and the super-Gaussian distribution but remains finite for Maxwellian distribution. χ is complex if the cutoff Maxwellian distribution is asymmetric. The Im(χ) approaches to different convergent values in both collisionless and strongly collisional limit, respectively. It yields an additional streaming heat flux in comparison with the symmetric cutoff Maxwellian distribution. Furthermore, due to the asymmetric distribution, there is a background heat flux q0 though there is no perturbation. The derived Landau-fluid closures are general for fluid moment models, and applicable for the cutoff Maxwellian distribution in an open magnetic field line region, such as the scape-off-layer of Tokamak plasmas, in the thermal quench plasmas during a tokamak disruption, and the super-Gaussian electron distribution function due to inverse bremsstrahlung heating in laser-plasma studies.

Transport coefficients of layered TiS3

We investigate the electrical and thermal transport coefficients of an emergent transition metal tri-chalcogenide, namely, ${\mathrm{TiS}}_{3}$. We implement an accurate description of the electron-phonon coupling and thus of the energy dependence of the relaxation time, which goes beyond the commonly used approximations. We find that standard calculation methods fail in accurately describing the thermoelectric transport properties of ${\mathrm{TiS}}_{3}$ and one needs to go beyond standard approximations to correctly investigate the thermoelectric performance of ${\mathrm{TiS}}_{3}$ and other 2D materials. By applying state-of-the-art ab initio methods, we conclude that ${\mathrm{TiS}}_{3}$ stands as a potential candidate for thermoelectric applications.

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.