Interface Observation of Aluminum-Coated Carbon Nanofibers

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Interface Observation of Aluminum-Coated Carbon Nanofibers Prepared by in situ Chemical Vapor Deposition Fumio Ogawa, and Chitoshi Masuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12956 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 8, 2017

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The Journal of Physical Chemistry

Interface observation of Aluminum-Coated Carbon Nanofibers Prepared by in situ Chemical Vapor Deposition Fumio Ogawa,a, * Chitoshi Masudab a

Department of Mechanical Engineering, College of Science and Engineering, Ritsumeikan University,

1-1-1, Nojihigashi, Kusatsu, Shiga, Japan (525-8577) b

Kagami Memorial Institute for Materials Science and Technology, Waseda University, 2-8-26,

Nishi-waseda, Shinjuku-ku, Tokyo, Japan (169-0051)

1 Environment ACS Paragon Plus


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ABSTRACT: The compatibility of carbon nanofibers (CNFs) with aluminum is important for the fabrication of CNF-reinforced aluminum matrix composites with superior mechanical and thermal properties. In this study, aluminum was deposited on CNFs by in situ chemical vapor deposition (CVD) utilizing I2 for the transportation of aluminum atoms. The microstructure of the aluminum-coated CNFs and the crystal structure of the aluminum coating layers were studied. When aluminum powder with ~3-µm-diameter particles, CNFs, and I2 powder were mixed in the molar ratio 1:1:0.1 and annealed at 500 °C for 48 h, the coating layer formed was polycrystalline and exhibited preferential orientation of the (0 0 2) and (1 1 1) aluminum planes; the grain size of aluminum in the coated layer was ~5 nm, the interface structure was governed by the CNF surface structure, and an amorphous region or aluminum oxycarbide (Al4O4C or Al2OC) was formed at the interface. When larger 106–180 µm particles of aluminum were used, a larger amount of I2 was required for the formation of the coating layer, and the interfacial reaction was mild because of the size of the aluminum powder used for the coating; aluminum was deposited and grown in a unidirectional way, and the coating layer consisted of larger aluminum grains. Nanosized aluminum grains can be deposited on CNFs by in situ CVD using iodine transport, which improves their compatibility with aluminum matrixes. The in situ CVD aluminum coating is beneficial for fabricating CNF-reinforced aluminum matrix composites with superior mechanical and thermal properties.


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INTRODUCTION Carbon nanofibers (CNFs) synthesized via the decomposition of hydrocarbons are an attractive reinforcement for composite materials owing to their outstanding mechanical properties and superior thermal conductivity.1,2 While numerous studies on the fabrication and properties of CNF-reinforced polymer matrix composites have been performed, relatively few studies have been reported on CNF-reinforced metal matrix composites. Aluminum matrix composites reinforced by CNFs are expected to be used as structural components in automobiles and aircraft because of their superior mechanical properties and high thermal conductivity.3-13 However, agglomeration of CNFs due to the weight difference between CNFs and aluminum powder, van der Waals forces10, and the poor wettability of CNFs by molten aluminum14 make the fabrication of composites difficult. Furthermore, the strength of the CNF/aluminum interface governs the overall mechanical performance of their composites; however, contact at the CNF/aluminum interface is usually weak.15 Therefore, tailoring interfaces is important for the fabrication of composites with superior properties. One solution to the aforementioned problems may be to coat reinforcements with a matrix metal. Matrix layers formed on the reinforcements are expected to improve the degree of dispersion of CNFs and the densification of composites, because matrix coating layers separate adjacent CNFs and fill the pores formed between CNFs. In previous studies, we proposed aluminum coating on CNFs by a simple in situ chemical vapor deposition (CVD) technique that utilizes the reaction of aluminum powder with I2 to transport aluminum atoms to the CNF surface.16,17 CVD is a preferred method for coating nanomaterials with aluminum. However, CVD is a relatively high-cost and complicated method. Therefore, CVD processes that generate gaseous species by in situ process are proposed. Leal-Cruz et al generated silicon fluoride (SiF4,



