Lateral Heat Spreaders Research Articles

Two-Dimensional Hexagonal Boron Nitride Nanosheets as Lateral Heat Spreader With High Thermal Conductivity

A highly thermally conductive heat spreader for applications in electronic devices is becoming increasingly demanding, and therefore the removal of excess heat requires an efficient heat dissipating device. Boron nitride nanosheets (BNNSs) were prepared as thermally conductive fillers using hexagonal boron nitride (h-BN) powder as raw material by a water exfoliation method. A composite film was prepared by vacuum filtration using cellulose nanofibers (CNFs) as the substrate with an in-plane thermal conductivity (TC) of 82.4 W m−1 K−1, thermal conductivity enhancement increasing by 9,486% compared to pure cellulose film. Thus, CNF/BNNS composite films are promising as effective thermal interface materials (TIMs) in electronic devices and electronic component applications.

Preserving thin layer of GO for direct growth of VG using a low-etch shielded PECVD

The growth of vertical graphene (VG) on thin graphene oxide (GO) may provide a semi-insulating VG-GO. Unfortunately, the highly energetic plasma will etch out the GO template layer. In this communication, we report the use of a meshed shield to lower the physical etching on the substrate during plasma-enhanced chemical vapor deposition (PECVD). The medium-temperature of 600 °C and high-power of 300 WRF managed to grow higt h-quality VG on GO during the shielded PECVD process. GO was only slightly reduced after the process. The synthesized VG-GO film is effective as a lateral heat spreader. The heating of the peripheral heat sink was increased from 1.35 to 8.14 KW−1 with the presence of a VG-GO film. The reduction in lateral thermal resistance was better than GO and VG alone.

Millefeuille-Inspired Thermally Conductive Polymer Nanocomposites with Overlapping BN Nanosheets for Thermal Management Applications

Increasing power density makes modern electronic devices and power equipment generate excess heat, which greatly restricts the applications of polymeric materials because of their poor thermal conductivity. In the present work, inspired by the structure and production process of millefeuille cakes, we show that electrostatic spraying of boron nitride nanosheets (BNNSs) onto electrospun poly(vinyl alcohol) (PVA) nanofibers can produce highly thermally conductive, electrically insulating, flexible, and lightweight nanocomposites via a scalable method of building a multilayer PVA/BNNS nanonetwork structure. The PVA/BNNS nanocomposites exhibit an ultrahigh in-plane thermal conductivity of 21.4 W/(m·K) at 22.2 vol % BNNS addition, realized by an orientated BNNS network structure with overlapping interconnections. The BNNS networks exhibit low thermal resistance and interfacial heat scattering between BNNSs. Moreover, for heat dissipation applications, the nanocomposites with an overlapping BNNS network show higher efficiency in dissipating hot spots than randomly dispersed BNNS or directly hot-pressed BNNS composites. These PVA/BNNS nanocomposites can be used as high-performance lateral heat spreaders in next-generation thermal management systems.

Preparation of Layered Polyethylene Oxide/rGO Composite: Flexible Lateral Heat Spreaders.

In this paper, high thermal conductive polyethylene oxide (PEO)/reduced graphene oxide (rGO) composite is prepared via large-scale green reduction. Flexible layered PEO/GO composites are pre-prepared in aqueous solution. It is demonstrated that PEO chains can form hydrogen bonds with GO. Being driven by hydrogen bonds, GO/PEO composites show homogeneous and lateral highly oriented structures, resulting in excellent mechanical properties. The pre-prepared composite films are large scale soaked into ascorbic acid solution. GO nanosheets in the matrix of the composites can be reduced by ascorbic acid. The results indicate that PEO chains can repair the damage of the films caused by the reduction process. Therefore, the films can maintain their original configuration and still keep excellent flexibility. By comparison, pristine GO films are totally destroyed when the same reduction is experienced. Due to the presence of PEO, the lateral highly oriented structure of the composite will not be damaged. After reduction, the thermal conductivity of the composite reaches to 12.03 W m−1 K−1 along the rGO nanosheet oriented direction.

