Boron Carbide Nanolumps on Carbon Nanotubes J

APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 3 21 JANUARY 2002

Boron carbide nanolumps on carbon nanotubes J. Y. Lao, W. Z. Li, J. G. Wen, and Z. F. Rena) Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467 ͑Received 5 September 2001; accepted for publication 9 November 2001͒ Boron carbide nanolumps are formed on the surface of multiwall carbon nanotubes by a solid-state reaction between boron and carbon nanotubes. The reaction is localized so that the integrity of the structure of carbon nanotubes is maintained. Inner layers of multiwall carbon nanotubes are also bonded to boron carbide nanolumps. These multiwall carbon nanotubes with boron carbide nanolumps are expected to be the ideal reinforcing ﬁllers for high-performance composites because of the favorable morphology. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1435062͔

Carbon nanotubes are expected to be the ideal reinforc- In this letter, we report the synthesis of BxC nanolumps ing fillers for composites because of their very high aspect on the surface of multiwall carbon nanotubes by a solid-state 1,2 ratio, large Young’s modulus, and low density. The bond- reaction of MgB2 and carbon nanotubes. Inner layers of mul- ing between coated nanomaterials and relatively ‘‘inert’’ car- tiwall carbon nanotubes are also bonded to boron carbide bon nanotubes is crucial for the mechanical performance of nanolumps through a possible covalent bonding. These mul- the composite. Matrix needs to stick to nanotubes strongly so tiwall carbon nanotubes with boron carbide nanolumps are that the load can be transferred to nanotubes to prevent the expected to be the ideal reinforcing fillers because of shape two surfaces slipping.3 Numerous experiments showed that modification of carbon nanotubes by nanolumps and the nanotubes were pulled out of the matrix instead of breaking bonding to inner layers of carbon nanotubes. inside it, suggesting that the interface between the matrix and The multiwall carbon nanotubes were grown by catalytic nanotubes was very weak so slipping happened first.4,5 In chemical vapor deposition method15 and purified by HF acid. 16 order to improve load transfer from matrix to nanotubes, MgB2 , a new superconducting material, is used as the methods such as chemical functionalization of the tube ends source of boron. Powder MgB2 with average grain size of and side walls were proposed and tried to enhance the bond- micrometer decomposes at an around 600 °C.17 Thermally ing between carbon nanotubes and matrix.6–8 But, no signifi- decomposed boron is more chemically reactive so the solid- cant improvement in mechanical properties has been ob- state reaction can be realized at relatively low temperature. served yet. Coating on multiwall and single-wall carbon The nanotubes were mixed gently with MgB2 powder first, nanotubes with metals and oxides have also been reported then wrapped by a Ta foil, and finally the whole assembly for applications such as heterogeneous catalysis and one- was placed in a ceramic tube furnace, pumped to below 0.5 dimensional nanoscale composite.9–11 However, the bonding Torr by mechanical pump. The sample was heated at between these coated materials and carbon nanotubes might 1100 °C to 1150 °C for 2 h. Microstructural studies were not be strong enough to have efficient load transfer. There- carried out by a JEOL JSM-6340F scanning electron micro- fore, a covalent bonding between these nanomaterials and scope ͑SEM͒ and JEOL 2010 transmission electron micro- multiwall carbon nanotube is most desirable. scope ͑TEM͒, respectively. The TEM is equipped with an Usually, only the outermost layer of multiwall carbon x-ray energy dispersive spectrometer ͑EDS͒. TEM specimen nanotubes contributes to the carrying load because the inter- was prepared by dispersing carbon nanotubes into acetone action between cylindrical graphene sheets is Van de Walls.12 solution by sonication and then putting a drop of the solution Multiwall carbon nanotubes broke under a ‘‘sword-in- on a holey carbon grid. sheath’’ failure mechanism under tensile stress.2 If the inner Figure 1͑a͒ shows a SEM image of the carbon nanotubes layers can also be bonded to matrix, load transfer is expected before the growth of boron carbide nanolumps, and Fig. 1͑b͒ to increase dramatically. In order to solve the problem, we is a SEM image showing nanolumps on the surface of mul- propose to use a solid-state reaction between boron and car- tiwall carbon nanotubes. These nanolumps tended to form ͑ ͒ bon nanotubes to form boron carbide BxC nanolumps on into a desired morphology, individual nanoparticles instead the surface of carbon nanotubes to improve the interaction of a homogeneous layer on the surface of multiwall carbon between nanotubes and matrix. Obviously, isolated nano- nanotubes. The average particle size of these lumps is about lumps are favorable than the uniformly coated layer. BxCisa 80 nm, which is two or three times of the average diameter covalent bonding compound, known as an important material of carbon nanotubes. The lump density on a carbon nanotube with outstanding hardness, excellent mechanical, thermal varies dramatically, with a spacing variation between adja- 13,14 and electrical properties. The bonding between BxC and cent nanolumps from 30 to 500 nm. carbon nanotubes can be also covalent bonding, if there is no Figures 2͑a͒ and 2͑b͒ are TEM images of these nano- secondary phase at the interface. lumps on multiwall carbon nanotubes in low and medium magnifications, respectively. Average particle size shown in ͒ ͑ ͒ a Author to whom all correspondence should be addressed; electronic mail: Fig. 2 a is about 50 nm, smaller than that shown in Fig. [email protected] 1͑b͒. As shown in these two images, the reaction between

