2020 Volume 61 Issue 4 Pages 626-631
To solve the problem of pore formation caused by the aggregation of carbon nanotubes (CNTs) in metal matrix composites, unidirectionally aligned CNT (CNT blocks) were used as the reinforcement material. CNT block preforms with high porosities were fabricated via a spacer method using electroless Cu-plated CNT blocks. During the preform manufacture, the thickness of the Cu layer was varied while maintaining a constant CNT block volume fraction (10%). In addition, CNT block-reinforced Al matrix (CNT block/Al) composites were manufactured using a low-pressure infiltration method at 0.1 MPa. The interface between the CNT blocks and the Al matrix and the reactivity of the Al matrix with the Cu layer were investigated. The composites with the CNT block:Cu ratios ≤6.8:3.2 showed an improved CNT block/Al interface. At the CNT block:Cu ratio of 3.7:6.3, an intermetallic compound, Al2Cu, was formed by the reaction between Cu and Al. Furthermore, the thermal conductivities of the fabricated CNT block/Al composites were determined.
Schematic diagram of CNT blocks/Al composite with different thickness of eletroless Cu plating layer.
Carbon nanotubes (CNTs) are considered to be ideal reinforcements for high-performance metal matrix composites (MMCs) because of their outstanding mechanical and thermal properties such as high tensile strength, high elastic modulus, high thermal conductivity (TC), and ultra-low thermal expansion.1–3) The TC of CNTs is 3000–6000 W·m−1·K−1, which is higher than that of carbon fibers (∼800 W·m−1·K−1) and about 10 times higher than those of Al (237 W·m−1·K−1) and Cu (386 W·m−1·K−1). However, dispersed CNTs always agglomerate and tangle together owing to their very strong van der Waals forces and high aspect ratios, and hence lead to the formation of numerous pores in CNT-reinforced composites. This limits the application of CNTs for the fabrication of MMCs. Accordingly, MMCs with dispersed CNTs cannot be effectively used as heat sink materials.4,5) CNTs can be easily aligned in a single direction (by van der Waals forces). Aligned CNTs (CNT blocks) show particle structures. Therefore, the fabrication of CNT block-reinforced MMCs can be simplified by using CNT blocks instead of CNTs (dispersion step can be avoided). The use of CNT blocks yields MMCs with fewer pores. Such CNT block-reinforced MMCs are considered to be potential candidates for heat sink applications.
The low-pressure infiltration (LPI) method has been used to fabricate MMCs with high volume fractions of reinforcements.6) In this method, a molten metal is infiltrated into a porous preform at a low pressure. To improve the wettability of CNT blocks and the molten metal, electroless Cu or Ni plating is often carried out as a surface treatment.4) In addition, the plating layer serves as the binder, facilitating the sintering of CNT blocks during the fabrication of preforms. In this study, preforms were fabricated using NaCl as the spacer. The sintering temperature of Ni (>1173 K) is higher than the melting point of NaCl (1074 K). Thus, electroless Cu plating was used in this study. As a binder, the Cu layer must be sufficiently thick. However, the Cu layer and molten Al decrease the TC of the resulting intermetallic compound (IMC).7) Therefore, the Cu layer significantly affects the microstructure and TC, and hence the performance of the MMC.
In this study, the CNT block-reinforced Al matrix (CNT block/Al) composites were fabricated using the LPI process, and the effects of the Cu layer on its microstructure and TC were investigated. The microstructures of the fabricated electroless Cu-plated CNT blocks and CNT block/Al composites were examined. Moreover, the effects of defects and IMCs on the TC of the CNT block/Al composites were investigated.