Highly thermal conductivity of CNF/AlN hybrid films for thermal management of flexible energy storage devices

As energy storage devices are becoming more highly integrated, it is inevitable that heat accumulation will occur under high power working conditions. Finding efficient thermal management materials for cooling down electronic components is an urgent problem for energy storage devices. In this work, a thermally conductive film with tailorable macroproperties is fabricated by using a simple vacuum filtration method, using cellulose nanofibrils as the polymer substrate and assembly with aluminum nitride nanosheets. The interaction between the cellulose nanofibrils and aluminum nitride nanosheets is studied, and the electronic components made using the composite exhibit excellent thermal conductivity, thermal stability and mechanical flexibility. A high thermal conductivity is achieved along the film surface (up to 4.20 W/mK for 25 wt.% of aluminum nitride). This green material can effectively promote potential applications as lateral heat spreaders in flexible energy storage devices and the thermal conductivity may facilitate the applications in thermal management.

A Paper-Like Inorganic Thermal Interface Material Composed of Hierarchically Structured Graphene/Silicon Carbide Nanorods.

With the increasing integration of devices in electronics fabrication, there are growing demands for thermal interface materials (TIMs) with high through-plane thermal conductivity for efficiently solving thermal management issues. Graphene-based papers consisting of a layer-by-layer stacked architecture have been commercially used as lateral heat spreaders; however, they lack in-depth studies on their TIM applications due to the low through-plane thermal conductivity (<6 W m-1 K-1). In this study, a graphene hybrid paper (GHP) was fabricated by the intercalation of silicon source and the in situ growth of SiC nanorods between graphene sheets based on the carbothermal reduction reaction. Due to the formation of covalent C-Si bonding at the graphene-SiC interface, the GHP possesses a superior through-plane thermal conductivity of 10.9 W m-1 K-1 and can be up to 17.6 W m-1 K-1 under packaging conditions at 75 psi. Compared with the current graphene-based papers, our GHP has the highest through-plane thermal conductivity value. In the TIM performance test, the cooling efficiency of the GHP achieves significant improvement compared to that of state-of-the-art thermal pads. Our GHP with characteristic structure is of great promise as an inorganic TIM for the highly efficient removal of heat from electronic devices.

Highly Thermally Conductive Composite Films Based on Nanofibrillated Cellulose in Situ Coated with a Small Amount of Silver Nanoparticles.

In this paper, a freestanding flexible nanofibrillated cellulose (NFC)/silver (Ag) composite film with high thermal conductivity (TC) was prepared using the NFC that was in situ coated with a small amount of Ag nanoparticles through mussel-inspired chemistry of dopamine. The results demonstrated that Ag nanoparticles were homogeneously coated on the surface of NFC nanofibers and their incorporation had little influence on the film-forming ability of NFC. The NFC decorated with Ag nanoparticles could easily form thermally conductive pathways in the composite films, and the resultant films containing only 2.0 vol % of Ag showed a high in-plane TC value of 6.0 W/(m·K), which was 4 times that of pure NFC film. Moreover, the composite films exhibited relatively high strength and flexibility. The highly thermally conductive NFC/Ag composite films possess potential applications as lateral heat spreaders in flexible electronic equipment.

