Anisotropic Thermal Transport Research Articles

Unexpected thermal transport properties of MgSiO3 monolayer at extreme conditions

The thermal transport properties of mantle minerals are of paramount importance to understand the thermal evolution processes of the Earth. Here, we perform extensively structural searches of two-dimensional MgSiO3 monolayer by CALYPSO method and first-principles calculations. A stable MgSiO3 monolayer with Pmm2 symmetry is uncovered, which possesses a wide indirect band gap of 4.39 eV. The calculations indicate the lattice thermal conductivities of MgSiO3 monolayer are 49.86 W (mK)−1 and 9.09 W (mK)−1 in x and y directions at room temperature. Our findings suggest that MgSiO3 monolayer is an excellent low-dimensional thermoelectric material with high ZT value of 4.58 from n-type doping in the y direction at 2000 K. The unexpected anisotropic thermal transport of MgSiO3 monolayer is due to the puckered crystal structure and the asymmetric phonon dispersion as well as the distinct electron states around the Fermi level. These results offer a detailed description of structural and thermal transport properties of MgSiO3 monolayer at extreme conditions.

Effect of anisotropic thermal transport on tearing mode stability in negative versus positive triangularity plasmas

The combined effects of anisotropic thermal transport and the plasma shaping, including negative triangularity, on the n = 1 (n is the toroidal mode number) tearing mode (TM) stability are numerically investigated utilizing the MARS-F code [Liu et al., Phys. Plasmas 7, 3681–3690 (2000)]. While varying the plasma boundary triangularity, the TM stability is found to be dictated by the competing effects of the Shafranov shift induced stabilization and the bad-curvature induced destabilization. The negative triangularity shape increases the Shafranov shift (stabilizing) in the plasma core but also enlarges bad-curvature regions (destabilizing) near the plasma edge, with the net effect being largely destabilizing for the TM as compared to the positive triangularity counter-part. Large negative triangularity however can also lead to more stabilization for the plasma core-localized TM. Anisotropic thermal transport reduces the stabilizing effect on the TM associated with the favorable averaged curvature, resulting in more unstable core-localized TMs in both negative and positive triangularity plasmas. But the opposite effect can also take place for the edge-localized TM in finite-pressure plasmas with negative triangularity.

Unraveling High Thermal Conductivity with In-Plane Anisotropy Observed in Suspended SiP2.

The anisotropic thermal transport properties of low-symmetry two-dimensional materials play an important role in understanding heat dissipation and optimizing thermal management in integrated devices. Examples of efficient energy dissipation and enhanced power sustainability have been demonstrated in nanodevices based on materials with anisotropic thermal transport properties. However, the exploration of materials with high thermal conductivity and strong in-plane anisotropy remains challenging. Herein, we demonstrate the observation of anisotropic in-plane thermal conductivities of few-layer SiP2 based on the micro-Raman thermometry method. For suspended SiP2 nanoflake, the thermal conductivity parallel to P-P chain direction (κ∥b) can reach 131 W m-1 K-1 and perpendicular to P-P chain direction (κ⊥b) is 89 W m-1 K-1 at room temperature, resulting in a significant anisotropic ratio (κ∥b/κ⊥b) of 1.47. Note that such a large anisotropic ratio mainly results from the higher phonon group velocity along the P-P chain direction. We also found that the thermal conductivity can be effectively modulated by increasing the SiP2 thickness, reaching a value as high as 202 W m-1 K-1 (120 W m-1 K-1) for κ∥b (κ⊥b) at 111 nm thickness, which is the highest among layered anisotropic phosphide materials. Notably, the anisotropic ratio always remains at a high level between 1.47 and 1.68, regardless of the variation of SiP2 thickness. Our observation provides a new platform to verify the fundamental theory of thermal transport and a crucial guidance for designing efficient thermal management schemes of anisotropic electronic devices.

Giant manipulation of thermal conductivity anisotropy in black phosphorene under external electric fields.