High-porosity preforms were fabricated using CNT blocks (density: 2.0 Mg·m−3, Zeon Nano Technology Co. Ltd., Japan) and NaCl particles with sizes in the range of 180–360 µm. The NaCl particles were used as the spacer to obtain continuous pores in the preform. Electroless Cu plating is a commonly used surface treatment method. Prior to the perform fabrication, the as-received CNT blocks were subjected to electroless Cu plating. The electroless Cu plating process involved sensitization, activation, and plating.4) To vary the thickness of the Cu layer, different plating times were used: 15, 30, 60, and 180 s (pH 12, 313 K). The electroless Cu-plated CNT blocks were buried in a resin and were then polished mechanically. The thicknesses of the Cu layers were measured using an image analysis software (Image Pro Plus 5.0). The preforms were fabricated as follows. First, the Cu-plated CNT blocks and NaCl particles were mechanically mixed (tilt of 45°) for 600 s using a glass rod. To maintain the infiltration conditions constant, the preform porosity was fixed at 90%. The electroless Cu-plated CNT block:NaCl particle volume ratio was fixed at 1:9. The resulting mixture was then placed in a graphite mold and compacted under a pressure of 60 MPa. Subsequently, the compacted mixture was sintered in Ar. The sintering process was carried out at 973 K for 1.8 ks. Finally, the sintered samples were immersed in distilled water for 48 h to dissolve NaCl particles followed by drying. The size of the preform was 10 × ø10 mm.
Pure Al (purity ≥99.7%) was used as the matrix. A pure Al ingot and the fabricated CNT block preform were placed into a cylindrical graphite die and then heated to 1037 K in Ar. The applied pressure and holding time were 0.1 MPa and 3.6 ks, respectively. The schematic of the fabrication process for the CNT block/Al composite is shown in Fig. 1. The microstructures of the CNT block/Al composites were observed using scanning electron microscopy (SEM; JEOL JXA8900; 15 kV). The elemental distribution of the composites was determined using an electron probe micro-analyzer (EPMA, JXA8900RL). X-ray diffraction (XRD; D/max-2500/PC, Japan) analysis was carried out using Cu Kα radiation (λ = 1.54056 Å) at a scanning speed of 1°/min over the 2θ range of 20°–90°. The TCs of the fabricated CNT block/Al composites were measured using the steady-state method.8)
Schematic image of fabrication process of CNT block/Al composites.
Figure 2 shows the morphologies of the as-received and electroless Cu-plated CNT blocks. As shown in Fig. 2(a), the CNT blocks showed a width of 100–200 µm and a thickness of 100–200 µm. Each CNT block consisted of uniformly and unidirectionally aligned CNTs. As can be observed from Figs. 2(b), 2(c), and 2(d), the electroless Cu-plated CNT blocks (plating times of 15–60 s) showed a smooth and uniform Cu layer. However, at the plating time of 180 s, dendritic growth occurred, making the surface of the Cu layer rough, as shown in Fig. 2(e). The thickness of the Cu layer was measured since it played an important role in the infiltration process. Figure 3 shows the cross-section of the electroless Cu-plated CNT blocks obtained at different plating times (15–180 s). The white regions represent the Cu layers. The CNT blocks showed unbonded parts. The thicknesses of the Cu layers formed at the plating times of 15, 30, 60, and 180 s were 2.60, 3.55, 5.21, and 9.25 µm, respectively (Figs. 3(a), (b), (c), and (d), respectively). The relation between the thickness of the Cu layer and the plating time is shown in Fig. 4. According to Fig. 4, this can be expressed as follows:
| \begin{equation} d = 0.63t^{0.52} \end{equation} | (1) |
SEM images of (a) as-received CNT blocks, (b), (c), (d), and (e) electroless Cu-plated CNT blocks of 15, 30, 60, and 180 s (inset for higher magnification of electroless Cu-plated layer).
BSE images of cross section of electroless Cu-plated CNT blocks with different plating times: (a) 15 s, (b) 30 s, (c) 60 s, and (d) 180 s.
Relation of thicknesses of Cu layers on electroless Cu-plated CNT blocks with different plating times.
