13060
2005, 109, 13060-13062 Published on Web 06/21/2005
From Al4B2O9 Nanowires to BN-Coated Al18B4O33 Nanowires J. Zhang, Y. Huang, J. Lin, X. X. Ding, Z. X. Huang, S. R. Qi, and C. C. Tang* Department of Physics, Central China Normal UniVersity, Wuhan, 430079 P. R. China ReceiVed: May 9, 2005; In Final Form: June 8, 2005
A novel route was proposed to completely coat aluminum borate nanowires by in situ providing the precursor for BN coating. Uniformly BN-coated Al18B4O33 nanowires could be obtained by the reaction of Al4B2O9 nanowires with ammonia at high temperature. The high-temperature unstable Al4B2O9 nanowires were converted into Al18B4O33 nanowires, simultaneously evaporated boron oxide. The reaction between the in situ generated vapors and ammonia ensures that the BN layers are attached tightly on the surface of the as-formed Al18B4O33 nanowires.
There has been a considerable effort in the aerospace community to develop high-temperature aluminum alloys capable of competing with titanium alloys on a specific strength basis. Reinforcing the aluminum alloys by incorporation of some whiskers with structural strength and chemical inertness provides the possibility to obtain composite materials with high mechanical strength. The most promising reinforcing additive to date is the SiC whisker which possesses exceptional toughness and strength.1 Considering the recent investigations on the mechanical properties that the strength value of SiC nanowires is a factor of 2 times the best observed previously SiC whiskers in micrometer diameter, SiC nanowires therefore are a more excellent candidate as a reinforcing element in composites, compared to SiC whiskers.2 However, it is the high cost of SiC whiskers or nanowires that precludes the possible extensive commercial application in airframe and spacecraft materials.3 Another difficulty is the serious interfacial reaction in the SiC/ Al system during the required high-temperature processing, which results in a significant strength and toughness degradation.4 Therefore, in recent years, another new oxide whisker/ nanowire composed of aluminum borate (Al18B4O33 or 9Al2O3‚ 2B2O3) has been paid much attention due to its high elastic modulus, tensile strength, and low thermal expansion coefficient.5 Although the aluminum borate also reacts with aluminum at high temperature, the interfacial reaction is generally not as severe as the SiC/Al reaction.6 Moreover, the low cost is incentive to adopt the whisker/nanowire for use as aluminum borate composites in aerospace community. There has been a great deal of research in depressing the chemical reaction between whiskers/nanowires and matrixes via various surface modification methods.7 From the idea that BN has unique chemical and physical properties such as low density, high melting point, chemical inertness, and high thermal conductivity in a wide range of temperature, BN coated SiC nanowires have been successfully fabricated, and further investigations exhibit that BN coating effectively improves the antioxidation ability and field emission characteristics of SiC nanowires.8 However, recent research indicated that a direct * To whom correspondence should be addressed. E-mail: admat@ phy.ccnu.edu.cn.
10.1021/jp052390n CCC: $30.25
nitrification to Al18B4O33 whiskers leads to a discontinuous artificial nitride layer with 50-nm thickness, and the whisker can still react with alumina alloys.9 Aimed at controlling the interfacial reaction between Al18B4O33 nanowires and Al alloy matrixes, in this letter, we report on a novel synthetic route to uniformly coated Al18B4O33 nanowires with BN by an ammoniathermal posttreatment route for high-temperature unstable Al4B2O9 (2Al2O3‚B2O3) nanowires. Experimental investigations indicated that the reaction precursors strongly control the composition and morphology of aluminum borate crystals.10-12 Al18B4O33 whiskers synthesized by heating boric acid-stabilized aluminum acetate are 10-20 µm in length and 2-3 µm in diameter. The diameter could be decreased to 400-800 nm if the whiskers were synthesized by the reaction of Al(OH)3 with B(OH)3. Commercially used Al18B4O33 whisker (ALBOREX) with the length of 5-15 µm were prepared via a flux method from Al2(SO4)3 and B(OH)3 in K2SO4 flux. Al18B4O33 nanowires with the diameter smaller than 100 nm can be obtained by adding 10 wt % of boron to Al2O3 and B2O3 mixture, whereas the direct reaction between Al and B2O3 at 850 °C leads to the formation of Al4B2O9 nanowires and alumina particles. In the present study, we selected the mixture of amorphous Al2O3, B, and graphite with the weight ratio 1:2:1 as the reaction precursor and then heated