Anisotropic Thermal Boundary Resistance across 2D Black Phosphorus: Experiment and Atomistic Modeling of Interfacial Energy Transport

Interfacial thermal boundary resistance (TBR) plays a critical role in near‐junction thermal management of modern electronics. In particular, TBR can dominate heat dissipation and has become increasingly important due to the continuous emergence of novel nanomaterials with promising electronic and thermal applications. A highly anisotropic TBR across a prototype 2D material, i.e., black phosphorus, is reported through a crystal‐orientation‐dependent interfacial transport study. The measurements show that the metal–semiconductor TBR of the cross‐plane interfaces is 241% and 327% as high as that of the armchair and zigzag direction‐oriented interfaces, respectively. Atomistic ab initio calculations are conducted to analyze the anisotropic and temperature‐dependent TBR using density functional theory (DFT)‐derived full phonon dispersion relation and molecular dynamics simulation. The measurement and modeling work reveals that such a highly anisotropic TBR can be attributed to the intrinsic band structure and phonon spectral transmission. Furthermore, it is shown that phonon hopping between different branches is important to modulate the interfacial transport process but with directional preferences. A critical fundamental understanding of interfacial thermal transport and TBR–structure relationships is provided, which may open up new opportunities in developing advanced thermal management technology through the rational control over nanostructures and interfaces.     

Polarized Light-Emitting Diodes Based on Anisotropic Excitons in Few-Layer ReS2

An on-chip polarized light source is desirable in signal processing, optical communication, and display applications. Layered semiconductors with reduced in-plane symmetry have inherent anisotropic excitons that are attractive candidates as polarized dipole emitters. Herein, the demonstration of polarized light-emitting diode based on anisotropic excitons in few-layer ReS₂, a 2D semiconductor with excitonic transition energy of 1.5–1.6 eV, is reported. The light-emitting device is based on minority carrier (hole) injection into n-type ReS₂ through a hexagonal boron nitride (hBN) tunnel barrier in a metal–insulator–semiconductor (MIS) van der Waals heterostack. Two distinct emission peaks from excitons are observed at near-infrared wavelength regime from few-layer ReS₂. The emissions exhibit a degree of polarization of 80% reflecting the nearly 1D nature of excitons in ReS₂.     

Site‐Selective and van der Waals Epitaxial Growth of Rhenium Disulfide on Graphene

The surface property of growth substrate imposes significant influence in the growth behaviors of 2D materials. Rhenium disulfide (ReS₂) is a new family of 2D transition metal dichalcogenides with unique distorted 1T crystal structure and thickness‐independent direct bandgap. The role of growth substrate is more critical for ReS₂ owing to its weak interlayer coupling property, which leads to preferred growth along the out‐of‐plane direction while suppressing the uniform in‐plane growth. Herein, graphene is introduced as the growth substrate for ReS₂ and the synthesis of graphene/ReS₂ vertical heterostructure is demonstrated via chemical vapor deposition. Compared with the rough surface of SiO₂/Si substrate with dangling bonds which hinders the uniform growth of ReS₂, the inert and smooth surface nature of graphene sheet provides a lower energy barrier for migration of the adatoms, thereby promoting the growth of ReS₂ on the graphene surface along the in‐plane direction. Furthermore, patterning of the graphene/ReS₂ heterostructure is achieved by the selective growth of ReS₂, which is attributed to the strong binding energy between sulfur atoms and graphene surface. The fundamental studies in the role of graphene as the growth template in the formation of van der Waals heterostructures provide better insights into the synthesis of 2D heterostructures.     