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SiF3, SiF2, SiF) vapor by in situ thermal decomposition of sodium hexafluorosilicate (Na2SiF6), then silicon nitride (Si3N4) was deposited by the reaction of the silicon fluoride vapor and nitrogen precursor vapor.18,19 TiO2-nanoribbons could be also synthesized by thermal decomposition of titanium solid precursor (K2TiF6) and its reaction with oxygen.20 Above mentioned processes are based on the in situ thermal decomposition of solid precursors. On the other hand, in Refs. 16, 17, iodine powder was vaporized in quartz tube and it reacted with aluminum powder to form aluminum source gas. Aluminum iodide vapor was formed by in situ reaction, and it was decomposed in quartz tube. Therefore, we call the technique in situ CVD method. Using this technique, aluminum was successfully deposited on CNFs, and properties of aluminum/CNF composites such as Vickers microhardness and thermal conductivity were improved owing to the aluminum coating layer formed on the CNFs.17 This is probably due to the strong adhesion and good thermal conductance at the CNF/coating interface obtained by in situ CVD of aluminum. However, the structures of the coating layer and CNF/coating interface remain open to investigation; in particular, their chemical and crystal structures play important roles in the fabrication and strengthening of composites. Therefore, here, we use high-resolution transmission electron microscopy (HRTEM) to make detailed observations of the aluminum coating layer and aluminum/CNF interface formed by in situ CVD.

EXPERIMENTAL SECTION Aluminum Coating of CNFs. Mixtures of CNFs (VGCF® Showa Denko Co., Ltd.; average diameter: 150 nm, average length: 10–20 µm), aluminum powder (Kojundo Chemical Laboratory Co., Ltd.; average diameter 3 µm), and I2 powder (Kanto Chemical Co., Inc.) were


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sealed into quartz tubes under vacuum. The ampoules were then annealed in an electric furnace. The detailed experimental procedure can be found in our previous study. The molar ratio of CNFs to aluminum powder to I2 was 1:1:0.1. They were heated in an electric furnace at 500 °C for 48 h. After annealing, the powders in the ampoules were recovered. The effect of the aluminum powder size on the structures of the coatings and interfacial layers was also investigated using aluminum powders with particle sizes ranging from 106 to 180 µm (Kojundo Chemical Laboratory Co., Ltd.) and CNF to aluminum to I2 molar ratios of either 1:1:0.1 or 1:1:0.2. Characterization of Aluminum-coated CNFs. Aluminum-coated CNFs were observed by field-emission transmission electron microscopy (FETEM) using a JEM-2100F (JEOL) equipped with an energy-dispersive x-ray spectrometry (EDX) system and a scanning transmission electron microscopy (STEM) unit. Before the observation, powder samples were dispersed in ethanol by ultrasonication and the dispersions were dropped onto Carbon-coated Cu grids. HRTEM samples for cross section observation were prepared by mixing CNFs with epoxy resin (Gatan G1) and sandwiching between silicon wafers.21 The sandwich was subsequently sliced, mechanically ground, polished, and finally finished by Ar+ ion etching using a JEOL IB09060CIS cryo ion slicer.

RESULTS AND DISCUSSION Pristine CNFs. Figure 1 shows TEM images of pristine CNFs having fiber-like structures (Figure 1 (a)), although some impurities with polygonal or hexagonal C structures resembling fullerenes can also be observed, as indicated by the white arrow. In addition, a junction point



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between CNFs can be found in Figure 1 (b). The dark contrast around the junction point is likely due to the catalyst used for CNF growth. Figure 2 shows the graphitic layers of a CNF with fiber-like structure. Graphitic layers were extracted from Figure 2 (a) using the Gatan DigitalMicrograph software. In Region (A), enclosed by a square, graphitic layers are continuous and straight. Almost no defects can be seen. The (0 0 2) plane spacing of the graphitic layers is 0.339 nm (Figure 2 (b)). On the other hand, in Region (B), the graphitic layers are wavy and exhibit a dislocated structure (Figure 2 (c)). It can also be seen that the CNFs are covered with amorphous C films that are reported to be reactive to aluminum.15

Figure 1. TEM images of pristine CNFs. (Scale bar 100 nm.)


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Figure 2. Graphitic layers of a CNF with fiber-like structure. (Scale bar 5 nm.)

