Enhanced through-plane thermal conductivity and mechanical properties of vertically aligned graphene nanoplatelet@graphite flakes reinforced aluminum composites
Graphical abstract
Introduction
Along with the rapid development of electronic devices towards miniaturization and high power density [[1], [2], [3]], the demand of electronic packaging materials with high heat dissipation capacity is increasing [[4], [5], [6], [7], [8]]. To meet this ever-increasing demand, it is critical to develop the thermal management materials with high thermal conductivity (TC) and low coefficient of thermal expansion (CTE) [9]. Pure metals such as Cu, Al, and Ag are traditional thermal management materials with relatively high TC, however, the application of these materials is restricted by their high CTE. By incorporating ceramic materials, such as BeO [10], AlN [11] and SiC [12] as reinforcements to metal, the metal matrix composites with both high TC and low CTE are obtained. Among these composites, aluminum matrix composites have attracted much attention due to its lightweight, high strength-to-weight ratio and excellent thermal properties [13,14]. However, Al composites reinforced by the reinforcements mentioned above still cannot meet the ever-increasing requirement for thermal dissipation.
In recent years, high-performances carbon materials, such as graphite(Gr) flake [15,16], diamond [[17], [18], [19]], carbon nanotube [[20], [21], [22]] and graphene [[23], [24], [25]], have been considered as reinforcements for thermal management. Among them, graphite flake has proven to be an effective heat dissipation material due to their low density, high TC, low CTE, low cost and great processability. For the Gr/Al composites consolidated by powder metallurgy, the in-plane TC increases from 324 to 783 W/mK with the Gr flakes content raising from 10 to 80 vol% [15]. For the Gr/Al composites prepared by squeezing casting, their TC can increase from 544 to 714 W/mK with the Gr flakes content raising from 40 to 70 vol% as a result of the preferred orientation of Gr flakes and the well-controlled interface reaction [16]. However, the TC of natural Gr flakes is largely anisotropic with the in-plane TC about 1000 W/mK [26] and through-plane TC < 10 W/mK [27]. As typical two-dimensional carbon materials, the Gr flakes can largely decrease the through-plane TC of Al (TC ~ 237 W/mK) composites. Xue et al. prepared the graphite/Al composites with SiC coated on graphite surface and the through-plane TC decreased from 60 to 43 W/mK with the Gr flakes content raising from 40 to 70 vol% [28]. Zhou et al. incorporated graphite flakes into Al alloy with Si particles as spacers by squeeze casting to obtain graphite/Al composites and the through-plane TC ranged from 80 to 50 W/mK with the Gr flakes content raising from 25 to 53 vol% [29]. The relatively lower through-plane TC limits the application of Gr/Al composites in areas requiring through-plane heat dissipation. For example, for composites joints and structural components in space industry, the high through-plane TC is expected to ensure efficient cooling and improve energy conversion [30]. Besides, the mechanical strength of Al composites may sharply drop due to the addition of Gr flakes. With the Gr flakes content raising from 40 to 70 vol%, the in-plane and through-plane flexural strength of Gr/Al composites prepared by squeezing casting decrease from 8.83 to 2.23 MPa and from 43.6 to 16.9 MPa, respectively [16]. Compared with Al matrix, the Gr/Al composite with much lower mechanical strength is not strong enough to keep the heat dissipation components away from thermo-mechanical damages [28]. Therefore, how to enhance the through-plane thermal conductivity and mechanical strength simultaneously remains a puzzling problem. Recently, in the field of polymer matrix composites, effective through-plane TC channels with high mechanical properties were constructed on carbon fiber and graphite block by growing CNT vertically [31,32]. Similarly, GNP with excellent thermal and mechanical properties can also be aligned vertically [33]. Besides, growing GNP vertically on the graphite was proved to be practicable [34]. However, there is little work on the Gr flakes modified with well-constructed through-plane TC channels reinforced aluminum matrix composites even metal matrix composites.
In this study, graphene nanoplatelet (GNP) was vertically grown on the surfaces of Gr flakes by chemical vapor deposition (CVD) to prepare the Gr(GNP) hybrid. 55 vol% Gr(GNP)/Al composites with different GNP contents were fabricated by means of vacuum hot-pressing sintering. Morphologies of Gr(GNP) were applied to investigate the effects of gas carbon sources on the growth density of GNP. Graphitization degree of Gr(GNP) was conducted to study the effect of carbon sources on the qualities of Gr(GNP). Microstructural characterizations of the fabricated Gr(GNP)/Al composites were used to study the effect of the orientations and distributions of Gr(GNP) in the Al matrix. Fractographies of the Gr/Al and Gr(GNP)/Al composites were obtained to study the effect of GNP on the flexural strength of Gr/Al. Nanoindentation of Gr/Al and Gr(GNP)/Al composites were used to study the effect of GNP on the interface of Gr/Al. Well-constructed through-plane channels in Gr(GNP) with varying GNP contents can effectively improve the thermal conductivities and mechanical properties of Gr(GNP)/Al composites, and make it a promising candidate in the thermal management applications.
Section snippets
Raw materials
Commercial Gr flakes (99.9% in purity, ~500 μm in diameter, Tianshengda Co. Ltd.) and pure Al powders (99.9% in purity, ~50 μm in diameter, Aladdin Co. Ltd.) were selected as the raw materials.
Preparation of Gr(GNP) hybrid
In order to effectively remove the impurities on the surface of Gr flakes, 50 g natural Gr flakes were cleaned ultrasonically for 30 min in acetone (200 ml) firstly and then absolute ethanol (200 ml), respectively. Afterwards, the filtered Gr flakes were placed in a vacuum drying oven at 80 °C for
Characterization of Gr flakes and Gr(GNP)
Fig. 2 shows the microstructures, XRD pattern and Raman spectra of Gr flakes. Gr flakes with a size of several hundred microns are irregular in shape and have a large number of edges, as shown in Fig. 2a. The sag and broken slices appearing on the surface of Gr flakes can provide the nucleation agent for the growth of GNP (Fig. 2b). From the XRD pattern of Gr flakes (Fig. 2c), two diffraction peaks are observed. The sharp diffraction peak attributed to (002) crystal planes happens at the
Conclusion
A type of carbon/metal composites, namely Gr(GNP)/Al composite with low anisotropy, excellent thermal conductivity and high mechanical properties strength, have been fabricated by the method of vacuum hot pressing sintering. GNP was synthesized directly on the surface of graphite flake by CVD method and distributed uniformly on the graphite flake. CH4 flow rate affects the morphology of the Gr(GNP) prepared. As CH4 flow rate increases, distribution uniformity of GNP gradually gets worse which
CRediT authorship contribution statement
Chaoyu Wang:Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing.Yishi Su:Methodology, Writing - review & editing.Qiubao Ouyang:Conceptualization, Funding acquisition.Di Zhang:Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the National Key Research and Development Program of China (No. 2018YFB0704400) and National Natural Science Foundation of China (No. 51671129, 51971132, 51501111, 51471106).
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