Carbon Nanotubes Grown on Graphite Films as Effective Interface

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Carbon Nanotubes Grown on Graphite Films as Effective Interface Enhancement for an Aluminum Matrix Laminated Composite in Thermal Management Applications Jing Chang,†,‡ Qiang Zhang,*,†,⊥ Yingfei Lin,§ Chang Zhou,† Wenshu Yang,† Liwen Yan,*,∥ and Gaohui Wu† †

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore § Guangdong Institute of Materials and Processing, Guangzhou 510650, P. R. China ∥ National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, P. R. China ⊥ Key Laboratory of Advanced Structure-Function Integrated Materials and Green Manufacturing Technology, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China

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S Supporting Information *

ABSTRACT: Uniform and dense carbon nanotubes (CNTs) were grown on the surface of the graphite film (GF) by a plasmaenhanced chemical vapor deposition process. The synthesized CNTs can act as a bridge between GF and Al matrix to enhance the interface performance and improve thermal properties of the GF/Al laminated composite simultaneously. A layer-by-layer CNTs−GF/Al composite with both increased mechanical property and thermal management capability was fabricated through an optimized pressure infiltration process, which was time- and energy-saving. The results show that the interface of the laminated composite is well bonded and no interface product such as Al4C3 is generated. Additional investigations reveal that the growth of CNTs is an effective way to improve the thermal conductivity and reduce the coefficient of thermal expansion of the GF reinforced Al composites. Overall, the best-performing CNTs−GF/Al composites with a CNTs−GF volume fraction of 51.42% show an increase of 47.99% in thermal conductivity and 26.44% in interlaminar shear strength, making them promising thermal management laminated materials. KEYWORDS: carbon nanotube, graphite film, thermal management application, interlaminar shear strength, pressure infiltration, laminated material

1. INTRODUCTION With the rapid development of high-tech technologies such as electronics, microelectronics, laser technology, and information technology, the power devices tend to be miniaturized and highly integrated. The power consumption and excessive heat of electronic equipments are increased dramatically, so the effective heat export has become a key issue in the electronic packaging design.1,2 Superior thermal management of power devices is urgent because the heat dissipation has become a technical bottleneck, restricting the upgrading of electronic technology.3−7 To solve the thermal management problems, the application of high thermal conductive materials is one of the most effective routes.8 In general, materials with high thermal conductivity (TC) are ceramics, metals, or composites, whereas the ceramic material is brittle and always need a © XXXX American Chemical Society

complex fabrication process and a high production cost. The TC of the metal material is outstanding, but the coefficient of thermal expansion (CTE) of metal is very high.9 The biggest characteristic of the composite material is that the function and performance of composite can be designed. It is reasonable to select the composition, content, or change the heat treatment state of the composite material to realize the adjustment of the composite material. Nowadays, metal-based composites applied in the field of electronic packaging thermal management are mainly SiCp/Al, diamond/Cu, diamond/Al, Cf/Al, and so on. However, with Received: July 28, 2018 Accepted: October 16, 2018 Published: October 16, 2018 A

DOI: 10.1021/acsami.8b12691 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

damages or defects of the GF. However, the physical method is mainly chemical vapor deposition (CVD)34,35 or chemical vapor impregnation36 method, in which the reaction condition is relatively mild. In this study, a novel CNTs−GF/Al composite was successfully fabricated by the pressure infiltration method and CNT-modified GFs were obtained through a plasmaenhanced CVD (PECVD) method with different fabrication parameters to optimize the morphology and scale of CNTs. Thermophysical properties of CNTs−GF/Al composites were carefully investigated. The CNT-modified GFs with good TC and low CTE were used in the composite to enhance the overall TC performance of the composites. The flexible CNTs can offer a support to improve the poor wettability between Al matrix and GFs, serve as a fine conductive substrate for enabling good TC contact between them, as well as effectively enhance the mechanical performance of the composite. The microstructure of the CNT-modified GFs and CNTs−GF/Al composite was characterized; interfacial microstructure and property of CNTs−GF/Al composites were also studied. The benefit of CNT-modified GFs on the composites was studied, and the TCs of the composites were compared with those of GF/Al composites. The results show that CNT-modified GFs are an effective way to improve the TC of the composites, especially the in-plane TC of the composites. Because of the high TC and unique laminated structure, the obtained CNTs− GF/Al composites display great potential in application as thermal management materials.