The graphitic layers around a junction point of pristine CNFs are shown in Figure 3. Extensive amounts of amorphous C and discontinuities in the CNF layers can be observed. These structures are likely to react with aluminum during the CNF coating treatment.



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Figure 3. Graphitic layers of CNFs at a junction point. (Scale bar 5 nm.)

Figure 4 shows the cross sections of a pristine CNF. It can be seen that the cross sections are not circular, but polygonal in shape (Figure 4 (a)). Figure 4 (b) shows a magnified image of the corner indicated by the arrow in Figure 4 (a). The imperfections in the graphitic layers can be seen in the higher magnification image of the region enclosed by the dashed square in Figure 4 (b), showing that the graphitic layers are partly separated from the CNF surface, as shown in the dashed circle in Figure 4 (c). Such structures may be susceptible to oxidation and are likely to react with aluminum.


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Figure 4. TEM and HRTEM images of a cross section of a pristine CNF. Scale bar (a)40 nm; (b) 20 nm.

Observation and Characterization Results of Aluminum-Coated CNFs. CNFs coated at 500 °C for 48 h using ~3-µm-diameter aluminum powder and a CNF:aluminum:I2 ratio of 1:1:0.1 are referred to as Sample 1. Figure 5 shows the analysis results of Sample 1 using FETEM. Figure 5 (a) shows the bright field image of a CNF in Sample 1. The hole indicated by an arrow was formed in the coated layer by exposure to the electron beam for EDX point analysis. Figure 5 (b) shows an EDX spectrum obtained from the point analysis. Al and O peaks were identified and a C peak with very low intensity was also detected. The Cu peak arises from



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the Cu TEM grid used. Figure 5 (c) also shows a bright field image of a CNF in Sample 1, and Figure 5 (d) shows an EDX map of Al atoms. It is evident that CNFs are practically homogeneously coated with an aluminum coating layer.

Figure 5. Results of analysis of sample 1 using FETEM: (a) Bright field image, (b) EDX spectrum obtained by point analysis, (c) Bright field image, (d) Result of Al atom EDX mapping of the area shown in (c). Scale bar (a)100 nm; (c, d) 50 nm.


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Figure 6 shows a coated layer attached to a CNF surface that resembles the coated layer shown in Figure 5 (a). Regions (A), (B), and (C) in Figure 6 indicate the interface between the CNF and the coated layer, and Regions (D) and (E) show only the coating layer.

Figure 6. HRTEM image of sample 1. (Scale bar 5 nm.)

Figure 7 (a) shows the fast Fourier transform (FFT) image of Region (A) in Figure 6.



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Figure 7. FFT and IFFT images of the square regions (A), (B), and (C) shown in Figure 6.

In Figure 7 (a), the points corresponding to CNF graphitic layers and interfacial phases can be identified and the points corresponding to interfacial phases are indicated by the white arrows. Figure 7 (b) shows the graphitic layers of the CNF reconstructed from the points shown in Figure 7 (a), where the black arrows indicate the edge of the graphitic layers. The crystal structure of the interfacial phase appears in Figure 7 (c), where the measured interplanar spacing of the


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interfacial phase coincides well with the (2 1 0) lattice spacing of aluminum oxycarbide (Al4O4C) (0.386 nm). It is apparent that the graphitic layers were slightly consumed during the coating treatment, probably due to the reaction with aluminum. Figure 7 (d) shows the FFT image of Region (B) shown in Fig. 6, in which the points corresponding to the crystal structure of the coated layer can be identified in addition to the points corresponding to the graphitic layers of the CNF. Figure 7 (e) shows the graphitic layers of the CNF in Region (B) in Figure 6. The graphitic layers have a wavy structure. The crystal structure for Region (B) appears in Figure 7 (f). The interplanar spacing of the grains coincides well with the (2 0 0) lattice spacing of aluminum (0.202 nm). Apparently, an amorphous region exists between the graphitic layers and the coating layer. It is thought that the reaction of amorphous C on the CNF surface with aluminum results in the formation of the amorphous region. The FFT image of Region (C) shown in Figure 6 is also present in Figure 7 (g) and clearly shows that a polycrystalline aluminum coating layer exists on the CNF. Figure 7 (h) illustrates the graphitic layers of the CNFs. The layers have a wavy and partly dislocated structure. The (0 0 2) plane spacing of the graphitic layers was 0.38 nm, which is considerably larger than that of pristine CNFs (0.339 nm). Nanosized aluminum grains are directly attached to the CNF in Region (C) (Figure 7 (i)), while an amorphous reaction product was formed between the surface of the CNF and the coated layer in Region (B) in Figure 6. Owing to the difference in surface energy between CNFs (200 mN/m) and aluminum (865 mN/m) 15, it is generally considered that a certain reaction occurs at CNF/aluminum interfaces. However, in Region (C), almost no reaction between CNF and aluminum was observed, and the broadening of the (0 0 2) lattice



