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carbon nanotube aerogel: A novel thermal interface material with highly thermal transport properties
Carbon 99 (2016) 222e228
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Carbon journal homepage: www.elsevier.com/locate/carbon
Super-elastic graphene/carbon nanotube aerogel: A novel thermal interface material with highly thermal transport properties Peng Lv, Xiao-Wen Tan, Ke-Han Yu, Rui-Lin Zheng, Jia-Jin Zheng, Wei Wei* School of Optoelectronic Engineering, Nanjing University of Posts & Telecommunications, Nanjing 210023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 April 2015 Received in revised form 9 December 2015 Accepted 10 December 2015 Available online 13 December 2015
Three-dimensional (3D) graphene structure exhibits promising potential in thermal interface materials (TIMs) due to the continuous network structure and the high thermal conductivity of graphene. Nevertheless, the very low density and the high porosity of the 3D graphene monoliths usually lead to poor thermal transport performance. To overcome these problems, we fabricated the graphene/carbon nanotube (Gr/CNT) aerogels by a synergistic assembly strategy. The entangled CNTs bond the graphene sheets together to avoid the sliding of them under compression and greatly enhance the elastic stiffness of cell walls, which brings the aerogels super-elasticity. Bearing a high compression strain of 80%, the continuous thermal transport paths in Gr/CNT aerogels are still preserved. Significantly increased thermal conductivity of Gr/CNT aerogels can be obtained by directly mechanical compression. Meanwhile, the thermal transport properties of Gr/CNT aerogels can be further improved by elevating their initial density. With an initial density of 85 mg cm3, a thermal conductivity up to 88.5 W m1K1 and a thermal interface resistance as low as 13.6 m m2KW1 were obtained, which outperforms other carbonbased TIMs reported previously. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Development of high-power density electronics made efficient thermal energy dissipation a critical issue for device lifetime and speed [1,2]. But gaps between the contact solid surfaces usually cause high interfacial thermal resistance [3]. Thermal interface materials (TIMs) inserted between the mating surfaces can conform well to surface asperities and reduce the temperature across the interface [4]. In the past decades, significant attentions have been focused on developing carbon-based TIMs due to the extraordinarily high theory thermal conductivity of carbon nano-materials [5e9]. Normally, for achieving high thermal transport properties, carbon-based TIMs should have the interconnected structure to ensure the continuous thermal transport, meanwhile own high flexibility for good contact with the mating surface. Three-dimensional (3D) graphene monoliths have been widely studied because it can assemble the individual graphene sheets with dramatic properties into a macroscopic functional architecture. In 3D graphene monoliths with interconnected structure, the
* Corresponding author. E-mail address: [email protected] (W. Wei). http://dx.doi.org/10.1016/j.carbon.2015.12.026 0008-6223/© 2015 Elsevier Ltd. All rights reserved.
heat fluxes conduct along the graphene sheets and come together at their junctions, then continue to transport to adjacent graphene sheets, which brings 3D graphene monoliths continuous thermal transport paths. And, the ultrahigh flexibility of graphene sheets in 3D graphene monoliths is beneficial to its matching to the mating surface [10]. Thus, 3D graphene monoliths show great promise in developing high performance TIMs. There are several literature reported the thermal transport properties of 3D graphene monoliths [10e14]. The measured thermal conductivities of graphene foam grown by chemical vapour deposition (CVD) were 1.7 W m1K1 (grown on nickel foam) [12] and 8.28 W m1K1 (grown on porous Al2O3) [11]. And some phase change materials were also introduced into graphene aerogel [10] and CVD-graphene foam [13], obtaining the thermal conductivities of 2.635 W m1K1 [10] and 0.44 W m1K1 [13], respectively. It is noteworthy that the thermal conductivity of these 3D graphene monoliths mentioned above is lower than that of other carbon-based TIMs [5e9] and far below that of single layer graphene [15]. The major reason is attributed to the very low graphene volume fraction and the ultrahigh porosity of 3D graphene monoliths [12]. Unfortunately, the most straightforward way, mechanical compression, is unable to decrease their porosity, since these 3D graphene monoliths easily collapse under pressure.
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Recently, some studies about 3D graphene monoliths with super-elasticity have attracted intense interests. The super-elastic graphene foams, aerogels and monoliths prepared by CVD method [16], freeze-casting strategy [17,18], 3D printing technique [19] and hydrothermal process [20] show excellent mechanical properties, especially the high compressibility (compressive strain up to 60%~90%). These progresses provide new insight and strategy for developing high performance TIMs: fabricating a novel superelastic graphene monolith with continuous thermal transport path even under high compressive deformation; then significantly improving the thermal conductivity of 3D graphene monoliths by simply mechanical compression. In this study, we fabricated the super-elastic graphene/carbon nanotube (Gr/CNT) aerogels by hydrothermal method and subsequent freeze-drying process. The aerogels with a 3D framework are constructed with cell walls of graphene sheets and entangled CNTs coating on them (Fig. 1). The entangled CNTs bond the graphene sheets together to avoid the sliding of them under compression and enhance the elastic stiffness of cell walls, which offers the asprepared aerogels super-elasticity. Resulting from the excellent mechanical properties, Gr/CNT aerogels can afford high compression while maintain the continuously thermal transport paths. The maximum thermal conductivity of Gr/CNT aerogel TIMs reaches up to 88.5 W m1K1 and the thermal interface resistance is as low as 13.6 mm2 KW1, which outperforms other carbon-based TIMs in literature.