Probing Distinctive Electron Conduction in Multilayer Rhenium Disulfide

Charge carrier transport in multilayer van der Waals (vdW) materials, which comprise multiple conducting layers, is well described using Thomas–Fermi charge screening (λ_TF) and interlayer resistance (R_int). When both effects occur in carrier transport, a channel centroid migrates along the c‐axis according to a vertical electrostatic force, causing redistribution of the conduction centroid in a multilayer system, unlike a conventional bulk material. Thus far, numerous unique properties of vdW materials are discovered, but direct evidence for distinctive charge transport behavior in 2D layered materials is not demonstrated. Herein, the distinctive electron conduction features are reported in a multilayer rhenium disulfide (ReS₂), which provides decoupled vdW interaction between adjacent layers and much high interlayer resistivity in comparison with other transition‐metal dichalcogenides materials. The existence of two plateaus in its transconductance curve clearly reveals the relocation of conduction paths with respect to the top and bottom surfaces, which is rationalized by a theoretical resistor network model by accounting of λ_TF and R_int coupling. The effective tunneling distance probed via low‐frequency noise spectroscopy further supports the shift of electron conduction channel along the thickness of ReS₂.     

一维高导热C/C复合材料的制备研究

以三种沥青作为基体前驱体, 实验室自制的AR中间相沥青基纤维为增强体, 通过500℃热压成型, 随后经炭化和石墨化处理制备出一维炭/炭(C/C)复合材料。研究了前驱体沥青种类和热处理温度对复合材料导热性能的影响, 并采用扫描电子显微镜和偏光显微镜对其石墨化样品的形貌和微观结构进行表征。结果表明; C/C复合材料在沿纤维轴向的室温热扩散系数和导热率均随热处理温度的升高而逐渐增大; 由AR沥青作为基体前驱体所制备的C/C复合材料具有更加明显的沿纤维轴向取向的石墨层状结构以及最好的导热性能, 其3000℃石墨化样品沿纤维轴向的室温热扩散系数和导热率分别达到594.5 mm²/s和734.4 W/(m·K)。     

Molecular Dynamics Simulations for Anisotropic Thermal Conductivity of Borophene

The present work carries out molecular dynamics simulations to compute the thermal conductivity of the borophene nanoribbon and the borophene nanotube using the Muller-Plathe approach. We investigate the thermal conductivity of the armchair and zigzag borophenes, and show the strong anisotropic thermal conductivity property of borophene. We compare results of the borophene nanoribbon and the borophene nanotube, and find the thermal conductivity of the borophene is orientation dependent.The thermal conductivity of the borophene does not vary as changing the width of the borophene nanoribbon and the perimeter of the borophene nanotube. In addition, the thermal conductivity of the borophene is not sensitive to the applied strains and the background temperatures.     

Giant Thermal Expansion in 2D and 3D Cellular Materials

When temperature increases, the volume of an object changes. This property was quantified as the coefficient of thermal expansion only a few hundred years ago. Part of the reason is that the change of volume due to the variation of temperature is in general extremely small and imperceptible. Here, abnormal giant linear thermal expansions in different types of two‐ingredient microstructured hierarchical and self‐similar cellular materials are reported. The cellular materials can be 2D or 3D, and isotropic or anisotropic, with a positive or negative thermal expansion due to the convex or/and concave shape in their representative volume elements respectively. The magnitude of the thermal expansion coefficient can be several times larger than the highest value reported in the literature. This study suggests an innovative approach to develop temperature‐sensitive functional materials and devices.     

纳米氧化钇稳定氧化锆/钛酸锶复合热电材料的热导率研究

通过添加纳米尺度的低热导率添加剂YSZ,形成钛酸锶复合热电材料,研究发现,虽然电子热导率有明显提高,但声子热导率降低得更加明显,进而导致总热导率显著下降。声子热导率的降低主要是因为纳米YSZ在钛酸锶体材料中弥散分布,声子在纳米YSZ与钛酸锶界面处被大量散射,导致声子平均自由程明显降低,进而导致声子热导率明显降低。     