Figures 5(a), 5(c), 5(e), and 5(g) show the microstructures of the CNT block/Al composites fabricated with different CNT block:Cu volume ratios, and Figs. 5(b), 5(d), 5(f), and 5(h) show the corresponding high-magnification SEM images. The dark and gray regions represent the CNT blocks and the Al matrix, respectively. The composites showed no pores. This indicates that the fabricated CNT block/Al composites showed high relative densities. At the interface between the CNT blocks and the Al matrix, light regions corresponding to the Cu layers were observed (Figs. 5(b), 5(d), and 5(f)). The composites shown in Figs. 5(b) and 5(d) showed cracks at the interface between the CNT blocks and the Al matrix. However, no cracks were observed at the CNT block/Al interface in the case of the composites shown in Figs. 5(f) and 5(h). During the infiltration process, the Cu layer reacted with molten Al. An amorphous layer is formed at the graphite/Al interface at 1033 K even in the absence of electroless plating.9) Therefore, in this study, the insufficient Cu layer thickness resulted in the reaction of the CNT blocks and molten Al at the CNT block/Al interface. This damaged the structure of the CNT blocks and resulted in the formation of cracks at the CNT block/Al interface. Therefore, for the formation of the CNT block/Al interface, the Cu layer must have a sufficient thickness. Figures 5(b) and 5(h) reveal the presence of unbonded parts in the CNT blocks. These unbonded parts were the same as those observed in Fig. 3. As can be observed from Figs. 5(g) and 5(h), the Al matrix of the composite with the CNT block:Cu volume ratio of 3.7:6.3 showed IMCs (light-gray regions). The elemental distribution of this sample is shown in Fig. 6. To identify the IMCs, EPMA was carried out on the region marked with a cross in Fig. 6(a). The Al:C atomic ratio of the IMC region was found to be approximately 2:1. As shown in Figs. 6(b)–6(d), the Al and Cu overlapped at the IMC regions, indicating the formation of Al–Cu IMCs. Thus, the IMC was identified to be Al2Cu. The XRD patterns of the CNT block/Al composites are shown in Fig. 7. Only the composite with the CNT block:Cu ratio of 3.7:6.3 exhibited the Al2Cu peak. The formation of Al2Cu can be attributed to the rough surface of the Cu layer, which facilitated the reaction between Cu and molten Al. According to the phase diagram of the Al–Cu system, Al2Cu was formed by crystallizing out of the liquid phase near the CNT blocks during the solidification process. In this sample, the fraction of Cu reacting with Al was higher than that in the other samples.10)
SEM images of microstructures fabricated CNT block/Al composites with different CNT block:Cu volume ratios: (a) 8.8:1.2, (c) 7.9:2.1, (e) 6.8:3.2, and (g) 3.7:6.3; (b), (d), (f), and (h) high-magnification images of (a), (c), (e), and (g), respectively.
Element distribution of fabricated CNT block/Al composite fabricated with condition of CNT block:Cu volume ratio of 3.7:6.3: (a) BSE image (inset for location and result of EPMA), (b) C element, (c) Al element, and (d) Cu element.
XRD patterns of fabricated CNT block/Al composites with different CNT block:Cu volume ratios.
The TCs of the CNT block/Al composites fabricated with different CNT block:Cu volume ratios are shown in Fig. 8. As can be observed from Fig. 8, with an increase in volume fraction of Cu, the TCs of the composites increased because of the improvement in the CNT block/Al interface structure. However, with an increase in the CNT block:Cu ratio to 3.7:6.3, the TC of the CNT block/Al composite decreased. This decrease in the TC can be attributed to the formation of Al2Cu, whose TC is approximately 60 W·m−1·K−1.7) This indicates that both the interfacial cracks and IMCs contributed to the decrease in the TC of the CNT block/Al composites. The TC of the CNT block/Al composite with the CNT block:Cu ratio of 6.8:3.2 was found to be 117 W·m−1·K−1, which is remarkably higher than that of a previously reported 10 vol% dispersed carbon nanofiber-reinforced Al matrix composite (35.5 W·m−1·K−1).11) This is because the CNT blocks used in our composite did not agglomerate. However, the TC of this composite was lower than those of Al (237 W·m−1·K−1) and Cu (385 W·m−1·K−1). This is because the thermal resistance at the CNT block/Al interface and the low real TC of the CNT blocks (compared to that of a single CNT). In the CNT blocks, the unbonded parts and thermal resistance at the CNT/CNT interface inhibited heat transfer, which resulted in a decrease in the real TC of the CNT blocks.12) Hence, CNT blocks were found to be a potential reinforcement for MMCs. However, the thermal resistance of the CNT block/Al interface and the unbonded parts of the CNT blocks in CNT block/Al composites should be improved.
TCs of fabricated CNT block/Al composites with different CNT block:Cu volume ratios.
CNT block preforms with high porosities were prepared by electroless Cu plating of CNT blocks for different plating times. CNT block/Al composites were successfully fabricated using the LPI method at 0.1 MPa. The main conclusions of the study are as follows:
