The former is usually carried out by the self-assembly and hydrothermal reduction of graphene oxide (GO) in aqueous solution followed by freeze-drying or air-drying to achieve the graphene framework, as shown in Figure 3a. So far, the approaches for the fabrication of a graphene framework can be mainly classified into two categories, the self-assembly method and the template-synthesis method. In recent years, the penetration of polymers into three-dimensional graphene frameworks has been considered as a promising solution to develop composites with improved thermal conductivity for TIM applications. Even so, the development of oriented alignment and surface modification technology of graphene filler may break this bottleneck. This less than satisfactory progress is mainly attributed to two factors, firstly the difficulty of graphene dispersion in a polymer matrix ( Figure 2b) as a result of the strong π–π interaction between graphene sheets, and secondly, the strong interfacial phonon scattering of the graphene and polymer leading to the low effective thermal conductivity of the graphene filler. One other thing to note is that there has been no obvious improvement in thermal conductivity of dispersed graphene/polymers during the last decade. However, such high graphene content usually results in high viscosity for printing operations and poor mechanical properties for the cured composites, hardly meeting the demands of current industry for practical applications. As seen from the table, to get the desired thermal conductivity beyond conventional TIMs (5 W/mK), 20–50 wt % of graphene loading in polymer is essential. A summary of thermal conductivity (κ) of the dispersed graphene/polymers is listed in Table 1. In earlier research, graphene-based TIMs were prepared through a solution or melt-blending process to disperse graphene sheets into a polymer matrix ( Figure 2a). Therefore, the development of new generational TIMs with improved thermal conductivity has drawn extensive attention and devotion for many researchers. However, with the shrinking feature size and improving power density of electronic devices, the accompanying thermal management has gone beyond the processing capacity of conventional TIMs. Conventional TIMs are made up of polymer matrices filled by thermally conductive materials (boron nitride, aluminum nitride, alumina, etc.) to achieve the thermal conductivity of 1–5 W/mK (50–70 wt % filler loading). In actual working status ( Figure 1), TIMs with high through-plane thermal conductivity and good compressibility bridge the heater and heat sink, filling the unavoidable air gaps between their mating interface. Particularly, thermal interface materials (TIMs) are an important and indispensable part for the efficient transfer/removal of generated heat from heaters (e.g., semiconductor chips) to avoid electronic devices working under overheating conditions. Thermal management has become a central issue to guarantee the reliability and service life of electrical and electronic devices, such as highly integrated CPUs, high-power LED lights and energy harvesting systems.
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