Anisotropic Thermal Conductivity Research Articles

Configuration-independent thermal invariants under flow reversal in thin vascular systems.

Modulating temperature fields is indispensable for advancing modern technologies: space probes, electronic packing, and implantable medical devices, to name a few. Bio-inspired thermal regulation achieved via fluid flow within a network of embedded vesicles is notably desirable for slender synthetic material systems. This far-reaching study-availing theory, numerics, and experiments-reveals a counter-intuitive yet fundamental property of vascular-based fluid-flow-engendered thermal regulation. For such thin systems, the mean surface temperature and the outlet temperature-consequently, the heat extracted by the flowing fluid (coolant)-are invariant under flow reversal (i.e. swapping the inlet and outlet). Despite markedly different temperature fields under flow reversal, our newfound invariance-a discovery-holds for anisotropic thermal conductivity, any inlet and ambient temperatures, transient and steady-state responses, irregular domains, and arbitrary internal vascular topologies, including those with branching. The reported configuration-independent result benefits thermal regulation designers. For instance, the flexibility in the coolant's inlet location eases coordination challenges between electronics and various delivery systems in microfluidic devices without compromising performance (e.g. soft implantable coolers for pain management). Last but not least, the invariance offers an innovative way to verify computer codes, especially when analytical solutions are unavailable for intricate domain and vascular configurations.

First-principles studies of the effects of vacancy defects on the properties of Ta2AlC

The effects of vacancy defects on the structural, electronic, mechanical, thermal and optical properties of Ta2AlC MAX-phase were investigated by density functional theory (DFT). The formation enthalpy and elastic properties indicate that all the structures in this calculation are thermodynamically and mechanically stable. The absence of atoms in Ta2AlC weaken the stiffness of the material, improve the plasticity, reduces the thermal conductivity and change the anisotropy of elastic and thermal conductivity. The Ta2AlC has a strong C–Ta covalent bond. The vacancy of Al atoms has no effect on the C–Ta bond, but the VTa, VC weaken the covalent bond severely. The vacancy defects improve the optical conductivity and reflectivity of the Ta2AlC in low energy region.

Magnetosonic Excitation of the Entropy Perturbations in a Plasma with Thermal Conduction Depending on Temperature

Nonlinear excitation of the entropy perturbations by magnetosonic waves in a uniform and infinite plasma model is considered. The wave vector of slow or fast mode forms an arbitrary angle θ (0≤θ≤π) with the equilibrium straight magnetic field, and all perturbations are functions of the time and longitudinal coordinate. Thermal conduction is the only factor which destroys isentropicity of wave perturbations and causes the nonlinear excitation of the entropy mode. A dynamic equation is derived which describes excitation of perturbation in the entropy mode in the field of dominant magnetosonic mode. Effects associatiated with temperature dependent and anisotropic thermal conduction are considered and discussed.

Mixed analytical model to estimate the anisotropic effective thermal conductivity and in-reactor performance of metallic microcell UO2

A mixed analytical model is utilized to estimate the effective thermal conductivity of microcell UO2 pellets. The proposed model consists of a mixture of parallel and series structural models that collectively take into account the unique cell geometry of the anisotropic metal network and the corresponding fractional contribution to the heat flow. This relatively simple structure model can explain the anisotropy of the thermal conductivity of UO2-Mo microcells, as observed in both experimental tests and numerical calculations. The physics-based mixed analytical model allows us to accommodate the effects of the UO2 burnup and wall structure changes on the effective thermal conductivity of microcell UO2 pellets. To validate the model's ability to predict the burnup dependence, the reduced fuel centerline temperature in a UO2 -5vol% Cr microcell under irradiation was estimated at selected burnup levels and compared to the temperature evolution observed in a Halden reactor test, in which the sample pellets were irradiated up to 16MWd/kgU. The comparison demonstrated that the mixed analytical model suitably predicts the fuel temperature changes in irradiated UO2 -5vol% Cr microcells. The model proposed here is a preliminary type based on limited experimental data, and the parameters of the model could be modified further with the accumulation of more measurement data.