2. Experimental details 2.1. Preparation of 3D Gr/CNT aerogel Graphene oxide (GO) was prepared by the modified Hummers method from natural graphite and got exfoliated GO sheets in their aqueous dispersions according to the previous process [21]. CNTs (diameter of 30e50 nm, length of 50e100 um) were functionalized by refluxing in the mixture of concentrated H2SO4 and HNO3 (3:1 by volume) for about 80 min, and then collected by repeated centrifuging and washing with deionized water. After gentle stirring, we obtained aqueous functionalized CNTs (fCNTs) dispersions. Typically, fCNT aqueous dispersion was added into GO aqueous dispersion with a mass ratio of GO:fCNTs ¼ 3:1. Gr/CNT hydrogel assembled with GO sheets and fCNTs was prepared using a hydrothermal reaction maintained at 180 C for 12 h in a Teflon-lined autoclave. After washing the as-obtained hydrogel with distilled water following with freeze-drying, the Gr/CNT aerogel was
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produced. A neat graphene aerogel without CNTs was also prepared in the same way for comparison. In addition, Gr/CNT aerogels with various initial densities were also prepared by controlling GO/fCNT concentrations. The as-prepared aerogels were defined as Gr/CNT-n (n ¼ 2, 3, 4, 5) based on the GO/fCNT concentration of 2, 3, 4 and 5 mg mL1, respectively. 2.2. Characterizations Scanning electron microscope (SEM) imaging was conducted on a Hitachi S4800 field-emission SEM system with a 5 kV accelerating voltage. Mechanical tests were done by a single-column system (Instron 5843) equipped with two flat-surface compression stages and 1 kN load cells. The initial density (ri) and the compression density (rc) of the samples were calculated as the mass of the samples divided by their corresponding volumes without and with compression, respectively. 2.3. Measurement of thermal transport properties The samples were prepared in cylindrical shape (~12 mm in diameter and 1.5 mm in height) for the measurement of thermal transport properties. The thermal resistance measurement system designed in accordance with the standard of American Society for Testing and Materials (ASTM) D5470. In a typical testing procedure, a sample was inserted into two parallel Cu blocks and compressed into a thin film with the affordable compressive strain. The thermal interface resistance of TIMs (RTIM) was measured by employing Eq. (1). And thermal conductivity (k) of TIMs and thermal contact resistance (Rc) across TIM/Cu interface were calculated by the simultaneous linear equations of Eq. (2) by varying the TIM thickness (l):
RTIM ¼
DT$ASample Q
RTIM ¼ 2RC þ
l k
(1)
(2)
Where ASample is the cross-sectional area of the samples, Q is the thermal flux through the sample, and DT is the temperature gradient across the sample, respectively. 3. Results and discussion The micro-morphologies of a neat graphene aerogel with initial
Fig. 1. Schematic illustration for the micro-structure of Gr/CNT aerogels and cartoon models of entangled CNTs preventing the sliding of overlapping graphene sheets and avoiding splitting of the wrapping graphene sheets. (A colour version of this figure can be viewed online).
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density of 25 mg cm3 (Fig. 2 aec) and the Gr/CNT-2 aerogel with initial density of 23 mg cm3 (Fig. 2 def) were observed under SEM. As shown in Fig. 2a, the neat graphene aerogel exhibits a 3D interconnected porous structure with the cell dimension in range of 2e10 mm, similar to those of previously reported graphene aerogels [22,23]. Each cell possesses stretched ligaments in the walls that are joined at the nodes or junctions. The zoom-in SEM images indicate that the cellar walls consist of partially overlapping (Fig. 2b) and wrapping graphene sheets (Fig. 2c). However, the p-p interactions between the adjacent graphene sheets are too weak to construct the macroscopically graphene aerogels with good mechanical strength. The graphene sheets easily move past each other when the graphene aerogels are under external force. After introducing CNTs into 3D graphene monoliths, the interconnected structure and the cell dimension remain nearly unchanged for Gr/CNT-2 aerogel (Fig. 2d). The CNTs are evenly distributed in the aerogel (Fig. 2e), which is attributed to CNTs well dispersed by acid-treatment and well combined with GO sheets during the hydrothermal process. The structural change of GO sheets and fCNTs before and after hydrothermal process is reflected by the X-ray diffraction spectra and Raman spectra (Fig. S1). It indicates that the hydrothermal process removes partial oxygen containing functional groups of GO and fCNTs, which would provide strong p-p interaction between graphene sheets and CNTs. Closer views (Fig. 2e and f) reveal the highly entangled CNTs covering the ligaments and junctions of cellar walls, which can avoid the sliding and splitting of graphene sheets and further enhance the stiffness of cell walls (The corresponding cartoons of the interconnections between CNTs and graphene sheets are displayed in Fig. 1). As shown in SEM images of the cross section (Fig. 2d) and the vertical section (Fig. S2) of Gr/CNT-2, the aerogels prepared by the hydrothermal process present an isotropic structure. This phenomenon is different from the anisotropy of the graphene monoliths prepared by freeze-casting process [17,18] but consistent with the isotropic graphene aerogels fabricated using hydrothermal strategy [20,24]. Thus, the Gr/CNT aerogels would present similar mechanical properties and thermal transport properties in the cross-plane direction and the in-plane direction. As aforementioned, the compressibility is of particular interest for the thermal transport properties of a 3D graphene structure. The compression experiments were performed to evaluate the mechanical performances of the neat graphene aerogel and the Gr/ CNT aerogel. As shown in Fig. 3a, both neat graphene aerogel and Gr/CNT aerogel with similar initial density can be squeezed into
pellet under certain pressure. Once the external pressure is removed, the neat graphene aerogel deforms permanently indicating a structural collapse. In contrast, the Gr/CNT aerogel is able to almost completely recover to its original shape rapidly. The compressive curves of the graphene aerogel and Gr/CNT-2 aerogel are shown in Fig. 3b. We could not measure during unloading of the graphene aerogel because they permanently deformed during loading to strain of 25%. SEM images of a neat graphene aerogel after compression shows that the cell walls are densely packed and are almost parallel from one to another (Fig. S3). The plastic deformation of the graphene aerogel prepared by the hydrothermal method is due to the lack of a restorative force to act on the graphene frameworks. Previous studies [17] suggest that when 3D graphene monoliths made of few layers of graphene is severely compressed, the intersheet van der Waals adhesion would overwhelm the elastic energy stored, preventing elastic recovery. In contrast, Gr/CNT-2 aerogel shows super-compressibility and can bear a compression strain as high as 80%, which close to the highest value (up to 90%) of 3D graphene monoliths reported previously [19,25,26]. Fig. S4 also shows its stressestrain curves at various maximum strains. Similar to other open-cell elastomeric foams [27e29], the loading process shows three distinct regions: strain<10%, nearly linear elastic regime, corresponding to bending of cell walls; 10% < strain<70%, relatively flat stress plateau, corresponding to elastic buckling of cell walls; and strain>70%, abrupt stress increasing regime, corresponding to densification of cells. Hysteresis loops appear due to dissipation of mechanical energy. And the stress keeps above zero until strain ¼ 0 suggesting the Gr/ CNT-2 aerogel can completely recover to its original volume. The super-elasticity is also reflected from the in situ observation of the full recovery of cell structure under large strain (insert of Fig. 3b). The super-elasticity of Gr/CNT aerogel is attributed to its unique hierarchical structure and the interaction between CNTs and graphene sheets. Firstly, the entangled CNTs attach on the graphene sheets and tightly bond them together, which ensure the deformation of cell walls rather than the sliding between them during the compression. It is similar to the flexible safety net used for preventing rock falling and landslip (Fig. S5). Secondly, the coating of CNTs reinforces the relatively flexible graphene substrate and endows their intrinsic elasticity [30] to the co-organized aerogel. When the aerogels are subjected to external stress, the load can effectively transfer from the loosely connected graphene sheets to the entangled CNT nets, which can greatly enhance the strength
Fig. 2. SEM images of 3D porous structure of (aec) neat graphene aerogel and (def) Gr/CNT-2 aerogel with similar initial density. Microscopic cellar wall architecture consisting of partially (b) overlapping graphene sheets and (c) wrapping graphene sheets for neat graphene aerogel, and entangled CNTs covering (e) overlapping graphene sheets and (f) wrapping graphene sheets for Gr/CNT-2 aerogel.