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͑ ͒ FIG. 3. a HRTEM image of a BxC nanolump on a multiwall carbon nanotube. ͑b͒ An enlarged image of the top part of ͑a͒. ͑c͒ A FFT image of ͑b͒. The simulated image ͓the inset of ͑b͔͒ and the indexing of the FFT image in ͑ ͒ c were carried out by using structural parameters of B4C and zone axis of ͓¯ ͔ ͑ ͒ ͑ ͒ ͑ ¯ ͒ 111 . d To show the twin boundaries along 101 or 011 planes of BxC. Main parameters for the simulated image ͓the inset of ͑b͔͒ are: spherical aberration coefﬁcientϭ0.5 mm, thicknessϭ10 nm, and defocusϭ50 nm.

FIG. 1. SEM images showing ͑a͒ multiwall carbon nanotubes before the 1100 °C– 1150 °C and was pumped out. But the existence of ͑ ͒ formation of BxC, and b BxC nanolumps on multiwall carbon nanotubes. boron can not be excluded because boron can not be detected by the energy dispersive x-ray system, since the low energy x-rays from boron atoms were absorbed by detector. boron and carbon nanotube is very much confined and the In order to find out whether the nanolumps are boron main structure of multiwall carbon nanotubes remains un- carbide, a high-resolution transmission electron microscopic changed. EDS analysis on the composition of these nano- ͑HRTEM͒ image of a nanolump is taken and shown in Fig. lumps shows that they contain only carbon. No Mg 3͑a͒. Clearly, the carbon nanotube nature has been preserved and B were detected. All the Mg from decomposition of after the reaction. The BxC nanolump is crystalline. Figure MgB2 becomes vapor at the reaction temperature of 3͑b͒ is an enlarged HRTEM image of the top part of Fig. 3͑a͒. Figure 3͑c͒ shows a fast-Fourier transformation ͑FFT͒ image of the HRTEM image shown in Fig. 3͑b͒. The diffraction pattern obtained from FFT ͓Fig. 3͑c͔͒ is indexed as one ͓¯ ͔ from zone axis 111 of B4C. Structure parameters of B4C for the indexing are space group R3m ͑166͒ and lattice parameters, aϭ0.56 nm, cϭ1.21 nm. As shown in Fig. 3͑b͒, the simulated HRTEM image using parameters defocus Ϫ30 nm and thickness 20 nm also matches with experimental image very well. Although no boron was detected by the EDS analysis, it is reasonable to draw a conclusion that these nanolumps are BxC, since both calculated HRTEM image and diffraction pattern match with experimental ones very well when using structural parameters of B4C. The ratio between boron and carbon in nanolumps may differ from B4C dramatically because boron and carbon atoms can easily sub- stitute each other. Twin boundaries were often observed in ͑ ͒ BxC nanolumps. As shown in Fig. 3 d , the twin boundary is along either ͑101͒ or ͑011¯ ͒ planes. ͑ ͒ Figure 4 a shows an interface between BxC nanolump and multiwall carbon nanotube. Part of the multiwall is clearly reacted with boron, so no lattice fringes of carbon nanotube can be observed at the bottom part of the BxC. The reaction area is localized only at the area where there is boron. No surface diffusion of boron is observed in this FIG. 2. TEM images of a multiwall carbon nanotube with BxC nanolumps in ͑a͒ low, and ͑b͒ medium magnification. solid-state reaction. From HRTEM images of Figs. 4͑a͒ and Downloaded 16 Jan 2002 to 136.167.171.102. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp 502 Appl. Phys. Lett., Vol. 80, No. 3, 21 January 2002 Lao et al.

tube morphology by these BxC nanolumps is expected to increase the load transfer between nanotubes and matrix. Moreover, inner layers of multiwall carbon nanotubes are also bonded to BxC nanolumps, so the inner layers can also contribute to load carrying, instead of only outmost layer. Due to the fact that the modified material is not enough for making a sample for mechanical property test, we do not have data to prove that these nanolumps as reinforcing fillers in composites improve the overall properties as expected. Nevertheless, we believe that optimization of the synthesis procedure will eventually lead us to large quantity so that test samples will be made and their properties will be studied and reported. In summary, BxC nanolumps are formed on the surface of carbon nanotubes by a solid-state reaction of boron and carbon nanotubes. The modification on morphology of carbon nanotube by these BxC nanolumps could increase the load transfer between carbon nanotubes and matrix. Inner layers of multiwall carbon nanotubes are also bonded to boron carbide nanolumps through a possible covalent bonding. All these make the multiwall carbon nanotubes with boron carbide nanolumps the potentially ideal reinforcing fillers. This work is partly supported by The U.S. Army Natick Soldier Systems Center under Grant No. DAAD16-00-C- 9227, and partly by the DOE under Grant No. DE-FG02- 00ER45805.

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