Three-dimensional interconnected networks for thermally conductive polymer composites: Design, preparation, properties, and mechanisms
Introduction
Advances in science and technology have been accompanied by the evolution of microelectronic devices toward miniaturization, high levels of integration, and high power, thus imposing new requirements on and posing new challenges for traditional heat conductors. Owing to the augmentation of the frequencies and powers of semiconductor chips and the dense packing of integrated circuits, electronic devices and their components emit a considerable amount of heat energy to their surroundings during operation. Unless the generated heat can be dissipated effectively, it will be concentrated in some small areas. Such local hotspots with extremely high temperatures will strongly influence the performance and service life of microelectronic devices. Consequently, efficient heat management systems are essential for lowering the temperature of the local hotspots in high-power electronics [[1], [2], [3]]. As the critical component of heat management systems, thermal interface materials (TIMs) play an important role in dissipating heat in electronic devices. Undoubtedly, overheated environments have limited the development of microelectronics and integrated circuits with higher powers and levels of integration. Thus, efficient thermal management systems are now emerging as a critical factor in realizing next-generation electronic devices.
Thermal management systems have become an integral part of the electronics and aerospace industries. As TIMs are the core components of thermal management systems, the thermal conductivity (k) of such a material directly determines the quality of the entire system. The excellent properties of organic polymers, such as electrical insulation, high flexibility, low weight, low cost, outstanding mechanical properties, and machinability, have enabled their wide use in various application fields over the past few decades [4]. However, multiple factors, including the amorphous arrangement and vibrations of the molecular chains, cause polymers to exhibit poor thermal conduction [5]. For most polymers, phonon thermal conduction is the dominant thermal conduction pathway. As the amorphous structure and vibrations of the macromolecular chains in polymers can induce massive phonon scattering, the vast majority of neat polymers are heat insulators or relatively poor heat conductors (k values of 0.1–0.5 W m−1 K−1) [[6], [7], [8]]. Therefore, pure polymers cannot facilitate heat dissipation to accelerate the minimization and integration of components in the modern electronics industry.
Phonons, being the quantized collective modes of crystal lattice vibrations, are the primary thermal energy carriers in insulators [9,10]. Heat transfer in materials with high crystallinity can be realized through lattice vibrations within the material [1,11]. As shown in Fig. 1a, the conduction of thermal energy in a polymer matrix can be regarded as a phonon transfer process. Because of the high disorder of polymer chains, the phonon scattering phenomenon is quite pronounced in polymers, resulting in ultralow k values. The scattering of phonons mainly occurs via collisions between phonons and at interfaces between impurities and lattice defects. To address the low thermal conduction of polymers caused by their disordered molecular chains and improve their thermal conduction capability, two strategies can be implemented: (i) engineer the morphology of the polymer chains or (ii) fill the polymer matrix with a highly thermally conductive filler [12,13]. Improving the orientation and crystallinity of the polymer chains can decrease the disorder of the polymer molecules and thus effectively reduce phonon scattering (Fig. 1b). Similarly, compounding with thermally conductive fillers can effectively improve the heat transfer properties of a polymer. The thermally conductive fillers connect to each other to form a continuous three-dimensional (3D) network in the polymer matrix, which provides a high-speed channel for the transmission of phonons. Thus, heat transfer can be accelerated along the 3D network with decreased phonon scattering (Fig. 1c). Compared with the morphological design of polymer molecular chains, compounding polymers with thermally conductive fillers is a more efficient and convenient approach.
Over the past few decades, considerable research has been dedicated to the fabrication of polymer-based TIMs by compounding with dispersed filler particles. However, owing to the huge interfacial thermal resistance at filler–filler and filler–matrix interfaces, the resulting polymer composites only exhibited moderate improvements in k, even with large filler ratios. Additionally, the poor particle dispersibility and large loading levels of dispersed fillers severely degrade the polymer properties. These factors have prevented conductive fillers from reaching their full potential for enhancing the heat conduction of polymers. Recently, more attention has been focused on the construction of thermally conductive networks. As shown in Fig. 2, compared with dispersed filler particles, a preconstructed 3D thermally conductive architecture is a good candidate for creating high-performance polymer-based TIMs because it ensures good filler distribution and forms an interconnected network in the polymer matrix, both of which minimize the unfavorable effect of interfacial thermal resistance at filler–filler interfaces. Moreover, with such architectures, the improvement can be maximized with a minimal filling ratio. The construction methods for 3D continuous thermally conductive networks include in situ chemical vapor deposition (CVD) growth, sol–gel methods, self-assembly, and template methods. Almost all thermally conductive fillers can form 3D thermally conductive frameworks, and the most widely used include graphene foams (GFs), carbon nanotube (CNT) frameworks, boron nitride (BN) networks, and hybrid architectures of several fillers [2,14,15]. The popularity of these materials is primarily due to their inherently good thermally conductivities, low weights, large specific surface areas, and excellent comprehensive performance. A percolated skeletal network provides sufficient phonon transfer pathways with abundant interfacial contact points, which can effectively weaken phonon interface scattering. As a result, constructing a continuous 3D architecture as the thermal enhancer in a polymer matrix is an effective strategy for preparing high-performance TIMs, and hence, has become a current research trend.
