Architecting Three-Dimensional Networks in Carbon Nanotube

Article pubs.acs.org/JPCC

Architecting Three-Dimensional Networks in Carbon Nanotube Buckypapers for Thermal Interface Materials Hongyuan Chen,†,‡ Minghai Chen,*,† Jiangtao Di,†,‡ Geng Xu,†,‡ Hongbo Li,† and Qingwen Li*,† †

Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China

‡

ABSTRACT: Carbon nanotube (CNT) buckypaper, which has large specific surface area and tunable network structures, shows great potential in the application of heat dissipation for high power electronic devices. In this article, we report that the heat conduction in a buckypaper depends greatly on CNT network formation, in which CNT structures, lengths, and orientations are important issues. The buckypaper composed of multiwalled CNTs with large diameter (around 50 nm) and suitable length (1− 10 μm) shows lower thermal impedance compared with those made by longer CNTs with smaller diameter. The thermal impedance of such buckypapers can be reduced to 0.27 cm2·K/W, lower than that of commercialized graphite foil and thermal grease. Thus, the buckypaper may serve as a promising candidate for advanced thermal interface materials. Detailed structural characterization indicates that the three-dimensional networks of buckypapers, with CNT orientations perpendicular to the surfaces, result in both the reduction of thermal contact resistance and the enhancement of heat conduction along the thickness.

1. INTRODUCTION As the power density of microelectronic devices has been increasing at a noticeable rate, chemically stable and highperformance thermal interface materials (TIMs) to dissipate heat efficiently from the devices are urgently needed.1,2 In general, the commercialized TIMs are often categorized into four groups: solder,3 thermal paste,4 phase-change material,5 and sheet material,6 all of which can be used to fill gaps between two hard surfaces, such as silicon or sapphire substrates for electronic devices and metal plates for heat sink (or heat spreader), in order to minimize the thermal contact resistance between them. However, TIMs still face challenges to meet the increasing requirements for high-power electronic devices in terms of long-term reliability, high thermal conductivity, low thermal contact resistance, and mechanical fatigue resistance. Carbon nanotube CNTs is a new alternative for excellent TIMs due to its ultrahigh axial thermal conductivity, which is more than 6000 W/mK7−9 for individual defect-free singlewalled CNT (SWCNT), much higher than widely used copper and diamond, along with other advantages beneficial for practical applications, such as low thermal expansion coefficient, high chemical stability, and corrosion resistance to many severe environments. As a result, CNTs were once mixed with © 2012 American Chemical Society

polymers to enhance their thermal conducting performance.10,11 Hu et al. reported that the thermal conductivity of silicon grease was enhanced by 70% after being composed with 3 vol % multiwalled CNTs (MWCNTs).12 Thang et al. reported that the working temperature of CPU could be reduced 5 °C by using the silicon grease with 2 wt % MWCNTs.13 However, the improvement was limited due to high interfacial thermal resistances between CNTs and polymer matrices.14−16 Meanwhile, vertically aligned CNT (VACNT) array films have attracted significant attention as TIMs,17−19 as the aligned structures of CNT arrays help utilize the high axial thermal conductivity of CNTs, principally allowing the heat to be drained out of the electronic device effectively and anisotropically. The thermal impedance of VACNT arrays was reported to drop to less than 0.2 cm2·K/W by tuning the growth process,20 being composed with copper,21 or using other substrates with high thermal conductivity.22 However, the existence of the substrates used for the array growth has constrained them to be directly and facilely used as TIMs. If the arrays were either peeled off from the substrate23 or filtrated Received: September 7, 2011 Revised: December 13, 2011 Published: January 18, 2012 3903