the development of high-power density electronic and microelectronic devices, relatively low TC of the SiCp/Al composite [the maximum reported TC of the SiCp/Al composite is 250 W/(m·K)] limits its application in thermal management.10−13 The carbon material is an ideal reinforcement candidate with high TC, especially the carbon materials with sp2 and sp3 hybridization, such as graphene, carbon nanotubes (CNTs), diamonds, and so forth. 14,15 The diamond/Al composite material has high TC [up to 600 W/ (m·K)] and low CTE, which could meet the requirements of high-power electronic packaging thermal management, but its poor processing and expensive production costs constrain its development.16,17 The dispersion and orientation of CNTs in the metal matrix have been technically difficult to keep in the laboratory stage and cannot be applied to large-scale industrial production.18 Cf/Al composites have high TC, especially the short carbon fiber/aluminum composites, but the preparation process of the composites is difficult and the price of carbon fiber is high.19 Electronic packaging and thermal management applications have an urgent and huge demand for highperformance new thermal conductive materials. Graphite crystals have a hexagonal planar network structure and can be divided into natural graphite and artificial graphite. The graphite film (GF) has an excellent in-plane TC of 1100− 1600 W/(m·K).20 Because of the excellent physical and thermal properties, the GF has attracted much attention in thermal management and has been widely used in mobile phones, computers, and other areas of heat dissipation.20−23 Comparing with other carbon materials, such as carbon fiber, graphene, CNT, and so forth, the potential GF is unfortunately rarely used as a reinforcement for the metal matrix composite (MMC). Although the microscale thickness of GF limits its direct application in electronic components, it can be combined with metal films to fabricate the GF/metal laminated composite. Furthermore, in the fabrication of GF/ metal laminated composites, the distribution and orientation of the GF reinforcements could be controlled easily by the layerby-layer stacking of GFs and metal foils, which is more convenient and effective than other carbon material reinforcements such as carbon graphite flakes,8,24 carbon fibers,19,25 CNTs,5,9,26 graphene,1,27 and so forth. However, as we all know, the compound of Al matrix and graphite arouse the problem of poor wettability (the contact angle between Al matrix and graphite is about 138°), which will result in the poor interface integration, pores and interface debonding of the composites, followed by the decrease of TC.8,28 Recently, the development of nanotechnology has provided new chances for improving traditional MMCs. A CNT is regarded as one of the most important reinforcements for composites because of its remarkable physical performance.18,27 What is more, the CNT array is also used in the surface modification for changing surface wettability and surface energy of carbon fiber or GF reinforcement.29−31 As for the laminated composites, the enhancements in strength, toughness, and z-axis properties such as interlaminar shear strength and TC are specially expected. Many previous research studies studied the improved interface and wetting performance of CNTs with the polymer material, whereas seldom studies about the CNT-modified GF improving the wetting performance with Al were reported. Generally, The CNT-modified GF can be achieved by chemical or physical methods. The chemical method mainly refers to chemical grafting32 or chemical oxidation,33 which will always cause

2. EXPERIMENTAL SECTION 2.1. Materials. GFs with 99.9% graphitization [TCs in xy and z direction were measured to be 1420 and 12 W/(m·K), respectively] and aluminum foils with purity of 99.9% and TC of 234 W/(m·K)37 were used in the preparation of the composites. GFs with an average thickness of 25 μm were purchased from Qingdao Donke Graphite Co. Ltd. Acetone, nitric acid, and nickel nitrate hexahydrate [Ni(NO3)2·6H2O] were purchased from Tianjin Fuchen Chemical Reagents Factory. The pure aluminum with a TC of 237 W/(m·K) was achieved from Aluminum Corporation of China, and the chemical composition was 0.12 wt % Si, 0.16 wt % Fe, 0.03 wt % Cu, 0.01 wt % Mg, 0.001 wt % Ca, and the balance Al. The aluminum foils with the thickness of about 29 μm were purchased from Jinan Longshan Aluminum Co. Ltd. All of the raw materials and chemicals were used as received without further purification. 2.2. Preparation of CNTs−GF Multiscale Architectures. GFs were cut into a specified size (70 × 45 mm2) and then washed with acetone solution at 80 °C to clean the surface impurities and dried at 60 °C in vacuum. Then, the GFs after drying were slightly oxidized by nitric acid for 3.5 h at 25 °C. Ni(NO3)2·6H2O acted as a catalyst was coated on the GFs by immersing the GFs into 0.01, 0.03, and 0.05 M acetone solution of Ni(NO3)2·6H2O at room temperature for 12 h. Then, GFs loaded with Ni(NO3)2·6H2O were moved into a PECVD reaction chamber; methane acted as a carbon resource and hydrogen used as reducing and carrier gas were inserted. The Ni(NO3)2·6H2Ocoated GFs were heated at 600 °C for 30 min under the protection of H2 with a flow rate of 20 sccm, and then Ni2+ was reduced to metallic Ni. Then, the GFs were heated to 700 °C with the rate of 5 °C/min. The switch of radio frequency was opened, and CNTs were grown onto the surface of GFs after introducing a mixture of methane/ hydrogen into the CVD reactor with a flow rate of 40/10 sccm. The operation time of radio frequency was controlled at 30 min. To achieve appropriate CNT loadings on the GFs, the catalyst concentration was controlled from 0.01 to 0.05 M. The reactor was cooled down to 25 °C in the H2 atmosphere. Finally, the CNTs−GF multiscale architectures were fabricated and taken out. B