Significantly reduced c-axis thermal diffusivity of graphene-based papers

Owing to their very high thermal conductivity as well as large surface-to-volume ratio, graphene-based films/papers have been proposed as promising candidates of lightweight thermal interface materials and lateral heat spreaders. In this work, we study the cross-plane (c-axis) thermal conductivity (kc) and diffusivity (αc) of two typical graphene-based papers, which are partially reduced graphene paper (PRGP) and graphene oxide paper (GOP), and compare their thermal properties with highly-reduced graphene paper and graphite. The determined αc of PRGP varies from (1.02 ± 0.09) × 10−7 m2 s−1 at 295 K to (2.31 ± 0.18) × 10−7 m2 s−1 at 12 K. This low αc is mainly attributed to the strong phonon scattering at the grain boundaries and defect centers due to the small grain sizes and high-level defects. For GOP, αc varies from (1.52 ± 0.05) × 10−7 m2 s−1 at 295 K to (2.28 ± 0.08) × 10−7 m2 s−1 at 12.5 K. The cross-plane thermal transport of GOP is attributed to the high density of functional groups between carbon layers which provide weak thermal transport tunnels across the layers in the absence of direct energy coupling among layers. This work sheds light on the understanding and optimizing of nanostructure of graphene-based paper-like materials for desired thermal performance.

Significant Enhancement of Thermal Conductivity in Nanofibrillated Cellulose Films with Low Mass Fraction of Nanodiamond.

High thermal conductive nanofibrillated cellulose (NFC) hybrid films based on nanodiamond (ND) were fabricated by a facile vacuum filtration technique. In this issue, the thermal conductivity (TC) on the in-plane direction of the NFC/ND hybrid film had a significant enhancement of 775.2% at a comparatively low ND content (0.5 wt %). The NFC not only helps ND to disperse in the aqueous medium stably but also plays a positive role in the formation of the hierarchical structure. ND could form a thermal conductive pathway in the hierarchical structures under the intermolecular hydrogen bonds. Moreover, the hybrid films composed of zero-dimensional ND and one-dimensional NFC exhibit remarkable mechanical properties and optical transparency. The NFC/ND hybrid films possessing superior TC, mechanical properties, and optical transparency can open applications for portable electronic equipment as a lateral heat spreader.

Thermal conduction across a boron nitride and SiO2 interface

The need for efficient heat removal and superior thermal conduction in nano/micro devices has triggered tremendous studies in low-dimensional materials with high thermal conductivity. Hexagonal boron nitride (h-BN) is believed to be one of the candidates for thermal management and heat dissipation due to its novel physical properties, i.e. as a thermal conductor and electrical insulator. Here we report the interfacial thermal resistance between few-layer h-BN and its SiO2 substrate using the differential 3ω method. The measured interfacial thermal resistance is around ~1.6 × 10−8 m2K W−1 for monolayer h-BN and ~3.4 × 10−8 m2K W−1 for 12.8 nm-thick h-BN in metal/h-BN/SiO2 interfaces. Our results suggest that the voids and gaps between the substrate and thick h-BN flakes limit the interfacial thermal conduction. This work provides a deeper understanding of utilizing h-BN flakes as a lateral heat spreader in electronic and optoelectronic nano/micro devices with further miniaturization and integration.

Layered nanofibrillated cellulose hybrid films as flexible lateral heat spreaders: The effect of graphene defect

The effect of graphene defects on the performance of polymer-based material was investigated to obtain thermal conductive and strong materials for various commercial portable electronics. The flexible lateral heat hybrid films based on graphene sheets (GSs) and nanofibrillated cellulose (NFC) were fabricated via a vacuum-assisted self-assembly technique. The hybrid film with low-defect density GSs exhibited high thermal conductivity (TC) of 6.75 W m−1 K−1 at 10 wt % GS loading. Compared with pure NFC film, the hybrid film showed excellent enhancement of 501.6%, which was 1.5× times higher than those with high-defect density GSs. The relationship between defect density of GSs and TC of hybrid films was consistent with the published simulation results. Meanwhile, the hybrid film with low-defect density GSs also exhibited higher toughness, tensile strength and Young's modulus than those with high-defect density GSs. NFC/GS hybrid films With high TC and mechanical properties can be used as a potential candidate for flexible lateral heat spreaders.