The thermal anisotropy of materials holds significant theoretical and practical implications in the domains of thermal transport and thermoelectricity. Black phosphorene, a novel two-dimensional (2D) semiconductor, is notable for its exceptional chemical and physical properties, attracting substantial attention for its thermal transport characteristics. Similar to other 2D materials, black phosphorene exhibits pronounced in-plane thermal anisotropy. Given its expanding applications in nanoelectronics, optoelectronics, and thermoelectrics, there is a growing need to manipulate its anisotropic thermal transport. Current methods for adjusting anisotropy or isotropy typically involve structural engineering or materials processing, which are often costly, time-consuming, and irreversible. In contrast, little progress has been made with methods that are intact, robust, and reversible. Driven by the intrinsic relationship between interatomic interaction-mediated phonon transport and electronic charges, we conduct a comprehensive investigation into the impact of an external electric field on the thermal transport properties of 2D black phosphorene using first-principles calculations and the phonon Boltzmann transport equation. Our findings reveal that applying an electric field in the Zigzag direction reduces the lattice thermal conductivity of black phosphorene, with the Zigzag direction being more responsive to the electric field than the Armchair direction. By adjusting the electric field to a maximum of E(f_xx) = 0.2 V Å-1, the anisotropic thermal conductivity of black phosphorene decreases by more than 28%, demonstrating effective manipulation of anisotropy. This significant transition in anisotropic thermal transport arises from the substantial reduction in thermal conductivity along the Zigzag direction at moderate electric field strengths. The underlying cause of this variation in anisotropy can be attributed to changes in group velocity, with the phonon lifetime serving as a scaling factor for reducing anisotropy. Analysis of the electronic structures shows that stronger electric fields induce more charges, enhancing the screening effect. The electric field significantly alters thermal conductivity by affecting bond ionicity and anharmonicity. Our study introduces a robust approach for tuning the anisotropy of phonon transport in materials using an external electric field, without altering the atomic structure, thus offering considerable advantages for applications in nanoelectronics and thermoelectric energy conversion.

A new mechanism for internal kink stabilization by plasma flow and anisotropic thermal transport

Stability of the internal kink (IK) mode is numerically investigated with inclusion of the anisotropic thermal transport effect and the toroidal plasma flow. It is found that anisotropic thermal transport, in combination with plasma flow, stabilizes the IK in two ways. One is direct stabilization of the mode synergistically with plasma flow. The other, indirect stabilization involves generation of a finite mode frequency in static plasmas by thermal transport, which in turn invokes wave-wave resonance damping of the mode via interaction with stable shear Alfvén waves. This second IK stabilization mechanism is further corroborated by examining the eigenmode structure, which peaks at the radial locations where the mode frequency matches that of the shear Alfvén wave. Finally, two branches of unstable IK are identified, with mode conversion occurring at certain plasma flow speed and thermal transport level. These findings provide new physics insights in the IK stability in tokamak fusion plasmas.

Three-sensor 2ω method with multi-directional layout: A general methodology for measuring thermal conductivity of solid materials

Anisotropic thermal transport plays a key role in both theoretical study and engineering practice of heat transfer, but accurately measuring anisotropic thermal conductivity remains a significant challenge. To address this issue, we propose the three-sensor 2ω method in this study, which is capable of accurately measuring the isotropic or anisotropic thermal conductivity of solid materials. In this method, several three-sensor groups following the design guidelines are fabricated upon the sample along different characteristic directions, and each group consists of three parallel metal sensors with unequal widths and distances optimally designed based on sensitivity analysis. Among the three sensors, the outer two serve as AC heaters and the middle one as a DC detector. The 2ω voltage signals across the detector in each three-sensor group are measured, and then the data are processed by the proposed Intersection Method to derive thermal conductivities along directions of interest. The application of the detector's 2ω instead of the heater's 3ω voltage signals eliminates errors introduced by the uncertainties of thermal resistance in superficial structures (metal layer, insulation layer, interface, etc.). Meanwhile, by replacing the fitting algorithm with the Intersection Method, the local optimum trap of multivariate fitting is avoided. To verify the accuracy and reliability, four typical monocrystalline semiconductors, i.e., Si, GaN, AlN, and β-Ga2O3, are measured, and the results are consistent with the literature. This method will provide a comprehensive and versatile solution for the thermal conductivity measurements of solid materials.

