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Acta Metall Sin  2017, Vol. 53 Issue (7): 869-878    DOI: 10.11900/0412.1961.2017.00015
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Effect of Graphite Flake Size on the Strength and Thermal Conductivity of Graphite Flakes/Al Composites
Xiaoyun LIU1,2,Wenguang WANG2,Dong WANG2,Bolv XIAO2,Dingrui NI2,Liqing CHEN1,Zongyi MA2()
1 State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China
2 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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Abstract  

Graphite flakes reinforced Al matrix composites (Gf /Al) with low density, good machining property and high thermal conductivity are considered an excellent heat sink materials used in electronic industry. When the composites are manufactured by liquid method such as liquid infiltration, it is easy to achieve a high thermal conductivity composite. However, the Al4C3 phase would be formed in the composite, which will decrease the corrosion properties of the composites. The powder metallurgy technique could avoid the formation of the Al4C3 phase. In this work, three seized graphite flakes (150, 300, 500 μm) were used to investigate the effect of the graphite flake size on the strength and thermal conductivity of Gf/Al alloy composites. The 50%Gf /Al alloy (volume fraction) composites were fabricated by the powder metallurgy technique. The density of all the three Gf /Al alloy composites were similar to the theoretical density. The graphite flakes had a well bonding with Al alloy matrix without cracks and pores. The (001)Gf basal plane of the graphite flakes were almost parallel to the circular plane (xy plane) of the composites ingot. However, for the small graphite flakes, their (001)Gf basal plane was not well parallel to the xy plane of the composite ingot due to the powder metallurgy process. For the large graphite flakes, they exhibited a good orientation in the xy plane of the composite ingot. The strength of the Gf /Al alloy composites decreased with the increase of the graphite flake size. For the 150 μm graphite flake, the bending strength of the Gf /Al alloy composite was 82 MPa. However, for the 500 μm graphite flake, the bending strength of the composite decreased to 39 MPa. Due to the low strength between the layers of the graphite flake, the cracks were prone to expand in the graphite flake. As the size of the graphite flake increased, this phenomenon became more obviously. It is easy to observe that the graphite flakes peeled off on the fracture surfaces. When the size of the graphite flake increased from 150 μm to 500 μm, the thermal conductivity increased by 63%. The highest thermal conductivity was 604 W/(mK). The interfacial thermal conductance (hc) of the composites were calculated by the Maxwell-Garnett type effective medium approximation model. The hc of 300 and 500 μm graphite flake Gf /Al alloy composites were slightly lower than the theoretical value (calculated by the acoustic mismatch model). However, the hc of the 150 μm graphite flake Gf /Al alloy composite was lower than that of the theoretical value. Besides the size of the graphite flakes, the shape, distribution and defect of the graphite flakes also influenced the thermal conductivity of the composites.

Key words:  graphite flake      aluminum matrix composite      thermal conductivity      mechanical property     
Received:  13 January 2017     
Fund: Supported by National Natural Science Foundation of China (Nos.U1508216 and 51271051)

Cite this article: 

Xiaoyun LIU,Wenguang WANG,Dong WANG,Bolv XIAO,Dingrui NI,Liqing CHEN,Zongyi MA. Effect of Graphite Flake Size on the Strength and Thermal Conductivity of Graphite Flakes/Al Composites. Acta Metall Sin, 2017, 53(7): 869-878.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00015     OR     https://www.ams.org.cn/EN/Y2017/V53/I7/869

Fig.1  SEM images of graphite flakes in different sizes

(a) 150 μm (b) 300 μm (c) 500 μm(d) side of the 500 μm graphite flake

Fig.2  XRD spectra of graphite flakes in different sizes
Fig.3  SEM images of graphite flake Gf /Al composites in different sizes

(a) 150 μm (b) 300 μm (c) 500 μm

Fig.4  Magnified SEM images of graphite flake Gf/Al composites in different sizes (Insets show the magnified images of Gf/Al interface)

(a) 150 μm (b) 300 μm (c) 500 μm

Fig.5  TEM (a) and HRTEM (b) images of the interface between graphite flake and Al matrix in 500 μm graphite flake Gf /Al composites
Graphite flake size Relative density Bending strength Thermal conductivity
μm (ρexp /ρthe) / % MPa Wm-1K-1
150 99.9 82 370
300 99.5 42 480
500 99.6 39 604
Table 1  Relative densities and properties of graphite flake Gf /Al composites in different sizes
Fig.6  Low (a, c, e) and high (b, d, f) magnified fracture SEM images of graphite flake Gf /Al composites in different sizes

(a, b) 150 μm (c, d) 300 μm (e, f) 500 μm

Material Density Thermal conductivity Specific heat Phonon velocity
kgm-3 Wm-1K-1 Jkg-1K-1 ms-1
Graphite 2200 1200 710 14800
Al 2700 180 895 3620
Table 2  Parameters of materials for AMM model[24,25]
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