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polished with 600 # and 1000 # sandpapers. The test was carried out on an Instron-5900 electronic testing system with the loading span of 10 mm and a loading rate of 2 mm/min. The ILSS of the CNTs−GF/ Al nanocomposite samples was calculated according to the equation τ = 3P/(4bh), in which P represented the fracture load, h was the thickness of the sample, b represented the width of the sample, and τ was the ILSS.

2.3. Fabrication of CNTs−GF/Al Laminated Nanocomposites. The CNTs−GF/Al nanocomposites were prepared through a pressure infiltration process, which is time- and energy-saving and environmentally friendly comparing with the traditional methods, such as powder metallurgy and hot pressing sintering. The detailed fabrication of the laminated CNTs−GF/Al composites is as follows: (1) aluminum foils were also cut into the specified size (70 × 45 mm2) as GFs. To remove the surface oxidation film, aluminum foils were dipped into 0.4 M NaOH aqueous solution for 2 min and then immersed in 20 vol % HNO3 solution for another 2 min. Finally, the aluminum foils were washed with ethanol and dried under vacuum. (2) The aluminum foils and CNTs−GFs were put into a graphite mold layer by layer to the top of the mold. Then, a graphite cover was installed on the top of the laminated films. The mold-loaded CNTs− GFs and aluminum foils were heated to 800 °C. (3) The extra bulk aluminum was heated to melt, degassed, and purified in an Al2O3 crucible. Then, the molten aluminum was added to the mold loaded with CNTs−GFs and aluminum foils, and suitable pressure was used to compel the molten aluminum to completely infiltrate into the CNTs−GFs. Here, the molten pure aluminum liquid was added to better recombine the CNTs−GF layers and the aluminum foils because the molten aluminum liquid could be impregnated into the preform consisting of CNTs−GFs and aluminum foils in the pressure infiltration process. (4) Finally, the laminated CNTs−GFs reinforced aluminum matrix composite blocks with the size of 45 × 70 × 4 mm3 were fabricated after the mold was kept with the pressure of 350 MPa at 800 °C for 5 min. To prepare the CNTs−GF/Al laminated composites with different amounts of CNTs−GF, the volume fraction of CNTs−GF was roughly controlled by stacking different layers of CNTs−GF and aluminum foil in the mold at first. Then, after the composite was fabricated, the real volume fraction of the CNTs−GF was different from the expected volume fraction because of the mold releasing and the extra aluminum liquid addition in the pressure infiltration process. Therefore, the density of the composite was tested by the drainage method after the CNTs−GF/Al composite was prepared. Then, according to the rule of mixture and the obtained density of the composite, the real volume fraction of CNTs−GF was calculated. 2.4. Characterization. The crystalline property and composition of the fabricated multiscale CNTs−GF structures were characterized by X-ray diffraction (XRD) using a Philip X’Pert X-ray diffractometer in the 2θ range from 10° to 80°. The molecular vibrations and crystal structures of the intermediate CNTs−GF products were analyzed by a Raman spectrophotometer (WITec Alpha 300R). The morphology of the CNTs−GF/Al laminated composites was observed with an optical microscope (GX53, OLYMPUS). The microstructures of the resulting multiscale CNTs−GF architectures and CNTs−GF/Al composites were observed using a FEI Helios Nanolab 600i field emission scanning electron microscope (FE-SEM) operated at 20 kV and a FEI Tecnai F30 transmission electron microscope (TEM) equipped with energy-dispersive spectroscopy operated at 200 kV. The TEM samples of the interface of the CNTs−GF/Al laminated composite were designed and prepared by a focused ion beam (FIB) system (Helios NanoLab 600i FIB/SEM) utilizing a beam of 30 kV Ga ions. The thermal diffusivity of the CNTs−GF/Al composite samples was tested using a NETZSCH LFA447 thermal analyzer at 25 °C. The Archimedes method was used to measure the density of the CNTs−GF/Al composite. The linear rule of mixture was applied to calculate the specific heat capacities of the samples.8 According to the equation λ = α·Cp·ρ, the TCs (λ) of the CNTs−GF/Al nanocomposites were calculated from the corresponding thermal diffusivities (α), specific heat capacities (Cp), and densities (ρ).38 The in-plane CTE of the CNTs−GF/Al nanocomposite samples was tested using a NETZSCH DIL 402C analyzer from 25 to 300 °C with a heating rate of 5 °C/min with the specimen size of 10 × 2 × 25 mm3. The interlayer shear strength (ILSS) of the CNTs−GF/Al nanocomposites was tested by a short-beam method according to the ASTMD 2344-2013 standard. The test samples with the size of 15 × 5 × 2 mm3 were prepared from the wire cutting process and further

