Ultrahigh Thermal Conductivity Research Articles

Temperature dependent elastic constants and thermodynamic properties of BAs: An ab initio investigation

We present an ab initio study of the temperature dependent elastic constants of boron arsenide, a semiconductor that exhibits ultra-high thermal conductivity and is under investigation for thermal management in electronics. We test the consistency of our predictions by computing the temperature dependent sound velocity of the longitudinal acoustic mode along the [111] direction and comparing with experiments. Furthermore, as a by-product, we present the room temperature phonon dispersions and the temperature dependent thermal expansion, isobaric heat capacity, and average Grüneisen parameter compared with the most updated experiments and previous calculations when available. Finally, we present the theoretical estimate of the temperature dependent mean square atomic displacements.

Fabrication of Cu/graphite film/Cu sandwich composites with ultrahigh thermal conductivity for thermal management applications

Effective thermal management of electronic integrated devices with high powder density has become a serious issue, which requires materials with high thermal conductivity (TC). In order to solve the problem of weak bonding between graphite and Cu, a novel Cu/graphite film/Cu sandwich composite (Cu/GF/Cu composite) with ultrahigh TC was fabricated by electro-deposition. The micro-riveting structure was introduced to enhance the bonding strength between graphite film and deposited Cu layers by preparing a rectangular array of micro-holes on the graphite film before electro-deposition. TC and mechanical properties of the composites with different graphite volume fractions and current densities were investigated. The results showed that the TC enhancement generated by the micro-riveting structure for Cu/GF/Cu composites at low graphite content was more effective than that at high graphite content, and the strong texture orientation of deposited Cu resulted in high TC. Under the optimizing preparing condition, the highest in-plane TC reached 824.3 W·m−1K−1, while the ultimate tensile strength of this composite was about four times higher than that of the graphite film.

Uniaxially stretched polyethylene/boron nitride nanocomposite films with metal-like thermal conductivity

Achieving ultra-high thermal conductivity while maintaining the electrical insulation of polymers is highly desirable for many applications such as thermal management and packaging. In this work, polyethylene/boron nitride nanoplatelets (PE/BNNP) nanocomposite film was produced through melt processing followed by uniaxial stretching. Microstructural analysis reveals that the stretched composite film features a co-continuous network structure which consists of oriented lamellae bridged by both stretched polymer chains and aligned BNNPs along the stretching direction. The resulting film exhibit a metal-like thermal conductivity as high as 106 W m−1 K−1 and it is believed that the unique network structure has enabled efficient phonon transfer across the film, resulting in superior thermal transporting performance. This work shines a light on the design and scalable manufacturing of high performance functional polymer-based composites for future thermal management applications.

Optical properties of cubic boron arsenide

The ultrahigh thermal conductivity of cubic boron arsenide (BAs) makes it a promising material for next-generation electronics and optoelectronics. Here, we report measured optical properties of BAs crystals, including the complex dielectric function, refractive index, and absorption coefficient in the ultraviolet, visible, and near-infrared wavelength range. The data were collected at room temperature using spectroscopic ellipsometry and transmission and reflection spectroscopy. We further calculated the optical response using density functional theory and many-body perturbation theory, considering quasiparticle and excitonic corrections. The computed values for the direct and indirect bandgaps (4.25 eV and 2.07 eV) agree well with the measured results (4.12 eV and 2.02 eV). Our findings pave the way for using BAs in future electronic and optoelectronic applications that take advantage of its demonstrated ultrahigh thermal conductivity and predicted high ambipolar carrier mobility.

