Mechanical and thermal properties of carbon nanotube/aluminum composites consolidated by spark plasma sintering
Highlights
► MWNTs/Al composites have been produced by spark plasma sintering. ► The highest tensile strength of 130 MPa was achieved at 0.5 wt.% MWNTs. ► The highest thermal conductivity of 199 W/m/K was achieved at 0.5 wt.% MWNTs. ► The enhanced mechanical and thermal properties are due to MWNTs bridging.
Introduction
Metal matrix composites (MMCs) with enhanced thermal conductivities have received substantial attention for packaging materials applications such as microelectronics and optical communications [1], [2], [3], [4]. Carbon nanotubes (CNTs) are one of novel reinforcements for composites due to their remarkable mechanical and thermal properties [5], [6], [7], [8], [9]. According to theoretical predictions and experimental measurements, the thermal conductivity of CNTs reaches as high as 3000–6600 W/m/K [10], [11]. Aluminum is one of the most important matrix materials for MMCs. Mechanical properties of CNTs reinforced Al matrix composites (CNTs/Al composites) have been intensively studied.
So far only a few reports have dealt with thermal conducting behavior of CNTs/Al composites. Bakshi et al. [12] produce a 10 wt.% CNTs/Al composite using plasma spraying and report a thermal conductivity of just 25.4 W/m/K, much less than that of pure Al. Yamanaka et al. [13] report that the thermal conductivity of CNTs/Al composites is decreased with increasing CNT content. Thermal conductivity reported for CNTs/Cu composites [14], [15] also shows a decreasing tendency compared with pure Cu. The thermal conductivity decline is mainly associated with CNTs agglomeration that has introduced thermal interface barriers into the composites. Two requirements should be satisfied in order to achieve high thermal conductivity in CNTs/metal composites – the composites have low interfacial thermal resistance and CNTs are homogeneously dispersed in metal matrix. The present study focuses on the thermal conducting behavior of aluminum matrix composites reinforced with multi-walled carbon nanotubes (MWNTs/Al composites) that are produced by spark plasma sintering (SPS). The SPS processing is a very useful technique for densifying unsinterable materials such as carbon nanotubes. Sound interfacial bonding between reinforcements and matrix can also be formed by strong spark plasma [5]. In order to generate homogeneous dispersion of carbon nanotubes in aluminum matrix, the powder mixture is carefully modified before SPS processing.
The purpose of this paper is to demonstrate the enhancement of both tensile strength and thermal conductivity by dispersing MWNTs in Al matrix. Theoretical models are combined to examine the contribution of dispersed carbon nanotubes to thermal conducting. It also suggests that the thermal conductivity of MWNTs/Al composites is reduced again by adding >1.0 wt.% MWNTs, owing to the difficulty in dispersing high fraction carbon nanotubes.
Section snippets
Experimental procedures
MWNTs (diameter 20–30 nm, length 10–30 μm) synthesized by a chemical vapor deposition method were used as the reinforcements. Gas atomized Al powders (particle size 3–5 μm) were employed as the matrix. The raw MWNTs were suspended in a nitric acid and then electromagnetically stirred in a water bath at 373 K for 6 h in order to remove the impurities and to disperse the MWNTs. After the acid treatment, the MWNTs were cleaned several times using deionized water and ethanol, before drying at 393 K for 2
Tensile strength
Fig. 2 shows the nominal stress–strain curves of the pure Al and MWNTs/Al composites. Compared with the pure Al, the elongation of the composites showed a sharp decline because of the adding of MWNTs. The tensile strength firstly increased and reached a maximum 130 MPa at 0.5 wt.% MWNTs, and then showed a decreasing trend with further increasing MWNTs content. The tensile strength was decreased to a level close to that of the pure Al when 2.0 wt.% MWNTs are added. The 0.5 wt.% MWNTs/Al composite
Conclusions
The 0–5.0 wt.% MWNTs-reinforced Al matrix composites were successfully produced by spark plasma sintering. The maximum thermal conductivity of 199 W/m/K and highest tensile strength of 130 MPa were obtained for 0.5 wt.% MWNTs addition. The enhanced thermal conductivity and tensile strength were ascribed to the bridging of CNTs in the composites. The thermal conductivity and tensile strength were decreased to lower than that of pure Al when >1.0 wt.% MWNTs are added. This is mainly due to CNTs
Acknowledgements
This work was financially supported by the Ministry of Education of China (No. 707007) and Beijing Municipal Commission of Education of China (No. 2008100071601Y). H.L.Z. was also supported by the Program for New Century Excellent Talents in University (No. NCET-10-0227).
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