Anisotropic Thermal Conductivity Research Articles

Multiple coherent amplitude modes and exciton-phonon coupling in quasi-one-dimensional excitonic insulator Ta2NiSe5.

An excitonic insulator (EI) is an intriguing correlated electronic phase of condensed excitons. Ta2NiSe5 is a model material for investigating condensed excitonic states. Herein, femtosecond pump-probe spectroscopy is used to study the coherent phonon dynamics and associated exciton-phonon coupling in single-crystal Ta2NiSe5. The reflectivity time series consists of exponential decay due to hot carriers and damped oscillations due to the Ag phonon vibration. Given the in-plane anisotropic thermal conductivity of Ta2NiSe5, coherent phonon oscillations are stronger with perpendicular polarization to its quasi-one-dimensional chains. The 1-, 2-, and 4-THz vibration modes show coherent amplitude responses in the EI phase of Ta2NiSe5 with increasing temperature, totally different from those of normal coherent phonons (the 3- and 3.7-THz modes). The amplitude modes at higher frequencies decouple with the EI order parameter at lower temperatures, as supported by theoretical analysis with a model Hamiltonian of the exciton-phonon coupling system. Our work provides valuable insights into the character of the EI order parameter and its coupling to multiple coherent amplitude modes.

Construction of brick/mortar-like graphene/thermoset composites with highly anisotropic thermal conductivity and strong electromagnetic interference shielding performance

A new fabricating strategy has been successfully employed to prepare brick/mortar-like graphene/thermoset composites with highly anisotropic thermal conductivity and excellent electromagnetic interference shielding performance.

Improvement of the anisotropic thermal conductivity of h-BN filled epoxy composites by changing the filler shape to spherical

Improving the thermal conductivity anisotropy of epoxy/h-BN composites by synthesizing spherical Fe–Bx through carbothermal reduction of iron oxide–boric oxide mixtures (Fe2O3–B2O3), resulting in the fabrication of spherical h-BN.

A Method for Accurate Measurement of Anisotropic Thermal Conductivity of Impregnated Electrical Windings

A Method for Accurate Measurement of Anisotropic Thermal Conductivity of Impregnated Electrical Windings

Coupling at the molecular scale between the graphene nanosheet and water and its effect on the thermal conductivity of the nanofluid.

Graphene nanofluid is a promising way to improve heat transfer in many situations. As a two-dimensional material, graphene's anisotropic thermal conductivity influences the heat transfer of nanofluids. In the present study, a nonequilibrium molecular dynamics (MD) simulation is adopted to study the interaction between graphene nanosheets (GNSs) and liquid water in water-based graphene nanofluids. Consequently, the coupling interaction between the orientation and length of GNSs and the thermal conductivity of nanofluids is then investigated. We discover that the molecular thermal coupling between GNSs and water can effectively influence the orientation angle of the GNSs. A preferential orientation angle of the GNSs inside the nanofluid is then observed during heat transfer. The preferential orientation angle decreases with the GNS length and has no apparent relation with the size of heat flux in this study. The overall thermal conductivity of the nanofluid decreases as the orientation angle of the GNS rises. Increasing the GNS length not only reduces the preferential orientation angle but also improves the thermal conductivity along the graphene length direction. The thermal conductivity of the nanofluid along the graphene length direction increases from 0.414 to 4.085 W m K-1 as the length increases from 103 to 3274 A. Our results provide the fundamental knowledge of the heat transfer performance of graphene nanofluids.

Polytelluride square planar chain-induced anharmonicity results in ultralow thermal conductivity and high thermoelectric efficiency in Al2Te5 monolayers.

