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Highly vertically thermally conductive silicon rubber (SiR) composites are widely used as thermal interface materials (TIMs) for chip cooling. Herein, inspired by water transport and transpiration of Moso bamboo-forests extensively existing in south China, and guided by filler self-assembly simulation, bamboo-forest-like heat conduction networks, with bamboo-stems-like vertically aligned polydopamine-coated carbon fibers (VA-PCFs), and bamboo-leaves-like horizontally layered Al2O3(HL-Al2O3), are rationally designed and constructed. VA-PCF/HL-Al2O3/SiR composites demonstrated enhanced heat conduction properties, and their through-plane thermal conductivity and thermal diffusivity reached 6.47 W (mK)-1 and 3.98 mm2 s-1 at 12 vol% PCF and 4 vol% Al2O3 loadings, which are 32% and 38% higher than those of VA-PCF (12 vol%) /SiR composites, respectively. The heat conduction enhancement mechanisms of VA-PCF/HL-Al2O3 networks on their SiR composites are revealed by multiscale simulation: HL-Al2O3 bridges the separate VA-PCF heat flow channels, and transfers more heat to the matrix, thereby increasing the vertical heat flux in composites. Along with high volume resistivity, low compression modulus, and coefficient of thermal expansion, VA-PCF/HL-Al2O3/SiR composites demonstrate great application potential as TIMs, which is proven using multiphysics simulation. This work not only makes a meaningful attempt at simulation-driven biomimetic material structure design but also provides inspiration for the preparation of TIMs.
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Compact reliable structure and strong electromechanical coupling are hot pursuits in piezoelectric vibration energy harvester (PVEH) design. PVEH with a static arc stopper makes piezoelectric stress uniformly distributed and widens the frequency band by collision but wastes space. This Article proposes a hinged PVEH with two arc mass stoppers (AS-H-PVEH). Two arc stoppers as movable masses increase the vibration energy and the effective electromechanical coupling coefficient to achieve strong electromechanical coupling. AS-H-PVEH generates a 4.1 mW power output at 11.6-12.0 Hz and 0.2 g. AS-H-PVEH sustains 4 g acceleration vibration for 10 min without attenuation. To offset the resonance frequency increase caused by arc contact, we discuss the magnetic coupling, and axial force effects are discussed. The design of the arc stopper radius, nonlinear electromechanical coupling model, and system parameter identification method are presented. The displacement varied mechanical quality factor and effective electromechanical coupling coefficient are considered in the modified model for the first time. The model obtained good agreement under experiments. The power generation and driven wireless sensor performance of AS-H-PVEH was verified. This research has important theoretical and application value for the performance optimization of PVEH with an arc stopper.
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With the rapid development of various fields, including aerospace, industrial measurement and control, and medical monitoring, the need to quantify flow velocity measurements is increasing. It is difficult for traditional flow velocity sensors to fulfill accuracy requirements for velocity measurements due to their small ranges, susceptibility to environmental impacts, and instability. Herein, to optimize sensor performance, a flexible microelectromechanical system (MEMS) thermal flow sensor is proposed that combines the working principles of thermal loss and thermal temperature difference and utilizes a flexible cavity substrate made of a low-thermal-conductivity polyimide/SiO2 (PI/SiO2) composite porous film to broaden the measurement range and improve the sensitivity. The measurement results show that the maximum measurable flow velocity can reach 30 m/s with a resolution of 5.4 mm/s. The average sensitivities of the sensor are 59.49 mV/(m s-1) in the medium-to-low wind velocity range of 0-2 m/s and 467.31 mV/(m s-1) in the wind velocity range of 2-30 m/s. The sensor proposed in this work can enable new applications of flexible flow sensors and wearable devices.