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Fig. 3. (a) Digital photographs show that the neat graphene aerogel collapses and the Gr/CNT aerogel recovers its original shape after compression. (b) Compressive stressestrain curves for neat graphene aerogel along the loading direction and for Gr/CNT-2 aerogel during loadingeunloading cycle. Inserts: SEM images showing the deformation process of Gr/ CNT-2 aerogel at 70% and back to 40%. Scale bars, 5 mm (c) stressestrain curves of several selected cycles on Gr/CNT-2 aerogel during repeated compression (the 1st, 2nd, 10th, 100th, 1000th cycle). (d) The corresponding maximum stress and energy loss coefficient for different cycles derived from (c). (A colour version of this figure can be viewed online).
and elastic stiffness of cell walls, allowing the van der Waals adhesion to be overcome by the elastic energy. Furthermore, the cyclic strain-stress curves of Gr/CNT-2 aerogel at strain up to 60% are shown in Fig. 3c. The stressestrain curves of the 1000th cycle are similar to that of the first cycle, with the exception that the maximum compressive stress decreases slightly to 76.3% of the original value. The ratio of energy dissipated during compression was calculated and presented in Fig. 3d. The first cycle yields an energy loss coefficient of 78% and this coefficient remains constant at 71% since the 10th cycle, indicating excellent mechanical robustness of the Gr/CNT aerogel. Thermal transport properties of the neat graphene aerogel and the Gr/CNT aerogel were also compared. The values of thermal interface resistance, thermal conductivity, and thermal contact resistance of the neat graphene aerogel (initial thickness of 1.5 mm) at the highest affordable strain (25%) are 379.4 mm2 kW1, 3.2 W m1K1, and 14.2 mm2 KW1, respectively. In comparison, the Gr/CNT-2 aerogel (initial thickness of 1.5 mm) at strain of 25% shows slightly higher thermal conductivity of 5.7 W m1K1, which may be attributed to the conductive bridging effect of CNTs between the graphene sheets. As being pointed out in previous studies, CNTs between adjacent graphitic layers can act as the additional heat flux paths and thus improve the thermal conductivity [3,31]. When the strain of Gr/CNT aerogel reached 80%, the thermal conductivity is as high as 42.3 W m1K1, and the thermal interface resistance and the thermal contact resistance are only 19.5 and 6.2 mm2 KW1, respectively. The significant improvement of thermal transport properties is mainly due to the increased density (increased from 23 to 159 mg cm3) of the Gr/CNT-2 aerogel under mechanical compression. Meanwhile, the continuous thermal transport paths of the Gr/CNT aerogel can be saved even under high compression, which is also very important for the high
thermal transport properties. As mentioned above, the thermal conductivity of 3D graphene monoliths is limited by the very low density [10e12,14]. To further improve the thermal transport properties of the super-elastic Gr/ CNT aerogels, it is feasible to prepare denser aerogels by increasing the initial density. Gr/CNT-2~5 aerogels with GO/fCNT concentration of 2e5 mg mL1 were prepared to investigate the effect of initial density on the thermal transport properties. The initial densities of Gr/CNT aerogels increase from 23 to 85 mg cm3 corresponding to GO/fCNT concentrations from 2 to 5 mg mL1 (Table 1). As shown in Fig. 4aec, with increasing GO/fCNT concentration, the porous structure becomes denser and the average pore size is smaller. And the 3D interconnected structure keeps well, even when GO/fCNT concentration is as high as 5 mg mL1. The stressestrain curves of Gr/CNT-3, -4 and -5 aerogels present the similar shapes with Gr/CNT-2 at strain of 80% (Fig. 4d). The maximum stress at 80% strain increases from 60.1 KPa for Gr/CNT-2 aerogel to 153.2 KPa for Gr/CNT-5 aerogel. The increase of stress is due to the smaller pore size of the aerogel providing higher stiffness and compressive strength. Fig. 4e presents the dependence of the compressive stress (corresponding to the maximum strains) on the aerogel initial density. At the low maximum strain (30%), the stress increases almost linearly with initial density; while for the high maximum strain (80%), the stress increases abruptly with the initial density. Such an abrupt rise can be attributed to cell wall densification, which is influenced by both the aerogel density and the applied strain. Table 1 shows the density change and the thermal transport properties of Gr/CNT aerogels at compressive strain of 80%. The compression densities of Gr/CNT aerogels are ~5 times higher than the initial densities, reaching 159e424 mg cm3. It is found that the thermal conductivity of Gr/CNT aerogels increases significantly
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Table 1 The initial density (ri) of various Gr/CNT aerogels, and their corresponding compression density (rc), thermal conductivity (k), thermal contact resistance (Rc) and thermal interface resistance (RTIM) at compressive strain of 80%. Samples
ri (mg cm3)
rc (mg cm3)
k (Wm1 K1)
Rc(mm2 KW1)
RTIM(mm2 KW1)
Gr/CNT-2 Gr/CNT-3 Gr/CNT-4 Gr/CNT-5
23 46 61 85
129 231 305 424
42.3 63.9 77.5 88.5
6.2 5.7 5.5 5.1
19.5 16.1 14.8 13.6
Fig. 4. SEM images of (a) Gr/CNT-3, (b) Gr/CNT-4 and (c) Gr/CNT-5 aerogels, respectively. (d) Stress-strain curves of Gr/CNT-3, -4 and -5 aerogels, respectively. (e) Density dependence of compressive stress. (A colour version of this figure can be viewed online).
