Cross-plane Thermal Conductivity Research Articles

Robust Thermal Transport across the Surface-Active Bonding SiC-on-SiC.

Surface-active bonding (SAB) is a promising technique for semiconductors directly bonding. However, the interlayer of the bonding interface and the reduced layer thickness may affect thermal transport. In this study, the temperature-dependent cross-plane thermal conductivity of 4H-SiC thin films and the effective thermal boundary resistance (TBReff) of the bonding SiC-on-SiC are measured by the multiple-probe wavelength nanosecond transient thermoreflectance (MW-TTR). The measured temperature-dependent cross-plane thermal conductivity of the 4H-SiC thin film exhibits good quantitative agreement with calculation by density functional theory (DFT) including higher-order four-phonon (4ph) scattering, especially at high temperatures (>400 K). The theoretical calculations indicate the non-negligible importance of 4ph scattering in 4H-SiC high-temperature applications, due to the significantly increasing 4ph scattering rate at increasing temperature and strong temperature dependence of 4ph scattering. The measured nonzero but small TBReff (2.33 + 0.43/-1.15 m2 K/GW) at the SiC-SiC interface is analyzed with molecular dynamics (MD) simulation, indicating that a strong bonding interface with an extremely thin interlayer is formed by the SAB process. Two-dimensional finite element simulations of the experimental equivalent structures are further investigated, and the significant effects (at least 19 °C) of TBReff on the maximum temperature (Tmax) are confirmed. This study provides insight into the fundamental phonon transport and interface thermal transport mechanism in SAB SiC-on-SiC and paves the way for improved 4H-SiC efficient device manufacturing and thermal management.

Effective cross-plane thermal conductivity of metal-dielectric multilayers at low temperatures

Heat transfer in layered metal-dielectric structures is considered theoretically based on an analytical solution of the Boltzmann transfer equation for the phonon distribution function. Taking into account the size effect, the problem of effective cross-plane thermal conductivity of structures containing two metal layers is analyzed in detail. If the thickness of the metal layers is less than the phonon mean free path, interlayer heat transfer is carried out predominantly by phonons, and the effective cross-plane thermal conductivity is determined by the reflection of phonons from the metal/dielectric interfaces. In the opposite case of thick metal layers, the effective cross-plane thermal conductivity is determined both by the thermal conductivity of the metal layers and by the thermal resistance of the dielectric layers. The results obtained are generalized to multilayer structures and superlattices.

Studies on thermoelectric properties of sonochemically exfoliated MoS2

2D transition metal dichalcogenides (TMDs) have gained immense attention as energy materials in the past decade owing to their unique structure and properties. As the need for energy is increasing at a great pace, the search for new and sustainable technologies and materials for them has increased. Thermoelectric electricity generation is one such sustainable solution as waste heat can be utilized to create electricity. Here in this article, the thermoelectric behaviour of MoS2 is studied which was exfoliated in mild/friendly solvents in a simple manner using the ultrasound energy. Such 2D materials have been studied for thermoelectric behaviour but their performance is restricted by the poor electrical conductivity. Hence, thermoelectric performance of MoS2 is often enhanced by preparing hybrids with other materials such as graphene, carbon fibre or by metal doping. However, herein the overall power factor was highly increased via a simple method attributing to the increased interlayer distance and phase change which was seen in XRD, Raman and XPS. MoS2 exfoliated in water resulted in power factor of 68.34 μW m−1K−2 at room temperature and highest power factor was obtained in case of exfoliation in DMF (310 μW m−1K−2). The cross-plane thermal conductivity also reduced about 5 times as compared to the bulk material.