3. RESULTS AND DISCUSSION A pressure infiltration method was carried out to prepare the layer-by-layer CNTs−GF/Al composites with the CNTmodified GF as multiscale reinforcement, and the schematic illustration of the fabrication process was shown in Figure 1.

Figure 1. Schematic drawing of the fabrication process of the CNTs− GF/Al laminated composite block.

To improve the interface wettability and integration between GF and Al matrix, CNTs were grown on the surface of the GF by a catalyst assistant PECVD method. GFs were immersed in the acetone solution of Ni(NO3)2·6H2O to allow the catalyst to be evenly distributed on the surface. Because of the hydrophobic surface of the GF, the acetone solution as the catalyst’s solvent guaranteed the better wettability and distribution of nickel nitrate on the surface of the GF. The concentration of the catalyst solution has a great effect on the dispersion of Ni2+ on the surface of GF, and the catalyst size directly determines the growth effect and diameter of the CNTs to some extents. The higher concentration of the catalyst solution always results in more agglomeration of the catalyst, which leads to lager diameter of the CNTs. Meanwhile, as the catalyst particle diameter increases, the diffusion path also expands, resulting in the formation of amorphous carbon rather than the formation of CNTs. From the results of our study, the appropriate loading concentration of the catalyst on the GF surface is of 0.05 M and the results are close to Zhao’s previous work.39 After the controlled growth of CNTs on the surface of the GF, the achieved CNTs−GFs with appropriate scale were used as effective enhancement to fabricate CNTs−GF/Al laminated composite with selected Al foils by the pressure infiltration process. Scanning electron micrographs of primary GF and CNTs grown on the GF (CNTs−GF architectures) with different concentrations of catalyst from 0 to 0.05 M are shown in Figure 2. From Figure 2a, the surface of the untreated GF was relatively smooth and no noticeable grooves could be observed. However, after the attachment of the transitionmetal catalyst and the heat treating with PECVD in CH4/H2 mixed atmosphere, CNTs were found to be uniformly grown on the surface of the GF. When the concentration of the catalyst was 0.01 M, as shown in Figure 2b, thinly scattered CNTs with inconsistent lengths were grown on the surface of GF with the PECVD operation time of 30 min in CH4/H2 C


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The XRD patterns and the Raman curves of the pure GF and CNTs−GF architectures were also collected, and the results are shown in Figure S2 and S3, respectively. There are two obvious peaks in both the pattern of GF and CNTs−GF samples (Figure S2). Two sharp diffraction peaks with the 2θ position at 26.38° and 54.54° were ascribed to the (002) and (004) crystal plane diffraction of hexagonal graphite, respectively. No impurity peaks could be observed in the XRD pattern of CNTs−GF samples, which shown that the CNTs with high purity were grown on the surface of the GF. However, comparing with that of pure GF, the relative intensity of the (002) diffraction peak in CNTs−GF was slightly lower, which demonstrated that the crystallinity degree of CNTs was lower than that of the GF substrate. In the Raman spectroscopy (Figure S3), as for the pure GF, only one peak at about ∼1580 cm−1 was observed and attributed to the tangential mode G-band, whereas for the CNTs−GF sample, both the D-band at about 1350 cm−1 and the G-band at ∼1580 cm−1 were displayed in the curve. The D-band in the spectrum corresponds to the breathing vibration of sp2 carbon atoms which revealing the disorder defects of the carbon materials, and the G-band in the curve relates to the interlayer displacement of the carbon materials. The ratio R (R = ID/ IG) from the intensity of D-band and G-band expressed the degree of the graphite ordering, and the smaller the R value represented the higher the graphite ordering degree.40 In the Raman spectra of pure GF, the almost nonexistent D-band indicated an extremely low R value, which further displayed the GF used in our experiment that had a high degree of graphitization and had no disorder defects. Although after the growth of CNTs on the surface of the GF, an obvious D peak was observed in the Raman spectra of CNTs−GF, showing that the CNTs fabricated by the PECVD method had much disorder defects. In order to confirm the structure of the CNTs clearly, Figure S4 shows the bright-field TEM image, high-resolution TEM (HRTEM) image, and selected-area electron diffraction (SAED) pattern of the CNTs prepared by the PECVD method. From Figure S4a,b, we could find that the fabricated carbonic materials on the surface of GF are multiwalled CNTs with wormlike structures and diameters of about 80 nm. It is a hollow tubular structure even though the wall of the CNT is thick. A high-resolution TEM image in Figure S4c shows part of the wall of single CNT. It is clear that the graphene layers in multiwalled CNTs have an irregular and freedom arrangement. The turbostratic graphene stacking in the multiwalled CNTs always leads to many defects in CNTs, which agrees with the result of Raman spectroscopy. The SAED pattern in Figure S4d reveals the crystal feature of the multiwalled CNTs and presents its (002), (100), and (110) diffraction rings clearly.