with the initial density. Gr/CNT-5 aerogel with initial density of 121 mg cm3 presents maximum value of 88.5 W m1K1. It means that the robust structure of Gr/CNT aerogels can keep the 3D continuous thermal transport paths well at high initial density. But the thermal transport properties of Gr/CNT aerogel with higher initial density (>85 mg cm3) are not considered in the present work. Because it requires too large pressure to compress denser aerogels to strain of 80% (Fig. 4e). The increase of initial density of Gr/CNT aerogels also leads to the slight decrease of thermal contact resistance across the sample/Cu interface (from 6.2 mm2 KW1 to 5.1 mm2 KW1), because the higher pressure favours the better contact. The thermal interface resistance, as the major parameter for comparing TIM thermal transport performances, is as low as 13.6 mm2 KW1 for the Gr/CNT-5 aerogel. Although there are several super-elastic 3D graphene monoliths have been reported previously, the densities of these 3D graphene monoliths (0.161 mg cm3 [32], 0.5e0.6 mg cm3 [17], 0.3e14 mg cm3 [25]) are not high enough to obtain high thermal transport properties even at the high compressive strain of 90%. The thermographic image of the Gr/CNT-5 aerogel inserted into two parallel Cu blocks and pressed under 153 kPa according to the one dimensional reference bar method (Fig. 5a) was photographed to directly characterize the thermal transport performance. Fig. 5b shows the comparison of thermographic images with and without Gr/CNT-5 aerogel under the same heat flow of 130 KWm2. The temperature difference is obvious at the CueCu dry contact interface as shown in the left of Fig. 5b, indicating poor thermal transport across the interface without TIMs. In comparison, the
temperature gradient almost disappears with the presence of Gr/ CNT-5 aerogel (right of Fig. 5b). Fig. 5c presents the plot of the temperature drop across the interface with and without Gr/CNT-5 aerogel. And the aerogel significantly decreases the interfacial temperature drop from 15 C (under dry contact) to only 3.2 C, indicating the effectiveness of Gr/CNT aerogel in the reduction of thermal interface resistance. The super-elastic Gr/CNT aerogels show excellent thermal transport properties than other carbon-based TIMs. It overcomes the drawbacks of brittle 3D graphene monoliths and provides much higher thermal conductivity than the reported value of the brittle 3D graphene monoliths [10e12]. Although the graphite-based TIMs own high thermal conductivity along the in-plane direction (>200 W m1K1), the thermal conductivity across the thickness direction of graphite-based TIMs (5e24.3 W m1K1) [31,33,34] is much lower than that of our Gr/CNT aerogels. By comparing the thermal transport properties of our Gr/CNT aerogels with the commercial available TIMs, the thermal interface resistance of the Gr/CNT-5 aerogel with compression thickness of 0.3 mm is much lower than that of graphite foil with the thickness of 0.115 mm (52 m m2KW1) [35] and silicon grease with thickness of 0.02 mm (27 m m2KW1) [35]. The thermal transport properties of the Gr/CNT-5 aerogel are even better than that of some best carbon-based TIMs reported previously, such as VACNT array, CNT buckypaper, and vertical graphene (Fig. 6). The relatively high thermal conductivity value of Gr/CNT-5 aerogel is attributed to that the density of the compressed aerogels (up to 424 mg cm3) is much larger than that of VACNT
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Fig. 5. (a) Thermal interface testing of Gr/CNT aerogel. (b) Thermographic images of the copper blocks under steady state cooling without and with Gr/CNT-5 aerogel. (c) Plots of the temperature drop across the interface without and with Gr/CNT-5 aerogel. (A colour version of this figure can be viewed online).
cell walls rather than sliding between them and enhance the elastic stiffness of the cell walls. The compression experiment results show that Gr/CNT aerogels can be compressed to 80% strain and are able to almost completely recover to its original shape. And the thermal conductivity of Gr/CNT-2 aerogel is much higher than that of neat graphene aerogel with similar initial density, which is attributed to the significantly increased density of the aerogels under compression. In addition, the thermal transport properties of Gr/CNT aerogels are improved by increases their initial density. A maximum thermal conductivity of 88.5 W m1K1 and a minimum thermal interface resistance of 13.6 mm2 KW1 of the Gr/CNT-5 aerogel with initial density of 85 mg cm3 are obtained. The comparison of thermal transport properties between Gr/CNT aerogels and other carbon-based TIMs indicates the advantages of Gr/CNT aerogel TIMs and suggested the prospective applications in advanced thermal management. Acknowledgement We acknowledge financial support by the National Natural Science Foundation of China (grant nos. 51503102 and 61274054), the Natural Science Foundation of Jiangsu Province, China (grant no. BK20140869) and NUPTSF (grant nos. NY214055 and NY213082). Fig. 6. Comparison of thermal transport properties of carbon-based TIMs reported previously (refs [8,32,35,37e39]) with the results in this work. (A colour version of this figure can be viewed online).
3
3
array (60 mg cm [36]) and CNT buckypaper (260 mg cm [35]), meaning more carbon nano-materials in unit volume participating in thermal transport. The relatively low thermal interface resistance of Gr/CNT aerogels results from ultrahigh flexibility of graphene sheets and CNTs beneficial to its matching to mating surface under pressure. As described in Eq. (2), TIMs with larger thickness should present higher thermal interface resistance. But the thermal interface resistance of the Gr/CNT-5 aerogel with compression thickness of 300 mm is even lower than that of CNT buckypaper with thickness of 56 mm [35] and that of CNT epoxy with thickness of 6 mm [36], and is similar with that of CNT array with thickness of 7 mm [37]. These excellent performances suggest the promising potentials of Gr/CNT aerogels in current demanding thermal management for electronic and photonics devices.
