High integration development of electronics requires materials possessing excellent thermal conductivity, electromagnetic interference (EMI) shielding, and electrical insulation. In this work, Fe2O3 particles are deposited on carbon fibers (CF) and then utilized as fillers ([email protected]2O3) in boron nitride/silicone rubber (BN/SR) to fabricate sandwich structured [email protected]2O3/(BN/SR) composites, herein, BN/SR as top & substrate layer, and [email protected]2O3 as middle layer. Orientation of BN in [email protected]2O3/(BN/SR) composites realizes excellent in-plane thermal conductivity coefficient (λ∥), and the core-sheath structure of [email protected]2O3 achieves good EMI shielding performance by the “absorption-reflection (transmittance)-reabsorption” process of electromagnetic waves, insulation modification of CF and the sandwich structure strengthen the electrical insulation. When the amount of BN and [email protected]2O3 are 20.6 wt% and 45.5 wt%, respectively, the λ∥, EMI shielding effectiveness, volume resistance and breakdown strength of [email protected]2O3/(BN/SR) composites reach 3.86 W/(m·K), 37.7 dB, 6.2 × 1014 Ω cm and 26.8 kV/mm, respectively, which are all higher than those of commonly fabricated CF/(BN/SR) composites with same amount of BN and CF (3.83 W/(m·K), 19.4 dB, 8.6 × 1013 Ω cm and 21.4 kV/mm). [email protected]2O3/(BN/SR) composites possess better cooling effect (5.6 °C) than that of commercial silicon grease (QM850) on the testing platform of computer's central processing unit, whose functions are more abundant and have wide application prospects in electronics. 

Flexible and insulating silicone rubber composites with sandwich structure for thermal management and electromagnetic interference shielding

We report on the acoustic attenuation in an AlPdMn icosahedral quasicrystal between 15 and 300 K, in the tens of GHz range. High frequency coherent longitudinal acoustic waves are excited and detected by an optical ``pump and probe'' technique, in an interferometric configuration. Our results reveal that, at high temperature $(Tg100\mathrm{K}),$ the acoustic attenuation is rather small, close to the attenuation in crystals and much smaller than in disordered solids. We stress that Akhieser processes (phonon-phonon interactions) must be active in quasicrystals and give the correct behavior and order of magnitude of the acoustic attenuation, at high temperature. At low temperature, departure from the standard behavior arising from phonon-phonon interaction is not understood.

Ultrasonic attenuation in a quasicrystal studied by picosecond acoustics as a function of temperature and frequency

Copper alloy–tungsten (CuA-W) material combination offers high-temperature damage resistance and excellent plasma radiation resistance. At present, there is a lack of knowledge on laser-based powder bed fusion (LPBF) additive manufacturing of CuA-W bimetallic structures. Here we show the influence of the build strategies (including directly bonding, using CuA-W powder mixture, and using the stainless steel (SS) interlayer) on the bonding of CuA to W. The experimental results show that pure CuA became spherical on the surface of solid W. The powder layers composed of the CuA-W mixture are thermally deformed after printing several layers. Using a SS transition layer between CuA and W, good bonding between the materials is achieved due to solid-state diffusion and grain boundary diffusion. We have also found that if the melting points of the two materials are significantly different, the printing of the high melting point material first will most likely make the material interface unmelted in multi-material LPBF. The material combination must have good wettability and a high element solid-state diffusion coefficient to achieve good bonding in such a condition. This study would be helpful for future investigation on multi-material AM involving a metallic material combination with significant difference in their melting points. 