High Dk and ultra-high thermal conductivity ST/μBN/polyolefin composites for microwave applications

In this work, we examined the structure, microwave dielectric, thermal and mechanical characteristics of composites containing SrTiO3/xμBN/polyolefin (with x at 0, 6, 8, 10 and 12 weight percent). SrTiO3 (ST) and micro boron nitride (μBN) powders were fabricated with 1,2-PB/EPDM/SBS (1,2-PB, EPDM and SBS are abbreviations of 1,2-polybutadienes, ethylene-propylene-dicyclopentadiene and styrene-butadiene-styrene triblock copolymer, respectively) by hot-press process after modified by Vinyltrimethoxysilane (VTMS) and 3-aminopropyltriethoxysilane (KH550), respectively. The ball mill and surface modification procedure enhanced the dispersion of the boron nitride in the solution and promoted compatibility between ceramic particles and polyolefin matrix. The incorporation of BN created more effective boron transfer pathways with ST particles, significantly improving the thermal conductivity of the composite. However, as μBN content increased, more ceramic agglomerates and pores were introduced, deteriorating dielectric, thermal and mechanical properties of the samples. The particle size test and XPS were conducted to determine the effect of surface modification on the dispersion and surface properties of ceramics. Cross-plane thermal conductivity, TGA and stress-strain curves were used to evaluate the thermal and mechanical properties of the composites. In all, a composite with high-Dk and high thermal conductivity was obtained when x = 10 wt.%: Dk = 17.4, Df = 0.0071 and λ = 1.784 W/(m*k), which is promising for microwave applications.

Cross-plane thermal transport in layered materials

The cross-plane (across-layers) phonon thermal transport of five diverse, layered semiconductors is investigated by accounting for higher-order four-phonon scattering, phonon renormalization, and multi-channel thermal transport. For materials having relatively large cross-plane thermal conductivity (AlB6, MoS2, and MoSi2N4), phonons contributing to cross-plane conductivity have an order of magnitude larger mean free path than that for the basal-plane thermal transport, whereas the opposite effect is observed for materials with low thermal conductivity (MoO3 and KCuSe). The contribution from the wave-like coherent transport channel is less than 5% in all considered materials. Our work unravels the contrasting role of nanostructuring on the basal- vs cross-plane thermal conductivity of low and high thermal conductivity layered materials.

Using transducerless time-domain thermoreflectance technique to measure in- and cross-plane thermal conductivity of nanofilms

Time-domain thermoreflectance (TDTR) and frequency-domain thermoreflectance techniques have been widely used to measure thermal properties. However, the existence of the metal sensor brings some limitations to the experimental measurement, such as temperature limits, disability to measure low in-plane thermal conductivity, in situ measurement cannot be achieved, etc. This paper proposes a transducerless time-domain thermoreflectance method to measure in- and cross-plane thermal conductivity of nanofilms, in which the optical absorption depth and thermal conductivity tensor are considered to establish a new differential equation that can describe the heat conduction process in multilayer structures. This thermal model can also calculate the effects of spot ellipticity and spot offset distance. Then, the analytical solution and relative deviation of this new model and the surface heat flow boundary model used in conventional TDTR are compared by calculating the phase signals. In terms of experimental measurement, this model is successfully used to derive cross- and in-plane thermal conductivity of PdSi and IrNiTa amorphous alloy nanofilms without a metal sensor.

Magnetism and Thermal Transport of Exchange-Spring-Coupled La2/3Sr1/3MnO3/La2MnCoO6 Superlattices with Perpendicular Magnetic Anisotropy.