Waffle-inspired thermal conductive silicone rubber composites with excellent electromagnetic shielding performance based on three-dimensional filler networks

Improving the thermal conductivity and electromagnetic shielding performance of silicone rubber composites is crucial for their application in electronic devices. In this work, inspired by the structure of waffle, a novel approach combining ice templating and mechanical vibration was employed to prepare reduced graphene oxide (rGO)/silicon carbide whisker (SiC) filler networks arranged in both horizontal and vertical directions. Silicone rubber composites were prepared by vacuum impregnation of silicone rubber into the three-dimensional filler networks. At the hybrid filler content of 3.88 vol%, the through-plane thermal conductivity reached 5.24 W/(m·K), exhibiting about 31 times higher than that of silicone rubber, and the anisotropy of thermal conductivity of the composite was only 1.35 due to the structural arrangement of the filler network in both vertical and horizontal directions. The total electromagnetic interference (EMI) shielding effectiveness of the composite with 3.88 vol% hybrid filler was 35.2 dB with low electromagnetic reflection, exhibiting good EMI shielding performance.

Excellent Thermal and Dielectric Properties of Hexagonal Boron Nitride/Phenolic Resin Bulk Composite Material for Heatsink Applications

In the present paper, we report polymer composites based on phenolic resin filled with hexagonal boron nitride; hot compression molding coupled with solution-based mixing were used to manufacture the composites. The paper presents experimental results on the physical and physicochemical properties of the obtained composites: thermal stability in air and argon, dielectric constant and dielectric loss tangent, active electrical resistance, thermal conductivity (mean and anisotropy), and mechanical strength. It is shown that the proposed technique of composite manufacturing, including the application of high-process pressures, makes it possible to obtain materials with high anisotropy of thermal conductivity, extremely high-filler content, and excellent dielectric properties, all of which are very important for prospective highly efficient lightweight heatsink elements for electronic devices. Experimental values of thermal conductivity and dielectric constant were analyzed using known mathematical models. Experimental values for thermal conductivities (up to 18.5 W·m−1·K−1) of composites at filler loadings of 65–85 vol.% are significantly higher than published data for bulk boron nitride/polymer composites.

Structural, electronic, thermal and mechanical properties of C60-based fullerene two-dimensional networks explored by first-principles and machine learning

Recent experimental reports on the realizations of two-dimensional (2D) networks of the C60-based fullerenes with anisotropic and nanoporous lattices represent a significant advance, and create exciting prospects for the development of a new class of nanomaterials. In this work, we employed theoretical calculations to explore novel C60-based fullerene lattices and subsequently evaluate their stability and key physical properties. After the energy minimization of extensive structures, we could detect novel 2D, 1D and porous carbon C60-based networks, with close energies to that of the isolated C60 cage. Density functional theory results confirm that the C60-based networks can exhibit remarkable thermal stability, and depending on their atomic structure show metallic, semimetallic or semiconducting electronic nature. Using the machine learning interatomic potentials, thermal and mechanical responses of the predicted nanoporous 2D lattices were investigated. The estimated thermal conductivity of the quasi-hexagonal-phase of C60 fullerene is shown to be in an excellent agreement with the experimental measurements. Despite of different atomic structures, the anisotropic room temperature lattice thermal conductivity of the fullerene nanosheets are estimated to be in the order of 10 W/mK. Unlike the majority of carbon-based 2D materials, C60-based counterparts noticeably are predicted to show positive thermal expansion coefficients. Porous carbon C60-based networks are found to exhibit superior mechanical properties, with tensile strengths and elastic modulus reaching extraordinary values of 50 and 300 GPa, respectively. The theoretical results presented in this work provide a comprehensive vision on the structural, energetic, electronic, thermal and mechanical properties of the C60-based fullerene networks.