Multi-material additive-manufacturing of tungsten - copper alloy bimetallic structure with a stainless-steel interlayer and associated bonding mechanisms

Rapid advancements in nanotechnologies for energy conversion and transport applications urgently require a further understanding of interfacial thermal transport and enhancement of interfacial thermal conductance (ITC). Sandwiching an intermediate material between two materials has led to a new insight on enhancing electron and phonon transportation, and is enabling the possible regulation of ITC, which has attracted increasing interest over the past decades. Herein, this strategy is named as thermal bonding, following the terminology of mechanical bonding and electrical bonding in microelectronics. The authors systematically summarize the interfacial thermal transport enhanced by thermal bonding from the category of interfacial thermal transport mechanisms, thermal bonding materials (TBMs), related applications, and potential challenges. Especially, the influence factors of interfacial heat transfer are highlighted, from three points, including interaction forces (van der Waals force, hydrogen bond, covalent bond, and metal bond), electron density, and phonon vibration density of states. The authors also outline and classify TBMs into metal bonding materials, organic bonding materials, and inorganic nonmetal bonding materials. As a novel and promising interface science and engineering field, it is believed that thermal bonding strategy will provide scientists and engineers more inspiration to understand interfacial thermal transport and solve the interfacial thermal challenges.

Bonding‐Enhanced Interfacial Thermal Transport: Mechanisms, Materials, and Applications

Controlling heat flow is a key challenge for applications ranging from thermal management in electronics to energy systems, industrial processing, and thermal therapy. However, progress has generally been limited by slow response times and low tunability in thermal conductance. In this work, we demonstrate an electronically gated solid-state thermal switch using self-assembled molecular junctions to achieve excellent performance at room temperature. In this three-terminal device, heat flow is continuously and reversibly modulated by an electric field through carefully controlled chemical bonding and charge distributions within the molecular interface. The devices have ultrahigh switching speeds above 1 megahertz, have on/off ratios in thermal conductance greater than 1300%, and can be switched more than 1 million times. We anticipate that these advances will generate opportunities in molecular engineering for thermal management systems and thermal circuit design. 

Electrically gated molecular thermal switch

Thermal boundary conductance is typically positively correlated with interfacial adhesion at the interface. Here, we demonstrate a counterintuitive experimental result in which a weak van der Waals interface can give a higher thermal boundary conductance than a strong covalently bonded interface. This occurs in a system with highly mismatched vibrational frequencies (copper/diamond) modified by a self-assembled monolayer. Using finely controlled fabrication and detailed characterization, complemented by molecular simulation, the effects of bridging the vibrational spectrum mismatch and bonding at the interface are systematically varied and understood from a molecular dynamics viewpoint. The results reveal that the bridging and binding effects have a trade-off relationship and, consequently, that the bridging can overwhelm the binding effect at a highly mismatched interface. This study provides a comprehensive understanding of phonon transport at interfaces, unifying physical and chemical understandings, and allowing interfacial tailoring of the thermal transport in various material systems.

/pdf/weaker-bonding-can-give-larger-thermal-conductance-at-highly-xk2dx6pgqy.pdf

Weaker bonding can give larger thermal conductance at highly mismatched interfaces

Thermoelectric materials are capable of directly converting heat into electricity by the Seebeck effect, which are applicable to a wide range of industrial applications. For semiconductor thermoelectrics, one of the most effective ways to enhance their performance is to use nanostructures to significantly reduce their lattice thermal conductivity, which is governed by lattice vibrations referred to as phonons, without dramatically sacrificing their electrical properties. Hence, the transport of phonons in nanostructured thermoelectrics is of crucial importance. This chapter stresses the basic concepts and fundamental principles of the transport of phonons in nanostructured materials. Starting with the concepts of phonons and phonon Boltzmann transport equation, we can derive the phonon gas model for thermal conductivity. Various scattering mechanisms of phonons that affect the thermal conductivity in nanostructured materials, well-established theoretical and experimental methods used for the investigation of phonon transport, and examples in terms of manipulating phonons with nanostructures in the field of thermoelectrics are described.

Thermal transport by phonons in thermoelectrics

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Weaker bonding can give larger thermal conductance at highly mismatched interfaces

Thermal transport by phonons in thermoelectrics