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spacing of CNF is likely to have bridged the gap between the surface energies of CNF and aluminum. Figure 8 (a) and Figure 8 (b) show the FFT image and the crystal structure of Region (D) in Figure 6, respectively, in which the coating layer consists of nanosized aluminum. The FFT image and the crystal structure of Region (E) in Figure 6 are shown in Figure 8 (c) and (d), respectively. At least four grains with sizes of ~5 nm are observed in Region (E) (Figure 8 (d)), all of which display the (0 0 2) planes of aluminum; the grain boundaries of these nanosized grains can be clearly identified in Figure 8 (d).


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Figure 8. FFT and IFFT images of the square regions (D) and (E) shown in Figure 6.

Figure 9 shows an HRTEM image of another CNF from sample 1, which has wavy and discontinuous graphitic layers, unlike those in Figure 6.

Figure 9. HRTEM image of sample 1. (Scale bar 5 nm.)

FFT images and crystal structures of Regions (A), (B), and (C) in Figure 9 are shown in Figure 10.



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Figure 10. FFT and IFFT images of regions (A), (B), and (C) in Figure 9.

For Region (A), points corresponding to the reaction product can be identified in the FFT image, in addition to those corresponding to aluminum grains, as indicated by the arrows in Figure 10 (a). In Figure 10 (b), aluminum grains of ~4 nm are observed, and the (0 0 2) plane and (1 1 1) plane (lattice spacing 0.23 nm) of aluminum can be identified. On the left side of the


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aluminum grains, a reaction product with an interplanar spacing of 0.275 nm can be observed. The interplanar spacings coincide well with the (1 0 0) lattice spacing of aluminum oxycarbide (Al2OC); therefore, it is thought that C atoms diffused from the CNF surface and reacted with aluminum to form aluminum oxycarbide. However, the effect of oxycarbide formation on nanofiber strength should be negligible, because only a small amount was formed. In the FFT image of Region (B) in Figure 9, the points corresponding to the interfacial phase can be identified (Figure 10 (c)), as indicated by the white arrows. Figure 10 (d) shows the graphitic layers of the CNF, and Figure 10 (e) shows the crystal structure of the interfacial phase reconstructed from the FFT image. The graphitic layers are likely to be consumed during the reaction, forming an interfacial phase with an interplanar spacing of 0.287 nm that coincides with the (0 0 2) lattice spacing of Al4O4C, confirming that a small amount of Al4O4C was formed. In Region (B), metallic aluminum grains were not observed, which is probably because of the presence of a large amount of amorphous or graphitic C that can easily detach from the CNF surface. In Region (C), metallic aluminum grains can be found in addition to the amorphous structure (Figure 10 (f)). Figure 10 (g) shows the crystal structure of Region (C), in which the interplanar spacing of the grains is 0.2 nm, equal to the (2 0 0) lattice spacing of aluminum. Comparison of Regions (A), (B), and (C) reveals that an amorphous layer exists between the metallic aluminum grains and the CNF. This amorphous interlayer bridges the gap between the surface energies of the CNF and the aluminum. Comparing the structures of the interfaces shown in Figure 6 and Figure 9, it is clear from the waviness and the dislocated structure of the CNF that a thick amorphous interlayer was formed in Figure 9 in contrast to that of the CNF in Figure 6. It is clear that the surface morphology of the CNF affected the formation of the interface.



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The results of observation of a cross section of an aluminum-coated CNF in Sample 1 are presented in Figure 11.

