Production of diamond/Cu composites with high thermal conductivity and low volume fraction using 3D direct ink writing technique

Dynamic viscosity of colloidal silica suspensions at low and high volume fractions

In this study, an advanced phase change material (PCM) was fabricated to combine the properties of a biobased thermoplastic polymer and the heat storage of PCM to overcome the limitations of the existing PCM composites. The novel PCM of erythritol (ET)-grafted polylactic acid (PLA) (ETPLA) was prepared by the in situ polymerization of ET and PLA. The thermally conductive ETPLA composites were fabricated by injection molding using a hybrid filler system of oxidized carbon fiber (CF-OH) and aluminum nitride (AlN), which showed a high through-plane thermal conductivity of 4.25 W/mK, a tensile strength of 16.4 MPa (1953% enhancement comparing pure ET), an elongation at break of 4.4% (189% enhancement comparing pure ET), and latent heat of 170.7 J/g. Thus, the ETPLA/CF-OH/AlN composites facilitate efficient thermal management to combine heat saving and heat dissipation through the large latent heat and high thermal conductivity. Thus, the fabricated PCM composites have potential applications in thermal management systems of next-generation electronic devices.

Highly thermally conductive, electrically insulating, thermal resistance polyimide-base composites with fibrillated carbon networks for thermal management applications

As the electronic industry moves toward higher power consumption, integrated functions and minimized geometry, one of the important challenges is the dramatically increasing power density. Thus, efficient thermal management has become a critical requirement for the design of modern electronic packages. A promising approach for this challenge is to find a high performance thermal interface materials (TIMs) made of a material with extremely high thermal conductivity. In this work, an innovative carbon-based nanomaterial, carbon nanoscroll (CNS) will be presented to yield extremely high thermal conductivity and have great potential as the component of TIMs in electronic packages. A CNS can be regarded as a monolayer graphene rolling up in a spiral form with a structure similar to a multi-walled carbon nanotube (CNT). Unlike the closed CNTs, CNS is topologically open-ended, like a Swiss roll. Using molecular dynamics (MD) simulations, the thermal conductivity of CNSs is investigated to be comparable to graphene, i.e. 3000– 5000 Wm−1K−1. Various factors that impact the thermal transport behavior of CNSs are investigated extensively. The MD simulation results show that the thermal conductivity of CNS is sensitive to the number of CNS walls, temperature, defects and functionalization. When the number of walls increases from 1 to 3, the thermal conductivity of CNSs is reduced by ∼8.9%. With environmental temperature rising from 300 K to 400K, the thermal conductivity of CNSs decreases by ∼16.5%. When the CNSs have single vacancy defects or functionalized hydrogen, their thermal conductivity decreases gradually with the higher densities of defects or functionalization. The results reveal that the vertical aligned CNSs can be superior to vertical aligned CNTs in serving as the thermal interface materials in electronic packages, due to their higher thermal conductivity. CNSs can also be used as superior thermally conductive fillers in polymeric TIMs. Using effective medium theory, the thermal conductivity of polymeric TIMs composited of epoxy resin matrix and CNS fillers is calculated. It is found that polymeric TIMs with epoxy resin matrix and 10% volume fraction of CNS fillers yield an effective thermal conductivity of ∼79 Wm−1K−1, which is one magnitude higher than the commonly used TIMs in current electronic packaging industry. The present work reveals new insights about the extremely high thermal conductivity of CNS and its great potential in improving the thermal management of electronic packages.

Research and development of polymer composites with high thermal conductivity and excellent electrical insulation are one of the research hotspots in thermal management. Although the thermal conductivity issue has been improved effectively with many methods, it remains challenging due to the interface thermal resistance issues and how to hold the electrical insulation performance at high thermal conductivity. Herein, MgAl layered double oxide/reduced graphene oxide (MgAl-LDO/rGO-s) with a hierarchical structure was innovatively used in the CNF thermal conductivity field. The lamellar structure and strong interfacial interactions between MgAl-LDO and rGO construct the continuous thermal conduction path for phonons transport, and the interface thermal resistance is effectively reduced. Meanwhile, the insulated MgAl-LDO forms a hierarchical structure on the surface of rGO by its positive charge characteristics and keeps excellent electrical insulation properties. Therefore, the CNF/MgAl-LDO/rGO-s composite films exhibit high in-plane thermal conductivity (8.7 W·m–1·K–1), electrical insulation (ρv > 1013 Ω·m), and excellent thermal stability. This work provides an interface engineering strategy for heat conduction and electrical insulation and broadens the upper limit application of thermally conductive polymer composites (TCPC) in thermal management.