dx.doi.org/10.1021/jp2086158 | J. Phys. Chem. C 2012, 116, 3903−3909

The Journal of Physical Chemistry C

Article

with polymers,24 their thermal conducting performance was found to become greatly worsened. Furthermore, the large-area growth and transferring of VACNT arrays onto other substrates for TIMs are still challenging.25,26 Actually, the CNTs in a vertical array tend to become collapsed and disordered after being compressed during the measurement of thermal impedance or packaging with electronic devices, typically with the array thickness decreased by 90%.27 As a result, CNT buckypaper, the membrane composed of CNT network,28 which is freestanding, robust, flexible, and easily adhered to many kinds of surfaces, has been considered as a competitive candidate for high-performance TIMs. Compared with graphite films, it may exhibit better thermal conducting performance not only in the plane but more importantly along the direction perpendicular to the plane of buckypaper, i.e., the thickness direction. It was reported that the in-plane thermal conductivity of the buckypaper with CNTs aligned in the plane could be higher than 300 W/mK,29 and the thermal conductivity along the thickness direction was in a range of 1−20 W/mK30−32 according to different preparation methods. Such a large difference indicates that the network structure of buckypapers is an important issue for tailoring it as an excellent TIM. Therefore, in the present work, we thoroughly investigate the influence of CNT structures including CNT diameters and lengths on the formation of the networks of buckypapers and their thermal conducting performance. The short MWCNTs with large diameter are found to favor the formation of threedimensional networks, enabling the buckypaper with desirable CNT orientations and thermal impedance. Enhancement of CNT orientation parallel to the thickness direction in the network plays an important role in decreasing the thermal impedance of the buckypaper down to 0.27 cm2·K/W, better than commercialized graphite foil and thermal conductive silicon grease. It implies that the buckypapers with a well designed network can serve as a freestanding and flexible thermal pad as excellent TIMs.

CNTs were sheared into CNT cotton again and dispersed into deionized water by ultrasonic treatment with the help of Tween-80 (as the dispersant). Then, the dispersed CNT solutions were filtered through a microporous cellulose filter membrane using vacuum filtration and washed by deionized water repeatedly to remove remnant dispersants. After dissolving the cellulose filter membrane by acetone, a freestanding buckypaper with a diameter of 40 mm was obtained. 2.3. Characterizations. The microstructures of the samples were characterized by scanning electron microscopy (SEM, Quanta 400 FEG, FEI) and high resolution transmission electron microscopy (HRTEM, Tecnai G2 F20 S-Twin, FEI). The Raman spectrum (LabRAM HR, HORIBA Jobin Yvon) was used to evaluate the quality of CNTs. The thermal conducting performance of buckypaper was measured by a thermal impedance measurement system (LW-9091IR, Longwin Science & Technology Corporation, Taiwan, China) designed according to ASTM D5470-06, and the applied pressure in the test was 40 psi (0.2758 MPa). Before measurement, the buckypapers were immersed by dimethyl silicon oil. The sheet resistances of buckypapers were tested by four-probe equipment (PM8, SUSS).

3. RESULTS AND DISCUSSION In order to prepare robust and conductive buckypapers, VACNT arrays with large tubular diameters (>30 nm) were intentionally synthesized by FCCVD. Figure 1 shows the morphological images of a typically as-grown VACNT array and the there-after prepared buckypaper. The MWCNTs were readily and massively formed on silicon wafers and the inner wall of a quartz tube by injecting toluene and 2% ferrocene into a CVD furnace at 750 °C. After the growth, the CNT deposit was peeled off in a large quantity from the substrate and the furnace tube (Figure 1a). It was then chunked into fine and homogeneous powder form by a muller for 5−10 min, ensuring the CNTs to be well dispersed in surfactant solution for film preparation. With this method, the CNT arrays with different heights can be formed by adjusting growth time, with a 1 mm tall array grown in a couple of 3 h (Figure 1b). As shown in Figure 1c,d, the CNTs show good alignment in the arrays with diameter around 30−100 nm, and they are multiwalled typically with more than 10 walls. The large G/D ratio of Raman spectrum (Figure 1e) indicates that the as-prepared CNTs are well crystallized, which favor the high thermal and electrical conductivity along the axial direction of individual CNTs. The buckypaper of 2 inch size was prepared by vacuum filtration of CNT dispersion, shown in Figure 1f. It is smooth on both sides and robust enough for further handling with mechanical strength around 10 MPa. Silicon oil was used to ensure a sufficient interfacial contact between the buckypaper and other substrate surfaces in the measurement setup. Figure 2a shows the thermal impedance variation of a 56 μm thick buckypaper before and after the infiltration of silicon oil. The pristine buckypaper showed a thermal impedance at 1.22 cm2·K/W. However, after the sample was immersed into silicon oil and lifted out, its thermal impedance drastically decreased to 0.51 cm2·K/W. On the basis of the relationship between thermal conductivity (k, including the influence of thermal contact resistance) and thermal impedance (R), which is formulated as k = d/R, where d is the thickness of buckypaper, the corresponding thermal conductivity of the buckypaper was increased from 0.46 W/mK