Figure 2. SEM images of (a) untreated GF and CNTs on the surface of GF obtained with the catalyst concentrations of (b) 0.01, (c) 0.03, and (d) 0.05 M. The insets in (c,d) are the corresponding highresolution SEM images, and the scale bar is 100 nm.

mixed atmosphere. With the increase of the catalyst concentration, the CNTs grown on the surface of the GF were increasingly intensive and gradually longer. As seen in Figure 2c, the uniform and dense CNT arrays with an average diameter of about 70 nm and a length of 1−2 μm were obtained with the catalyst concentration of 0.03 M. In this condition, the CNTs had a wormlike shape and an unsmooth surface. As the catalyst concentration reached to 0.05 M (Figure 2d), it was observed that the surface of the GF was covered with a layer of more excellent and homogeneous CNTs with an average diameter of 80 nm and a length of 3−4 μm. In addition, the CNTs prepared in this condition had a relatively smooth surface comparing with those in Figure 2c. The growth direction of the CNTs on the surface of the GF by the PECVD method was confirmed by the SEM images of the cross section of CNTs−GF structures. As shown in Figure S1, the CNTs fabricated by the PECVD method with different catalyst concentrations were vertically grown on the surface of the GF. In summary, CNTs with different lengths and distribution densities could be controlled grown on the surface of the GF by adjusting the concentration of the transitionmetal catalyst. CNTs−GF architectures fabricated with the catalyst concentration of 0.05 M were selected as an effective reinforcement for the layer-by-layer aluminum matrix thermal management material.

Figure 3. (a) Optical microscope image of the cross section of laminated CNTs−GF/Al composites, (b) high-resolution optical microscope image of the cross section of laminated CNTs−GF/Al composites, and (c) XRD pattern of the laminated CNTs−GF/Al nanocomposites. D


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Figure 4. TEM images of the CNTs−GF/Al laminated nanocomposites. (a) Bright-field TEM image of the interface area of CNTs−GF/Al. (b) Bright-field TEM image of GF in the composites, the inset is the SAED pattern of GF in laminated composites. (c) Bright-field TEM image of the CNTs in the composites, the inset is the SAED pattern of the CNTs in composites. (d) Bright-field TEM image of the Al matrix in the composites, the inset is the SAED pattern of the Al matrix in the composites. (e) HRTEM photograph of the interface between CNTs and Al matrix. (f) FFT pattern from the interface area in (e).

The CNTs−GF reinforced aluminum laminated nanocomposites were fabricated through a pressure infiltration process. Figure 3a,b presents the typical optical microscope photograph of the obtained CNTs−GF/Al composites with the CNTs−GF volume fraction of 51.42 vol %. It could be seen that the CNTs−GF/Al composites had an obvious lamellar microstructure, and both the CNTs−GFs and the aluminum matrix were well arranged in parallel and perpendicular to the pressing direction as expected. The XRD pattern of the CNTs−GF/Al laminated composite with a CNTs−GF volume fraction of 51.42 vol % was shown in Figure 3c. The diffraction peaks of both Al and C could be observed. Two characteristic diffraction peaks at about 2θ = 26.5° and 54.7° could be assigned to C(002) and C(004) crystal planes from CNTs−GF, respectively. The peaks at about 2θ = 38.5° and 44.7° were ascribed to (111) and (200) crystal planes of Al, respectively. In general, the interfacial product Al4C3 always exists at the interface of GF reinforced aluminum composites prepared by the pressure infiltration method under atmospheric pressure. However, only the diffraction peaks of Al and C could be observed from the XRD pattern of our CNTs−GF/Al laminated composites, and there was no evidence of Al4C3 generation at the interface of GF and aluminum. The result of no Al4C3 interfacial product was further examined by HRTEM shown in Figure 4. In order to better understand the interface morphology and further confirm the interfacial configurations of the CNTs− GF/Al laminated composites, the interface structure of the composites is also characterized by (HR)TEM and the images are shown in Figure 4. Figure 4a presents the overview TEM photograph of the interface area of CNTs−GF/Al composites, and it can be found that the CNTs are embedded between the GF and Al matrix. Figure 4b shows the TEM image of the GF in the composites, and the inset shows the corresponding SAED pattern. The GF has an obvious lamellar microstructure, and the clear diffraction spots reveal that the GF has a high degree of crystallinity and graphitization. The bright-field TEM image of CNTs in the Al matrix is shown in Figure 4c. CNTs