Photoluminescence mapping and time-domain thermo-photoluminescence for rapid imaging and measurement of thermal conductivity of boron arsenide

Cubic boron arsenide (BAs) is attracting greater attention owing to the recent experimental demonstration of ultrahigh thermal conductivity κ higher than 1000 W/m·K. However, its bandgap has not been settled and a simple yet effective method to probe its crystal quality is missing. Furthermore, traditional κ measurement methods are destructive and time consuming, thus they cannot meet the urgent demand for fast screening of high κ materials. After we experimentally established 1.82 eV as the indirect bandgap of BAs and observed room-temperature band-edge photoluminescence, we developed two new optical techniques that can provide rapid and non-destructive characterization of κ with little sample preparation: photoluminescence mapping (PL-mapping) and time-domain thermo-photoluminescence (TDTP). PL-mapping provides nearly real-time image of crystal quality and κ over mm-sized crystal surfaces; while TDTP allows us to pick up any spot on the sample surface and measure its κ using nanosecond laser pulses. These new techniques reveal that the apparent single crystals are not only non-uniform in κ but also are made of domains of very distinct κ. Because PL-mapping and TDTP are based on the band-edge PL and its dependence on temperature, they can be applied to other semiconductors, thus paving the way for rapid identification and development of high-κ semiconducting materials.

Advanced flexible rGO-BN natural rubber films with high thermal conductivity for improved thermal management capability

Driven by increasingly severe cooling issue of modern electronic devices, the design of heat-dissipation films with ultrahigh in-plane thermal conductivity (TC) has achieved considerable development. However, the problems of the poor flexibility and low out-plane TC remain outstanding, significantly restricting the large-scale application of the films. Herein, we developed a novel GO-assisted gelation method combined with facile hot compression to fabricate the highly flexible rGO-BN-NR composite films with an outstanding heat dissipating performance. The as-prepared films, at a BN loading of 250phr, demonstrated both superior in-plane TC (16.0 W/(m·K)) and impressive elongation at break (113%). In addition, the composite films also exhibited excellent flame-retardant ability and pronounced antistatic performance. More importantly, to accommodate the applied characteristics of thermal interface materials, the orienting direction of the composites can be easily transferred to achieve a high vertical TC. The strong cooling capability of the rGO-BN-NR composite films was directly certified by thermal infrared imaging and finite element simulation, indicating a broad and bright application for thermal management in a variety of emerging electronic devices.

Self-driven Liquid Metal Mollusk Oscillations

Abstract Room temperature liquid metal (galinstan, made up of gallium, indium and tin) is driven to oscillate by the phase change of water in the self-excited oscillating heat pipe (OHP) charged with liquid metal and water. The cross section of the channel is square with the dimension of 3×3 mm2. Liquid metal is a high-performance coolant with ultrahigh thermal conductivity (nearly 27 times higher than water). During the operation of OHP, liquid metal is pushed up by the vapor expansion in the evaporator section, and moves back by the vapor contraction and gravity effects. Then, liquid metal is driven to oscillate continuously between evaporator and condenser in the OHP. Surface tension of galinstan is 10 times higher than water, resulting in a ball shape of liquid metal in the channels. And there is a thin water film existing between liquid metal and inner surface. During the oscillating motion, liquid metal presents the characteristics of a mollusk. Soft liquid metal is extended into metal filament, or breaks up into small liquid metal balls. Then, small liquid metal balls coalesce into a long liquid metal slug, resulting in surface waves due to the released surface energy during the coalescence process. The deformation and deformation recovery of the liquid metal is the result of competition between deforming pressure forces and the reforming surface tension forces. Oscillations of liquid metal will generate flow field disturbance in the base fluid of water and further enhance the heat transfer performance in two-phase flow systems. [This research work was supported by the National Natural Science Foundation of China under Grant No. 21606034 and Office of Naval Research under Grant No. N00014-19-1-2006.]

Advanced Materials for Heat Energy Transfer, Conversion, Storage and Utilization

Advanced Materials for Heat Energy Transfer, Conversion, Storage and Utilization

A cool way to use isotopes

Thermal Conductivity Thermal management of electronics requires materials that can efficiently remove heat. Several promising materials have been found recently, but diamond remains the bulk material with the highest thermal conductivity. Chen et al. found that isotopically pure cubic boron nitride has an ultrahigh thermal conductivity, 75% that of diamond. Using only boron-11 or boron-10 allows the crystal vibrations that carry heat to move more efficiently through the material. This property could be exploited for better regulating the temperature of high-power devices. Science , this issue p. [555][1] [1]: /lookup/doi/10.1126/science.aaz6149

Boosting the Heat Dissipation Performance of Graphene/Polyimide Flexible Carbon Film via Enhanced Through-Plane Conductivity of 3D Hybridized Structure.