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High-efficient heat dissipation plays critical role for high-power-density electronics. Experimental synthesis of ultrahigh thermal conductivity boron arsenide (BAs, 1300 W m-1K-1) cooling substrates into the wide-bandgap semiconductor of gallium nitride (GaN) devices has been realized. However, the lack of systematic analysis on the heat transfer across the GaN-BAs interface hampers the practical applications. In this study, by constructing the accurate and high-efficient machine learning interatomic potentials, we perform multiscale simulations of the GaN-BAs heterostructures. Ultrahigh interfacial thermal conductance of 260 MW m-2K-1 is achieved, which lies in the well-matched lattice vibrations of BAs and GaN. The strong temperature dependence of interfacial thermal conductance is found between 300 to 450 K. Moreover, the competition between grain size and boundary resistance is revealed with size increasing from 1 nm to 1000 µm. Such deep-potential equipped multiscale simulations not only promote the practical applications of BAs cooling substrates in electronics, but also offer approach for designing advanced thermal management systems.
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The negative Poisson's ratio (NPR) effect usually endows materials with promising ductility and shear resistance, facilitating a wider range of applications. It has been generally acknowledged that alloys show strong advantages in manipulating material properties. Thus, a thought-provoking question arises: how does alloying affect the NPR? In this paper, based on first-principles calculations, we systematically study the NPR in two-dimensional (2D) GaN and AlN, and their alloy of AlxGa1-xN. It is intriguing to find that the NPR in AlxGa1-xN is significantly enhanced compared to the parent materials of GaN and AlN. The underlying mechanism mainly originates from a counter-intuitive increase of the bond angle θ. We further study the microscopic origin of the anomalies by electron orbital analysis as well as electron localization functions. It is revealed that the distribution and movement of electrons change with the applied strain, providing a fundamental view on the effect of strain on lattice parameters and the NPR. The physical origin as revealed in this study deepens the understanding of the NPR and shed light on the future design of modern nanoscale electromechanical devices with fantastic functions based on the auxetic nanomaterials and nanostructures.
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The two-dimensional (2D) MA2Z4 family has received extensive attention in manipulating its electronic structure and achieving intriguing physical properties. However, engineering the electronic properties remains a challenge. Herein, based on first-principles calculations, we systematically investigate the effect of biaxial strains on the electronic structure of 2D Rashba MoSiGeN4 (MSGN), and further explore how the interlayer interactions affect the Rashba spin splitting (RSS) in such strained layered MSGN systems. After applying biaxial strains, the band gap decreases monotonically with increasing tensile strains but increases when the compressive strains are applied. An indirect-direct-indirect band gap transition is induced by applying a moderate compressive strain (<5%) in the MSGN systems. Due to the symmetry breaking and moderate spin-orbit coupling (SOC), the monolayer MSGN possesses an isolated RSS near the Fermi level, which could be effectively regulated to the Lifshitz-type spin splitting (LSS) by biaxial strain. For instance, the LSS â RSS â LSS transformation of the Fermi surface is presented in the monolayer and a more complex and changeable LSS â RSS â LSS â RSS evolution is observed in bilayer and trilayer MSGN systems as the biaxial strain varies from -8% to 12%, which actually depends on the appearance, variation, and vanish of the Mexican hat band in the absence of SOC under different strains. The contribution of the Mo-dz2 orbital hybridized with the N-pz orbital in the highest valence band plays a dominant role in band evolution under biaxial strains, where the RSS â LSS evolution corresponds to the decreased Mo-dz2 orbital contribution. Our study highlights the biaxial strain controllable RSS, in particular the introduction and even the evolution of LSS near the Fermi surface, which makes the strained MSGN systems promising candidates for future applications in spintronic devices.
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Anisotropic heat transfer is crucial for advanced thermal management in nanoelectronics, optoelectronics, thermoelectrics, etc. Traditional approaches modifying thermal conductivity (κ) mostly adjust the magnitude but disregard anisotropy. Herein, by solving the Boltzmann transport equation from first principles, we report κ anisotropy modulation by alloying gallium nitride (GaN) and aluminum nitride (AlN). The alloyed Al0.5Ga0.5N demonstrates reversed κ anisotropy compared to the parent materials, where the preferred thermal transport direction shifts from cross-plane to in-plane. Moreover, the κ anisotropy (κin-plane/κcross-plane) in the Al0.5Ga0.5N alloy is enhanced to 1.63 and 1.51 times that in bulk GaN and AlN, respectively, which can be further enhanced by increased temperature. Deep analysis attributes the alloying reversed κ anisotropy of Al0.5Ga0.5N to the structure distortion-driven phonon group velocity, as well as phonon anharmonicity. The alloying reversed κ anisotropy as reported in this study sheds light on future studies in advanced heat dissipation and intelligent thermal management.