4. Conclusions Gr/CNT aerogels with super-elasticity were prepared by a synergistic assembly strategy. The highly entangled CNTs were introduced into 3D graphene monoliths to make sure the deformation of
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Super-elastic graphene/carbon nanotube aerogel: A novel thermal interface material with highly thermal transport properties Peng Lv, Xiao-Wen Tan, Ke-Han Yu, Rui-Lin Zheng, Jia-Jin Zheng, Wei Wei* School of Optoelectronic Engineering, Nanjing University of Posts & Telecommunications, Nanjing 210023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 April 2015 Received in revised form 9 December 2015 Accepted 10 December 2015 Available online 13 December 2015
Three-dimensional (3D) graphene structure exhibits promising potential in thermal interface materials (TIMs) due to the continuous network structure and the high thermal conductivity of graphene. Nevertheless, the very low density and the high porosity of the 3D graphene monoliths usually lead to poor thermal transport performance. To overcome these problems, we fabricated the graphene/carbon nanotube (Gr/CNT) aerogels by a synergistic assembly strategy. The entangled CNTs bond the graphene sheets together to avoid the sliding of them under compression and greatly enhance the elastic stiffness of cell walls, which brings the aerogels super-elasticity. Bearing a high compression strain of 80%, the continuous thermal transport paths in Gr/CNT aerogels are still preserved. Significantly increased thermal conductivity of Gr/CNT aerogels can be obtained by directly mechanical compression. Meanwhile, the thermal transport properties of Gr/CNT aerogels can be further improved by elevating their initial density. With an initial density of 85 mg cm3, a thermal conductivity up to 88.5 W m1K1 and a thermal interface resistance as low as 13.6 m m2KW1 were obtained, which outperforms other carbonbased TIMs reported previously. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Development of high-power density electronics made efficient thermal energy dissipation a critical issue for device lifetime and speed [1,2]. But gaps between the contact solid surfaces usually cause high interfacial thermal resistance [3]. Thermal interface materials (TIMs) inserted between the mating surfaces can conform well to surface asperities and reduce the temperature across the interface [4]. In the past decades, significant attentions have been focused on developing carbon-based TIMs due to the extraordinarily high theory thermal conductivity of carbon nano-materials [5e9]. Normally, for achieving high thermal transport properties, carbon-based TIMs should have the interconnected structure to ensure the continuous thermal transport, meanwhile own high flexibility for good contact with the mating surface. Three-dimensional (3D) graphene monoliths have been widely studied because it can assemble the individual graphene sheets with dramatic properties into a macroscopic functional architecture. In 3D graphene monoliths with interconnected structure, the
* Corresponding author. E-mail address: [email protected] (W. Wei). http://dx.doi.org/10.1016/j.carbon.2015.12.026 0008-6223/© 2015 Elsevier Ltd. All rights reserved.
heat fluxes conduct along the graphene sheets and come together at their junctions, then continue to transport to adjacent graphene sheets, which brings 3D graphene monoliths continuous thermal transport paths. And, the ultrahigh flexibility of graphene sheets in 3D graphene monoliths is beneficial to its matching to the mating surface [10]. Thus, 3D graphene monoliths show great promise in developing high performance TIMs. There are several literature reported the thermal transport properties of 3D graphene monoliths [10e14]. The measured thermal conductivities of graphene foam grown by chemical vapour deposition (CVD) were 1.7 W m1K1 (grown on nickel foam) [12] and 8.28 W m1K1 (grown on porous Al2O3) [11]. And some phase change materials were also introduced into graphene aerogel [10] and CVD-graphene foam [13], obtaining the thermal conductivities of 2.635 W m1K1 [10] and 0.44 W m1K1 [13], respectively. It is noteworthy that the thermal conductivity of these 3D graphene monoliths mentioned above is lower than that of other carbon-based TIMs [5e9] and far below that of single layer graphene [15]. The major reason is attributed to the very low graphene volume fraction and the ultrahigh porosity of 3D graphene monoliths [12]. Unfortunately, the most straightforward way, mechanical compression, is unable to decrease their porosity, since these 3D graphene monoliths easily collapse under pressure.
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Recently, some studies about 3D graphene monoliths with super-elasticity have attracted intense interests. The super-elastic graphene foams, aerogels and monoliths prepared by CVD method [16], freeze-casting strategy [17,18], 3D printing technique [19] and hydrothermal process [20] show excellent mechanical properties, especially the high compressibility (compressive strain up to 60%~90%). These progresses provide new insight and strategy for developing high performance TIMs: fabricating a novel superelastic graphene monolith with continuous thermal transport path even under high compressive deformation; then significantly improving the thermal conductivity of 3D graphene monoliths by simply mechanical compression. In this study, we fabricated the super-elastic graphene/carbon nanotube (Gr/CNT) aerogels by hydrothermal method and subsequent freeze-drying process. The aerogels with a 3D framework are constructed with cell walls of graphene sheets and entangled CNTs coating on them (Fig. 1). The entangled CNTs bond the graphene sheets together to avoid the sliding of them under compression and enhance the elastic stiffness of cell walls, which offers the asprepared aerogels super-elasticity. Resulting from the excellent mechanical properties, Gr/CNT aerogels can afford high compression while maintain the continuously thermal transport paths. The maximum thermal conductivity of Gr/CNT aerogel TIMs reaches up to 88.5 W m1K1 and the thermal interface resistance is as low as 13.6 mm2 KW1, which outperforms other carbon-based TIMs in literature.