The development of materials with efficient heat dissipation capability has become essential for next-generation integrated electronics and flexible smart devices. Here, a 3D hybridized carbon film with graphene nanowrinkles and microhinge structures by a simple solution dip-coating technique using graphene oxide (GO) on polyimide (PI) skeletons, followed by high-temperature annealing, is constructed. Such a design provides this graphitized GO/PI (g-GO/PI) film with superflexibility and ultrahigh thermal conductivity in the through-plane (150 ± 7 W m-1 K-1 ) and in-plane (1428 ± 64 W m-1 K-1 ) directions. Its superior thermal management capability compared with aluminum foil is also revealed by proving its benefit as a thermal interface material. More importantly, by coupling the hypermetallic thermal conductivity in two directions, a novel type of carbon film origami heat sink is proposed and demonstrated, outperforming copper foil in terms of heat extraction and heat transfer for high-power devices. The hypermetallic heat dissipation performance of g-GO/PI carbon film not only shows its promising application as an emerging thermal management material, but also provides a facile and feasible route for the design of next-generation heat dissipation components for high-power flexible smart devices.

A 3D interconnected Cu network supported by carbon felt skeleton for highly thermally conductive epoxy composites

Effective heat removal through polymer composites has become a crucial issue in thermal management. Constructing a 3D interconnected filler network in polymer matrix is generally considered to be one of the most effective strategies for enhancing thermal conductivity of polymer-based composites. Herein, a unique 3D interconnected Cu network was successfully fabricated to enhance the thermal transfer properties of epoxy composites. We used Carbon felt (CFelt) as 3D skeleton to electroplate Cu on the CFelt surface for constructing the 3D Cu film network, which can serve as continuous heat conductive highways. The highly thermally conductive epoxy composites were then prepared by impregnating epoxy resin into the 3D Cu-plated CFelt (Cu-CFelt) network. Taking advantage of the 3D interconnected Cu heat transfer channel, an ultrahigh thermal conductivity of 30.69 W m−1 K−1 was achieved, which exhibited nearly 140 times higher than that of pure epoxy (0.22 W m−1 K−1) and 110 times higher than that of CFelt/epoxy composite (0.28 W m−1 K−1). And our composite exhibited outstanding thermal behaviors during heating and cooling processes. Additionally, the obtained composite maintained good mechanical properties and presented superior electrical conductivity of 7.49 × 104 S cm−1. Our work reveals that the novel 3D Cu-CFelt/epoxy composites have strong application prospects in thermal management materials.

Diffusion behaviors of energy and momentum for 1D single-walled carbon nanotubes

The single-walled carbon nanotube (CNT) is a quasi-1D material with ultra high thermal conductivity. It is known that the heat conduction is closely related with the energy diffusion and the information of heat conduction can be obtained by examining the energy diffusion behavior. The correlated energy distribution CE(i,t) calculated in diffusion process gives the whole spatial distribution of energy spreading at every correlation time which is much better than the traditional heat conduction method where only a number of thermal conductivity is provided. In previous study the anomalous super-diffusion of energy has been reported for CNT via a non-equilibrium diffusion method. Here we apply the more sophisticate equilibrium diffusion method to investigate the energy diffusion behavior in CNT with much longer length up to m with much higher accuracy. At room temperature, the super-diffusion of energy described by the mean square displacement of energy is found to be not far away from ballistic diffusion which is consistent with the experimentally measured ultra high thermal conductivity for CNT. With the equilibrium diffusion method, the diffusion of momentum has also been performed for CNT and ballistic diffusion is found which can be viewed as a support for anomalous super energy diffusion. The momentum diffusion can also give accurate information of sound velocity which is determined as m s−1 at room temperature. In particular, the very weak temperature coefficient of sound velocity can be obtained as m (. The normalized temperature coefficient of sound velocity can be related with the normalized temperature coefficient of Raman frequency shifts experimentally measured as . The obtained value via the momentum diffusion method is consistent with the experimental measurements.