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Insulating thermally conductive polymer composites are in great demand in integrated-circuit packages, for efficient heat dissipation and to alleviative short-circuit risk. Herein, the continuous oriented hexagonal boron nitride (h-BN) frameworks (o-BN@SiC) were prepared via self-assembly and in situ chemical vapor infiltration (CVI) interface welding. The insulating o-BN@SiC/epoxy (o-BN@SiC/EP) composites exhibited enhanced thermal conductivity benefited from the CVI-SiC-welded BN-BN interface. Further, multiscale simulation, combining first-principles calculation, Monte Carlo simulation, and finite-element simulation, was performed to quantitatively reveal the effect of the welded BN-BN interface on the heat transfer of o-BN@SiC/EP composites. Phonon transmission in solders and phonon-phonon coupling of filler-solder interfaces enhanced the interfacial heat transfer between adjacent h-BN microplatelets, and the interfacial thermal resistance of the dominant BN-BN interface was decreased to only 3.83 nK·m2/W from 400 nK·m2/W, plunging by over 99%. This highly weakened interfacial thermal resistance greatly improved the heat transfer along thermal pathways and resulted in a 26% thermal conductivity enhancement of o-BN@SiC/EP composites, compared with physically contacted oriented h-BN/EP composites, at 15 vol % h-BN. This systematic multiscale simulation broke through the barrier of revealing the heat transfer mechanism of polymer composites from the nanoscale to the macroscale, which provided rational cognition about the effect of the interfacial thermal resistance between fillers on the thermal conductivity of polymer composites.
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High thermal conductivity is of great interest due to the novel applications in high-performance heat dissipation for microelectronic devices. Two-dimensional (2D) materials with graphene as a representative have attracted tremendous interest due to the excellent properties, where C23is an emerging 2D allotrope of carbon with a large bandgap. In this paper, by solving the Boltzmann transport equation based onstate-of-the-artfirst-principles calculations, the C23is predicted to have an ultrahigh thermal conductivity of 2051.47 Wm-1K-1, which is on the same order of magnitude as graphene. Based on the comparative analysis among C23, graphene, and penta-graphene, it is shown that the unique spatial structure and the orbital hybridization of C23lead to weak anharmonicity, which results in the large relaxation time of phonons and finally results in ultrahigh thermal conductivity. Our study is expected to promote the comprehensive understanding of thermal transport in C23and shed light on future exploration of novel materials with high thermal conductivity.
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Reducing thermal conductivity (κ) is of great significance to lots of applications, such as thermal insulation, thermoelectrics, etc. In this study, we propose an effective approach for realizing low κ by introducing lone-pair electrons or making the lone-pair electrons stereochemically active through bond nanodesigning. By cutting at the (111) cross section of the three-dimensional cubic boron arsenide (c-BAs), the κ is lowered by more than 1 order of magnitude in the resultant two-dimensional graphene-like BAs (g-BAs). The underlying mechanism of activating lone-pair electrons is analyzed based on the comparative study on the thermal transport properties and electronic structures of g-BAs, c-BAs, graphene, and diamond (c-BAs â g-BAs vs diamond â graphene). The proposed approach for realizing low κ and the underlying mechanism uncovered in this study would largely benefit the design of advanced thermal functional materials, especially in future research involving novel materials for energy applications.
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With the development of chip technology, the density of transistors on integrated circuits is increasing and the size is gradually shrinking to the micro-/nanoscale, with the consequent problem of heat dissipation on chips becoming increasingly serious. For device applications, efficient heat dissipation and thermal management play a key role in ensuring device operation reliability. In this review, we summarize the thermal management applications based on 2D materials from both theoretical and experimental perspectives. The regulation approaches of thermal transport can be divided into two main types: intrinsic structure engineering (acting on the intrinsic structure) and non-structure engineering (applying external fields). On one hand, the thermal transport properties of 2D materials can be modulated by defects and disorders, size effect (including length, width, and the number of layers), heterostructures, structure regulation, doping, alloy, functionalizing, and isotope purity. On the other hand, strain engineering, electric field, and substrate can also modulate thermal transport efficiently without changing the intrinsic structure of the materials. Furthermore, we propose a perspective on the topic of using magnetism and light field to modulate the thermal transport properties of 2D materials. In short, we comprehensively review the existing thermal management modulation applications as well as the latest research progress, and conclude with a discussion and perspective on the applications of 2D materials in thermal management, which will be of great significance to the development of next-generation nanoelectronic devices.