2. EXPERIMENTAL SECTION 2.1. Materials. In this study, large-diameter MWCNTs (diameter larger than 30 μm) in a form of VACNT arrays were synthesized by the method of floating catalyst chemical vapor deposition (FCCVD).33,34 The products were scratched away from the inner wall of the quartz tube. As contrastive materials, SWCNTs (diameter 1−2 nm, length 5−30 μm) and smalldiameter MWCNTs (diameter ∼10 nm, length around 10 μm) were purchased from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences (Chengdu, PRC) and Cnano Technology Ltd. (Beijing, PRC), respectively. Silicon grease (IDEAL@IDL-280 Heatsink Compounds) and graphite foil, typical commercialized TIMs, were purchased from Shenzhen Futian district BUBEEN electronic firm (Shenzhen, PRC) and Shenzhen Kua Yue Electron Co., Ltd. (Shenzhen, PRC), respectively. Other chemical reagents were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, PRC). The cellulose filter membranes with a pore diameter of 0.45 μm were purchased from Hangzhou ANOW Microfiltration Co., Ltd. (Hangzhou, PRC). 2.2. Preparations. The buckypapers were prepared by a vacuum filtration method. In a typical process, the pristine CNTs were sheared into CNT cotton by high-speed shearing, and then immersed into the solution of hydrochloric acid (5 mol/L) for 48 h to remove catalyst particles. The purified 3904



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Regarding the contribution of silicon oil, the buckypapers of different thicknesses were prepared and tested under identical conditions, and the dependence of their thermal impedances on thickness is shown in Figure 2b according to the following equation:35 RTIM =

d BLT + R c1 + R c2 = TIM + R c kTIM kTIM

(1)

(where RTIM is the thermal impedance of the TIM, which can be directly measured; Rc is the thermal contact resistances of the TIM with the two bonding surfaces; BLT (bond line thickness), the distance of the TIM between the two opposite bounding surfaces in the direction of heat flow, here can be considered as the thickness of the TIM (dTIM) for the buckypaper is solid state; and kTIM is the thermal conductivity of the TIM without the influence of thermal contact resistance.) The thermal contact resistance (Rc) and thermal conductivity (kTIM) can be obtained by the intercept and the slope of a plot of RTIM vs dTIM shown in Figure 2b, respectively. It is revealed that the introduction of silicon oil causes the correlation between RTIM and dTIM to be more linear and the Rc of the impregnated buckypaper to drop to 0.35 cm2·K/W by a degree of more than 50% compared with 0.81 cm2·K/W of the bare buckypaper. Furthermore, it also shows that the kTIM of the buckypaper increases from 1.59 W/mK to 4.02 W/mK with the impregnation of poorly thermal conductive silicon oil. Therefore, it can be inferred that the role of silicon oil is more likely to help enhance intertube thermal contacts in the CNT network and the surface thermal contact between the buckypaper and the holders, and thus decrease the thermal impedance of the buckypaper. Under sufficient contact by silicon oil infiltration, we observed that the thermal transport properties of buckypapers depended greatly on the formation of CNT network structures. Previously, under similar measurement according to ASTM D5470-06, Prasher et al. reported that the thermal conductivity of a CNT mat, which was prepared by simply pressing CNT powder on a plate, was lower than 0.2 W/mK, although the CNTs were in a similar diameter range from 60 to 100 nm with length about 5−15 μm.36 Such CNT network was thus proposed as a thermal insulator. Differently, our buckypapers