The multifunctional composite with high thermal conductivity and electromagnetic interference (EMI) shielding properties is of great practical application value to many fields [1]. In the past decade, functional fillers were usually incorporated into polymer-based composites to improve their thermal and electrical conductivity. However, these composites only exhibit high horizontal thermal conductivity, limiting their application scenarios, mainly because of poor control over the vertically aligned conductive network. Here, an NG/TPU composite with a high vertical orientation structure is fabricated through a facile laminar hot pressing and wire cutting process. The NG/TPU composite exhibits anisotropic thermal conductivity with thermal conductivity of 26.79 Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 4.03 Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> in the vertical and horizontal directions, respectively; meanwhile, due to the excellent electrical conductivity of natural graphite flakes, the high filling amount ensures that the material has excellent EMI shielding performance in the horizontal direction, up to 50.75 dB. The NG/TPU composites with both excellent thermal conductivity and EMI shielding performance have broad application prospects in EMC (electromagnetic compatibility) and thermal management. Besides, this work provides a promising industrialization strategy to build a vertical functional network for filler-type composites.

In this study, silica aerogel microspheres were prepared through sol–gel method with tetraethyl orthosilicate as the silicon source. Thermal insulation coatings were prepared by mixing silica aerogel and acrylic emulsion. The relationships between the thermal conductivity and volume fractions, densities, sizes, and interfaces of silica aerogel microspheres were investigated through scanning electron microscopy and thermal conductivity analysis. The thermal insulation mechanism of composite coating was discussed in detail. Results showed that the aggregations prevented the decrease of thermal conductivity in the coating when the volume fraction of silica aerogel microspheres was lower than 30 %. However, the pores in the coating reduced the thermal conductivity when the volume fraction was higher than 30 %. The porosity of silica aerogel increased with the declining density, which improved the thermal insulation performance of silica aerogel and reduced the thermal conductivity of the coating. The thermal conductivity of the coating with large microspheres was lower than that with small microspheres at low volume fractions. However, the thermal conductivity of the coating with small silica aerogel microspheres was low because of their large interfacial thermal resistance at high volume fractions. Wetting agents were beneficial in improving the compatibility of hydrophobic aerogel microspheres and polymer, improving the volume fractions of silica aerogel microspheres in the coating, and reducing the thermal conductivity of the coating.

Overheating is a crucial problem limiting the reliability and performance of electronic systems. Since these systems have become continuously smaller, more powerful and sophisticated in recent decades, they produce much larger amounts of heat. As such, effective thermal management, in other words, thermal interface materials (TIMs) used in these devices for heat dissipation has gained utmost significance in terms of lifetime and reliability. Thermal greases, polymer-based composites, phase change materials (PCMs) and solders are currently the most commonly used TIMs. In this work, we describe next-generation metal matrix-based nanocomposite TIMs produced by chemical integration of organic ligand-functionalized boron nitride nanosheets (BNNS) into copper matrix. These TIMs possess much higher thermal conductivity than existing solder TIMs and comparable mechanical compliance with existing epoxy-based TIMs. BNNS, prepared by mechanically assisted cleavage of h-BN flakes through ultrasonic dispersion, was functionalized with three groups of organic ligands through nucleophilic addition to form functionalized BNNS (f-BNNS). These ligand groups were categorized by their chain type, and compatibility with the BNNS and metal matrix, and the functionalization was verified via Fourier Transform Infrared (FTIR) Spectroscopy. Then, the f-BNNS was dispersed in a copper matrix using a novel electrocodeposition method where copper and f-BNNS are deposited on a cathode in the presence of an electric field and potential difference to form nanocomposite TIMs. The thermal properties of the nanocomposite TIMs were investigated using the laser flash analysis, and the thermal conductivies were found to be between 160 W.m−1.K−1 and 320 W.m−1.K−1. Once the thermal properties were measured, mechanical properties of the developed TIMs were investigated via nanoindentation technique and tensile testing. The elastic moduli of the nanocomposite TIMs were determined to be as low as 14 GPa as these values were much smaller than that of the electroplated pure copper thin films (99–125 GPa). There was nearly a six-fold reduction in elastic modulus of copper matrix in presence of functionalized BNNSs. The hardness value corresponding to this sample was about 0.18 GPa, which is about five-to-fifteen times lower than the measured hardness values of pure copper (1.1–2.8 GPa). Briefly, BNNS were functionalized with various ligands, and it was shown that electrocodeposited nanocomposite TIMs with thermal conductivies as high as 320 W.m−1.K−1 and elastic modulus values less than 20 GPa can be produced. Considering two extreme cases of epoxy-based and pure copper shim TIMs, our results are significant in advancing the current state of art for TIMs.