Improved Thermal Anisotropy of Multi-Layer Tungsten Telluride on Silicon Substrate.

WTe2, a low-symmetry transition metal dichalcogenide, has broad prospects in functional device applications due to its excellent physical properties. When WTe2 flake is integrated into practical device structures, its anisotropic thermal transport could be affected greatly by the substrate, which matters a lot to the energy efficiency and functional performance of the device. To investigate the effect of SiO2/Si substrate, we carried out a comparative Raman thermometry study on a 50 nm-thick supported WTe2 flake (with κzigzag = 62.17 W·m-1·K-1 and κarmchair = 32.93 W·m-1·K-1), and a suspended WTe2 flake of similar thickness (with κzigzag = 4.45 W·m-1·K-1, κarmchair = 4.10 W·m-1·K-1). The results show that the thermal anisotropy ratio of supported WTe2 flake (κzigzag/κarmchair ≈ 1.89) is about 1.7 times that of suspended WTe2 flake (κzigzag/κarmchair ≈ 1.09). Based on the low symmetry nature of the WTe2 structure, it is speculated that the factors contributing to thermal conductivity (mechanical properties and anisotropic low-frequency phonons) may have affected the thermal conductivity of WTe2 flake in an uneven manner when supported on a substrate. Our findings could contribute to the 2D anisotropy physics and thermal transport study of functional devices based on WTe2 and other low-symmetry materials, which helps solve the heat dissipation problem and optimize thermal/thermoelectric performance for practical electronic devices.

Direct Measurements of Anisotropic Thermal Transport in γ‐InSe Nanolayers via Cross‐Sectional Scanning Thermal Microscopy

AbstractVan der Waals (vdW) atomically thin materials and their heterostructures offer a versatile platform for the management of nanoscale heat transport and the design of novel thermoelectrics. These require the measurement of highly anisotropic heat transport in vdW‐based nanolayers, a major challenge for nanostructured materials and devices. In the present study, a novel effective method of cross‐sectional scanning thermal microscopy was used to map and quantify the anisotropic heat transport in nanoscale thick layers of vdW materials and the material‐substrate interfaces. This technique measures the heat conducted into a vdW crystal via the nanoscale apex of a heat‐sensitive probe. The crystal is nano‐polished via Ar ion beams generating an oblique nearly atomically flat surface. By measuring the thermal conductance variation as a function of increasing layer thickness, the transition between the cross‐plane and in‐plane heat transport (defined by heat conductivity anisotropy) is acquired. By using an analytical model validated by finite element simulations, anisotropic thermal transport in a gamma indium selenide crystal nano‐thin flake on a Si substrate was studied, obtaining results corresponding to anomalously low anisotropic thermal conductivities of kxy = 2.16 Wm−1 K−1 in‐plane and kz = 0.89 Wm−1 K−1 cross‐plane confirming its potential for thermoelectric applications.

Anisotropic Phonon Scattering and Thermal Transport Property Induced by the Liquid-like Behavior of AgCrSe2.

Superionic conductors exhibiting a periodic crystalline lattice and liquid-like ionic conductivity have emerged as promising materials in energy-conversion devices. Herein, we have investigated the interplay among anharmonic lattice dynamics, thermal conduction, and ultrafast atomic diffusion across the superionic transition of AgCrSe2. We show that the thermal conductivity (κ) contributions from convection and conduction-convection interactions increase simultaneously due to the gradual fluidization of Ag atoms, leading to a temperature-independent κ in the superionic state. We demonstrate a non-Peierls type thermal transport behavior induced by the strong lattice anharmonicity of Ag atoms, which promotes a nontrivial wave-like phonon tunneling in the normal state of AgCrSe2. Our current fluctuation analysis demonstrates an anisotropic phonon-liquid scattering behavior that the in-plane nondispersive transverse acoustic (TA) phonons near the zone boundary collapse, while the zone center and boundary TA phonons in the direction perpendicular to the liquid-like flow of Ag atoms survive.

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.