Precise Electrical Machine Stator Winding Modeling for Thermal Analysis of Efficient Cooling Concepts

<div>The current development of electric and hybrid electric vehicles has drawn more attention toward the development of electrical machines with high power densities. Though highly efficient, these machines heat up significantly during operation. By design, state-of-the-art water jacket cooling concepts remove the heat mainly through high internal thermal resistances of the electrical machine. The resulting maximum temperatures in the end winding region limit the achievable machine power output. In this study, alternative cooling concepts are presented, which efficiently use the existing heat conduction paths of an electric machine. For this purpose, two modeling methods for the stator windings were developed: a high-resolution approach that considers each individual wire and an abstract approach that uses zones of constant anisotropic thermal conductivity to specify the heat flow in the windings. Both models were used in conjugate heat transfer simulations of a long-term thermal test of the electrical machine integrated in the BMW i3. For both models the validation showed a very good agreement of simulated and measured temperatures. An evaluation of both methods showed the abstract approach to be more efficient than other simulation methods used in the current R&D. Its application for alternative cooling concepts revealed the necessary heat transfer coefficients at different fluid temperatures for a sole convective cooling of the end windings. However, it could be found that a homogeneous temperature distribution in the stator of the machine can only be achieved if a combination of water jacket cooling and convective end winding cooling is used.</div>

3D strongly anisotropic intrinsic thermal conductivity of polypropylene separator

In this work, the thermal conductivity in every direction is reported for the first time for the dry separator component made out of polypropylene (PP) that is used in lithium-ion batteries. Herein, a PP separator film is studied dry and in vacuum, as an example of polyolefins separators. These separators are crucial for performance parameters, including cycle life, thermal management, energy and power density, and safety. Hence, a better understanding of the thermal conductivity k in different in-plane directions (parallel and normal direction to the stretching direction in fabrication) as well as the out-of-plane direction is crucial to the overall performance. k of dry PP separator, measured in vacuum, is found four times bigger in the parallel direction (0.217 W⋅m−1⋅K−1) than that in the normal direction (0.043 W⋅m−1⋅K−1), all for the in-plane direction. This strong anisotropy can be explained by the macroscopic and microscopic anisotropic structure and intrinsic anisotropic thermal conductivity of aligned PP. Moreover, the out-of-plane k of the dry PP separator is measured to be 0.036 and 0.025 W⋅m−1⋅K−1 in the air and vacuum, respectively, suggesting good parallel structure of PP in the thickness direction.

Unusual thermal transport in molecular crystals

Phonon transport plays a vital role in various applications of molecular crystals, such as vibrational energy transfer in energetic materials during thermally- or shock-induced processes and in radiative cooling through infrared emissions in delignified and densified wood. Unlike inorganic crystals, molecular crystals are generally composed of large numbers of atoms in the repeating cell, a wide variety of force interactions ranging from long range electrostatic and London dispersion to short-range bonds of covalent or ionic character, and an accompanying broad-based vibrational spectrum with optic branches near 100 THz. Here, we examine the behavior of thermal carriers in four materials – silicon, Cs2PbI2Cl2, α-RDX, and cellulose I β– using multiple thermal transport models including the phonon gas, Cahill-Watson-Pohl, Allen-Feldman, and Wigner models. We show that three unusual mechanisms are remarkably present in molecular crystals: 1) wave-like carriers due to coupling of disparate modes are the dominant contributors to thermal transport in some complex crystals, 2) the thermal transport can occur through two, and as many as four, distinct phonon channels with each channel representing coupled modes through which varying degrees of Zener-like tunneling occur, and 3) anisotropy of thermal conductivity in some materials can only occur through direction-dependent tunneling. The mechanisms are responsible for ∼40, 75, and 80% of the total thermal conductivity in Cs2PbI2Cl2, α-RDX, and cellulose I β, respectively. The findings open new directions for controlling thermal transport in materials through chemical functionalization and molecular design principles.

Shear-induced anisotropy of effective thermal conductivity in granular packings

This work explores shear-induced anisotropy of the effective thermal conductivity (ETC) in granular packings when subjected to quasistatic shear deformation. The discrete element method is adopted to track the position, force, and contact condition of every particle in the packing during the shear process. The ETC of the packing at different shear strains is evaluated using the particle-resolved finite element method. The simulation results show that the anisotropy ratio of the ETC can reach 40% as the shear strain increases to 0.11. The increase of the anisotropy mainly results from the variation of the network formed by the contacts that exist in two consecutive shear steps. It is also proved that contacts with large contact radius and strong thermal conductance are preferentially aligned with the direction of the applied deformation. The non-uniform distribution of the contact radius and the preferential orientation of the contacts contribute to the anisotropy of the thermal conductivity. Finally, by grouping the contacts in a packing into sub-networks with either weak or strong contact, it is shown that the thermal conductivity anisotropy of a packing is mainly caused by strong contacts.