with a diameter of 80 nm are uniformly embedded in the Al matrix. The inset shows the corresponding SAED pattern of the CNTs, in which (002), (100), and (110) diffraction rings could be observed. Figure 4d shows the TEM image of the Al matrix, and the inset reveals the corresponding SAED pattern, in which the clear diffraction spots of (200), (220), and (020) can be found. As the HRTEM photograph of the interface area of CNTs−GF/Al nanocomposites shown in Figure 4e, a distinguished and clear interface is observed along the CNTs/ Al interface. The indexed fast Fourier transform (FFT) results of the interface area in Figure 4e are shown in Figure 4f. The FFT result contains the central diffuse ring and the clear diffraction spots of the Al matrix. The FFT result reveals that an amorphous phase exists in the interface of CNTs/Al. Several previous literatures reported that the amorphous interface layers have been existed in diamond/Al, graphite flakes/Al, and CNTs/Cu composites.41,42 Moreover, no interfacial reaction product such as Al4C3 could be detected along the well-integrated interface of the composite. Therefore, the XRD analysis results and the TEM observation results demonstrate that no Al4C3 reaction products generated in the interface area of CNTs−GF/Al composites which may be beneficial to facilitate heat conduction and achieve superior mechanical performance. As we all know, carbon/Al composites such as CF/Al, diamond/Al, and graphite flakes/Al composites prepared by the gas pressure infiltration method often generate interfacial product Al4C3.43−45 The chemical reaction equation in forming the Al4C3 is 4Al + 3C = Al4C3. The standard free energy of Al4C3 formation reaction is negative, which shows a tendency of the generation of Al4C3 in the interface. However, the amount of the Al4C3 interfacial product is determined by kinetics.46,47 The reaction rate of Al4C3 production is determined by atomic diffusion, which can be described as X = (2Kt)1/2, where X is the thickness of the reaction layer, K represents the reaction rate constant related with temperature, and t represents the reaction time.46,47 It is obvious that the generation of interfacial product Al4C3 is determined by the E


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layer. Thus, the interlaminar shear strength of the composites and the thermal conduction between the GF layer and Al layer are further improved. The comparison of the thermal properties between CNTs− GF/Al and GF/Al layer-by-layer composites is shown in Table

reaction time (t) and reaction temperature (T) simultaneously. In other words, with the pressure infiltration technique in our fabrication process, the use of high-speed cooling rate makes the reaction time shorter, and thus the formation of Al4C3 at the interface can be suppressed. For the laminated materials, the interlaminar shear strength (ILSS) is an important indicator to reflect the interface performance between the layers. Figure 5a,b shows the

Figure 6. TC of CNTs−GF/Al and GF/Al composites in the (a) xy and (b) z directions and (c) CTE of CNTs−GF/Al and GF/Al laminated composites.

Figure 5. (a) Interlaminar shear strength test results and (b) bending load−displacement curves of the different composites with the CNTs−GF volume fractions of 17.81 vol % (S1), 31.45 vol % (S2), 51.42 vol % (S3) and GF volume fraction of 51.42 vol %.

