RGO and Three-Dimensional Graphene Networks Co-modified TIMs with High Performances
© The Author(s). 2017
Received: 10 July 2017
Accepted: 29 August 2017
Published: 6 September 2017
With the development of microelectronic devices, the insufficient heat dissipation ability becomes one of the major bottlenecks for further miniaturization. Although graphene-assisted epoxy resin (ER) display promising potential to enhance the thermal performances, some limitations of the reduced graphene oxide (RGO) nanosheets and three-dimensional graphene networks (3DGNs) hinder the further improvement of the resulting thermal interface materials (TIMs). In this study, both the RGO nanosheets and 3DGNs are adopted as co-modifiers to improve the thermal conductivity of the ER. The 3DGNs provide a fast transport network for phonon, while the presence of RGO nanosheets enhances the heat transport at the interface between the graphene basal plane and the ER. The synergy of these two modifiers is achieved by selecting a proper proportion and an optimized reduction degree of the RGO nanosheets. Moreover, both the high stability of the thermal conductivity and well mechanical properties of the resulting TIM indicate the potential application prospect in the practical field.
KeywordsThermal Interface Materials Graphene Thermal Boundary Resistance
Graphene-assisted thermal interface materials (TIMs) have attracted increasing attention because of their high thermal and mechanical performances [1–5]. Kim et al. reported that the resulting thermal conductivity is 1400% higher than the pristine epoxy resin (ER), and Joen’s group found that a 10 wt% additional graphene filler will bring about a high thermal conductivity (~ 2 W/mK) [3, 4]. However, considering the theoretical thermal conductivity of this unique material is as high as 5000 W/mK , the reported results are far from satisfactory. Although graphene is expected to act as the fast transport channel for phonon in the TIMs during the thermal transport process, the nano-scaled RGO sheets lack a continuous structure to form the transport network. Moreover, overmuch interfaces of the RGO nanosheets lead to a high total thermal boundary resistance (Kapitza scattering), which results in a strong phonon scattering . At last, the high defect density of the RGO nanosheets due to the violent oxidation-reduction processes also brings about an extra thermal resistance source (shortening the mean free path of phonon, Umklapp scattering) .
In order to give full play to the high thermal conductivity of the adopted graphene, high-quality three-dimensional graphene networks (3DGNs) prepared by chemical vapor deposition method have been adopted to hybridize with ER by our group . The better thermal and mechanical properties of the 3DGNs-ER (compared with that of the RGO-based sample) manifest the fatal significance of the low defect density and the continuous construction of the employed graphene . On the other hand, originating from the absence of surface functional groups of the 3DGNs, a bottleneck, a bed contact between the 3DGNs and ER (a poor wettability of the 3DGNs), is revealed with the ongoing study. Based on our recent report, a moderated amount of surface defects of the 3DGNs can play as a positive role to improve the contact between the graphene basal plane and matrix [10, 11]. However, some tedious adjustment processes including a precise CH4 flow and a strict cooling rate of the substrate are needed during the CVD procedure . Therefore, an idea on combining the RGO nanosheets and 3DGNs to utilize their advantages is naturally presented.
In this study, the RGO nanosheets and 3DGNs are adopted as fillers to enhance the thermal performances of the resulting ER. The specific functions of these two modifiers are discussed and proven. On the one hand, the 3DGNs provide a fast transport network, increasing the average mean path of phonons. On the other hand, the RGO nanosheets on the 3DGN surface improve the contact at the interface of the graphene basal plane and ER remarkably, which depresses the interface scattering of phonons. The further improvement of the resulting thermal performance resulting from the synergy of the RGO nanosheets and 3DGNs indicates that utilizing graphene with an optimizing manner is a useful strategy to prepare the high-performance TIMs.
Nickel foam with 300 gm−2 in areal density and 12 mm in thickness was purchased from Haobo Co., Ltd. (Shenzhen, China) and used as a template to fabricate the 3DGNs. Ethanol, HCl, FeCl3, and poly(methyl methacrylate) (PMMA, average molecular mass 996,000, 4% in ethyl lactate) were obtained commercially from the Beijing chemical reagent plant (Beijing, China). Ethyl lactate, natural graphite, poly(methyl methacrylate), and acetone were received from Aladdin Co., Ltd. Polytetrafluoroethylene (PTFE) and sodium dodecyl benzene sulfonate were purchased from the Huangjiang Co., Ltd. (Dongguan, China). ER and curing agent were purchased from Sanmu Co. Ltd. (Suzhou, China). Deionized water (resistivity 18 MΩcm) was utilized to prepare all aqueous solutions.