Superlattices (SLs) comprising layers of a soft ferromagnetic metal La2/3Sr1/3MnO3 (LSMO) with in-plane (IP) magnetic easy axis and a hard ferromagnetic insulator La2MnCoO6 (LMCO, out-of-plane anisotropy) were grown on SrTiO3 (100)(STO) substrates by a metalorganic aerosol deposition technique. Exchange spring magnetic (ESM) behavior between LSMO and LMCO, manifested by a spin reorientation transition of the LSMO layers towards perpendicular magnetic anisotropy below TSR = 260 K, was observed. Further, 3ω measurements of the [(LMCO)9/(LSMO)9]11/STO(100) superlattices revealed extremely low values of the cross-plane thermal conductivity κ(300 K) = 0.32 Wm-1K-1. Additionally, the thermal conductivity shows a peculiar dependence on the applied IP magnetic field, either decreasing or increasing in accordance with the magnetic disorder induced by ESM. Furthermore, both positive and negative magnetoresistance were observed in the SL in the respective temperature regions due to the formation of 90°-Néel domain walls within the ESM, when applying IP magnetic fields. The results are discussed in the framework of electronic contribution to thermal conductivity originating from the LSMO layers.

Non-Linear Optics at Twist Interfaces in h-BN/SiC Heterostructures.

Understanding the emergent electronic structure in twisted atomically thin layers has led to the exciting field of twistronics. However, practical applications of such systems are challenging since the specific angular correlations between the layers must be precisely controlled and the layers have to be single crystalline with uniform atomic ordering. Here, an alternative, simple, and scalable approach is suggested, where nanocrystallinetwo-dimensional (2D) film on 3D substrates yields twisted-interface-dependent properties. Ultrawide-bandgap hexagonal boron nitride (h-BN) thin films are directly grown on high in-plane lattice mismatched wide-bandgap silicon carbide (4H-SiC) substrates to explore the twist-dependent structure-property correlations. Concurrently, nanocrystalline h-BN thin film shows strong non-linear second-harmonic generation and ultra-low cross-plane thermal conductivity at room temperature, which are attributed to the twisted domain edges between van der Waals stacked nanocrystals with random in-plane orientations. First-principles calculations based on time-dependent density functional theory manifest strong even-order optical nonlinearity in twisted h-BN layers. This work unveils that directly deposited 2D nanocrystalline thin film on 3D substrates could provide easily accessible twist-interfaces, therefore enabling a simple and scalable approach to utilize the 2D-twistronics integrated in 3D material devices for next-generation nanotechnology.

Temperature and Thickness Dependence of the Thermal Conductivity in 2D Ferromagnet Fe3GeTe2.

The emergence of symmetry-breaking orders such as ferromagnetism and the weak interlayer bonding in van der Waals materials offers a unique platform to engineer novel heterostructures and tune transport properties like thermal conductivity. Here, we report the experimental and theoretical study of the cross-plane thermal conductivity, κ⊥, of the van der Waals two-dimensional (2D) ferromagnet Fe3GeTe2. We observe an increase in κ⊥ with thickness, indicating a diffusive transport regime with ballistic contributions. These results are supported by the theoretical analyses of the accumulated thermal conductivity, which show an important contribution of phonons with mean free paths between 10 and 200 nm. Moreover, our experiments show a reduction of κ⊥ in the low-temperature ferromagnetic phase occurring at the magnetic transition. The calculations show that this reduction in κ⊥ is associated with a decrease in the group velocities of the acoustic phonons and an increase in the phonon-phonon scattering of the Raman modes that couple to the magnetic phase. These results demonstrate the potential of van der Waals ferromagnets for thermal transport engineering.

A Molecular Dynamics Simulation Study of In- and Cross-Plane Thermal Conductivity of Bilayer Graphene.

Efficient thermal management of modern electronics requires the use of thin films with highly anisotropic thermal conductivity. Such films enable the effective dissipation of excess heat along one direction while simultaneously providing thermal insulation along the perpendicular direction. This study employs non-equilibrium molecular dynamics to investigate the thermal conductivity of bilayer graphene (BLG) sheets, examining both in-plane and cross-plane thermal conductivities. The in-plane thermal conductivity of 10 nm × 10 nm BLG with zigzag and armchair edges at room temperature is found to be around 204 W/m·K and 124 W/m·K, respectively. The in-plane thermal conductivity of BLG increases with sheet length. BLG with zigzag edges consistently exhibits 30-40% higher thermal conductivity than BLG with armchair edges. In addition, increasing temperature from 300 K to 600 K decreases the in-plane thermal conductivity of a 10 nm × 10 nm zigzag BLG by about 34%. Similarly, the application of a 12.5% tensile strain induces a 51% reduction in its thermal conductivity compared to the strain-free values. Armchair configurations exhibit similar responses to variations in temperature and strain, but with less sensitivity. Furthermore, the cross-plane thermal conductivity of BLG at 300 K is estimated to be 0.05 W/m·K, significantly lower than the in-plane results. The cross-plane thermal conductance of BLG decreases with increasing temperatures, specifically, at 600 K, its value is almost 16% of that observed at 300 K.