Hierarchical thermal-conductive polymer nanocomposites for thermal management

Managing heat in electrical conductors is a major challenge to meet the demands for sustainable energy use and electrical reliability, most notably power electronics and energy-critical electrical machines. Achieving such disparate functionalities, such as high temperature thermal and electrical reliability, require rational design and manufacturing of thermal conductor material and its hierarchical structures. Here we present hierarchical thermal-conductive nanocomposites, consisting of nanostructured ceramic conformal coating and aligned ultrahigh molecular weight polyethylene fiber, to tailor heat dissipation in electric conductors. The hybrid aligned thermal interface exhibits a highly desirable temperature dependent anisotropic high thermal conductivity with 0.98 W m−1K−1 and dielectric strength with 3.4. In addition, electrically insulating thermal interfaces demonstrate high-performing and reliable electrical systems under the dynamic load conditions. The surface temperature of heterogeneous ceramic-polymer encapsulated conductor is 17.8 °C lower than that of polymer-encapsulated conductor at the same electrical load. The findings shown here hold great promises for directing heat extraction in electrical machine systems, advancing thermal management for emerging electronic applications.

3D-Printed Polyamide 12/Styrene-Acrylic Copolymer-Boron Nitride (PA12/SA-BN) Composite with Macro and Micro Double Anisotropic Thermally Conductive Structures.

Anisotropic thermally conductive composites are very critical for precise thermal management of electronic devices. In this work, in order to prepare a composite with significant anisotropic thermal conductivity, polyamide 12/styrene-acrylic copolymer-boron nitride (PA12/SA-BN) composites with macro and micro double anisotropic structures were fabricated successfully using 3D printing and micro-shear methods. The morphologies and thermally conductive properties of composites were systematically characterized via SEM, XRD, and the laser flash method. Experimental results indicate that the through-plane thermal conductivity of the composite is 4.2 W/(m·K) with only 21.4 wt% BN, which is five times higher than that of the composite with randomly oriented BN. Simulation results show that the macro-anisotropic structure of the composite (caused by the selective distribution of BN) as well as the micro-anisotropic structure (caused by the orientation structure of BN) both play critical roles in spreading heat along the specified direction. Therefore, as-obtained composites with double anisotropic structures possess great potential for the application inefficient and controllable thermal management in various fields.

Robust multifunctional composite films with alternating multilayered architecture for highly efficient electromagnetic interference shielding, Joule heating and infrared stealth

Developing ultrathin electromagnetic interference (EMI) shielding materials with high-efficiency EMI shielding effectiveness (SE), anisotropic thermal conductivity and robust mechanical performance is urgently required but challenging to satisfy the integration and miniaturization of modern flexible electronics. Herein, an alternating multilayered film containing alternating CoFe2O4@MXene/cellulose nanofibers (CNF) layers and silver nanowires (AgNWs)/CNF layers was successfully fabricated via a facile alternating vacuum-assisted filtration strategy. Profiting from the rational arrangement of CoFe2O4@MXene/CNF layers and AgNWs/CNF layers, electric/magnetic coupling effect and unique “absorption-reflection-reabsorption” shielding behavior among the alternating multilayered films, the resultant composite film with alternating six-layered architecture realizes an ultra-high EMI SE of 87.8 dB with low electromagnetic waves reflection effectiveness of 5.6 dB at a thickness of only 0.1 mm. The film remains reliable EMI shielding performance (over 97% EMI SE retention) after undergoing persistently physical damage and prolonged chemical attack. Meanwhile, owing to the construction of overlapping AgNWs network, the obtained film acquires outstanding in-plane thermal conductivity (6.875 W/(m·K)) and controllable Joule heating performance (steady temperature over 90 °C at applied voltage of 3.0 V within 20 s). The inferior through-plane thermal conductivity (0.689 W/(m·K)) caused by the design of multilayered structure guarantees the remarkable infrared stealth performance. Additionally, the good interfacial interaction endows the prepared film with robust mechanical properties (tensile strength of 183.2 MPa and fracture strain of 5.4%). This work not only offers a creative avenue for designing and preparing multifunctional EMI shielding composite films with excellent mechanical properties, but also broadens the applications of EMI shielding composite films in military fields, artificial intelligence and wearable electronics.