Herein, we review numerous studies pertaining to the preparation of heat-conductive polymers and polymer composites with high k values by filling with thermally conductive fillers in the form of conventional dispersed filler particles or interconnected 3D networks. Although there are many reviews of thermally conductive polymer-based composites [8,16,17], systematic assessments of the effect of 3D networks on improving the k values of polymers and comparisons between the two aforementioned compounding strategies are rare. In view of this, it is essential to review the advances in 3D framework-strengthened polymer composites. Here, we provide an overview of the recent literature on enhancing thermal conduction via 3D framework-improved polymers and discuss the advantages of this strategy over the traditional mixed filling method. Additionally, the preparation methods and factors that influence the properties of polymer composites with different types of 3D structures are examined. A brief overview of the existing 3D heat-conducting frameworks is shown in Fig. 3.
Section snippets
Intrinsic thermally conductive polymers
In practice, the inferior k of polymers is one of the major technological barriers for advancements in electronic devices. Improving the heat transfer performance of polymers is crucial and has become a long-standing hot research topic. Generally, disordered polymer molecular chains and weak molecular interactions are responsible for the poor k values of polymers, as shown in Fig. 4a, b. The amorphous structure and the vibrations of polymer chains can considerably reduce the mean free path of
Polymer-based composites with dispersed fillers
Compared with improving the thermal conduction of polymers by engineering the morphology of their molecular chains, blending polymers with thermally conductive fillers is considered more efficient and convenient, and is applicable to almost all polymers. Owing to advances in nanotechnology, various new conductive fillers of different types, morphologies, sizes, and properties have been produced, which provides more options for the preparation of thermally conductive polymer composites. The
Polymer-based composites with preconstructed 3D frameworks
According to Maxwell’s mixing theory, single-walled CNTs (SWCNTs) should improve the k value of a polymer by 50-fold at a volume fraction of 1%. However, such improvements cannot be achieved in practice, as the experimentally observed improvement is at least an order of magnitude less than expected [85]. This contrast results from the huge thermal resistance inside the composite, which mainly consists of the thermal resistance at the filler–polymer interface and the interfacial resistance
In situ construction of 3D interconnected networks
The formation of an interconnected network is essential for improving the heat transfer ability of polymers. While using preconstructed 3D networks as fillers is an effective method for forming continuous heat conduction networks in polymer matrices, other methods, such as adding a large volume fraction of particle fillers, are not widely successful. However, another simple and scalable approach has been proposed for preparing thermally conductive polymer composites via in situ construction of
Summary and outlook
Flexible, lightweight, and highly thermally conductivity polymer-based TIMs are significant for the development of next-generation high-power and highly integrated electronic devices. Because of the revolutionary progress in nanomaterials and materials processing technology, a variety of novel thermally conductive fillers and 3D architectures have been rapidly developed, providing numerous possibilities for the preparation of high-performance polymer-based TIMs. In this review, we provide
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
This work was financially supported by National Key R&D Program of China (No. 2016YFA0202302), National Natural Science Funds for Distinguished Young Scholars (No. 51425306), the State Key Program of National Natural Science Foundation of China (No. 51633007), National Natural Science Foundation of China (Nos. 51773147, 51803151 and 51973152), National Outstanding Youth Talent Program (2019), and Scientific and Technological Commission of China.
Fei Zhang received his BS degree (2012) and MS degree (2015) in the Department of Chemistry and Chemical Engineering from Hunan University of Science and Technology. He obtained his PhD degree from the group of Prof. Wei Feng at the School of Materials Science and Engineering, Tianjin University in 2019. He held an academic position at The Chinese University of Hong Kong, Shenzhen in 2020. His current research is focus on the field of carbon based thermal conductive composite and its integrated
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Fei Zhang received his BS degree (2012) and MS degree (2015) in the Department of Chemistry and Chemical Engineering from Hunan University of Science and Technology. He obtained his PhD degree from the group of Prof. Wei Feng at the School of Materials Science and Engineering, Tianjin University in 2019. He held an academic position at The Chinese University of Hong Kong, Shenzhen in 2020. His current research is focus on the field of carbon based thermal conductive composite and its integrated applications.
Prof. Yiyu Feng is a research professor at the School of Materials Science and Engineering, Tianjin University. He obtained his PHD degree form Tianjin University in 2009 and held an academic position at Tianjin University in 2009. He has authored and co-authored over 90 academic articles and reviews. Currently, his research is focused on solar-thermal conversion and thermal interfacial materials and composites, as well as their application in heat-control systems.
Prof. Wei Feng is a full professor at School of Materials Science and Engineering of Tianjin University (China). He obtained his Ph.D. from the Xi′an Jiaotong University (China) in 2000. Then, he worked at Osaka University and Tsinghua University as a Japan Society for the Promotion of Science (JSPS) fellow and a postdoctoral researcher, respectively. His research interests include photo-responsive organic molecules or their derivatives, thermal-conductive and high-strength carbon-based composites, and two-dimensional fluorinated carbon materials or polymers. He was a recipient of the program for New Century Excellent Talents in University of Ministry of Education of China (2006), the “Ten-Thousand Talent Program” leading innovators of China (2018), the Young and middle-aged leading scientists of Ministry of Science and Technology of China (2017), the “131” Innovative Talent of Tianjin (2015), and senior visiting scholar of JSPS (2008). He has authored or coauthored 160 peer-reviewed SCI publication and recently published articles/reviews in Chem. Soc. Rev., Nat. Commun., Prog. Mater. Sci., Adv. Funct. Mater., Adv. Sci., J. Mater. Chem. A, etc., as a corresponding author. He holds over 60 Chinese patents and received the First Prize of Technical Innovation Award in Tianjin at 2015 and 2017, and the Second Prize of Natural Science Award in Tianjin at 2007 and 2011. As for the professional offices and services, he is serving as member of Science & Technology Commission of Ministry of Education, Evaluation Expert of National Found Program, and Chairman of Committee on thermal conductive Composite Materials, Chinese Society for Composite Materials.