2. Experimental details 2.1. Preparation of 3D Gr/CNT aerogel Graphene oxide (GO) was prepared by the modified Hummers method from natural graphite and got exfoliated GO sheets in their aqueous dispersions according to the previous process [21]. CNTs (diameter of 30e50 nm, length of 50e100 um) were functionalized by refluxing in the mixture of concentrated H2SO4 and HNO3 (3:1 by volume) for about 80 min, and then collected by repeated centrifuging and washing with deionized water. After gentle stirring, we obtained aqueous functionalized CNTs (fCNTs) dispersions. Typically, fCNT aqueous dispersion was added into GO aqueous dispersion with a mass ratio of GO:fCNTs ¼ 3:1. Gr/CNT hydrogel assembled with GO sheets and fCNTs was prepared using a hydrothermal reaction maintained at 180 C for 12 h in a Teflon-lined autoclave. After washing the as-obtained hydrogel with distilled water following with freeze-drying, the Gr/CNT aerogel was
223
produced. A neat graphene aerogel without CNTs was also prepared in the same way for comparison. In addition, Gr/CNT aerogels with various initial densities were also prepared by controlling GO/fCNT concentrations. The as-prepared aerogels were defined as Gr/CNT-n (n ¼ 2, 3, 4, 5) based on the GO/fCNT concentration of 2, 3, 4 and 5 mg mL1, respectively. 2.2. Characterizations Scanning electron microscope (SEM) imaging was conducted on a Hitachi S4800 field-emission SEM system with a 5 kV accelerating voltage. Mechanical tests were done by a single-column system (Instron 5843) equipped with two flat-surface compression stages and 1 kN load cells. The initial density (ri) and the compression density (rc) of the samples were calculated as the mass of the samples divided by their corresponding volumes without and with compression, respectively. 2.3. Measurement of thermal transport properties The samples were prepared in cylindrical shape (~12 mm in diameter and 1.5 mm in height) for the measurement of thermal transport properties. The thermal resistance measurement system designed in accordance with the standard of American Society for Testing and Materials (ASTM) D5470. In a typical testing procedure, a sample was inserted into two parallel Cu blocks and compressed into a thin film with the affordable compressive strain. The thermal interface resistance of TIMs (RTIM) was measured by employing Eq. (1). And thermal conductivity (k) of TIMs and thermal contact resistance (Rc) across TIM/Cu interface were calculated by the simultaneous linear equations of Eq. (2) by varying the TIM thickness (l):
RTIM ¼
DT$ASample Q
RTIM ¼ 2RC þ
l k
(1)
(2)
Where ASample is the cross-sectional area of the samples, Q is the thermal flux through the sample, and DT is the temperature gradient across the sample, respectively. 3. Results and discussion The micro-morphologies of a neat graphene aerogel with initial
Fig. 1. Schematic illustration for the micro-structure of Gr/CNT aerogels and cartoon models of entangled CNTs preventing the sliding of overlapping graphene sheets and avoiding splitting of the wrapping graphene sheets. (A colour version of this figure can be viewed online).
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density of 25 mg cm3 (Fig. 2 aec) and the Gr/CNT-2 aerogel with initial density of 23 mg cm3 (Fig. 2 def) were observed under SEM. As shown in Fig. 2a, the neat graphene aerogel exhibits a 3D interconnected porous structure with the cell dimension in range of 2e10 mm, similar to those of previously reported graphene aerogels [22,23]. Each cell possesses stretched ligaments in the walls that are joined at the nodes or junctions. The zoom-in SEM images indicate that the cellar walls consist of partially overlapping (Fig. 2b) and wrapping graphene sheets (Fig. 2c). However, the p-p interactions between the adjacent graphene sheets are too weak to construct the macroscopically graphene aerogels with good mechanical strength. The graphene sheets easily move past each other when the graphene aerogels are under external force. After introducing CNTs into 3D graphene monoliths, the interconnected structure and the cell dimension remain nearly unchanged for Gr/CNT-2 aerogel (Fig. 2d). The CNTs are evenly distributed in the aerogel (Fig. 2e), which is attributed to CNTs well dispersed by acid-treatment and well combined with GO sheets during the hydrothermal process. The structural change of GO sheets and fCNTs before and after hydrothermal process is reflected by the X-ray diffraction spectra and Raman spectra (Fig. S1). It indicates that the hydrothermal process removes partial oxygen containing functional groups of GO and fCNTs, which would provide strong p-p interaction between graphene sheets and CNTs. Closer views (Fig. 2e and f) reveal the highly entangled CNTs covering the ligaments and junctions of cellar walls, which can avoid the sliding and splitting of graphene sheets and further enhance the stiffness of cell walls (The corresponding cartoons of the interconnections between CNTs and graphene sheets are displayed in Fig. 1). As shown in SEM images of the cross section (Fig. 2d) and the vertical section (Fig. S2) of Gr/CNT-2, the aerogels prepared by the hydrothermal process present an isotropic structure. This phenomenon is different from the anisotropy of the graphene monoliths prepared by freeze-casting process [17,18] but consistent with the isotropic graphene aerogels fabricated using hydrothermal strategy [20,24]. Thus, the Gr/CNT aerogels would present similar mechanical properties and thermal transport properties in the cross-plane direction and the in-plane direction. As aforementioned, the compressibility is of particular interest for the thermal transport properties of a 3D graphene structure. The compression experiments were performed to evaluate the mechanical performances of the neat graphene aerogel and the Gr/ CNT aerogel. As shown in Fig. 3a, both neat graphene aerogel and Gr/CNT aerogel with similar initial density can be squeezed into
pellet under certain pressure. Once the external pressure is removed, the neat graphene aerogel deforms permanently indicating a structural collapse. In contrast, the Gr/CNT aerogel is able to almost completely recover to its original shape rapidly. The compressive curves of the graphene aerogel and Gr/CNT-2 aerogel are shown in Fig. 3b. We could not measure during unloading of the graphene aerogel because they permanently deformed during loading to strain of 25%. SEM images of a neat graphene aerogel after compression shows that the cell walls are densely packed and are almost parallel from one to another (Fig. S3). The plastic deformation of the graphene aerogel prepared by the hydrothermal method is due to the lack of a restorative force to act on the graphene frameworks. Previous studies [17] suggest that when 3D graphene monoliths made of few layers of graphene is severely compressed, the intersheet van der Waals adhesion would overwhelm the elastic energy stored, preventing elastic recovery. In contrast, Gr/CNT-2 aerogel shows super-compressibility and can bear a compression strain as high as 80%, which close to the highest value (up to 90%) of 3D graphene monoliths reported previously [19,25,26]. Fig. S4 also shows its stressestrain curves at various maximum strains. Similar to other open-cell elastomeric foams [27e29], the loading process shows three distinct regions: strain<10%, nearly linear elastic regime, corresponding to bending of cell walls; 10% < strain<70%, relatively flat stress plateau, corresponding to elastic buckling of cell walls; and strain>70%, abrupt stress increasing regime, corresponding to densification of cells. Hysteresis loops appear due to dissipation of mechanical energy. And the stress keeps above zero until strain ¼ 0 suggesting the Gr/ CNT-2 aerogel can completely recover to its original volume. The super-elasticity is also reflected from the in situ observation of the full recovery of cell structure under large strain (insert of Fig. 3b). The super-elasticity of Gr/CNT aerogel is attributed to its unique hierarchical structure and the interaction between CNTs and graphene sheets. Firstly, the entangled CNTs attach on the graphene sheets and tightly bond them together, which ensure the deformation of cell walls rather than the sliding between them during the compression. It is similar to the flexible safety net used for preventing rock falling and landslip (Fig. S5). Secondly, the coating of CNTs reinforces the relatively flexible graphene substrate and endows their intrinsic elasticity [30] to the co-organized aerogel. When the aerogels are subjected to external stress, the load can effectively transfer from the loosely connected graphene sheets to the entangled CNT nets, which can greatly enhance the strength
Fig. 2. SEM images of 3D porous structure of (aec) neat graphene aerogel and (def) Gr/CNT-2 aerogel with similar initial density. Microscopic cellar wall architecture consisting of partially (b) overlapping graphene sheets and (c) wrapping graphene sheets for neat graphene aerogel, and entangled CNTs covering (e) overlapping graphene sheets and (f) wrapping graphene sheets for Gr/CNT-2 aerogel.