1 and Figure 6. The TC of the composites is calculated as the formula

interlaminar shear strength results and load−displacement curves of the different composite samples with CNTs−GF volume fractions of 17.81 vol % (S1), 31.45 vol % (S2), and 51.42 vol % (S3) and a GF volume fraction of 51.42 vol %. Obviously, the interlaminar shear strength of CNTs−GF/Al composites displays a gradual decline with increasing volume fraction of CNTs−GFs. The ILSS of 17.81 vol % CNTs−GF/ Al is 104.2 MPa, whereas the ILSS of 51.42 vol % CNTs−GF/ Al is 70.3 MPa. Besides, the ILSS of CNTs−GF/Al composites is higher than that of GF/Al composites with the same reinforcement volume fraction. Owing to the growth of CNTs on the surface of the GF, it is harder for the CNTs−GF to slide under the compression comparing with the pure GF, which increases the ILSS of CNTs−GF/Al composites by 26.44% comparing with that of GF/Al. From the load−displacement curves in Figure 5b, the CNTs−GF/Al sample shown a higher bending load at the failure point compared to the GF/Al composite, which demonstrated that the CNTs−GF/Al nanocomposite had better resistance to deformation. The specific values of the ILSS of CNTs−GF/Al composites are also shown in Table 1. The growth of CNTs on the surface of GF increased the contact area of GF and Al matrix effectively and interlocked with the Al matrix, which leads to a much stronger interface cohesion between GF and Al matrix. To reveal the positive interface connection effect of CNTs in the composites, the energy-dispersive spectroscopy (EDS) mapping of the CNTs/Al interface is shown in Figure S5. Only the C and Al elements can be detected, and the CNTs are embedded uniformly in the Al matrix, which results in the enhanced interface combination between the GF layer and Al

λ = αρC

(1)

where λ is the TC, α represents the thermal diffusivity, ρ represents the density, and C is the specific heat capacity of the composite. The thermal diffusivity of the CNTs−GF/Al and GF/Al samples is tested by the laser flash method. According to the experimental results (see Table 1), the xy direction TC of CNTs−GF/Al with the CNTs−GF volume fraction of 51.42 vol % is 1042.0 W/(m·K), whereas the z direction TC is 47.0 W/(m·K). As for the GF/Al composites with a GF volume fraction of 51.42 vol %, the xy direction TC is 704.1 W/(m·K) and the z direction TC is 24.9 W/(m·K). TCs in z direction are much lower than those in xy direction for both the CNTs−GF/Al and GF/Al layer-by-layer composites. That is because the thermal contact resistances in z direction are much larger than those in xy direction for both the laminated CNTs−GF/Al and GF/Al composites. As long as there is a contact interface in the direction of heat conduction, there will be a thermal contact resistance which will affect the heat conduction in this direction. As schematic illustrated in Figure S6, the interface contact area in z direction is much larger than that in xy direction for the GF/Al composites. As for the CNTs−GF/Al laminated composites, the thermal contact resistance induced by the interface contact between CNTs and GF, CNTs and Al matrix, and CNTs each other was also mainly distributed along z direction. In addition, only part of the thermal contact resistance caused by the interface contact between CNTs each other distributed in the xy plane. Moreover, the heat transfer in plane is much easier than out of plane in both the GF and Al foil.

Table 1. Mechanical and Thermal Properties of CNTs−GF/Al Laminated Composites TC [W/(m·K)] sample

VCNTs−GF (%)

ρ (g/cm3)

Cp (J/(g·K))

xy

z

CTE (ppm/K)

ILSS (MPa)