Two-dimensional hexagonal boron nitride as lateral heat spreader in electrically insulating packaging

The need for electrically insulating materials with a high in-plane thermal conductivity for lateral heat spreading applications in electronic devices has intensified studies of layered hexagonal boron nitride (h-BN) films. Due to its physicochemical properties, h-BN can be utilised in power dissipating devices such as an electrically insulating heat spreader material for laterally redistributing the heat from hotspots caused by locally excessive heat flux densities. In this study, two types of boron nitride based heat spreader test structures have been assembled and evaluated for heat dissipation. The test structures separately utilised a few-layer h-BN film with and without graphene enhancement drop coated onto the hotspot test structure. The influence of the h-BN heat spreader films on the temperature distribution across the surface of the hotspot test structure was studied at a range of heat flux densities through the hotspot. It was found that the graphene-enhanced h-BN film reduced the hotspot temperature by about 8–10 °C at a 1000 W cm−2 heat flux density, a temperature decrease significantly larger than for h-BN film without graphene enhancement. Finite element simulations of the h-BN film predict that further improvements in heat spreading ability are possible if the thermal contact resistance between the film and test chip are minimised.

Carbon Nanorings: 3D Bridged Carbon Nanoring/Graphene Hybrid Paper as a High‐Performance Lateral Heat Spreader (Small 46/2015)

Carbon Nanorings: 3D Bridged Carbon Nanoring/Graphene Hybrid Paper as a High‐Performance Lateral Heat Spreader (Small 46/2015)

3D Bridged Carbon Nanoring/Graphene Hybrid Paper as a High-Performance Lateral Heat Spreader.

Graphene paper (GP) has attracted great attention as a heat dissipation material due to its unique thermal transfer property exceeding the limit of graphite. However, the relatively poor thermal transfer properties in the normal direction of GP restricts its wider applications in thermal management. In this work, a 3D bridged carbon nanoring (CNR)/graphene hybrid paper is constructed by the intercalation of polymer carbon source and metal catalyst particles, and the subsequent in situ growth of CNRs in the confined intergallery spaces between graphene sheets through thermal annealing. Further investigation demonstrates that the CNRs are covalently bonded to the graphene sheets and highly improve the thermal transport in the normal direction of the CNR/graphene hybrid paper. This full-carbon architecture shows excellent heat dissipation ability and is much more efficient in removing hot spots than the reduced GP without CNR bridges. This highly thermally conductive CNR/graphene hybrid paper can be easily integrated into next generation commercial high-power electronics and stretchable/foldable devices as high-performance lateral heat spreader materials. This full-carbon architecture also has a great potential in acting as electrodes in supercapacitors or hydrogen storage devices due to the high surface area.

Nanofocus diamond X-ray windows: Thermal modeling of nano-sized heat source systems

An analytical thermal model is proposed to study heat transfers occurring at high power density in X-ray tubes with micron to submicron sized source. The use of a simple analytical approach instead of a complex numerical simulation allows readily modeling of more and more challenging systems such as multi-source X-ray tubes. By significantly reducing the computing time, it enables a wider parameter range evaluation for engineering phase. We focused our work on tubes integrating a transmission window that is mainly edge-cooled. Our approach can be generally used in cylindrical lateral heat spreaders with distributed small sized source systems. The model enables an efficient estimation of temperature distributions for a large range of parameters and source designs and is, for instance, well-suited to described nanosized heat source systems. It is developed from an electrostatic analogy with the point charge particle model and uses of a series of virtual sources. A multiscale resolution of the heat equation is proposed, hence providing the temperature distribution at any point within the whole system. Moreover, the non-linearity of equations caused by temperature-dependent thermal conductivities is solved by using Kirchhoff's transformation, giving a more realistic approach of heat conduction in diamond X-ray windows, where temperature in excess of 1000°C can be encountered. The influence of convective and radiative transfers has been discussed and the physical accuracy of the predicted temperature is controlled by adjusting the number of virtual sources of the model.