Figure 11. (a) Bright field image of a cross section of a CNF in sample 1, (b) EDX mapping result for C atoms, (c) EDX mapping result for Al atoms, and (d) EDX mapping result for O atoms.


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In Figure 11 (a), the CNF is embedded in an epoxy resin and covered with a coating layer that contains aluminum atoms. The in situ CVD coating used in this study utilizes the reaction of aluminum powder and I2 vapor. Aluminum iodide vapor transports aluminum atoms that are deposited by the following reaction:

9AlIሺgሻ → AlIଷ ሺgሻ + Alଶ I଺ ሺgሻ + 6Alሺsሻ

(1)

However, the sintering of aluminum powder during the coating treatment hinders the formation of a perfectly homogeneous coating of aluminum on the CNF surface. Therefore, the coating layer is only present in the upper part of Figure 11 (a), (b), (c), and (d). This is a shortcoming of the coating technique used in this study and needs to be improved to achieve more homogeneous aluminum coatings on CNFs. Figure 12 shows an FETEM image of the region enclosed by a white dashed square in Figure 11 (a); Figure 12 is a reversed image of the region.



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Figure 12. HRTEM image of a cross section of a CNF in sample 1. (Scale bar 5 nm.)

The boundary of the coating layer is indicated by a white dashed line. The crystal structures in Regions (A), (B), (C), and (D) in Figure 12 are shown in Figure 13.


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Figure 13. HRTEM showing the crystal structure of regions (A), (B), (C), and (D) in Figure 12.

In Figure 13 (a), the interplanar spacings of the crystal structures in Region (A) coincide well with the lattice spacing of the (2 2 2) plane of Al4O4C (0.211 nm). In Region (B), almost no crystal structure was observed, and therefore, it is thought that this region consists of amorphous matter. Figure 13 (b), (c), (d) show crystal structure in Region (C). Figure 13 (b) shows the (1 0 4) plane of α-Al2O3 (lattice spacing 0.255 nm). It is suggested that part of the oxycarbide was likely transformed into α-Al2O3 because of the reaction with gaseous CO.17 Figure 13 (c) and (d) show the (2 0 0) and (1 1 1) planes of metallic aluminum, respectively. In Region (D), aluminum oxycarbide (Al4O4C) with a (0 2 0) lattice spacing of 0.455 nm was directly grown on the surface of the CNF (Figure 13 (e)). It is apparent from the comparison of Figure 4 (c) and Figure 13 (e) that C atoms separated from the defect at the corner of the CNF and reacted with aluminum to



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form Al4O4C. In regions other than (A), (C), and (D), the coating layer consists of amorphous matter containing C, Al, and O atoms. This is probably because of the oxidation of the coating layer after the coating treatment or during preparation of the TEM sample. We next discuss CNFs coated under different coating conditions. Aluminum powder with particle diameters of 106 to 180 µm was used as the aluminum source, and the annealing temperature and time was maintained at 500 °C and 48 h, respectively. When the molar ratio of aluminum powder to CNF to I2 was 1:1:0.1, almost no coating layer was observed. This is because the surface area of the aluminum powder decreased compared with the aluminum powder with 3-µm particle diameter, and because the aluminum iodide vapor pressure required to deposit aluminum could not be reached. When the molar ratio of aluminum powder to CNF to I2 was 1:1:0.2, a coating layer was observed; this sample is called sample 2. It is apparent from this result that the amount of I2 governs the result of the aluminum deposition. Figure 14 shows the HRTEM images of a CNF and a coating layer in Sample 2.

Figure 14. HRTEM image and IFFT images of sample 2. (Scale bar 5 nm.)