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Nanoclusters like fullerenes as the unit to build intriguing two-dimensional (2D) topological structures is of great challenge. Here we propose three bridged fullerene monolayers and comprehensively investigate the novel fullerene monolayer (α-C60-2D) as synthesized experimentally [Hou et al. Nature 2022, 606, 507-510] by state-of-the-art first-principles calculations. Our results show that α-C60-2D has a direct band gap of 1.55 eV close to the experimental value, an optical linear dichroism with strong absorption in the long-wave ultraviolet region, a small anisotropic Young's modulus, a large hole mobility, and an ultrahigh Seebeck coefficient at middle-low temperatures. It is unveiled that the anisotropic optical, mechanical, electrical, and thermoelectric properties of α-C60-2D originate from the asymmetric bridging arrangements between C60 clusters. Our study promises potential applications of monolayer fullerene networks in lots of fields.
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Two-dimensional materials have attracted significant research interest due to the fantastic properties that are unique to their bulk counterparts. In this paper, from the state-of-the-art first-principles, we predicted the stable structure of a monolayer counterpart of γ-CuI (cuprous iodide) that is a p-type wide bandgap semiconductor. The monolayer CuI presents multifunctional superiority in terms of electronic, optical, and thermal transport properties. Specifically, the ultralow thermal conductivity of 0.116 W m-1 K-1 is predicted for monolayer CuI, which is much lower than those of γ-CuI (0.997 W m-1 K-1) and other typical semiconductors. Moreover, an ultrawide direct bandgap of 3.57 eV is found in monolayer CuI, which is even larger than that of γ-CuI (2.95-3.1 eV), promising for applications in nano-/optoelectronics with better optical performance. The ultralow thermal conductivity and direct wide bandgap of monolayer CuI as reported in this study would promise its potential applications in transparent and wearable electronics.
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Thermal transport plays a key role in the working stability of gallium nitride (GaN) based optoelectronic devices, where doping has been widely employed for practical applications. However, it remains unclear how doping affects thermal transport. In this study, based on first-principles calculations, we studied the doping effect on the thermal transport properties of GaN by substituting Ga with In/Al atoms. The thermal conductivities at 300 K along the in-plane(out-of-plane) directions of In- and Al-doped GaN are calculated to be 7.3(8.62) and 12.45(11.80) W m-1 K-1, respectively, which are more than one order of magnitude lower compared to that of GaN [242(239) W m-1 K-1]. From the analysis of phonon transport properties, we find that the low phonon group velocity and small phonon relaxation time dominate the degenerated thermal conductivity, which originated from the strong phonon anharmonicity of In/Al-doped GaN. Furthermore, by examining the crystal structure and electronic properties, the lowered thermal conductivity is revealed lying in the strong polarization of In-N and Al-N bonds, which is due to the large difference in electronegativity of In/Al and N atoms. The results achieved in this study have guiding significance to the thermal transport design of GaN-based optoelectronic devices.
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The two-dimensional (2D) materials, represented by graphene, stand out in the electrical industry applications of the future and have been widely studied. As commonly existing in electronic devices, the electric field has been extensively utilized to modulate the performance. However, how the electric field regulates thermal transport is rarely studied. Herein, we investigate the modulation of thermal transport properties by applying an external electric field ranging from 0 to 0.4 V Å-1, with bilayer graphene, monolayer silicene, and germanene as study cases. The monotonically decreasing trend of thermal conductivity in all three materials is revealed. A significant effect on the scattering rate is found to be responsible for the decreased thermal conductivity driven by the electric field. Further evidence shows that the reconstruction of internal electric field and generation of induced charges lead to increased scattering rate from strong phonon anharmonicity. Thus, the ultralow thermal conductivity emerges with the application of external electric fields. Applying an external electric field to regulate thermal conductivity illustrates a constructive idea for highly efficient thermal management.