Figure 1. As-formed CNTs by floating CVD method are collected in a powder form (a); the SEM images of the CNTs at low (b) and high magnification (c); typical HRTEM image of the CNTs showing that the nanotubes are multiwalled (d); Raman characterization of the asformed CNTs (e); the resultant buckypaper prepared by vacuum filtration is robust and typically with diameter around 2 inches (f).

to 1.09 W/mK with the impregnation of silicon oil. As the thermal conductivity of silicon oil is only 0.15 W/mK, much lower than that of the buckypaper, such thermal conductivity enhancement cannot be simply explained as a result of the presence of silicon oil.

Figure 2. Comparison on the thermal conductivities and thermal impedances of the buckypaper with and without silicon oil (a); the fitted plots of the thermal impedance of buckypaper against its thickness with and without the existence of silicon oil (b). 3905



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form a relatively loose network structure with some CNTs extruding out of the buckypaper surface, its thermal impedance drops to 1.58 cm2·K/W. The buckypaper composed of largediameter MWCNTs, however, exhibits a very porous structure with more CNTs stretching out along the thickness direction and shows the lowest thermal impedance around 0.53 cm2·K/ W. As a result, it infers that the large-diameter MWCNTs present a smaller aspect ratio and greater stiffness compared with SWCNTs and small-diameter MWCNTs, leading to more nanotubes sitting through or extruding out of the planar buckypaper membrane composed by bended and overlapped tubes during the preparation of buckypapers by filtration, which may eventually favor the heat conduction along the thickness direction. Thus, the large-diameter MWCNTs grown by FCCVD appear to be more suitable as the raw materials for producing buckypapers as TIMs, and they were intentionally used for the following studies. Based upon the above result, we assume that the CNT length may further take an effective role in tailoring the network structures and the thermal conducting performance of the buckypapers. The buckypapers composed of the large-diameter MWCNTs with different length distributions were thus prepared. The shortening of MWCNTs was carried out by ultrasonicating MWCNT dispersion for different times. Figure 4a shows the MWCNT length distribution derived from 4 different ultrasonic treatment times under the same power. By statistic evaluation with more than one hundred tubes in SEM images of dispersed CNTs, it is revealed that about 30% of MWCNTs are longer than 20 μm, 52% of the MWCNTs are in a length range from 10 to 20 μm, and 18% are shorter than 10 μm. After ultrasonic treatment for 30 min, the percentage of MWCNTs with length distribution longer than 20 μm and between 10 and 20 μm was changed to 19% and 61%, respectively, indicating an obvious shortening effect. When further prolonging the ultrasonic treatment time to 120 min, the MWCNTs were massively shortened to less than 10 μm. However, further prolonging the ultrasonication time to 180 min did not lead to a successive shortening of MWCNTs. Figure 4b shows the effect of ultrasonication time on thickness change of the buckypaper before and after the removal of pressure loading. As the thermal impedance of buckypapers was measured under the pressure of 40 Psi according to ASTM D5470-06, the buckypapers were always thinned to a degree after the compressive loading was removed. The more planar network structure tends to induce a more

prepared by film filtration showed much improved thermal conductivity, as the pristine entangled CNTs were better stretched and interwoven with each other through dispersion and filtration process, and the formed CNT network structures affected the heat conduction of buckypapers. The network formation of one-dimensional CNTs will inevitably rely much on their sizes. In order to understand the effect of CNT diameter on network morphology and the heat conduction of the buckypapers, three kinds of CNTs with tubular diameter distribution around 1 (SWCNTs), 10, and 50 nm (both MWCNTs) were chosen to make a comparative study. As shown in Figure 3a, the resultant buckypapers

Figure 3. Dependence of the thermal impedance of buckypapers on CNT diameter (a); SEM images showing the surface morphology of the three kinds of buckypapers prepared with SWCNT (b), smalldiameter MWCNTs (c), and large-diameter MWCNTs (d), respectively.

