Thermal Properties of Carbon Nanotube–Copper Composites for Thermal Management Applications
© The Author(s) 2010
Received: 20 October 2009
Accepted: 5 March 2010
Published: 19 March 2010
Carbon nanotube–copper (CNT/Cu) composites have been successfully synthesized by means of a novel particles-compositing process followed by spark plasma sintering (SPS) technique. The thermal conductivity of the composites was measured by a laser flash technique and theoretical analyzed using an effective medium approach. The experimental results showed that the thermal conductivity unusually decreased after the incorporation of CNTs. Theoretical analyses revealed that the interfacial thermal resistance between the CNTs and the Cu matrix plays a crucial role in determining the thermal conductivity of bulk composites, and only small interfacial thermal resistance can induce a significant degradation in thermal conductivity for CNT/Cu composites. The influence of sintering condition on the thermal conductivity depended on the combined effects of multiple factors, i.e. porosity, CNTs distribution and CNT kinks or twists. The composites sintered at 600°C for 5 min under 50 MPa showed the maximum thermal conductivity. CNT/Cu composites are considered to be a promising material for thermal management applications.
KeywordsMetal–matrix composites Carbon nanotubes Spark plasma sintering Thermal conductivity
Carbon nanotubes (CNTs) possess a unique structure and properties that have attracted significant interest worldwide [1, 2]. Research in potential CNTs application fields including energy storage, molecular electronics, nanoprobes, sensor and composite materials has developed very quickly and shows a promising future .
When single-wall carbon nanotubes (SWCNTs) or multi-wall carbon nanotubes (MWCNTs) are incorporated in numerous composite systems, including the metal, polymer and ceramic as matrices, newly excellent mechanical, electrical and thermal properties of the composites can be obtained. Over the past decade, a larger portion of researches have been focused on the development of CNT-reinforced polymer [4–6] and ceramic [7–9]-based composites. The incorporation of CNTs into metal–matrix has become a new focus in recent years [10–13]. One main application of CNTs is as a reinforcing agent to improve the mechanical properties of the metal–matrix composites (MMCs) by taking advantage of their outstanding mechanical properties such as elastic modulus (~1 TPa) and tensile strength (~150 GPa), which is a research hot spot at present. Cha et al.  reported a 200% increase in the yield strength in 10 vol.% CNT-reinforced Cu composites. Noguchi et al.  found a sevenfold increase in compressive yield strength in 1.6 vol.% CNT/Al composites. Laha et al.  obtained a 71.8% increase in microhardness in CNT/6061Al composites when 10 wt. % CNTs was added. Carreno-Morelli et al.  showed that the Young’s modulus was increased by 9% in CNT/Mg composites with addition of 2 wt. % CNTs. As for these CNT/metal composites, the orientation of the CNTs, homogeneity of the composite, nanotube aspect ratio and the volume fraction of nanotubes are expected to have significant influences on the properties of the nanocomposite [14, 15]. Controlling such factors to obtain an exceptional composite is very challenging. Furthermore, given the excellent thermal properties of CNTs like extraordinarily low coefficient of thermal expansion (≈0 10−6/K)  and ultra-high thermal conductivity (3,000–6,000 W/mK)  in combination with the superior electrical and thermal conductivities of the copper, CNTs-reinforced Cu matrix (CNT/Cu) composites are very attractive to meet the increasing demands for high performance thermal management materials used in heat sinks and electronic packages. However, most effects have been made to investigate the fabrication process and mechanical properties of CNT/Cu composites [10, 18–21]. Up to now, little work has been reported on the thermal properties of CNT/Cu composites. Therefore, the study of their thermal properties has become an important issue that needs to be addressed.
In this study, a dispersion of CNTs in the Cu powders is attempted to use a novel mixing procedure called particle-compositing process. As will be demonstrated in this paper, it is successful to generate composite powders with homogenous distribution of CNTs. The composite powders are consolidated into CNT/Cu composites by using spark plasma sintering (SPS) technique. A main attention is paid for the first time to evaluate the thermal conductivity of CNT/Cu composites.
Materials used in this study were high-purity (99.9%) electrolytic copper powders with 5–10 μm in diameter and multi-walled carbon nanotubes (simply called CNTs hereafter). The average diameter and length of CNTs are 10–30 nm and 5–15 μm, respectively. As-received CNTs were purified in the concentrated nitric acid by performing the ultrasonic cleaning, then filtered and washed with deionized water and dried at 120°C.