Interfacial Thermal Conductance across Room-Temperature-Bonded GaN/Diamond Interfaces for GaN-on-Diamond Devices.

The wide bandgap, high-breakdown electric field, and high carrier mobility makes GaN an ideal material for high-power and high-frequency electronics applications, such as wireless communication and radar systems. However, the performance and reliability of GaN-based high-electron-mobility transistors (HEMTs) are limited by the high channel temperature induced by Joule heating in the device channel. Integration of GaN with high thermal conductivity substrates can improve the heat extraction from GaN-based HEMTs and lower the operating temperature of the device. However, heterogeneous integration of GaN with diamond substrates presents technical challenges to maximize the heat dissipation potential brought by the ultrahigh thermal conductivity of diamond substrates. In this work, two modified room-temperature surface-activated bonding (SAB) techniques are used to bond GaN and single-crystal diamond. Time-domain thermoreflectance (TDTR) is used to measure the thermal properties from room temperature to 480 K. A relatively large thermal boundary conductance (TBC) of the GaN/diamond interfaces with a ∼4 nm interlayer (∼90 MW/(m2 K)) was observed and material characterization was performed to link the interfacial structure with the TBC. Device modeling shows that the measured TBC of the bonded GaN/diamond interfaces can enable high-power GaN devices by taking full advantage of the ultrahigh thermal conductivity of single-crystal diamond. For the modeled devices, the power density of GaN-on-diamond can reach values ∼2.5 times higher than that of GaN-on-SiC and ∼5.4 times higher than that of GaN-on-Si with a maximum device temperature of 250 °C. Our work sheds light on the potential for room-temperature heterogeneous integration of semiconductors with diamond for applications of electronics cooling, especially for GaN-on-diamond devices.

Boron arsenide heterostructures: lattice-matched heterointerfaces and strain effects on band alignments and mobility

BAs is a III–V semiconductor with ultra-high thermal conductivity, but many of its electronic properties are unknown. This work applies predictive atomistic calculations to investigate the properties of BAs heterostructures, such as strain effects on band alignments and carrier mobility, considering BAs as both a thin film and a substrate for lattice-matched materials. The results show that isotropic biaxial in-plane strain decreases the band gap independent of sign or direction. In addition, 1% biaxial tensile strain increases the in-plane electron and hole mobilities at 300 K by >60% compared to the unstrained values due to a reduction of the electron effective mass and of hole interband scattering. Moreover, BAs is shown to be nearly lattice-matched with InGaN and ZnSnN2, two important optoelectronic semiconductors with tunable band gaps by alloying and cation disorder, respectively. The results predict type-II band alignments and determine the absolute band offsets of these two materials with BAs. The combination of the ultra-high thermal conductivity and intrinsic p-type character of BAs, with its high electron and hole mobilities that can be further increased by tensile strain, as well as the lattice-match and the type-II band alignment with intrinsically n-type InGaN and ZnSnN2 demonstrate the potential of BAs heterostructures for electronic and optoelectronic devices.

Ultrahigh thermal conductivity in isotope-enriched cubic boron nitride.

Materials with high thermal conductivity (κ) are of technological importance and fundamental interest. We grew cubic boron nitride (cBN) crystals with controlled abundance of boron isotopes and measured κ greater than 1600 watts per meter-kelvin at room temperature in samples with enriched 10B or 11B. In comparison, we found that the isotope enhancement of κ is considerably lower for boron phosphide and boron arsenide as the identical isotopic mass disorder becomes increasingly invisible to phonons. The ultrahigh κ in conjunction with its wide bandgap (6.2 electron volts) makes cBN a promising material for microelectronics thermal management, high-power electronics, and optoelectronics applications.