Quantifying anisotropic thermal transport in two-dimensional perovskite (PEA2PbI4) through cross-sectional scanning thermal microscopy

In this paper, we investigated the anisotropic thermal transport in two-dimensional (2D) perovskite (phenethylammonium lead iodide) nanolayers through a measurement technique called cross-sectional scanning thermal microscopy. In this method, a target perovskite layer on a substrate was oblique polished with an Ar ion beam to create a low-angle wedge with nanoscale roughness that is followed by high-vacuum scanning thermal microscopy to obtain the thermal conductance map as a function of local thickness. The experimentally obtained data were processed with an analytical model and validated by the finite elemental analysis simulation to quantify the in-plane $({k}_{l,xy})$ and cross-plane thermal conductivities $({k}_{l,z})$ of the 2D perovskite from a single set of measurements with nanoscale resolution. We obtained ultralow thermal conductivity $({k}_{l}=0.25\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1})$ for the 2D perovskite along with an anisotropy $({k}_{l,\phantom{\rule{0.16em}{0ex}}xy}=0.45\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}\mathrm{and}\phantom{\rule{0.16em}{0ex}}{k}_{l,\phantom{\rule{0.16em}{0ex}}z}=0.13\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1})$ linked to the unique structure of the perovskite and different phonon lifetimes and group velocities for in-plane and out-of-plane directions. The results that are available are essential for the thermal management of 2D perovskite-based optoelectronic devices and potential thermoelectric applications of these materials.

Curvature and van der Waals interface effects on thermal transport in carbon nanotube bundles

A van der Waals (vdW) heterostructure, can be used in efficient heat management, due to its promising anisotropic thermal transport feature, with high heat conductance in one direction and low conductance in the rest. A carbon nanotube (CNT) bundle, can be used as one of the most feasible vdW heterostructures in a wide range of nanoscale devices. However, detailed investigations of heat transport in CNT bundles are still lacking. In this paper, we study heat transport in different CNT bundles—homogeneous bundles consisting of the one CNT radius (curvature) and inhomogeneous bundles constructed from different CNTs with different curvatures. We also investigate the comparison between two possible thermostatting configurations: the two ends connected (TEC) case in which there is at least a direct covalently connected path between the hot and cold heat baths, and the one end connected (OEC) case in which the system can be divided at least into two parts, by a vdW interacting interface. Nonequilibrium molecular dynamics simulations have been carried out for a wide range of configurations and curvature differences. We find that, in homogeneous bundles, by increasing the number of outer CNTs, the heat conductance increases. In inhomogeneous bundles, the total heat flux shows dependence on the difference between the curvature of the core and outer CNTs. The less the difference between the curvature of the core and the outer CNTs, the more the thermal conductance in the system. By investigating the spectral heat conductance (SHC) in the system, we found that a larger curvature difference between the core and outer CNTs leads to a considerable decrease in the contribution of 0–10 THz phonons in the bundled zone. These results provide an insightful understanding of the heat transport mechanism in vdW nano-heterostructures, more important for designing nanoelectronic devices as well as systems in which asymmetry plays a significant role.

Anisotropic Thermal Conductivity of Inkjet-Printed 2D Crystal Films: Role of the Microstructure and Interfaces.

Two-dimensional (2D) materials are uniquely suited for highly anisotropic thermal transport, which is important in thermoelectrics, thermal barrier coatings, and heat spreaders. Solution-processed 2D materials are attractive for simple, low-cost, and large-scale fabrication of devices on, virtually, any substrate. However, to date, there are only few reports with contrasting results on the thermal conductivity of graphene films, while thermal transport has been hardly measured for other types of solution-processed 2D material films. In this work, inkjet-printed graphene, h-BN and MoS2 films are demonstrated with thermal conductivities of ∼10 Wm-1K-1 and ∼0.3 Wm-1K-1 along and across the basal plane, respectively, giving rise to an anisotropy of ∼30, hardly dependent on the material type and annealing treatment. First-principles calculations indicate that portion of the phonon spectrum is cut-off by the quality of the thermal contact for transport along the plane, yet the ultra-low conductivity across the plane is associated with high-transmissivity interfaces. These findings can drive the design of highly anisotropic 2D material films for heat management applications.