Offset supper-cell model of polymer composites with oriented anisotropic fillers for thermal conductivity prediction considering shape factor

Thermal conductivity of polymer composites with oriented anisotropic fillers is commonly predicted based on the simplified cuboid unit-cell models. However, the models are too simple to reveal the real thermal conductivity of composites without absolutely aligned fillers. Here, a super-cell model with offset fillers based on a hexagonal prism primitive-cell after considering shape factor [Chem. Eng. J. 2022, 441, 136104.] is established to predict thermal conductivity as close to reality as possible for polymer composites with oriented fibrous, wirelike, and flake fillers. The impact of shape factor, fillers’ intrinsic thermal conductivity, size and loading, polymer's intrinsic thermal conductivity, and thermal interface resistance between polymer and fillers, on the anisotropic thermal conductivity of polymer composites are systematically evaluated by performing finite element numerical simulation. The model is applied to predict the anisotropic thermal conductivity of polymer based highly through-plane thermally conductive thermal interface materials, and highly in-plane thermally conductive thermal diffusion materials, which are in excellent agreement with the experimental ones. This work not only provides a practical means to predict the thermal conductivity of the oriented anisotropic fillers /polymer composites, but also has the guiding significance for designing thermal management materials.

Cross-plane thermal conductivity of h-BN thin films grown by pulsed laser deposition

The distinguished properties of hexagonal boron nitride (h-BN), specifically its atomically smooth surface, large critical electric field, and large electronic bandgap, make it ideal for thin film microelectronics and as an ultrawide bandgap semiconductor. Owing to weak van der Waals interactions between layers, h-BN exhibits a significant degree of anisotropic thermal conductivity. The in-plane thermal conductivity of h-BN has extensively been studied, yet the only measured data of cross-plane thermal conductivity (k⊥) are for exfoliated h-BN films. Exfoliation from bulk crystals is not a sustainable method for scalable production of h-BN due to its low repeatability, low yield, poor control of sample thickness, and limitation to small areas. Thus, it is necessary to investigate the thickness-dependence of k⊥ for thin films grown by a practical growth method, such as pulsed laser deposition (PLD), which enables the production of reliable and large-area h-BN films with a control of film thickness. We grew h-BN using PLD at 750 °C and observed a decreasing trend of k⊥ as thickness increases from 30 to 300 nm, varying from ∼1.5 to ∼0.2 W/(m K). We observed a relatively high k⊥ value for h-BN at a thickness of 30 nm, providing insight into the k⊥ of PLD-grown films suitable for electronics applications.

Design of Bioinspired Highly Aligned Bamboo-Mimetic Metamaterials With Structural and Functional Anisotropy

Metamaterials have received widespread attention for their unique properties and broad application prospects. With inspiration from bamboo, metamaterials can be rationally designed to achieve anisotropic functionality. Using a combination of 3D printing and subsequent hot pressing of dual-curing composites, we describe a simple method for creating well-aligned SiC whisker (SiCw) composites that mimic bamboo and have achievable anisotropic thermal conductivity (TC), dielectric properties (DP), and flashover performances (FP). The anisotropic structure is proved by scanning electron microscope and X-ray diffraction observations. At the low loading of 8 vol% SiCw, the axial thermal conductivity of the structured composites can reach 0.655 W m-1 K-1, whereas the radial thermal conductivity is only 0.281 W m-1 K-1, with a high anisotropic TC ratio of 2.33. Strong anisotropy in the dielectric constant was also observed with the permittivity of 134 along the axial direction versus that of 65 along the radial direction at 1 kHz, with a high anisotropic DP ratio of 2.06. Notably, metamaterials exhibit significant anisotropy in both their surface conductivity and flashover voltage, with a high conductivity ratio of ~10,000. The flashover voltage in the diametrical direction is 6 times that in the axial direction and 60% higher than that of the neat resin. This study shows that this approach is anticipated to offer a new path for the creation of high-performance functionally anisotropic materials to meet the diversified requirements of modern electronics.