Metallic nanowires are widely used in energy conversion and storage, especially in the thermal management area, because of their high specific surface area, rich active sites, and high thermal conductivity. Metallic nanowires, such as copper or silver nanowires, are extensively applied to prepare the next-generation thermal interface materials with excellent thermal conductivity, light weight, high strength and ductility. Metallic hollow nanowires, which hold the typical one-dimension hollow nanostructures, have high axial thermal conductivity to prepare advanced thermal interface materials applied in thermal management and waste heat recovery of high-power microelectronic devices. Thermal conductivity is one of the most important indicators to assess the thermal performance of thermal interface materials. Over the past decades, many studies in both theory and experiment have been carried out to evaluate the thermal conductivity of solid nanowires. Molecular dynamics (MD) simulation has been applied to calculate the thermal conductivity of single nanowires, single core-shell nanowires and super-lattice nanowires. Meanwhile, advanced measuring techniques, including 3ω method, Raman spectroscopy and T-type method, have been invented and developed to measure the thermal conductivity of single nanowires. However, investigations on the thermal conductivity of metallic hollow nanowires are limited. Considering the difficulty in the fabrication and thermal conductivity measurement of single hollow metallic nanowires, creating a theoretical thermal conductivity model is urgently required. This work developed the electrical thermal conductivity model, phonon thermal conductivity model and phonon specific heat model of metallic nanowires to study the size effect on the mean free path, group velocity and specific heat capacity of the material. This study also proposed the effective thermal conductivity model of metallic hollow nanowire. These models have been used to study the effect of the both length and thickness of the metallic hollow nanowire on the effective thermal conductivity as well as the influence of the wall thickness on the electronic and phonon thermal conductivity. Finally, the mechanism of size effect on the thermal conductivity was discussed, and a reasonable interpretation based on the developed model was also proposed. Results show that an exact thermal conductivity model, validated by the experimental data from open-reported literature, was established with a correlation coefficient high than 90%. The size effect on the thermal conductivity of both hollow copper nanowire and solid copper nanowire was observed with the increased length and thickness. The thermal conductivity of solid copper nanowire was about 1.2 times higher than that of the hollow copper nanowire with the same length of 800 nm. In detail, the electronic thermal conductivity of solid copper nanowire was nearly 18.7% higher than that of hollow copper nanowire, while their phonon thermal conductivities almost remained unchanged. The size effect on the specific heat of hollow copper nanowire was also observed. The thermal conductivity of the hollow copper nanowire was 1.6 times higher than that of bulk copper and 1.2 times higher than that of a solid copper nanowire with the similar thickness.

High thermal conductivity polymer matrix composites have become an urgent need for the thermal management of modern electronic devices. However, increasing the thermal conductivity of polymer-based composites typically results in loss of lightweight, flexibility and electrical insulation. Herein, the polyvinyl alcohol (PVA)/PVA-chitosan-adsorbed multi-walled carbon nanotubes/PVA (PVA/CS@MWCNTs) composite films with a sandwich structure were designed and fabricated by a self-construction strategy inspired by the surface film formation of milk. The obtained film simultaneously possesses high thermal conductivity, electrical insulation, and excellent flexibility. In this particular structure, the uniform intermediate layer of PVA-CS@MWCNTs contributed to improving the thermal conductivity of composite films, and the PVA distributed on both sides of the sandwich structure maintains the electrical insulation of the films (superior electrical resistivity above 1012 Ω·cm). It has been demonstrated that the fillers could be arranged in a horizontal direction during the scraping process. Thus, the obtained composite film exhibited high in-plane thermal conductivity of 5.312 W·m−1·K−1 at fairly low MWCNTs loading of 5 wt%, which increased by about 1190% compared with pure PVA (0.412 W·m−1·K−1). This work effectively realizes the combination of high thermal conductivity and excellent electrical insulation, which could greatly expand the application of polymer-based composite films in the area of thermal management.