A biomimetic non-woven fabric with passive thermal-insulation and active heat-recovering

The fiber-based porous materials illustrate the advantages on thermal insulation because of the limited heat convection by porous morphology and the phonon scattering at multi-scale interfaces. However, there is still space for improving thermal insulation by restricting the thermal radiation. In this work, inspired by the architecture configuration of “black body”, a double-wall carbon nanotube (DWCNT) non-woven fabric (CNF) with a gradient-pore configuration is developed to trigger the multiple reflective mechanism of infrared rays, which facilitates the infrared shielding ability, and leads to the ultra-low cross-plane thermal conductivity (k⊥) of 0.022 W m−1 K−1 at room temperature. As a result, the CNF shows a better steady and dynamic thermal-insulation performance than the commercial silica aerogel. In addition to passively insulate heat, the CNF can act as a power generator by leveraging the temperature difference to trigger the thermoelectric effect. A proof-of-concept CNF-based thermoelectric module yields a maximum output power of 42 nW at ∆T=20 K. Moreover, the CNF demonstrates the great mechanical durability, wearability and electrothermal effect, suggesting a promising candidate as a smart textile for personal thermal management.

Twist-Dependent Anisotropic Thermal Conductivity in Homogeneous MoS2 Stacks

Thermal transport property of homogeneous twisted molybdenum disulfide (MoS2) is investigated using non-equilibrium molecular dynamics simulations with the state-of-art force fields. The simulation results demonstrate that the cross-plane thermal conductivity strongly depends on the interfacial twist angle, while it has only a minor effect on the in-plane thermal conductivity, exhibiting a highly anisotropic nature. A frequency-decomposed phonon analysis showed that the cross-plane and in-plane thermal conductivity of MoS2 are dominated by the phonons with frequencies below 12.5 THz and 7.5 THz, respectively. As the interfacial twist angle increases, these low-frequency phonons significantly attenuate the phonon transport across the interface, leading to impeded cross-plane thermal transport. However, the in-plane phonon transport is almost unaffected, which allows for maintaining high in-plane thermal conductivity. Furthermore, our study revealed a strong size dependence for both cross-plane and in-plane thermal conductivities due to the influence of low-frequency phonons in MoS2. The maximum thermal conductivity anisotropy ratio is estimated as ∼400 for twisted MoS2 from our simulation, which is in the same order of magnitude as recent experimental results (∼900). Our study highlights the potential of twist engineering as a tool for tailoring the thermal transport properties of layered materials.

Pressure-induced tuning of thermal transport in carbon-based composites: Directional control of heat dissipation

Aggressive miniaturization in the electronics industry demands more efficient thermal management for electronic devices. Employing the excluded volume approach, we enhanced thermal conductivity (TC) and manipulated heat dissipation in a polymer composite loaded with a thermally conductive filler—graphene nanoplatelets (GNP)—alone or supplemented with a high-volume filler (diamonds). This yielded a maximum bulk TC of 4.5 W m−1K−1 (no compression). The heat dissipation direction could be controlled by compressing (≥50 bars) the wet composite before curing, resulting in an enhancement of the cross-plane thermal conductivity (for thermal-interface materials) to 12.3 W m−1K−1. For non-compressed samples, in-plane thermal conductivity (for circuit board conformal coatings) was dominant (8 Wm−1K−1). Our holistic approach meets diverse thermal management needs.