Two-dimensional (2D) metal chalcogenides provide rich ground for the development of nanoscale thermoelectrics, although achieving optimal thermoelectric efficiency is still a challenge. Here, we leverage the unique chemistry of tellurium (Te), renowned for its hypervalent bonding and catenation abilities, to tackle this challenge as manifested in Al2Te3 and Al2Te5 monolayers. While the former forms a straightforward covalent Al-Te network, the latter adopts a more intricate bonding mechanism, enabled by eccentric features of Te chemistry, to maintain charge balance. In Al2Te5, a square planar chain (SPC) known as polytelluride [Te3]2- is neutralized by the covalently bonded [Al2Te2]2+ framework. The hypervalent nature of Te results in bizarre Born effective charges of 7 and -4 for adjacent Te atoms within the square planar chain, a feature that induces significant anharmonicity in Al2Te5 monolayers. Enhanced anharmonic lattice vibrations and the accordion pattern bestow glass-like, strongly anisotropic thermal conductivity to the Al2Te5 monolayer. The calculated κL values of 1.8 and 0.5 W m-1 K-1 along the a- and b-axes at 600 K are one order of magnitude lower than those of Al2Te3, and even lower than monolayers that contain heavy cations like Bi2Te3. Moreover, the tellurium chain dominates the valence band maximum and conduction band minimum of Al2Te5, leading to a high valley degeneracy of 10, and thus a high power factor and figure of merit (zT). Using rigorous first-principles calculations of electron relaxation time, the estimated hole-doped and electron-doped zT of, respectively, 1.5 and 0.5 at 600 K is achieved for Al2Te5. The pioneering zT of Al2Te5 compared to that of Al2Te3 is rooted merely in its amorphous-like lattice thermal transport and its polytelluride chain. These findings underscore the importance of aluminum telluride and polymeric-based inorganic compounds as practical and cost-effective thermoelectric materials, pending further experimental validation.

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.

Tailoring anisotropic thermal conductivity of 2D aramid nanoribbon-based dielectrics with potential high-temperature capacitive energy storage

Tailoring the high anisotropic thermal conductivity of polymer dielectrics for enhanced high-temperature breakdown strength with potential capacitive energy storage.

Extreme in-plane thermal conductivity anisotropy in Rhenium-based dichalcogenides

Anisotropies in thermal conductivity are important for thermal management in a variety of applications, but also provide insight on the physics of nanoscale heat transfer. As materials are discovered with more extreme transport properties, it is interesting to ask what the limits are for how dissimilar the thermal conductivity can be along different directions in a crystal. Here we report on the thermal properties of rhenium-based transition metal dichalcogenides (TMDs), specifically rhenium disulfide (ReS2) and rhenium diselenide (ReSe2), highlighting their extraordinary thermal conductivity anisotropy. Along the basal crystal plane of ReS2, a maximum of 169±11 W mK−1 is detected along the b-axis and a minimum of 53±4 W mK−1 perpendicular to it. For ReSe2, the maximum and minimum values of 116±3 W mK−1 and 27±1 W mK−1 are found to lie 60° and 150° away from the b-axis, along the polarization direction of some of the principal Raman modes. These measurements demonstrate a remarkable anisotropy of 3.2 × and 4.3 × in the conductivity within the crystal basal planes, respectively. The through-plane thermal conductivities, recorded at 0.66±0.01 W mK−1 for ReS2 and 2.31±0.01 W mK−1 for ReSe2, highlight the impact of their layered structures, contributing to notably high in-plane to through-plane thermal conductivity ratios of 256 × for ReS2 and 50 × for ReSe2. This research demonstrates the unique thermal properties that these comparatively underexplored TMDs have, shedding light on the need for further exploration into the intricate thermal behavior of such materials, while underscoring their potential significance for future applications in the fields of semiconductor devices and nanotechnology.

Asymmetrical LaO9 polyhedron inducing anisotropic positive thermal expansion and moderate lattice thermal conductivity in LaBO3 with isolated planar [BO3] group

Thermal physical properties of inorganic borates are important in application of optical device working under high temperature. Herein, we investigate the temperature-dependent thermoelasticity, thermal expansion(α), and lattice thermal conductivity(κ) of simple orthohomibic LaBO3 with a two dimension like crystal structure though first principles for the first time. The calculated rate of temperature-dependent second elastic constant C11 is significantly larger than that of C22 and C33, and the theoretical slopes of corresponding bulk, shear, and Young's moduli in the range of 300–900 K are −0.0289 GPa•K−1, −0.0156 GPa•K−1, and −0.0398 GPa•K−1, respectively. The predicted α is positive and increases with the rise of temperature. The anisotropic ratio αc/αa or αc/αb is approximately 2 at 300 K, which might be attributed to the difference of elastic compliance S12 and S13. Besides, the anisotropic lattice thermal conductivity κa, κb and κc of LaBO3 is predicted to be 1.43, 1.40 and 0.80 W/m-K at 300 K via Debye-Callaway model. Most importantly, the atomic projected Grüneisen parameters γ reveal that asymmetrical LaO9 polyhedra in the crystal lattice dominate the thermal expansion and lattice thermal conductivity. Our results indicate that LaBO3 has good thermal physical properties working under high temperature.