A highly thermostable and transparent lateral heat spreader based on silver nanowire/polyimide composite

Heat accumulation is a severe problem for high-power light-emitting diodes (LEDs). Here, we introduce a thermostable and transparent lateral heat spreader as an additional heat-escaping channel of an LED chip to improve the thermal management of LED devices. The lateral heat spreader was prepared based on a silver nanowire (AgNW)/polyimide composite comprising a thin polyacrylate layer soldering a conductive AgNW network to confine the nanowires to the surface of a polyimide film and to obtain low contact resistance between the nanowires. The AgNW/polyimide composite film has a figure-of-merit sheet resistance of 7 ohm sq−1 with 76% transmittance at 550 nm. After heating at 200 °C for 168 h, the sheet resistance increases to 16 ohm sq−1. The thermal conductivity and thermal diffusivity are 130.2 W m−1 K−1 and 60.5 mm2 s−1, respectively, which are comparable to those of a commercial copper foil. A demonstration shows the core temperature in a thermal diffusion apparatus can be lowered by 9 °C. The experimental data combined with computational simulation indicate the Joule heating could be drawn away efficiently along the lateral heat spreader.

Thermally reduced graphene oxide films as flexible lateral heat spreaders

A thermally reduced graphene oxide film (r-GOF), with tailorable micro-structures and macro-properties, is fabricated by annealing a filtrated graphene oxide film (GOF) in a confined space. The structural evolution of the film at different annealing temperatures is systematically investigated, and further correlated to the thermal conductivity and mechanical performances. With the increase of temperature, more oxygen-containing functional groups are removed from the film by a simultaneous conversion from sp3 to sp2 carbon in the graphitic lattice. As the temperature reached 1200 °C, the r-GOF achieves an ultrahigh thermal conductivity of ca. 1043.5 W m−1 K−1, while 1000 °C is a critical temperature in enhancing the thermal conductivity. Moreover, G1200 exhibits excellent mechanical stiffness and flexibility with a high tensile strength (13.62 MPa) and Young's modulus (2.31 GPa). The combined conductivity and mechanical performances render the r-GOFs promising materials as flexible lateral heat spreaders for electronics.

Graphene: Hierarchical Graphene–Carbon Fiber Composite Paper as a Flexible Lateral Heat Spreader (Adv. Funct. Mater. 27/2014)

Graphene: Hierarchical Graphene–Carbon Fiber Composite Paper as a Flexible Lateral Heat Spreader (Adv. Funct. Mater. 27/2014)

Hierarchical Graphene–Carbon Fiber Composite Paper as a Flexible Lateral Heat Spreader

As a low dimensional crystal, graphene attracts great attention as heat dissipation material due to its unique thermal transfer property exceeding the limit of bulk graphite. In this contribution, flexible graphene–carbon fiber composite paper is fabricated by depositing graphene oxide into the carbon fiber precursor followed by carbonization. In this full‐carbon architecture, scaffold of one‐dimensional carbon fiber is employed as the structural component to reinforce the mechanical strength, while the hierarchically arranged two‐dimensional graphene in the framework provides a convenient pathway for in‐plane acoustic phonon transmission. The as‐obtained hierarchical carbon/carbon composite paper possesses ultra‐high in‐plane thermal conductivity of 977 W m−1 K−1 and favorable tensile strength of 15.3 MPa. The combined mechanical and thermal performances make the material highly desirable as lateral heat spreader for next‐generation commercial portable electronics.

Heat Transport in Graphene Interconnect Networks With Graphene Lateral Heat Spreaders

We simulated heat propagation in the integrated graphene heat spreaders within the interconnect hierarchy. In the considered design, the graphene layers perform the dual functions of interconnects and heat spreaders. We investigated Joule heating effects within the chip with graphene interconnect networks and heat spreaders. Numerical solutions for direct current and heat propagation equations were found using the finite-element method. The simulation results showed that the use of graphene as interconnects as well as heat spreaders lowers the maximum temperature of the chip. The maximum temperature of the chip was studied as a function of the interconnect current and thickness of few-layer graphene. Our results are important for design of graphene-based thermal and electrical interconnect networks in the next generations of integrated circuits and 3-D electronics.