The preparation of the RGO nanosheets and 3DGNs has been reported by our group [12–14], and more details are provided in the Supplementary materials. The RGO-3DGNs-ER composite was fabricated by a two-step method. Firstly, the combination of the RGO nanosheets and 3DGNs is achieved by a simple hydrothermal method. A certain amount of the RGO nanosheets and 3DGNs was added into 50 ml deionized water, and a 30-min ultrasonic process is carried out. After that, 1 mg sodium dodecyl benzene sulfonate was added, and then the mixture was moved into a Teflon vessel for hydrothermal reaction at 80 °C for 6 h. Then, the resulting material was washed with deionized water for three times, and the RGO nanosheets were loaded on the surface of the 3DGNs. Secondly, the preparation of the RGO-3DGNs-ER is similar with our reported 3DGNs-ER . Briefly, a certain amount of prepared RGO-3DGNs was put into a mold, and the ER including the curing agent was dropped on the solid surface. After dropping a layer of the ER, the RGO-3DGNs was added again. The two steps are repeated for three or four times. The dropped ER penetrates into the porous RGO-3DGNs by capillary effect. Finally, the RGO-3DGNs-ER mixture was cured at 110 °C for 3 h.
Morphology of the TIMs was obtained by a scanning electron microscope (SEM, FEI Sirion 200 scanning electron microscope working at 5 kV) and transmission electron microscope (TEM, JEM-2100F, operated at an accelerating voltage of 20 kV). Atomic force microscopy (AFM) results were recorded by Nanoscope IIIa (Digital Instrument, USA) and E-Sweep (Seiko, Japan) in tapping mode. Scanning Raman spectra were recorded by LabRam-1B Raman microspectrometer at 532 nm (Horiba Jobin Yvon, France). X-ray photoelectron spectroscopy (XPS) measurements were performed on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer). Fourier transform infrared spectroscopy (FTIR) curves were measured on IR Prestige-21 system (PerkinElmer). Mechanical properties of these composites were recorded by a Triton DMTA (Triton Instrument, UK) instrument. The Tg and storage modulus were measured at a frequency of 1 Hz and a heating rate of 5 °C min−1 according to ASTM1640 and analyzed in the tensile mode. The dimensions of the samples were 2 × 4 cm. Laser flash analysis and differential scanning calorimetry were used to analyze the thermal transport performance of the fabricated composites.
Results and Discussion
As last, the mechanical properties of these TIMs are also recorded. The corresponding performances including ultimate strengths and stretching limits of them are listed in Additional file 1: Table S1. Both the 3DGNs-ER and RGO-3DGNs-ER samples display the high mechanical strength because the continuous 3D structure of the 3DGNs is beneficial to keeping the outstanding intrinsic mechanical property of the graphene. After comparing the performances of the 3DGNs-ER and RGO-3DGNs-ER samples, it can be inferred again that the RGO nanosheets are loaded on the surface of the 3DGNs rather than dispersed in the ER matrix because the influence from the added RGO nanosheets can be ignored.
The RGO nanosheets and 3DGNs co-modified ER has been prepared to prepare the TIMs. The advantages of the RGO nanosheets and 3DGNs can give full play to loading the RGO nanosheets on the surface of 3DGNs (by a hydrothermal process) rather than dispersing in the ER matrix. The presence of the 3DGNs not only provides a fast transport network for phonons but also acts as a scaffold for the RGO nanosheets. On the other hand, the surface functional groups of the RGO nanosheets enhance the close contact between the graphene basal plane and ER at their interface, which offsets the poor wettability of the 3DGNs. Therefore, the thermal performance of the resulting TIM is enhanced significantly (a high thermal conductivity ~ 4.6 W/mK is achieved when a 9 wt% 3DGNs and 1 wt% RGO nanosheets are added, which is 10 and 36% higher than those cases of 10 wt% 3DGNs and 10 wt% RGO nanosheet added samples), and a well stability of the thermal performance of the resulting TIM is revealed under high temperature (at 100 °C, the decrease of the thermal conductivity is less than 25%). Moreover, the excellent mechanical properties including high ultimate strength and stretch limits indicate the potential promising prospect of the presented TIM.
This work is supplied by National Natural Science Foundation of China (51506012) and Natural Science Foundation of Jiangsu Province (BK20150266).
This manuscript is written by TB and WH. The preparation of samples is performed by WZ and LS. The characterization of samples and preparing experiments are made by MT, YH, and LX. The analysis and discussion of the obtained results are carried out by TB, WZ, MT, and HW. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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