Highly thermally conductive composites of graphene fibers

Phase change materials (PCMs) are regarded as promising candidates for realizing zero-energy thermal management of electronic devices owing to their high thermal storage capacity and stable working temperature. However, PCM-based thermal management always suffers from the long-standing challenges of low thermal conductivity and liquid leakage of PCMs. Herein, a dual-encapsulation strategy to fabricate highly conductive and liquid-free phase change composites (PCCs) for thermal management by constructing a polyurethane/graphite nanoplatelets hybrid networks is reported. The PCM of polyethylene glycol (PEG) is first infiltrated into the cross-linked network of polyurethane (PU) to synthesize hybridized semi-interpenetrated composites (PEG@PU), and then incorporated with reticulated graphite nanoplatelets (RGNPs) via pressure-induced assembly to fabricate highly conductive PCCs (PEG@PU-RGNPs). The hybrid networks enable the PCCs to show excellent mechanical strength, liquid-free phase change, and stable thermal property. Notably, the dual-encapsulated PCCs exhibit high thermal and electrical conductivities up to 27.0 W m-1 K-1 and 51.0 S cm-1 , superior to the state-of-the-art PEG-based PCCs. Furthermore, the PCC-based energy device is demonstrated for efficient battery thermal management toward versatile demands of active preheating at a cold environment and passive cooling at a hot ambient. Overall, this work provides a promising route for fabricating highly conductive and liquid-free PCCs toward thermal management.

Highly thermally conductive and mechanically robust composite of linear ultrahigh molecular weight polyethylene and boron nitride via constructing nacre-like structure

<p indent="0mm">In the 5G era, the effective thermal management has become more demanding due to the ever-rising integration of electronic devices. High thermal conductivity materials play a crucial role in the field of thermal management, for example, thermal interface materials (TIMs) are used to fill the gap between the electronic chip and the heat sink to improve thermal transfer. In recent years, polymers have become a popular choice for thermal conductive materials because of their light, economical and excellent insulation and processability. To improve the thermal conductivity of materials, inorganic fillers with high thermal conductivity are generally composited with polymers. With the merits of high thermal conductivity, desirable chemical stability, carbon nanotubes (CNTs) are considered to have broad application potential in thermal conductive composites. Simple composition methods failed to increase thermal conductivity of composite materials to expected levels due to the large interfacial thermal resistance between CNTs and polymers and the disorderly distribution of CNTs in polymers. Therefore, reasonable design of polymer composites filled with CNTs is the key to achieving high thermal conductivity. This review mainly introduces the application of CNTs in thermal conductive polymer composites. Based on the existing theoretical research on thermal conductivity of composites and the application of molecular dynamics, more feasible strategies for improving the thermal conductivity of polymer composites filled with CNTs have been proposed. The approaches that improve the thermal conductivity of composites are mainly introduced from three aspects. (1) The intrinsic thermal conductivity of CNTs is an important factor affecting the thermal conductivity of polymer-based composites. CNTs and their macroscopic bulk materials both have excellent thermal conductivity. However, the thermal conductivity test results of the macroscopic materials of CNTs (such as CNTs fibers, arrays, and films) showed that the thermal conductivity of the macroscopic materials of CNTs was much smaller than that of single CNTs due to impurities, defects and inter-tube contact thermal resistance. Studies have shown that purification of CNTs and reduction of intertubular contact thermal resistance can improve the intrinsic thermal conductivity of CNTs. (2) From a microscopic perspective, phonons, the quantized energy of lattice vibration, are the main mechanism of heat conduction in most carbon fillers and polymers. Phonon scattering occurs in the process of phonon transfer, including the scattering between phonons and the scattering at the interface caused by defects and impurities, resulting in thermal resistance. The bonding strength of fillers and polymer interfaces are the crucial factors affecting the transmission of phonons. Hence, the thermal conductivity of composites could be effectively enhanced by surface treatment of CNTs, including covalent functionalization and non-covalent functionalization. Studies have shown that the functionalization can enhance the interfacial interaction between CNTs and polymers, while improving the dispersion of CNTs in polymers. (3) According to the thermal conduction network theory, the key to improving the thermal conductivity is whether the fillers can form a large number of continuous thermal conduction paths in the polymers and maintain a stable existence. However, high CNTs content usually affects the comprehensive properties of composites. To solve this problem, the arrangement and distribution of CNTs in the polymer should be improved to construct more heat conduction pathways, which can achieve high thermal conductivity at a low filling content. Here we introduce some effective methods, including the synergy effect, field orientation and the construction of 3D network structures. In this review, the characteristics and improvement effects of different technical approaches are summarized, which provides a reference for the research and application of CNT-filled polymer-based composites with high thermal conductivity. Finally, the future development prospects of carbon nanomaterial-filled polymer composites are discussed from perspectives of theoretical research, experimental design and engineering application.

Enhanced thermal conductivity in oriented cellulose nanofibril/graphene composites via interfacial engineering