Thermal Conductivity during Phase Transitions

Thermal conductivity is a very basic property that determines how fast a material conducts heat, which plays an important and sometimes a dominant role in many fields. However, because materials with phase transitions have been widely used recently, understanding and measuring temperature‐dependent thermal conductivity during phase transitions are important and sometimes even questionable. Here, the thermal transport equation is corrected by including heat absorption due to phase transitions to reveal how a phase transition affects the measured thermal conductivity. In addition to the enhanced heat capacity that is well known, it is found that thermal diffusivity can be abnormally lowered from the true value, which is also dependent on the speed of phase transitions. The extraction of the true thermal conductivity requires removing the contributions from both altered heat capacity and thermal diffusivity during phase transitions, which is well demonstrated in four selected kinds of phase transition materials (Cu₂Se, Cu₂S, Ag₂S, and Ag₂Se) in experiment. This study also explains the lowered abnormal thermal diffusivity during phase transitions in other materials and thus provides a novel strategy to engineer thermal conductivity for various applications.     

Estimation of the Mean Thermal Conductivity of Anisotropic Materials

An estimation method of the plane directional thermal conductivity of fibrous insulations using the cyclic heat method and the transient hot-wire method is proposed. By assuming that the thermal conductivity _h of anisotropic materials measured by the transient hot-wire method is equivalent to that of the isotropic materials which have the same bulk density and specific heat c as the anisotropic materials, the thermal conductivity _h is shown to be equal to , which is a geometrical mean of the thermal conductivities in the direction of the plane _x and the thickness _y of the anisotropic materials. For an alumina silica blanket (=125 kg·m^–3), the thermal conductivities _h , _x , and _y were measured in the temperature range between –140 and 300°C using the transient hot-wire method for _h and the cyclic heat method for _x and _y . In the same way, the thermal conductivities _h , _x , and _y of a rock wool (=121 kg·m^–3) insulation were also measured in the temperature range, 100 to 600°C. From a comparison of the measured results with the estimated values of _x , it is confirmed that the proposed method can estimate the measured values reasonably well.     

霜层综合导热系数的研究

提出了霜层综合导热系数概念。认为霜层导热系数除了应考虑霜层结构复杂性还应计及蒸汽凝华潜热释放的影响。从霜层能量平衡分析出发，导出霜层综合导热系数是霜层有效导热系数和蒸汽有效导热系数之和。据此计算的霜层综合导热系数与实验数据符合较好。     

热障涂层热导率的研究进展

简要回顾了热障涂层体系的发展,讨论了氧化锆陶瓷材料的传热规律,包括涂层微观结构、陶瓷成分等因素的影响.同时指出了改进陶瓷涂层热导率的方法和开发适用于更高温度下的陶瓷涂层材料的指导原则,并详细介绍了改善热障涂层热导率的研究现状.     

具有低热导率的Skutterudite类新型热电材料

结合声子散射机制介绍了近来报道的几种降低skutterudite类热电材料热导率的途径，为研究和发展skutterudite类热电材料提供一些思路和方法。     

In‐Plane Anisotropic Thermal Conductivity of Few‐Layered Transition Metal Dichalcogenide Td‐WTe2

2D Td‐WTe₂ has attracted increasing attention due to its promising applications in spintronic, field‐effect chiral, and high‐efficiency thermoelectric devices. It is known that thermal conductivity plays a crucial role in condensed matter devices, especially in 2D systems where phonons, electrons, and magnons are highly confined and coupled. This work reports the first experimental evidence of in‐plane anisotropic thermal conductivities in suspended Td‐WTe₂ samples of different thicknesses, and is also the first demonstration of such anisotropy in 2D transition metal dichalcogenides. The results reveal an obvious anisotropy in the thermal conductivities between the zigzag and armchair axes. The theoretical calculation implies that the in‐plane anisotropy is attributed to the different mean free paths along the two orientations. As thickness decreases, the phonon‐boundary scattering increases faster along the armchair direction, resulting in stronger anisotropy. The findings here are crucial for developing efficient thermal management schemes when engineering thermal‐related applications of a 2D system.