Thermal metamaterials with nonconformal geometry

Thermal metamaterials have garnered significant attention for their potential to manipulate heat flow, leading to the development of various thermal metadevices such as thermal cloaks, concentrators, and rotators. However, the theoretical study of thermal metadevices with nonconformal geometry remains limited due to design and fabrication challenges. This letter proposes a method for designing and manufacturing nonconformal thermal metamaterials using the conformal discrete theory to simplify the anisotropic thermal conductivity tensors. The method involves 3D printing three thermal metadevices (a thermal cloak, concentrator, and rotator) with complex nonconformal geometry. Simulation and experimental results demonstrate the successful implementation of cloaking, concentrating, and rotating functionalities. Moreover, the thermal metadevices still maintained thermal functionality well under the condition of omnidirectional heat flow. This work provides guidance for the design and manufacture of nonconformal thermal metamaterials, as well as their potential applications in other fields such as electrics/magnetics, electromagnetics/optics, and acoustics.

Implications of phonon anisotropy on thermal conductivity of fluorite oxides

Fluorite oxides are attractive ionic compounds for a range of applications with critical thermal management requirements. In view of recent reports alluding to anisotropic thermal conductivity in this face-centered cubic crystalline systems, we perform a detailed analysis of the impact of direction-dependent phonon group velocities and lifetimes on the thermal transport of fluorite oxides. We demonstrate that the bulk thermal conductivity of this class of materials remains isotropic despite notable anisotropy in phonon lifetime and group velocity. However, breaking the symmetry of the phonon lifetime under external stimuli including boundary scattering present in nonequilibrium molecular dynamics simulations of finite size simulation cell gives rise to apparent thermal conductivity anisotropy. We observe that for accurate determination of thermal conductivity, it is important to consider phonon properties not only along high symmetry directions commonly measured in inelastic neutron or x-ray scattering experiments but also of those along lower symmetry. Our results suggests that certain low symmetry directions have a larger contribution to thermal conductivity compared to high symmetry ones.

Fast heat transfer simulation for laser powder bed fusion

Accurate and fast modeling of the temperature distribution and phase transitions in laser powder bed fusion is a major milestone in achieving its quality assurance. Commonly referred to as digital twin technology, the goal is to find agile, fast-to-compute but also sufficiently accurate simulators that can replicate the 3D printing process while enhancing the quality of its outcomes. In this work, we propose a surrogate model for the nonlinear heat transfer equation coupled with subspace projection and randomized sketching, that exploits the accuracy and explainability of finite element time-domain simulation with the computational efficiency of Monte Carlo sampling, applied to the modality of laser powder bed fusion. Focusing on tackling the high-dimensionality imparted from the finite element approximation and the nonlinearity in the governing equations, our surrogate relies on low-dimensional projection with subspace selection and subsequently sub-samples the Picard iterations utilized to solve the projected non-linear system of equations. The projection bases are generated in the process of simulation by combining previous temperature profiles and locally deployed anisotropic Gaussian functions, while the sketching process utilizes efficient sampling without replacement based on approximate optimal sampling distributions. Both the projection and the sketching are designed to implement alongside the printing process, which makes the proposed surrogate capable of handling different process parameters without requiring prior computations offline. A series of numerical experiments are presented to validate the surrogate’s accuracy and reduction in compute time compared to high-fidelity finite element simulations. Although the achieved speed-up can be as a high ten, computational times are still orders of magnitude away from what would be required for real-time computations. The presented methodology allows to handle different printing attributes (laser power and scan speed) and arbitrary thermal conductivity anisotropy.