Study of twisting of β-Ga2O3 crystals based on optical absorption and thermal conductivity anisotropy in the crystals grown by the Czochralski method

The effects of internal radiation and thermal conductivity anisotropy in β-gallium oxide crystals on heat and mass transfer in the Czochralski method were numerically studied. Specifically, we calculated the temperature and velocity distributions in a crystal and/or melt and the interface shape between the crystals and melt for the Czochralski method. We used the Rosseland approximation to include the internal radiation in the crystals to calculate the heat transfer in the crystal. Moreover, the thermal conductivity anisotropy in the crystals was considered to study the asymmetric temperature distribution.The results indicated that the temperature gradient in the semitransparent crystal was smaller than that in the opaque crystal. This result is attributed to the absorption of the outgoing heat flux from the interface between the crystal and melt in the crystal. We studied the asymmetric temperature distributions in the crystals caused by thermal conductivity anisotropy as a function of the crystal absorption coefficient.The results also indicated that the interface shape between a semitransparent crystal and the melt was convex to the melt, whereas the shape between an opaque crystal and the melt was almost flat.

Simulation of the temperatures in the permanently shadowed region of the Moon's south pole and data validation

The permanently shadowed regions (PSRs) at the lunar poles without direct solar illumination maintain low temperatures throughout the year, making them optimal environments for the preservation of water ice. This paper investigates the diurnal and seasonal temperature variations of the PSRs within craters at the lunar south pole. The craters de Gerlache, Shoemaker, Amundsen, and Wiechert E are selected to explore how latitudes and shapes impact PSR temperature. Using the ray tracing approach, the shadow effect of terrain on the scattered sunlight and thermal radiation is calculated. PSR temperatures are simulated with the one-dimensional heat conduction equation. The thermal emissivity of the lunar surface is studied. Simulations with anisotropic emissivity are closer to the Diviner data in PSRs than the simulations with isotropic emissivity. In addition, by comparing the simulations to the Diviner data, it is found that the thermal conductivity at the extremely low temperatures in PSRs might be lower than that at high temperatures outside the PSRs. Simulations of the selected craters are used to study the influence of polar day phenomena occurring at high-latitude craters during the summer and the effect of direct solar illumination at the floors of low-latitude craters on the temperature distribution of PSRs. The influence of crater shape is analyzed as well by investigating the temperature distributions within PSRs of flat-floored and bowl-shaped craters. With the analysis of the heat flux propagation distances, emission angles, and incident angles, this study quantitatively elucidates the spatial and temporal variations in PSR temperatures. To explain why the Diviner data are lower than the simulation results at the solar-illuminated crater walls, an undulating rough surface is generated. The locations of the Sun and the observer on opposite sides of the rough surface may decrease the observed infrared brightness temperature according to the simulations. The PSR peak temperatures simulated with the anisotropic emissivity and thermal conductivity for PSRs are used to search for low-temperature areas suitable for preserving water ice within de Gerlache and Shoemaker.

Enhancing cryogenic thermal conductivity of epoxy composites through the incorporation of boron nitride nanosheets/nanodiamond aerogels prepared by directional‐freezing method

AbstractIn the realm of electronic circuits, materials with high thermal conductivity are sought after as ideal packaging components. It is crucial that such materials not only possess excellent thermal conductivity, but also maintain desirable dielectric properties. To this end, boron nitride nanosheets (BNNS) and BNNS/nanodiamond (ND) aerogels with a three‐dimensional (3D) structure using the ice‐template method were prepared. Subsequently, these aerogels were utilized as a framework to produce 3D‐BNNS/epoxy and 3D‐BNNS/ND/epoxy composites via the vacuum‐assisted impregnation method, which exhibit enhanced anisotropic thermal conductivity. Furthermore, the thermal conductivity exhibited by the 3D‐BNNS/epoxy composites outperforms those containing randomly distributed BNNS. The experiment also reveals sintering the aerogel at elevated temperatures to remove the PVA can significantly improve the thermal conductivity exhibited by the 3D‐BNNS/epoxy and 3D‐BNNS/ND/epoxy composites. Importantly, the 3D‐BNNS/ND/epoxy composites illustrate exceptional thermal conductivity values of 1.326 W/(m·K), indicating a remarkable synergistic effect of BNNS and ND in improving thermal conductivity. It is noteworthy to state that the relative thermal conductivity ratio of the 3D‐BNNS/epoxy and 3D‐BNNS/ND/epoxy composites to pure epoxy resin was markedly higher at lower temperatures than the values measured at room temperature, signifying their superior heat transfer performance at lower temperatures. In addition, the composites depict excellent dielectric properties and lower coefficients of thermal expansion values. Finally, the 3D‐BNNS/ND/epoxy composites also have significantly improved Tg (132.5 and 152.6°C). The high‐performance polymer composites described in this study are envisaged for implementation within the realm of electronic packaging materials.