Tunable anisotropic in-plane thermal transport of multilayer graphene induced by 2D empty space: Insights from interfaces

The emergence of two-dimensional empty space (2D-ES) not only enriches the means of van der Waals integration, but also provides a new and reliable solution for structural design-driven performance modulation. Here, by applying the concept of 2D-ES-based periodic structure design to multilayer graphene, the large-range tunable in-plane anisotropic phonon thermal transport behavior was discovered by extensive molecular dynamics simulations. Through a series of in-depth frequency-dependent and in-and-out of plane decomposition phonon analysis, it is found that 2D-ES and its interfaces with different periodic properties exhibit exactly opposite effects on phonon thermal transport along two in-plane orientations, which is the fundamental reason for the existence of the above-mentioned anisotropic thermal transport and its modulation. These findings provide new insights into the realization of in-plane anisotropic thermal transport in quasi-2D materials, which may further inspire novel thermal management strategies.

Layered bismuth copper oxychalcogenides as advanced photothermal materials for efficient interfacial solar desalination

Interfacial solar desalination is a promising method to sustainably address global water scarcity. Thus far, it has been a challenge to find photothermal materials with both efficient light absorption and low thermal conductivity. This study is the first to explore a layered oxychalcogenide of bismuth copper oxysulfide (BiCuSO, BCSO) nanosheets as a promising photothermal absorber material for interfacial solar desalination. We show that the two-dimensional layered crystal structure of BCSO enables anisotropic thermal transport. At the same time, the nanosheet morphology induces nanoporous coating due to a nano-clustering effect when it is coated on substrates, leading to a rough surface, nano-porosity, and substantial heat confinement. Significantly, BCSO nanosheets show attractive properties such as intense light absorption (up to 1040 nm), low thermal conductivity (0.099 W/m·K), strong heat confinement and excellent wettability. With capillary water supply and porous polyurethane structure, BCSO nanosheets demonstrated a high water-evaporation rate (1.77 kg/m2·h), solar-to-thermal conversion efficiency (96 %), and stable long-term evaporation performance under simulated sunlight irradiation. Furthermore, condensed vapors from seawater met the World Health Organization's drinking standard, proving the strong ion rejection capability of BCSO. Our results provide new insights into developing efficient photothermal materials for enhanced interfacial solar-powered desalination systems.

Toroidal modeling of energetic passing particle drift kinetic effects on tearing mode stability

Drift kinetic effects of the neutral beam injection induced passing energetic particles (EPs) on the linear stability of the n = 1 tearing mode (TM) (with the dominant poloidal harmonic of m = 2) are numerically investigated utilizing the MARS-K code (Liu et al 2008 Phys. Plasmas 15 112503), in a tokamak plasma with finite equilibrium pressure and anisotropic thermal transport. In the low plasma pressure regime, it is found that co- (counter-) passing EPs stabilize (destabilize) the TM, agreeing with previous studies. However, as the plasma pressure increases beyond a critical value, it is found that co-passing EPs also destabilize the mode. An in-depth analysis reveals that the net effect of co-passing EPs is a result of competition between the stabilizing contribution from the non-adiabatic drift kinetic terms and the destabilizing contribution associated with adiabatic terms, with the latter becoming more dominant at higher equilibrium pressure. Non-perturbative magnetohydrodynamic-kinetic hybrid modeling also finds that co- and counter-passing EPs modify the TM eigenfunction differently, with the counter-passing EPs enhancing the sideband harmonics. Furthermore, effects of the plasma resistivity and toroidal rotation, as well as that of the equilibrium distribution of EPs in the particle pitch angle space, are also investigated, showing asymmetric results on the TM stability between the co- and counter-passing EPs. The first order finite orbit width correction is found to be stabilizing with co-passing EPs and destabilizing with counter-passing particles. Finally, drift resonances between passing EPs and the TM induce finite frequency to the mode and generate finite net torques inside the plasma, due to the neoclassical toroidal viscosity and the Reynolds stress associated with 3D perturbations.