Molecular dynamics study on the thermal conductivity and ballistic resistance of twisted graphene

The twist as a new degree of freedom induces several angle dependent properties in twisted graphene, such as electronic and optical properties. However, the ballistic resistance and thermal conductivity of twisted graphene remain not well understood and experimentally challenging. Therefore, the effect of interlayer twist angle on the thermal conductivity and ballistic resistance of bilayer graphene (BLG) has been explored using molecular dynamics simulation herein. Results show that the thermal conductivity of graphene is very sensitive to the twist angle of the interlayer plane. As the twist angle increases from 0° to 60°, the in-plane thermal conductivity of twisted bilayer graphene (t-BLG) first decreases and then increases with a periodic interval of 30°, exhibiting an asymmetric double V-shaped curve. And the cross-plane thermal conductivity of t-BLG gradually first decreases from 0° to 30° and then increases from 30° to 60°, also showing an asymmetric V-shaped curve. In contrast, the interlayer twist angle has a limited influence on the ballistic resistance of t-BLG. With the change of the twist angle, the ballistic limit velocity does not change much, and the fluctuation range is less than 1.56%; the relative kinetic energy also changes little with the fluctuation less than 4%. Such a slight difference is insufficient to make a significant change in its ballistic resistance and its impact resistance is mainly affected by the number of membrane layers. Each additional layer increases its ballistic limit speed by about 8%. This study will provide some theoretical guidance for the study of heat dissipation in the anti-ballistic process of twisted graphene and the design of advanced protective materials.

Spiderweb-inspired fabrication of 3D MXene hybrid filler with multilayer and multiscale features for multifunctional polydimethylsiloxane composites

Polymer-based composite with high thermal conductivity and excellent fire safety is urgently desired and required in the field of thermal management of advanced electronic devices. In this study, inspired by the structure of a spiderweb, we reported a polymer composite with three-dimensional (3D) interconnected spiderweb-like structures by assembling adsorption of transition metal carbides/nitrides (MXene) and diethylenetriaminepenta(methylene-phosphonic acid) coordinated cobalt ions complex. In this composite, MXene combined with the coordination complex forms a 3D network structure with multi-layer/-scale features, which enables high cross-plane thermal conductivity (∼0.59 K w-1m−1) and excellent flame retardancy, especially including effective inhibition of toxic CO and asphyxiating CO2 production. Notably, the 3D MXene networks embedded in the polymer matrix impart the composite with an intelligent damage detection function that is able to detect the depth of cracks. This bioinspired strategy provides a promising approach to creating high-performance polymer-based composites.

A universal law for the pattern evolution of fullerene-based sandwiches

Fullerene-based sandwiches have excellent controllable physical properties such as cross-plane thermal conductivity and charge transfer, which provide new possibilities for advancing the field of thermoelectricity. Because the pattern of the fullerene cluster impacts the physical properties of the sandwiches, it is important to understand the mechanisms for their structural transformations. In the present work, we find that the graphene/fullerene/graphene sandwich structure transforms among three configurations in an ideal environment, depending on the fullerene to graphene area ratio. Molecular dynamics simulations show that there are two critical values for the area ratio. The fullerene pattern transforms from circular to rectangular at the first critical area ratio of 1π. The critical value of 1π is successfully derived by comparing the geometrical perimeter of the circular and rectangular shapes for the fullerene cluster without any physical parameters. At the second critical area ratio, the graphene layers surrounding the fullerene cluster become separated due to the competition between the bending energy and the cohesive energy. The vacuum space is predicted to be 34 Å based on the analytic model. These findings provide fundamental insights into the mechanisms driving structural transformation of fullerene-based sandwiches, which will enable the selection of appropriate structures and materials to guide the design of future electronic and energy storage applications.

