Thermal properties of epoxy resin based thermal interfacial materials by filling Ag nanoparticle-decorated graphene nanosheets

https://doi.org/10.1016/j.compscitech.2016.01.011Get rights and content

Abstract

Epoxy resin based thermal interfacial materials (TIMs) with high thermal conductivity have been obtained by filling Ag nanoparticle-decorated graphene nanosheets (GNSs) as thermal conductive fillers. The thermal conductivity (k) enhancement of epoxy resin based TIMs increases with the thermal filler loading. The more decoration of Ag nanoparticles on the GNS surfaces, the higher thermal conductivity enhancement of epoxy resin based TIM is. It is proposed that the bigger Ag nanoparticles acting as “spacers” increase the distance between the graphene sheets more than the smaller ones. It is not easy for graphene sheets to form stacked graphitic structures and the high specific surface area as well as other unique properties exhibited by 2D graphene are retained. Furthermore, the larger particle size is desired to minimize the scattering of phonons because of low interfacial thermal barrier. The obvious enhancement of thermal properties should be also attributed to the high intrinsic k of graphene and the effective thermal conductive networks forming by graphene and Ag nanoparticles. The synergistic effects including the stronger phonon Umklapp scattering, better phonon transmission trough the interfaces, decreasing Kapitza resistance, and decreasing ability of heat transfer by electrons result in the slight variation of k with the temperature. The weak temperature dependence of k is beneficial for TIM applications and can be obtained by controlling the addition of hybrid thermal fillers and quantity of decorated silver nanoparticles.

Introduction

Graphene nanosheet (GNS) as one of nanostructure carbon materials exhibits a unique structure of two-dimensional sheet composed of sp2-bonded carbon atoms with one-atomic thickness [1]. GNSs have large specific surface area, extraordinary physical properties [2], and strong interactions towards metals [3], [4], [5]. Which make it possible that GNSs can be used as supporting materials for dispersing the metal nanoparticles directly on their surfaces. Because of their extraordinary electrical and thermal conductivity (k), thermal stability, and excellent mechanical strength, GNSs and its derivatives are important thermal conductive filler materials for polymer composites [6], [7]. This kind of polymer composites with high k are urgently needed for the progress in information, communication, and energy storage technologies. The rapidly increasing power densities in electronics makes it a crucial issue to efficiently remove heat for the performance and reliability of modern electronic, optoelectronic, photonic devices, and systems [8], [9]. In the thermal management, it is essential to apply a special material, thermal interface materials (TIMs), between heat sources and heat sinks [10], [11], [12], [13]. The attempts of utilizing highly thermal conductive nanomaterials, for example, GNSs as fillers in TIMs, may not lead to practical applications because on drying the graphene dispersion, the isolated sheets aggregate and form an irreversibly precipitated agglomerate, like other dispersions of nanomaterials with high aspect ratios. And the advantage caused by the ultrahigh surface area of 2D GNS is lost. As a result, the aggregated GNS behaves similar to the particulate graphite platelets with relatively low surface area [14]. Si et al. has verified that the introduction of nanoparticles into the dispersion of graphene sheets impeded the formation of a stacked graphitic structure [14]. The metal nanoparticles functioning as a “spacer” increase the distance between the graphene sheets to several nanometers, thereby making both the faces of graphene accessible even in its dry state. It can be deduced that the metal nanoparticles decorating on GNS surfaces also can act as spacers to ensure that the high specific surface area as well as other unique properties exhibited by 2D graphene is retained when it is dispersed into polymer materials to form TIMs. Decoration of GNSs with metal nanoparticles (NPs) has also been extensively presented using reducing agents in solution or external electron sources [15], [16], [17], [18]. Those widely spread methods do not allow good reproducibility in the fabrication process in terms of number of layers, stacking on the transducer when drying or homogeneous transducer coverage, and thus do not lend themselves easily to mass production [19]. Lin et al. [20] reported a rapid, solventless, and readily scalable method to prepare various metal nanoparticle-decorated carbon nanotubes from the thermal decomposition of metal acetate/carbon nanotube solid mixtures without the use of any reducing agent. This simple but effective “mix-and-heat” method was also successfully applied to various other carbon substrates with the use of many different metal acetates. This reported method makes it possible to prepare TIMs with Ag-GNSs as thermal conductive fillers in mass production.

In this paper, “mix-and-heat” method is applied to prepare Ag-GNSs. Different loadings of Ag nanoparticles on the GNS surfaces are studied. The scanning electronic microscopy (SEM) is applied to obtain the morphology of Ag-GNSs. X-ray diffraction is also employed to characterize the obtained product. The obtained product as thermal conductive fillers is used to prepare epoxy resin based TIMs. The k of TIM samples is tested by using thermal conductivity meter (C-THERM TCI). The results show that the Ag-GNSs are perfect thermal conductive fillers due to lower loading of fillers can make great k enhancement of epoxy resin based TIMs. The weak temperature dependence of k is beneficial for TIM applications and can be obtained by controlling the addition of hybrid thermal fillers and the loading of silver nanoparticles.

Section snippets

Synthesis of thermal conductive fillers

The Ag-GNSs as thermal conductive fillers are synthesized by using the reported method [20]. In this report it has realized the decoration of Ag nanoparticles on the surfaces of carbon nanotube, carbon nanofiber, expanded graphite, and carbon black. It also deduces that other metal nanoparticles such as Au, Ni, Co, and Pd also can decorate the surfaces of the mentioned carbon materials. Typically, a certain proportion of silver acetate powders and GNSs with a total weight of 5 g (0.5 mol % Ag

Morphology of Ag nanoparticle-decorated GNSs

The formation of Ag nanoparticles on the GNS surfaces is achieved by using a straightforward “mix-and-heat” process [20] and the SEM images of Ag-GNS nanohybrid with different Ag loadings are shown in Fig. 1. Fig. 1(a) clearly shows that the GNSs are multilayer ones, which supply wide surfaces for depositing Ag nanoparticles. As shown in Fig. 1(b) the average size of the Ag nanoparticles is about 30 nm except several bigger ones as the Ag loading is 0.5 mol %. With the increase of Ag loading,

Conclusion

In summary, epoxy resin based TIMs with strong enhancement of k have been prepared by using graphene decorated with silver nanoparticles as hybrid thermal fillers. The physical processes of the thermal filler loading, quantity of decorated silver nanoparticles, and temperature resulting in such k are complicated. The obtained results make it possible to develop high-efficiency TIMs for advanced electronics. The obtained results are also very essential for thermal management of advanced

Acknowledgments

This work was supported by the National Science Foundation of China (51306109& 51176106), the Basic Research Foundation of Shanghai Science and Technology Committee (12JC1404300), and Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.

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