Abnormal in-plane thermal conductivity anisotropy in bilayer α-phase tellurene

In layered materials, it is well known that the cross-plane or cross-chain (CC) thermal conductivity is lower than that of the in-plane or along-chain direction due to weaker bonding in the cross-plane or CC directions. In this work, surprisingly, we predict using first-principles that the CC thermal conductivity κ⊥ is higher than the along-chain thermal conductivity κ∥ with a κ⊥/κ∥ ratio of 1.174 in bilayer α-phase tellurene (2L-αTe), an emerging 2D material that has recently drawn extensive interest due to its intriguing electronic, thermoelectronic, and piezoelectronic properties. Also, the room-temperature κ⊥ and κ∥ of 2L-αTe are 5.69 W/mK and 4.85 W/mK, which are 251% and 31% enhanced from that of bulk Te, respectively. A detailed analysis of lattice structure, phonon spectra, and electron properties of 2L-αTe and bulk Te is provided. We find that the larger lattice shrink in CC direction and the change in band angle lead to stiffened phonons and weaker anharmonicity, which in turn yield higher phonon group velocity and longer phonon lifetime in CC direction for 2L-αTe, resulting in the enhanced thermal conductivity and the anomolus thermal anisotropy. Crystal orbital Hamiltan population (COHP) and electron localization function (ELF) analysis demonstrate the enhancement of the strength of “covalent-like quasi-bonding” (CLQB) in CC direction of 2L-αTe, which is accompanied by the lattice shrink in CC direction and the change of band angle. Our study identifies a case of unexpected anisotropic thermal transport in 2D materials, and sheds light on the improvement of thermal conductivity isotropy in 2D materials for the thermal management of electronic devices.

Direction-Dependent Thermoelectric Properties of a Layered Compound In2Te5 Single Crystal

We examine the anisotropic electrical and thermal transport properties in single crystals of In2Te5 belonging to the monoclinic space group C12/c1 with the temperature gradient applied parallel (||) and perpendicular (\(\perp \) ) to the crystallographic c-axis of the crystals. A systematic investigation of structural, electrical, and thermal properties suggests the role of layered structure in this material in guiding its thermoelectric behavior. The thermal conductivity along the c-axis (κ||c) was found to be smaller by a factor of 2 compared to the thermal conductivity along the direction perpendicular to the c-axis (κ\(_{\perp{c}}) \) over the entire temperature range. In contrast, the Seebeck coefficient along the c-axis (S||c) was found to be higher than its value along the direction perpendicular to the c-axis \((S_{\perp{c}}) \). At room temperature, the figure of merit zT||c is found to be four times larger as compared to the figure of merit \(zT_{{\perp}c} \). Our results provide insights into how the resistivity, thermal conductivity, and thermopower depends on the crystalline anisotropy and its impact on the overall zT.

Anisotropic thermal transport in twisted bilayer graphene

Recently, twisted bilayer graphene (TBLG) has attracted enormous attention owing to its peculiar electronic properties. In this work, the anisotropic thermal conductivity of TBLG is comprehensively investigated. It is reported that interlayer twisting can be a practical approach for thermal transport regulation with high accuracy. A strong non-monotonic correlation between anisotropic thermal conductivity and twisting angles is revealed. Extensive phonon behavior analyses reveal the physical mechanism. The anisotropic thermal transport in TBLG is explained by the calculated phonon density of states (PDOS). Meanwhile, the phonon spectra and phonon relaxation times extracted from spectral energy density (SED) profiles explain the decreasing trend of thermal conductivity with increasing twisting angles. The increase in thermal conductivity is attributed to the combined effects of twist and anisotropy. The reported anisotropic thermal conductivity is important to the thermal modulation and our analyses provide a valuable complement to the phonon studies of TBLG.

Effect of Four-Phonon Scattering on Anisotropic Thermal Transport in Bulk Hexagonal Boron Nitride by Machine Learning Interatomic Potential

Effect of Four-Phonon Scattering on Anisotropic Thermal Transport in Bulk Hexagonal Boron Nitride by Machine Learning Interatomic Potential