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The coating layer shown in Figure 14 (a) is thin, but relatively wide compared to that in Sample 1. The graphitic layers of the CNF in Region (A), which are continuous and straight, are shown in Figure 14 (b). Almost no damage due to the coating treatment is seen. The crystal structures of Region (B) are shown in Figure 14 (c). The (1 0 4) plane of α-Al2O3 (lattice spacing: 0.255 nm) can be observed around the interface between the CNF and the Al coating layer, which is thought to be an interlayer. The coating layer was metallic aluminum displaying (2 0 0) planes. The aluminum crystal structure in Region (C) is shown in Figure 14 (d). The (2 0 0) planes of the aluminum coating layer shown in Figure 14 (c) and (d) are parallel, and it is thought that the aluminum coating layer grew in a unidirectional way. In Region (D), almost no crystal structure of the coating layer can be identified, and amorphous matter is likely to exist. It is apparent that a pit is formed on the CNF by the coating treatment, as indicated by the white arrow. It is evident that the tip of the CNF is reactive15, and that the reaction layer is easily formed; however, only a small amount of graphitic layers were consumed in the reaction, and the CNF was not completely converted to the reaction product of aluminum and C, as reported by He et al.22, which is attributed to a CNF structure with a minimal number of defects. In Region (E), the (2 0 0) plane of aluminum can be identified on the right panel of Figure 14 (e). Apparently, a large amount of amorphous interlayer was formed around the tip of the CNF. Here, we compare the microstructure of aluminum-coated CNFs with the interfacial structure in aluminum matrix composites reinforced by CNFs or CNTs. Sasaki23 reported the interfacial structure in CNF-reinforced aluminum matrix composites fabricated by sintering. An amorphous interlayer was formed at the interface under 873 K and was determined to comprise Al, O, and C. The amorphous layer was thicker in the interface between wavy and discontinuous C structures and aluminum than in the interface between aluminum and linear C structures. When the



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composite was fabricated at 933 K, crystalline aluminum carbide (Al4C3) was formed. Kwon7,24 reported the interfacial structure of CNT-reinforced aluminum matrix composites in which CNTs were converted to Al4C3, even below the melting point of aluminum. In this study, an aluminum layer was formed on the CNFs by the transportation of aluminum atoms by vaporized aluminum iodide. Aluminum was deposited at the atomic level, followed by an interfacial reaction that led to the formation of an amorphous region or aluminum oxycarbides. However, the formation of the interlayer was limited owing to the low temperature used for the coating treatment, and the conversion of CNFs to Al4C3 did not occur. Therefore, the strength degradation of CNFs because of the coating treatment is expected to be negligible, and CNFs are strongly interlocked in the aluminum matrix owing to the aluminum oxycarbide or amorphous interlayer. In our previous study, we reported that the density, Vickers microhardness, and thermal conductivity of composites reinforced by aluminum-coated CNFs were improved. In addition, the pressure-less infiltration of pure aluminum into preforms that contained aluminum-coated CNF under high purity nitrogen environment was enabled owing to aluminum coating on CNFs. CNFs were stable after infiltration at 800 °C, and the CNFs maintained their original shape and structure. The aforementioned enhancement of the composite properties and stability of the aluminum-coated CNFs can be explained by the results of this study. We are, therefore, convinced that the in situ CVD technique is an effective method to fabricate CNF-reinforced aluminum matrix composites with controlled microstructure and superior mechanical and physical properties.


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CONCLUSIONS Aluminum coating of CNFs by in situ CVD was performed, and detailed HRTEM observations of the coating layer and interface were made. Aluminum was deposited on the CNFs at the atomic level, and nanosized aluminum grains of ~5 nm were formed. The interfacial structure was governed by the surface structure of the CNFs. Wavy and discontinuous graphitic layers led to the formation of thick interlayers comprising aluminum oxycarbide (Al4O4C or Al2OC) or amorphous materials. It was found that the aluminum powder particle size used for the coating treatment governs the morphology of the coating layer and the nature of the interfacial reaction between aluminum and CNFs. HRTEM observation thoroughly revealed the characteristic of aluminum coating layer and interface formed via in situ CVD, and the results justify that the aluminum coating of CNFs by in situ CVD is an effective method for the fabrication of aluminum matrix composites reinforced by CNFs.

Corresponding Author *Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.



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ACKNOWLEDGMENT This work was partly supported by Grants-in-Aid for Scientific Research (Nos. 18560668 and 22560685) from the Ministry of Education, Culture, Sports, Science and Technology. Financial support from the Light Metal Educational Foundation Inc. and Japan Aluminum Association are also greatly acknowledged.

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Interface Observation of Aluminum-Coated Carbon Nanofibers

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