S1 S2 S3 GF/Al

17.81 31.45 51.42 51.42

2.54 2.42 2.26 2.27

0.866 0.840 0.807 0.849

543.2 730.8 1042.0 704.1

25.2 39.7 47.0 24.9

20.0 18.2 17.4 19.2

104.2 79.5 70.3 55.6

F


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ACS Applied Materials & Interfaces From Figure 6a,b, the TC of the CNTs−GF/Al composite is significantly higher than that of GF/Al in both xy and z directions because of the embedment of CNTs in the interface of GF and Al foils. The xy direction TC of CNTs−GF/Al with the CNTs−GF volume fraction of 51.42 vol % is 1042.0 W/ (m·K), which is increased by 47.99% comparing with that of GF/Al composites. The enhancement of the TC of CNTs− GF/Al composites in xy direction could be attributed to three reasons. First, the multiwalled CNTs have an extremely high intrinsic TC up to 3000 W/(m·K) in the axial direction.48 The intrinsic high TC and the arrangement of CNTs approximately parallel with the GF substrate mainly determined the increase of xy direction TC of CNTs−GF/Al. Additionally, the CNTs between GF and Al layers interconnected with each other and part of them embedded in the Al matrix, forming a threedimensional network structure which effectively increased the heat transfer path of the CNTs−GF/Al composites in the xy direction. Third, even though the thermal contact resistance in xy direction was also increased due to the interface contact of CNTs between each other, the effect of thermal conduction caused by the CNTs with high TC was far greater than thermal resistance caused by the increased interface contact in the xy plane. However, in the z direction, the TC is increased from 24.9 W/(m·K) of GF/Al to 47.0 W/(m·K) of CNTs−GF/Al with the reinforcement volume fraction of 51.42 vol %. The improvement of TC in the z direction is limited comparing with that in the xy direction for CNTs−GF/Al composites because the intrinsic TC of CNTs in radial direction is much lower than that in the axial direction49 and the CNTs in our work are arrayed approximately parallel with the GF surface. In addition, the thermal contact resistance was increased in z direction because of the increased interface contact between CNTs and GF, CNTs and Al matrix, and CNTs each other. The enhancement in TC of CNTs−GF/Al layer-by-layer composites in z direction comparing with GF/Al composites is also mainly attributed to the growth of CNTs. On the one hand, the intrinsic high TC of CNTs increased the overall TC of the CNTs−GF/Al composites in z direction. On the other hand, the effective interface bridging and enhanced interface combination between the GF layer and Al layer by the CNTs greatly improved the heat transfer between the layers in z direction. Moreover, no interfacial reaction product such as Al4C3 is generated in the interfaces of CNTs−GF/Al composites, which is beneficial to heat conduction between the layers along z direction. From the CTE results shown in Figure 6c, the CTE of CNTs−GF/Al is lower than that of GF/ Al, which further indicates that the CNTs−GF/Al composites have superior thermal performance comparing with the GF/Al composites. To better understand the TC of CNTs−GF/Al laminated composites in our study, Figure 7 displays the comparison of the TC between CNTs−GF/Al layer-by-layer composites and other C/Al composites applied in the thermal management area reported in recent literatures.8,12,20,37,44 From Figure 7, the CNTs−GF/Al laminated composites fabricated by the pressure infiltration method in our work present much higher in-plane TC than the other C/Al composites. The enhancement of thermal conduction property of CNTs−GF/Al layerby-layer composites is mainly due to the embedding of the CNTs in the interface of GFs and Al foils. On the one hand, the CNTs between GFs and Al foils have a high intrinsic TC.50,51 On the other hand, the growth of CNTs effectively

Figure 7. Comparison of in-plane TC of CNTs−GF/Al layer-by-layer composites and other C/Al composites reported in recent literatures.

improves the interface bonding strength and reduces the interface thermal resistance, which is beneficial for thermal conduction. In addition, no interfacial products such as Al4C3 are generated in our short-time pressure infiltration fabrication, which also contributes to the high TC of CNTs−GF/Al layerby-layer composites.

4. CONCLUSIONS It was demonstrated that the CNTs with controllable size could be grown on the GF through the PECVD process, which further enhanced the performance of the layer-by-layer CNTs−GF/Al composite. CNTs−GF/Al layer-by-layer composites were prepared through a pressure infiltration process that is short in time, energy-saving, and no interface products such as Al4C3 are generated. The embedding of CNTs in the interface of GFs and Al foils resulted in enhanced interlaminar shear strength with 26.44% increase in the value. The presence of CNTs in the composites also had significantly improved the thermal property of the GF reinforced Al matrix layer-by-layer composite. Comparing with the original GF/Al composites, the TC of CNTs−GF/Al nanocomposites in the xy direction was increased by 47.99%. Moreover, the CTE of CNTs−GF/ Al composites was decreased by 10%. In summary, an ideal layer-by-layer thermal management material, CNTs−GF/Al laminated composites with both increased mechanical strength and enhanced TC [1042 W/(m·K)], was first fabricated. The excellent thermal property of the CNTs−GF/Al composite promises the potential application in large-scale thermal management area and will attract more academic and industrial research interests.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b12691. SEM images of cross section of CNTs−GF architectures prepared by the PECVD method; XRD patterns of GF and CNTs−GF architectures; Raman spectra of GF and CNTs−GF architectures; bright-field TEM image of CNTs and single CNTs, HRTEM image of part of the single CNT, and SAED pattern of the CNTs; brightfield TEM image of CNTs embedded in Al matrix, highangle annular dark field TEM image of CNTs embedded in Al matrix, and EDS elemental mapping of Al and C of CNTs/Al; and schematic illustration of the effect of different thermal contact resistance in xy and z directions on the TC of GF/Al and CNTs−GF/Al layer-by-layer composites in xy and z directions (PDF) G


Research Article


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.Z.). *E-mail: [email protected] (L.Y.). ORCID

Jing Chang: 0000-0002-1255-9421 Liwen Yan: 0000-0001-7042-1802 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (nos. 51771063, 51571069, and 51501047) and the China Postdoctoral Science Foundation (nos. 2016M590280 and 2017T100240).

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Carbon Nanotubes Grown on Graphite Films as Effective Interface

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