Exceptionally high thermal conductivity of thermal grease: Synergistic effects of graphene and alumina
Introduction
Recently, with the miniaturization and increasing power of electronic devices, the dissipation power density increases rapidly in these downscaled devices. The heat dissipation has become one of the most critical problems that limit the performance, power, reliability and further miniaturization of electronic devices [1], [2], [3]. It is well-known that the reliability of devices is exponentially dependant on the operating temperature of the junction, whereby a small difference in operating temperatures can result in a two times reduction in the lifespan of a device [4]. Therefore the heat generated from the devices should be removed as quickly and effectively as possible to maintain the temperatures of the devices at a desired level. Traditionally, heat sinks are used to dissipate the heat from the devices. However, the performance of a heat sink to dissipate heat is sharply limited due to interfacial thermal resistance arising from the mismatch of non-surface flatness and surface roughness of both the devices and the heat sink [5], [6].
For the sake of reduction of interfacial thermal resistance, suitable thermal interface materials (TIMs) are utilized to bond the contact surfaces of heat sink and devices [7]. An ideal TIMs possessing high thermal conductivity and strong deformability must conform to the surface topography of the mating surfaces and can take the place of air and fill in the gaps at the interface [8], [9], [10]. Because the thermal conductivity of TIMs is much higher than the replaced air (which is only 0.026 W/m K), the interfacial thermal resistance is reduced giving rise to low temperature of the electronic component junction [2]. Currently, thermal interface materials can be classified as thermal greases, elastomeric thermal pads, solders and phase change materials [7], [10], [11], among which thermal greases, a type of thixotropic pastes with high thermal conductivity, can fill the gaps of the mating surface more effectively. Thermal greases are generally made of two primary components i.e. a polymer base and ceramic fillers. Silicone is commonly used as the base for its good thermal stability, wetting characteristics and low modulus of elasticity [12], [13]. Ceramic fillers [13], [14], [15], [16] such as alumina, aluminum nitride, boron nitride, silicon carbide, which are highly thermally conductive but electrically insulative, have been used in this system.
Graphene is a two-dimensional nanocarbon material with the hexagonal packed structure compromised of sp2-hybridized carbon atoms. Graphene and its derivatives have attracted intensive interest in many fields due to their exceptional physical and chemical properties [17], [18], [19], such as high electrical conductivity [20], superb thermal conductivity [21] and high mechanical strength [22], [23], etc. The superb thermal conduction property of graphene discovered by Balandin et al. [21], revealed the thermal conductivity values in the range from 2000 W/mK to 5200 W/m K near room temperature. This discovery has opened a new window for graphene's applications in thermal management fields. Yu et al. [24] explored the graphite nanoplatelet (GNP)-epoxy composite thermal interface materials and attributed the outstanding thermal properties of TIMs to a favorable combination of the large aspect ratio, two-dimensional geometry, stiffness, and low thermal interface resistance of the GNPs. Shahil et al. [25] synthesized polymer composites filled with graphene and multilayer graphene. It could lead to an extremely strong enhancement of the cross-plane thermal conductivity of the composites. The modeling results suggested that these TIMs outperform those with carbon nanotubes or metal nanoparticles owing to graphene's large aspect ratio and low Kapitza resistance at the graphene–matrix interface. The thermal conductivity of graphene prepared by different methods differs greatly. Yu et al. [26] reported silicone thermal greases based on graphene prepared by different procedures, that an enhancement of thermal conductivity was obtained. Theoretical model analyzing validated that graphene is an effective thermally conducting filler to let grease have high thermal conductivity with low filler content. Goli et al. [27] fabricated graphene-enhanced hybrid phase change materials for thermal management of Li-ion batteries, which showed record-high thermal conductivity enhancement. Based on the rapid growth of interest in the thermal properties of graphene, Balandin [28] reviewed the thermal properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. He also described the prospects of applications of graphene for thermal management of electronics.
The idea of using hybrid fillers containing two or more traditional fillers has already been explored in some literature. It has been demonstrated that the improvements of composite performances can be achieved by combining the advantages of each filler [29], [30]. For example, the complex systems of boron nitride and multi-walled carbon nanotube [31], graphite nanoplatelet and carbon nanotube [32], alumina and zinc oxide [6], graphene and silver particle [33] have been used for improving the thermal conductivity of thermal interface materials. Up to now, study on the thermal conductive silicone grease reinforced with hybrid filler consisted of graphene and alumina as we know it has not been reported. Generally speaking, TIMs are electrical insulating materials to prevent short circuit of electronic devices. Novel thermal grease with high thermal conductivity and electric insulation is developed by addition of graphene and alumina in this study. Due to the synergistic effects of graphene sheets and alumina particles, a significant thermal conductivity enhancement is observed.
Section snippets
Materials and sample preparation
The α-Al2O3 with a purity of 99.5% and average particle sizes of 0.7 and 5 μm were used as received from Nippon Steel Materials Co., Ltd., Japan. The silicone oil and flake graphite were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China and used directly without further purification. The other reagents were analytical grade and also purchased from Sinopharm Chemical Reagent Co., Ltd.
The graphite oxide was prepared by the improved Hummers' method [34], and then the graphene
Morphology of samples
The size and the shape of fillers have an effect on thermal conductivity of composite. The size and the shape of different Al2O3 particles were observed by scanning electron microscope, as shown in Fig 1. The Al2O3 particles are almost spherical with average particle sizes of 5 (Fig. 1a) and 0.7 μm (Fig. 1b), respectively. As seen from the SEM image in Fig. 1c, a fluffy and crumpled morphology of graphene is viewed, which also demonstrates thin and wrinkled platelets transparent to electrons.
Conclusion
Novel thermal grease was prepared by mixing graphene and Al2O3 particles in silicone base, in which a synergistic improvement on thermal conductive properties was observed. The maximum thermal conductivity of the novel thermal grease was 3.45 W/m K and was significantly improved compared with the thermal grease without graphene. Meanwhile an enhancement of thermal conductivity of 2553% was obtained with respect to the silicone base. We proposed a correction theoretical predictive model by
Acknowledgments
The work was supported by National Natural Science Foundation of China (51476094, 51106093 & 51306109), the Basic Research Foundation of Shanghai Science and Technology Committee (12JC1404300), Innovation Program of Shanghai Municipal Education Commission (14ZZ168 & 14cxy37), Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning and the Key Subject of Shanghai Second Polytechnic University (No.4, Material Science, XXKPY1302).
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