Stability and Ly α emission of Cold Stream in the Circumgalactic Medium: impact of magnetic fields and thermal conduction

ABSTRACT Cold streams of gas with temperatures around 104 K play a crucial role in the gas accretion on to high-redshift galaxies. The current resolution of cosmological simulations is insufficient to fully capture the stability and Ly α emission characteristics of cold stream accretion, underscoring the imperative need for conducting idealized high-resolution simulations. We investigate the impact of magnetic fields at various angles and anisotropic thermal conduction (TC) on the dynamics of radiatively cooling streams through a comprehensive suite of two-dimensional high-resolution simulations. An initially small magnetic field ($\sim 10^{-3} \, \mu\rm G$), oriented non-parallel to the stream, can grow significantly, providing stability against Kelvin–Helmholtz instabilities and reducing the Ly α emission by a factor of <20 compared to the hydrodynamics case. With TC, the stream evolution can be categorized into three regimes: (1) the Diffusing Stream regime, where the stream diffuses into the surrounding hot circumgalactic medium; (2) the Intermediate regime, where TC diffuses the mixing layer, resulting in enhanced stabilization and reduced emissions; (3) the Condensing Stream regime, where the impact of magnetic field and TC on the stream’s emission and evolution becomes negligible. Extrapolating our findings to the cosmological context suggests that cold streams with a radius of $\le 1 \rm \, {\rm kpc}$ may fuel galaxies with cold metal-enriched magnetized gas ($B \sim 0.1\!-\!1 \, \mu \rm G$) for a longer time, leading to a broad range of Ly α luminosity signatures of $\sim 10^{37}\!-\!10^{41}\, \rm \, erg \, s^{-1}$.

Tuning the Through-Plane Lattice Thermal Conductivity in van der Waals Structures through Rotational (Dis)ordering.

It has recently been demonstrated that MoS2 with irregular interlayer rotations can achieve an extreme anisotropy in the lattice thermal conductivity (LTC), which is, for example, of interest for applications in waste heat management in integrated circuits. Here, we show by atomic-scale simulations based on machine-learned potentials that this principle extends to other two-dimensional materials, including C and BN. In all three materials, introducing rotational disorder drives the through-plane LTC to the glass limit, while the in-plane LTC remains almost unchanged compared to those of the ideal bulk materials. We demonstrate that the ultralow through-plane LTC is connected to the collapse of their transverse acoustic modes in the through-plane direction. Furthermore, we find that the twist angle in periodic moiré structures representing rotational order provides an efficient means for tuning the through-plane LTC that operates for all chemistries considered here. The minimal through-plane LTC is obtained for angles between 1 and 4° depending on the material, with the biggest effect in MoS2. The angular dependence is correlated with the degree of stacking disorder in the materials, which in turn is connected to the slip surface. This provides a simple descriptor for predicting the optimal conditions at which the LTC is expected to become minimal.

Collisional cooling in confined gases at cryogenic temperatures: Time dependent near-wall accumulation and gas dynamics

The heat transfer characteristics of a confined gas are often quantified by anisotropic thermal conductivity and are affected by near-wall accumulation. However, in non-equilibrium conditions, where gas and walls are at different temperatures, transport coefficients, and state variables become dynamic, thus offering an interesting aspect of heat transfer, which is investigated in the present work. Here, we focus on the cooling of confined He by collisions with cold metal walls at cryogenic temperatures (30 - 150 K) in pristine and mixed states with a secondary gas at atmospheric pressure. The effects of wall temperature (Tw), type, and mass (m⁎) of the secondary gas on the cooling rate constant (k) and subsequently on the near-wall accumulation of He are investigated through non-equilibrium molecular dynamics simulations. The cooling curves of the confined gases are found to obey Newton's law of cooling. To probe the cooling process further, the temporal evolution of the near-wall accumulation is analyzed and found to be significant for lower Tw, which, in turn, promotes heat transfer at the interface through gas-wall and gas-gas collisions. In a gaseous mixture, such accumulations are found to be smaller for a heavier secondary gas, which leads to the inhibition of gas-wall collisions. Furthermore, the kinetic energy transfer between He and a considerably heavier gas particle is inefficient, resulting in the slower cooling of He, as observed in He-Ne and He-Ar mixtures. The quantity k is additionally affected by the interaction strength and thermal conductivity of the secondary gas, including the initial temperature difference between the wall and the gas (ΔT). The results of this study find their applications in the fields of microelectronics and microplasmas.