Engineering Heat Transport Across Epitaxial Lattice-Mismatched van der Waals Heterointerfaces.

Artificially engineered 2D materials offer unique physical properties for thermal management, surpassing naturally occurring materials. Here, using van der Waals epitaxy, we demonstrate the ability to engineer extremely insulating thermal metamaterials based on atomically thin lattice-mismatched Bi2Se3/MoSe2 superlattices and graphene/PdSe2 heterostructures with exceptional thermal resistances (70-202 m2 K/GW) and ultralow cross-plane thermal conductivities (0.012-0.07 W/mK) at room temperature, comparable to those of amorphous materials. Experimental data obtained using frequency-domain thermoreflectance and low-frequency Raman spectroscopy, supported by tight-binding phonon calculations, reveal the impact of lattice mismatch, phonon-interface scattering, size effects, temperature, and interface thermal resistance on cross-plane heat dissipation, uncovering different thermal transport regimes and the dominant role of long-wavelength phonons. Our findings provide essential insights into emerging synthesis and thermal characterization methods and valuable guidance for the development of large-area heteroepitaxial van der Waals films of dissimilar materials with tailored thermal transport characteristics.

Stacking and thickness effects on cross-plane thermal conductivity of hexagonal boron nitride

Recently, the in-plane thermal transport in van der Waals (vdW) materials such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs) has been widely studied. Whereas, the cross-plane one is far from sufficient. Based on the non-equilibrium molecular dynamics simulations and Boltzmann transport equation, here we reveal the stacking and thickness effects on the cross-plane thermal conductivity (κ) of h-BN. We find that κ can be significantly modulated by both the stacking structure and thickness (d) of h-BN, which is unexpected from the viewpoint of its smooth in-plane structure and weak interlayer interaction. In the small thickness region (d < 6 nm), κ of h-BN at room temperature significantly increases with thickness, following a power law of κ∝dβ withβ = 0.84, 0.66, and 0.92 for AA', AB, and AB' stacking structures, respectively. Moreover, κ of AB' structure reaches up to 60% larger than that of AA' and AB structures with d = 5.3 nm, showing the remarkable stacking effect. We also find that the stacking effect on κ changes dramatically with d increasing, where AA' stacking has the largest κ with d > 200 nm. We finally clarify that such exotic stacking and thickness dependence of κ is owing to the competing effect of excited number of phonons and phonon relaxation time, both of which directly affect the thermal conductivity. Our findings may provide new insights into the cross-plane thermal management in vdW materials.

A dual-domain 3 ω method for measuring the in-plane thermal conductivity of high-conductive thin films

The thermal conductivity measurement of films with submicrometer thicknesses is difficult due to their exceptionally low thermal resistance, which makes it challenging to accurately measure the temperature changes that occur as heat flows through the film. Thus, specialized and sensitive measurement techniques are required. 3ω method is a widely used and reliable tool for measuring the thermal conductivity of films. However, the high in-plane thermal conductivity in thin films results in rapid heat dissipation across the thin film, resulting in poor measurement sensitivity and making it difficult to accurately measure the temperature gradient with the traditional 3ω method. Also, the traditional 3ω method requires cross-plane thermal conductivity to derive the in-plane counterpart. Here, we introduce a dual-domain 3ω method that adopts AC-modulated heating and electrode arrays facilitating surface temperature profiling: (1) the sensitivity was significantly improved due to the employment of low-thermal-conductivity-substrate, and (2) cross-plane thermal conductivity is not required for the analysis of in-plane counterpart. This measurement platform allows us to control heat penetration in depth via varied heating frequencies as well as spatial temperature detection through laterally distributed electrodes on the thin film surface. By utilizing the described method, we have determined the in-plane thermal conductivity of a copper film, having a thickness of 300 nm, which was found to be 346 Wm−1K−1 and validated by the Wiedemann–Franz law.

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.