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In recent years, the energy crisis and global warming have been urgent problems that need to be solved. As is known, thermoelectric (TE) materials can transfer heat energy to electrical energy without air pollution. High-throughput calculations as a novel approach are adopted by screening promising TE materials. In this paper, we use first-principles calculations combined with the semiclassical Boltzmann transport theory to estimate the TE performance of monolayer Ir2Cl2O2 according to the prediction that Ir2Cl2O2 has potential as a good TE material via high-throughput calculations. The low thermal conductivities of 1.73 and 4.68 W mK-1 of Ir2Cl2O2 along the x- and y-axes are calculated, respectively, which exhibits the strong anisotropy caused by the difference in group velocities of low-frequency phonon modes. Then, the electronic transport properties are explored, and the figure of merit ZT is eventually obtained. The maximum ZT value reaches 2.85 (0.40) along the x-axis (y-axis) at 700 K, revealing that the TE properties of the Ir2Cl2O2 monolayer are highly anisotropic. This work reveals that the anisotropic layer Ir2Cl2O2 exhibits high TE performance, which confirms that it is feasible to screen excellent TE materials via high-throughput calculations.
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The negative Poisson's ratio (NPR) is a novel property of materials, which enhances the mechanical feature and creates a wide range of application prospects in lots of fields, such as aerospace, electronics, medicine, etc. Fundamental understanding on the mechanism underlying NPR plays an important role in designing advanced mechanical functional materials. However, with different methods used, the origin of NPR is found different and conflicting with each other, for instance, in the representative graphene. In this study, based on machine learning technique, we constructed a moment tensor potential for molecular dynamics (MD) simulations of graphene. By analyzing the evolution of key geometries, the increase of bond angle is found to be responsible for the NPR of graphene instead of bond length. The results on the origin of NPR are well consistent with the start-of-art first-principles, which amend the results from MD simulations using classic empirical potentials. Our study facilitates the understanding on the origin of NPR of graphene and paves the way to improve the accuracy of MD simulations being comparable to first-principle calculations. Our study would also promote the applications of machine learning interatomic potentials in multiscale simulations of functional materials.
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With the miniaturization and integration of nanoelectronic devices, efficient heat removal becomes a key factor affecting their reliable operation. Two-dimensional (2D) materials, with high intrinsic thermal conductivity, good mechanical flexibility, and precisely controllable growth, are widely accepted as ideal candidates for thermal management materials. In this work, by solving the phonon Boltzmann transport equation (BTE) based on first-principles calculations, we investigated the thermal conductivity of novel 2D layered MSi2N4 (M = Mo, W). Our results point to a competitive thermal conductivity as large as 162 W m-1 K-1 of monolayer MoSi2N4, which is around two times larger than that of WSi2N4 and seven times larger than that of monolayer MoS2 despite their similar non-planar structures. It is revealed that the high thermal conductivity arises mainly from its large group velocity and low anharmonicity. Our result suggests that MoSi2N4 could be a potential candidate for 2D thermal management materials.
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Two-dimensional Mg2C, one of the typical representative MXene materials, is attracting lots of attention due to its outstanding properties. In this study, we find the thermal conductivity of monolayer Mg2C is more than 2 orders of magnitude lower than graphene and is even lower than MoS2 despite the relatively lighter atoms of Mg and C. Based on the comparative analysis with graphene, silicene, and MoS2, the underlying mechanism is found lying in the unique arrangement of atoms (lighter atoms in the middle plane) and large electronegativity difference in Mg2C. The phonon anharmonicity is strong due to the resonant bonding. In addition, dual band gaps emerge in the phonon dispersion of Mg2C, which limit the phonon-phonon scattering and reduce the phonon relaxation time. This study reveals a new mechanism responsible for low thermal conductivity, which would be helpful for designing thermal functional materials and pave the way for applications in thermoelectrics.