Electronic thermal conductivity of few-layer black phosphorus

The theoretical investigations of anisotropic electronic thermal conductivity of nanoscale black phosphorus along zigzag and armchair directions are presented. Electrons in black phosphorus assumed to be scattered by in-plane linear dispersion phonons and out-of-plane quadratic dispersion phonons via deformation potential couplings. The individual contributions from in-plane phonon and out-of-plane phonon processes to the electronic thermal conductivity show dominant role of in-plane phonon processes. The anisotropic behaviour of electronic thermal conductivity is noticed and found to be sensitive to the carrier density and temperature. Role and relative importance of anisotropic electronic thermal conductivity in the analysis of experimental thermal conductivity data of nanoscale black phosphorus samples is investigated.

Topology optimization of controllable porous microstructure with maximum thermal conductivity

Porous structures are lightweight and thus possess tailorable thermophysical properties through topological design. A multi-constraint topology optimization scheme is developed for designing 2D periodic lattice material with controllable porosity and optimal thermal conductivity. The porosity, pore size and specific surface area are modulated by imposing local density constraints, and thus the structure-determined thermal conductivity can be systematically investigated. The isotropic porous structures after optimization show effective thermal conductivity close to the Hashin-Shtrikman theoretical bound. The specific surface area of base cell is enlarged with imposing local density constraint. In addition, microstructure with anisotropic thermal conductivity can be also obtained. The samples of lattice structure are additively manufactured via selective laser melting, and the thermal conductivities are experimentally validated with deviation within 10 %. The proposed porous structures with targeted porosity have potential application in skeleton embedded with phase change materials.

Theoretical Investigation of the Lattice Thermal Conductivities of II-IV-V2 Pnictide Semiconductors.

Ternary pnictide semiconductors with II-IV-V2 stoichiometry hold potential as cost-effective thermoelectric materials with suitable electronic transport properties, but their lattice thermal conductivities (κ) are typically too high. Insights into their vibrational properties are therefore crucial to finding strategies to reduce κ and achieve improved thermoelectric performance. We present a theoretical exploration of the lattice thermal conductivities for a set of pnictide semiconductors with ABX2 composition (A = Zn, Cd; B = Si, Ge, Sn; and X = P, As) using machine-learning-based regression algorithms to extract force constants from a reduced number of density functional theory simulations and then solving the Boltzmann transport equation for phonons. Our results align well with available experimental data, decreasing the mean absolute error by ∼3 W m-1 K-1 with respect to the best previous set of theoretical predictions. Zn-based ternary pnictides have, on average, more than double the thermal conductivity of the Cd-based compounds. Anisotropic behavior increases with the mass difference between A and B cations, but while the nature of the anion does not affect the structural anisotropy, the thermal conductivity anisotropy is typically higher for arsenides than for phosphides. We identify compounds such as CdGeAs2, for which nanostructuring to an affordable range of particle sizes could lead to κ values low enough for thermoelectric applications.

Controlling heat capacity in a thermal concentrator using metamaterials: Numerical and experimental studies

We developed two thermal concentrators, one with a 2D design of uniform thickness and another with a 3D design, using the coordinate transformation technique and metamaterials. By structuring the thermal conductor, we were able to achieve the desired local density-heat capacity product ρC and anisotropic thermal conductivities. Homogenized thermal conductivities were obtained from the boundary heat fluxes of the unit cells computed with finite element simulations and taking into account the cylindrical symmetry. We fabricated the 3D concentrator using 3D metal printing and used a thermal camera for characterization. By designing the unit cells to obtain the (ρC) values given by the coordinate transformation, the time evolution characteristics of the metadevice are closer to those of an ideal concentrator when compared to devices that solely consider anisotropic conductivities.