Ultra-high lattice thermal conductivity and the effect of pressure in superhard hexagonal BC2N

Using first-principles methods to calculate thermomechanical properties of BC2N, we investigate the effect of pressure on its high thermal conductivity and show that its thermal expansion matches that of Si, making it a good candidate as a heat sink for electronic devices.

High thermal conductivity driven by the unusual phonon relaxation time platform in 2D monolayer boron arsenide.

The cubic boron arsenide (BAs) crystal has received extensive research attention because of its ultra-high thermal conductivity comparable to that of diamond. In this work, we performed a comprehensive study on the diffusive thermal properties of its two-dimensional (2D) counterpart, the monolayer honeycomb BAs (h-BAs), through solving the phonon Boltzmann transport equation combined with first-principles calculation. We found that unlike the pronounced contribution from out-of-plane acoustic phonons (ZA) in graphene, the high thermal conductivity (181 W m−1 K−1 at 300 K) of h-BAs is mainly contributed by in-plane phonon modes, instead of the ZA mode. This result is explained by the unique frequency-independent ‘platform’ region in the relaxation time of in-plane phonons. Moreover, we conducted a comparative study of thermal conductivity between 2D h-BAs and h-GaN, because both of them have a similar mass density. The thermal conductivity of h-BAs is one order of magnitude higher than that of h-GaN (16 W m−1 K−1), which is governed by the different phonon scattering process attributed to the opposite wavevector dependence in out-of-plane optical phonons. Our findings provide new insight into the physics of heat conduction in 2D materials, and demonstrate h-BAs to be a new thermally conductive 2D semiconductor.

Ultrahigh Thermal Conductivity of Interface Materials by Good Wettability of Indium and Diamond

Ultrahigh Thermal Conductivity of Interface Materials by Good Wettability of Indium and Diamond

High Thermal Conductivity and Anisotropy Values of Aligned Graphite Flakes/Copper Foil Composites.

Much attention has been paid to graphite flakes/copper (GFs/Cu) composites for thermal management due to their remarkable thermal properties. Most studies focus on the interface interaction between GFs and Cu in composites. However, controlling the orientation of GFs still remains a challenge. Herein, we report a reliable method to ensure consistent orientation of GFs in the composites. Firstly, the disorder GFs were well arranged on the surface of copper foil by tape casting process in the casting machine. Then highly aligned GFs/Cu composites were fabricated by hot pressing process in a vacuum hot-pressing furnace, with the volume fraction of graphite from 30% to 70%. The SEM images show that the obtained GFs/Cu composites presented a layer-by-layer structure or network structure with a different content of GFs. The thermal conductivity of GFs/Cu composites exhibited an extreme anisotropy due to the highly aligned GFs. The ultrahigh thermal conductivity of GFs/Cu composites with 70 vol% GFs reached 741 W/(m·K), while through-plane thermal conductivity was just 42 W/(m·K). The alignment of GFs and interfacial thermal resistance were deeply analyzed and a thermal conductivity model for GFs/Cu composites was established. Our work provides a new idea to significantly enhance the thermal transportation performance of GFs/Cu composites by well controlled alignment of GFs in Cu matrix.

Self-Assembled Boron Nitride Nanotube Reinforced Graphene Oxide Aerogels for Dielectric Nanocomposites with High Thermal Management Capability.

Thermally conductive polymeric composites are highly promising in current energy devices such as light-emitting diodes, integrated circuits, and solar cells to achieve appropriate thermal management. However, the introduction of traditional thermoconductive fillers into a polymer usually results in low thermal conductivity enhancement. Here, an ideal dielectric epoxy nanocomposite with ultrahigh thermal conductivity is successfully fabricated using three-dimensional interconnected boron nitride nanotube reinforced graphene oxide nanosheet (3D-BNNT-GONS) aerogels as fillers. The nanocomposite exhibits a nearly 20-fold increase in thermal conductivity with only 11.6 vol % loading fraction. Meanwhile, the nanocomposite possesses excellent insulation performance, including low dielectric constant, low dielectric loss, and high breakdown strength. A heating and cooling process reveals that the nanocomposite has a fast response of surface temperature, indicating high thermal management capability.