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Fig. 3. (a) Digital photographs show that the neat graphene aerogel collapses and the Gr/CNT aerogel recovers its original shape after compression. (b) Compressive stressestrain curves for neat graphene aerogel along the loading direction and for Gr/CNT-2 aerogel during loadingeunloading cycle. Inserts: SEM images showing the deformation process of Gr/ CNT-2 aerogel at 70% and back to 40%. Scale bars, 5 mm (c) stressestrain curves of several selected cycles on Gr/CNT-2 aerogel during repeated compression (the 1st, 2nd, 10th, 100th, 1000th cycle). (d) The corresponding maximum stress and energy loss coefficient for different cycles derived from (c). (A colour version of this figure can be viewed online).
and elastic stiffness of cell walls, allowing the van der Waals adhesion to be overcome by the elastic energy. Furthermore, the cyclic strain-stress curves of Gr/CNT-2 aerogel at strain up to 60% are shown in Fig. 3c. The stressestrain curves of the 1000th cycle are similar to that of the first cycle, with the exception that the maximum compressive stress decreases slightly to 76.3% of the original value. The ratio of energy dissipated during compression was calculated and presented in Fig. 3d. The first cycle yields an energy loss coefficient of 78% and this coefficient remains constant at 71% since the 10th cycle, indicating excellent mechanical robustness of the Gr/CNT aerogel. Thermal transport properties of the neat graphene aerogel and the Gr/CNT aerogel were also compared. The values of thermal interface resistance, thermal conductivity, and thermal contact resistance of the neat graphene aerogel (initial thickness of 1.5 mm) at the highest affordable strain (25%) are 379.4 mm2 kW1, 3.2 W m1K1, and 14.2 mm2 KW1, respectively. In comparison, the Gr/CNT-2 aerogel (initial thickness of 1.5 mm) at strain of 25% shows slightly higher thermal conductivity of 5.7 W m1K1, which may be attributed to the conductive bridging effect of CNTs between the graphene sheets. As being pointed out in previous studies, CNTs between adjacent graphitic layers can act as the additional heat flux paths and thus improve the thermal conductivity [3,31]. When the strain of Gr/CNT aerogel reached 80%, the thermal conductivity is as high as 42.3 W m1K1, and the thermal interface resistance and the thermal contact resistance are only 19.5 and 6.2 mm2 KW1, respectively. The significant improvement of thermal transport properties is mainly due to the increased density (increased from 23 to 159 mg cm3) of the Gr/CNT-2 aerogel under mechanical compression. Meanwhile, the continuous thermal transport paths of the Gr/CNT aerogel can be saved even under high compression, which is also very important for the high
thermal transport properties. As mentioned above, the thermal conductivity of 3D graphene monoliths is limited by the very low density [10e12,14]. To further improve the thermal transport properties of the super-elastic Gr/ CNT aerogels, it is feasible to prepare denser aerogels by increasing the initial density. Gr/CNT-2~5 aerogels with GO/fCNT concentration of 2e5 mg mL1 were prepared to investigate the effect of initial density on the thermal transport properties. The initial densities of Gr/CNT aerogels increase from 23 to 85 mg cm3 corresponding to GO/fCNT concentrations from 2 to 5 mg mL1 (Table 1). As shown in Fig. 4aec, with increasing GO/fCNT concentration, the porous structure becomes denser and the average pore size is smaller. And the 3D interconnected structure keeps well, even when GO/fCNT concentration is as high as 5 mg mL1. The stressestrain curves of Gr/CNT-3, -4 and -5 aerogels present the similar shapes with Gr/CNT-2 at strain of 80% (Fig. 4d). The maximum stress at 80% strain increases from 60.1 KPa for Gr/CNT-2 aerogel to 153.2 KPa for Gr/CNT-5 aerogel. The increase of stress is due to the smaller pore size of the aerogel providing higher stiffness and compressive strength. Fig. 4e presents the dependence of the compressive stress (corresponding to the maximum strains) on the aerogel initial density. At the low maximum strain (30%), the stress increases almost linearly with initial density; while for the high maximum strain (80%), the stress increases abruptly with the initial density. Such an abrupt rise can be attributed to cell wall densification, which is influenced by both the aerogel density and the applied strain. Table 1 shows the density change and the thermal transport properties of Gr/CNT aerogels at compressive strain of 80%. The compression densities of Gr/CNT aerogels are ~5 times higher than the initial densities, reaching 159e424 mg cm3. It is found that the thermal conductivity of Gr/CNT aerogels increases significantly
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Table 1 The initial density (ri) of various Gr/CNT aerogels, and their corresponding compression density (rc), thermal conductivity (k), thermal contact resistance (Rc) and thermal interface resistance (RTIM) at compressive strain of 80%. Samples
ri (mg cm3)
rc (mg cm3)
k (Wm1 K1)
Rc(mm2 KW1)
RTIM(mm2 KW1)
Gr/CNT-2 Gr/CNT-3 Gr/CNT-4 Gr/CNT-5
23 46 61 85
129 231 305 424
42.3 63.9 77.5 88.5
6.2 5.7 5.5 5.1
19.5 16.1 14.8 13.6
Fig. 4. SEM images of (a) Gr/CNT-3, (b) Gr/CNT-4 and (c) Gr/CNT-5 aerogels, respectively. (d) Stress-strain curves of Gr/CNT-3, -4 and -5 aerogels, respectively. (e) Density dependence of compressive stress. (A colour version of this figure can be viewed online).