filtrated from the three kinds of CNT dispersions, which were prepared with the same amount of CNTs and same procedure (all CNT solutions were ultrasonically dispersed for 60 min, and the buckypaper samples were all impregnated with silicon oil before thermal impedance testing), showed similar thickness (around 80 μm) but different thermal impedance. The buckypaper composed of large-diameter MWCNTs exhibited lower thermal impedance compared with that composed of small-diameter MWCNTs (diameter ≈ 10 nm) or SWCNTs. Figure 3b−d shows the SEM images of the surface morphology of the three kinds of buckypapers. It is clearly seen that the buckypaper composed of SWCNTs tends to exhibit a denser and planar CNT distribution with thermal impedance at 2.89 cm2·K/W. In contrast, the small-diameter MWCNTs prefer to

Figure 4. Variation of CNT length distribution with ultrasonication time (a); Effect of ultrasonication time on the variation of thickness of the buckypapers (b). 3906



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collapsed and thinned membrane under pressure. As shown in Figure 4b, after the test, the buckypaper became obviously thinner compared with its original thickness, and under shortterm ultrasonication, the thickness variation before and after the test varied greatly with ultrasonication time. An increase in the ultrasonication time led to the decrease of thickness variation, confirming that the planar membrane composed of longer nanotubes preferred to be more thinned and less compression resistant. However, when the ultrasonic time was longer than 100 min, the thickness variation became almost diminished, implying that the compression resistance was increased with the shortening of CNTs. The density of the buckypaper (after the test of thermal impedance) also increases from 0.26 g/cm3 (0 min) to 0.42 g/cm3 (180 min) by shortening the length of MWCNTs to different extents, though they are also smaller than the density of individual MWCNTs37 (2.0 g/cm3). It can be thus deduced that with prolonging the ultrasonication time, the MWCNTs are shortened and densely packed during filtration, consequently leading to the buckypapers being more compact. Importantly, the CNT shortening can also lead to their orientation variation during the network formation. Figure 5

Figure 6. Effect of CNT length modulated by ultrasonication time on the thermal impedance and thermal conductivity in the thickness direction of buckypapers.

increase of ultrasonication time. The thermal impedance of buckypapers drastically decreases when the CNT length is shortened, and then reduces gradually to a steady value (0.39 cm2·K/W) when prolonging the ultrasonic treatment time. The derived thermal conductivity along the thickness direction presents an opposite variation tendency. As stated above, the ultrasonication of CNT dispersion may lead to the shortening of CNTs and therefore their denser assembly and orientation change during the network formation. The improvement of heat conduction in buckypapers by prolonging ultrasonication time may not be simply attributed to the increase of CNT packing density in the buckypaper, as the density has already been eliminated upon calculating the thermal conductivity as the form of dTIM. Furthermore, shortening CNTs in a steady CNT network structure could decrease the thermal conductivity of the network.38 Thus, the transition of the CNT network structure from a planar network to a three-dimensional network along with shortening CNT length could be the key factor for the improvement of the calculated thermal conductivity along the thickness direction of the MWCNT buckypapers. As the axial thermal conductivity of MWCNT is more than 1000 W/mK but its radial thermal conductivity is fewer than 10 W/mK,39 when the MWCNTs dominate a planar network in the buckypaper, the heat conduction perpendicular to CNT orientation will be highly prohibited. Additionally, the planar orientation of MWCNTs in the buckypaper tends to result in large contact areas with high thermal contact resistance.40 Consequently, it leads to a low thermal conductivity along the thickness direction of the buckypaper. By contrast, the three-dimensional CNT network structure seems more beneficial for heat conduction through the buckypaper, i.e., between the two opposite surfaces. Those CNTs with the orientation along the thickness direction of the buckypapers are somewhat similar to the CNTs in vertical arrays as TIMs. They may not only enhance the thermal conducting performance along the thickness direction of buckypapers but also help improve the compact contact between the two surfaces and therefore reduce the thermal contact resistance at the surface−buckypaper interface. The further shortening of CNT length could slightly decrease the calculated thermal conductivity of the buckypaper only after the formation of a steady three-dimensional network38 (ultrasonication for more than 120 min) as is shown in Figure 6. Under the circumstances, the decrease of the thermal impedance should only be attributed to the decrease of the