The characteristics of all the samples investigated in this work
The microstructure characterization was carried out on a Zeiss Supra55 field emission scanning electron micrograph (FESEM) and an optical microscope. Densities of the consolidated composite specimens were obtained using Archimedes’ method with distilled water as the medium. The theoretical densities of pure copper (8.96 g/cm3) and CNT (1.75 g/cm3, provided by the manufacturer) were used to calculate the relative density of products. The laser flash method was used to measure thermal diffusivity (α) in the JR-3 thermal physical testing instrument. The sample with the size of Ф 10 mm × 3 mm was placed in a chamber, the front of the sample was heated by a laser beam, and the temperature on the rear was recorded by an infrared recorder. The overall measurement confidence interval was assessed as ±2% of the measured value. The specific heat of the specimens was measured by a differential scanning calorimeter using high-purity single-crystal alumina as a reference material. Thermal conductivity can be then calculated based on the equation K = αρC p, where K, ρ and C p represent thermal conductivity, bulk density and specific heat of the sample, respectively. All measurement results were summarized in Table 1.
Results and discussion
A considerable number of papers [4, 5, 8, 9] have reported that the incorporation of CNTs into polymers or ceramics can obtain the composites with largely enhanced thermal conductivity. However, it is surprising to find in Table 1 that the thermal conductivities for all the CNT/Cu composites with addition of CNTs are more or less lower than that of CNT-free specimen no matter what the sintering condition is, although the thermal conductivity of MWCNTs (3,000 W/mK)  is nearly tenfold higher than that of pure copper sample (331 W/mK). It is worth noting that the powder sintered copper with polycystic structure has much lower thermal conductivity than the monocrystal copper (402 W/mK) due to the existence of grain boundary and defects in polycrystals . The presence of an interfacial thermal resistance between the CNT and the surrounding Cu matrix is believed to be a main reason in causing the measured thermal conductivity to be much less than expected. This interfacial thermal resistance acts as a barrier to the heat flow and thus decreases the overall conductivity. The reported interface resistance values (R K) across the interface of CNT/polymer cover the range of 2.9–8.3 × 10−8 m2K/W [26, 27], and these values are of the same order of magnitude as those in CNT/ceramic composites . So far, the value of R K accounting for the CNT/metal composites has not been reported to our knowledge. Here, we use an effective medium approach to estimate the R K in CNT/Cu composites based on the thermal conductivity measurements.
The CNT/Cu composites have attracted the interest of many researchers for their excellent mechanical properties. On the other hand, it is of interest to point out that CNT/Cu composites may find thermal management applications based on their superior thermal properties. In these applications, there are two main requirements: a high thermal conductivity and a low coefficient of thermal expansion (CTE) similar to that of semiconductor materials. The present study suggests that the incorporation of CNTs can still maintain the high thermal conductivity of the copper via the prescribed fabrication procedures. Actual measurements of CNT/Cu composites show a maximum thermal conductivity of 328 W/mK, which is much better than that of commercialized W-Cu(Mo) or SiC/Al(Cu) materials . With respect to the CTE, although the investigations into the CTE of CNT/Cu composites have not been reported, a recent research on the CTE of CNT/Al composites demonstrated that the addition of 15 vol.% CNTs to Al reduced the CTE of Al matrix by as much as 65% . It is noted that the CTE of copper is 30% lower than that of aluminum; one can expect that the addition of CNTs in Cu matrix would lead to a further reduced CTE. Therefore, the high thermal conductivity and low CTE will push CNT/Cu composites up in the list of candidates being considered for thermal management applications. However, the high price of CNT and fabrication limitations may still be an issue for their applications.
The CNT/Cu composite powders with homogeneously dispersed CNTs were fabricated by means of a novel particles-compositing process, which were fully densified by subsequent SPS technique. The thermal conductivity of the composites was not enhanced by the incorporation of CNTs. Besides the effect of sintering condition, the existence of interface thermal resistance between the CNT and the Cu matrix was considered to be the main reason for this unexpected low thermal conductivity. The thermal conductivity predictions obtained from the effective medium approach revealed that only small interface thermal conductance can cause a significant degradation in thermal conductivity for CNT/Cu composites. The composites sintered at 600°C for 5 min under 50 MPa showed the maximum thermal conductivity. At lower sintering temperatures (below 600°C) or pressure (below 50 MPa), the increases in both temperature and pressure enhanced the composite conductivity attributed to porosity removal. At higher temperatures above 600°C, the composite conductivity decreased with increasing sintering temperature and holding time, and this degradation was enhanced with the increase in CNT content. It was suggested that the presence of CNTs segregation formed in the matrix grain rearrangement process may be a main disadvantage for resulting in the reduction in thermal conductivity at higher temperature. The high pressure (60 MPa) reduced the composite conductivity due to the inducement of massive kinks or twists in CNTs. The present study implies that the CNT/Cu composites may have potential applications in the field of thermal management.
This study was financially supported by National Natural Science Fund of China (No. 50971020) and National 863 Plan Project of China (No. 2008AA03Z505).
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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