with the initial density. Gr/CNT-5 aerogel with initial density of 121 mg cm3 presents maximum value of 88.5 W m1K1. It means that the robust structure of Gr/CNT aerogels can keep the 3D continuous thermal transport paths well at high initial density. But the thermal transport properties of Gr/CNT aerogel with higher initial density (>85 mg cm3) are not considered in the present work. Because it requires too large pressure to compress denser aerogels to strain of 80% (Fig. 4e). The increase of initial density of Gr/CNT aerogels also leads to the slight decrease of thermal contact resistance across the sample/Cu interface (from 6.2 mm2 KW1 to 5.1 mm2 KW1), because the higher pressure favours the better contact. The thermal interface resistance, as the major parameter for comparing TIM thermal transport performances, is as low as 13.6 mm2 KW1 for the Gr/CNT-5 aerogel. Although there are several super-elastic 3D graphene monoliths have been reported previously, the densities of these 3D graphene monoliths (0.161 mg cm3 [32], 0.5e0.6 mg cm3 [17], 0.3e14 mg cm3 [25]) are not high enough to obtain high thermal transport properties even at the high compressive strain of 90%. The thermographic image of the Gr/CNT-5 aerogel inserted into two parallel Cu blocks and pressed under 153 kPa according to the one dimensional reference bar method (Fig. 5a) was photographed to directly characterize the thermal transport performance. Fig. 5b shows the comparison of thermographic images with and without Gr/CNT-5 aerogel under the same heat flow of 130 KWm2. The temperature difference is obvious at the CueCu dry contact interface as shown in the left of Fig. 5b, indicating poor thermal transport across the interface without TIMs. In comparison, the
temperature gradient almost disappears with the presence of Gr/ CNT-5 aerogel (right of Fig. 5b). Fig. 5c presents the plot of the temperature drop across the interface with and without Gr/CNT-5 aerogel. And the aerogel significantly decreases the interfacial temperature drop from 15 C (under dry contact) to only 3.2 C, indicating the effectiveness of Gr/CNT aerogel in the reduction of thermal interface resistance. The super-elastic Gr/CNT aerogels show excellent thermal transport properties than other carbon-based TIMs. It overcomes the drawbacks of brittle 3D graphene monoliths and provides much higher thermal conductivity than the reported value of the brittle 3D graphene monoliths [10e12]. Although the graphite-based TIMs own high thermal conductivity along the in-plane direction (>200 W m1K1), the thermal conductivity across the thickness direction of graphite-based TIMs (5e24.3 W m1K1) [31,33,34] is much lower than that of our Gr/CNT aerogels. By comparing the thermal transport properties of our Gr/CNT aerogels with the commercial available TIMs, the thermal interface resistance of the Gr/CNT-5 aerogel with compression thickness of 0.3 mm is much lower than that of graphite foil with the thickness of 0.115 mm (52 m m2KW1) [35] and silicon grease with thickness of 0.02 mm (27 m m2KW1) [35]. The thermal transport properties of the Gr/CNT-5 aerogel are even better than that of some best carbon-based TIMs reported previously, such as VACNT array, CNT buckypaper, and vertical graphene (Fig. 6). The relatively high thermal conductivity value of Gr/CNT-5 aerogel is attributed to that the density of the compressed aerogels (up to 424 mg cm3) is much larger than that of VACNT
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Fig. 5. (a) Thermal interface testing of Gr/CNT aerogel. (b) Thermographic images of the copper blocks under steady state cooling without and with Gr/CNT-5 aerogel. (c) Plots of the temperature drop across the interface without and with Gr/CNT-5 aerogel. (A colour version of this figure can be viewed online).
cell walls rather than sliding between them and enhance the elastic stiffness of the cell walls. The compression experiment results show that Gr/CNT aerogels can be compressed to 80% strain and are able to almost completely recover to its original shape. And the thermal conductivity of Gr/CNT-2 aerogel is much higher than that of neat graphene aerogel with similar initial density, which is attributed to the significantly increased density of the aerogels under compression. In addition, the thermal transport properties of Gr/CNT aerogels are improved by increases their initial density. A maximum thermal conductivity of 88.5 W m1K1 and a minimum thermal interface resistance of 13.6 mm2 KW1 of the Gr/CNT-5 aerogel with initial density of 85 mg cm3 are obtained. The comparison of thermal transport properties between Gr/CNT aerogels and other carbon-based TIMs indicates the advantages of Gr/CNT aerogel TIMs and suggested the prospective applications in advanced thermal management. Acknowledgement We acknowledge financial support by the National Natural Science Foundation of China (grant nos. 51503102 and 61274054), the Natural Science Foundation of Jiangsu Province, China (grant no. BK20140869) and NUPTSF (grant nos. NY214055 and NY213082). Fig. 6. Comparison of thermal transport properties of carbon-based TIMs reported previously (refs [8,32,35,37e39]) with the results in this work. (A colour version of this figure can be viewed online).
3
3
array (60 mg cm [36]) and CNT buckypaper (260 mg cm [35]), meaning more carbon nano-materials in unit volume participating in thermal transport. The relatively low thermal interface resistance of Gr/CNT aerogels results from ultrahigh flexibility of graphene sheets and CNTs beneficial to its matching to mating surface under pressure. As described in Eq. (2), TIMs with larger thickness should present higher thermal interface resistance. But the thermal interface resistance of the Gr/CNT-5 aerogel with compression thickness of 300 mm is even lower than that of CNT buckypaper with thickness of 56 mm [35] and that of CNT epoxy with thickness of 6 mm [36], and is similar with that of CNT array with thickness of 7 mm [37]. These excellent performances suggest the promising potentials of Gr/CNT aerogels in current demanding thermal management for electronic and photonics devices.
4. Conclusions Gr/CNT aerogels with super-elasticity were prepared by a synergistic assembly strategy. The highly entangled CNTs were introduced into 3D graphene monoliths to make sure the deformation of
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