Figure 5. Surface (a,b,d,e) and cross-section (c,f) morphologies of asprepared buckypapers composed of the MWCNTs treated by ultrasonication for 0 min (a−c) and 180 min (d−f).

shows the surface and cross-section morphologies of two typical buckypapers composed by the pristine MWCNTs and the short MWCNTs ultrasonically treated for 180 min, respectively. As shown in Figure 5a−c, the long CNTs prefer to lie and assemble along the x and y directions, forming the buckypaper in a planar network structure. The shortened CNTs, on the contrary, as shown in Figure 5d−f, present an enhanced orientation along the thickness direction, with CNTs more densely packed to favorably form a three-dimensional network structure. It is reasonable that short CNTs are more rigid and liable to orientate them parallel to the flow direction of the dispersion upon filtration. In contrast, the pristine long CNTs with large aspect ratio (about 400 for an individual MWCNT with the diameter of 50 nm and the length of 20 μm) are flexible and therefore favorable to bend and collapse into a planar network structure during filtration (see Figure 5c). Figure 6 shows the variation of the thermal impedance and derived thermal conductivity of the buckypapers with the 3907



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buckypaper thickness (also be illuminated as the density) of the samples. To verify the efficiency of our buckypapers for practical application as TIMs, we compared it with the two widely used TIMs, including the commercialized silicon grease (IDEAL@ IDL-280, with Al2O3 and ZnO particles as the thermal conducting fillers) and graphite foil under similar measurement condition. As shown in Table 1, the buckypaper with a

thickness (μm)

mass (g)

density (g/cm3)

thermal impedance (cm2·K/W)

buckypaper silicon grease graphite foil

51 ∼20 115

0.015

0.47 2.50 1.79

0.27 0.29 0.52

0.125

thickness of 51 μm (prepared by dispersing 30 mg of MWCNTs by ultrasonic treatment for 150 min) exhibits the thermal impedance at 0.27 cm2·K/W, lower than that of silicon grease (0.29 cm2·K/W), whose thickness was around 20 μm, and also much lower than that of graphite foil with the thickness of 115 μm (0.52 cm2·K/W). As a result, the threedimensional structured buckypaper is promising, as it is much lighter and more chemically inert, and can be easily tailored to any shapes, thickness, and sizes; more importantly, it provides a large room to be modified by surface treatment for further performance improvement.

4. CONCLUSIONS In this work, the effects of CNT diameter and length on the network formation of buckypapers by vacuum filtration and their thermal transport properties were thoroughly investigated. The CNT architecture in the buckypaper is found to be a key issue to affect the thermal conductivity of buckypapers. An optimized three-dimensional network structure in the buckypaper, which can be constructed by tuning the length of raw MWCNTs, is proposed for a high-performance TIM. This buckypaper is lighter, flexible, and environmentally stable with lower thermal impedance compared to that of commercialized silicon grease and graphite foil, which makes it a promising candidate for the next generation of high-performance TIMs.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 512 62872577. Fax: +86 512 62872552. E-mail: [email protected] (Q.W.L.); [email protected] (M.H.C.).

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REFERENCES

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Table 1. Performance Comparison between the Buckypaper and Two Commercialized TIMs materials

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ACKNOWLEDGMENTS

We acknowledged the funding support by the Hundred Talent Program for Q.W.L. and Knowledge Innovation Program (KJCX2.YW.M12) by Chinese Academy of Sciences, International Collaboration Project (2009DFB50150), and National Basic Research Program of (2010CB934700) by Ministry of Science and Technology, the Key Program of the National Science Foundation of China (No.10834004). We also acknowledged Dr. Xuan Wang and Dr. Lu Zou in Honeywell (China) Ltd. for their help of testing thermal impedances. 3908



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Architecting Three-Dimensional Networks in Carbon Nanotube

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