it to 1100 °C in an argon flow. After reacting for 2 h, the argon was replaced by oxygen to remove the remaining graphite and B. Figure 1a shows the X-ray diffraction pattern (XRD, Cu KR) of the as-prepared product, which can be indexed as an orthorhombic structure with the lattice parameters of a ) 1.457, b ) 1.512, and c ) 0.548 nm, respectively. The pattern matches well with those assigned to Al4B2O9 concerning both the reflection profile and intensity (JCPDS 09-0158). Scanning electron microscopy (SEM) examinations indicate that the product consists of abundant straight nanowires with the widths ranging between 20 and 80 nm and lengths of the order of several micrometers, and some woollike aggregates involving amorphous boron oxide were found (Figure 1b). The amorphous materials could be dissolved fully into ethanol, as evidenced from the transmission electron © 2005 American Chemical Society
Letters
J. Phys. Chem. B, Vol. 109, No. 27, 2005 13061
Figure 1. Al4B2O9 nanowires. (a) XRD pattern, displaying a pure orthorhombic structure. (b) SEM image, showing a nanowire-like structure and some amorphous aggregates. (c, d) Low-magnification TEM image of the nanowires with smooth surface; and of the nanowires degraded under TEM beam irradiation heating; (inset) EDS showing chemical composition of Al4B2O9 and SAED pattern recorded along [010] zone axis, before irradiation. (e) High-resolution TEM images shows the BOx fragments occurred after irradiation.
microscopy (TEM) observations (Figure 1, panels c and d), which pure nanowires with smooth surfaces can be observed for the TEM sample prepared by ultrasonically ethanol dispersion method. A typical energy-dispersive X-ray spectra (EDS) taken from an individual nanowire is shown in the inset of Figure 1d, displaying the characteristic peaks from Al, B, and O with the molar intensity ratio of O/Al ∼ 2.3. This suggests an Al4B2O9 composition of the nanowires. Further evidence can be obtained from the corresponding selected area electron diffraction (SAED) taken from the same nanowire (inset, Figure 1d). The pattern can be indexed as the Al4B2O9 recorded from [010] zone axis. The fitting lattice parameters are same as calculated from XRD measurement. The axis direction of the Al4B2O9 nanowires is along [001]. Our extensive TEM observations indicate that no metal impurities could be found on their tips, suggest a self-catalytic growth mechanism.12 During TEM examination with the 300 kV electron beams, we found that the as-observed Al4B2O9 nanowires are unstable in the process of the electron irradiation-induced heating. The nanowires easily degrade, depended on the beam dose, focus area and observation time, as evidenced from the gradually changed morphology of the Al4B2O9 nanowires under irradiation. As shown in Figure 1d and 1e, the starting nanowires with smooth surfaces are gradually coarsened and numerous fragments with amorphous structures appear on their surfaces. EDS showed that the O/Al ratio gradually reduced and was stable at a constant ∼1.8 (Al18B4O33) finally. The experimental observation is in agreement with the Al2O3-B2O3 phase diagram,[13]
which Al4B2O9 is a low-temperature stable phase and will be converted to Al18B4O33 at high temperature. It is noteworthy mentioning that there must be boron oxide evaporation during the conversion. This suggests a possible route to in situ formation of the BN coating by the reaction of the B2O3 with ammonia during the conversion. Considering that the evaporated B2O3 has a relatively fixed concentration and is closely near to the nanowires, the conversion will ensure a tight BN coating with uniform thickness, according to the following reaction route:
9Al4B2O9 f 2Al18B4O33 + 5B2O3
(1)
B2O3 + 2NH3 f 2BN + 3H2O
(2)
The prediction was verified by slowly heating the as-synthesized Al4B2O9 nanowires to 1200 °C for 4 h in an ammonia flow. Figure 2a shows the XRD pattern of the product, which can be well indexed as the mixed phase of an orthorhombic Al18B4O33 and a hexagonal BN. The measured pattern is in agreement with the reported XRD patterns (JCPDS 32-003 for Al18B4O33 and JCPDS 26-0773 for BN) concerning both peak intensity and position. No Al4B2O9 phase could be found from the pattern. SEM examinations show that the product contains the nanowirelike structure with the same morphology as the starting Al4B2O9 nanowires and BN platelike aggregates, which result from the conversion product of the starting amorphous boron oxide substance. Extensive TEM observations establish that all of the
13062 J. Phys. Chem. B, Vol. 109, No. 27, 2005
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Figure 2. BN coated Al18B4O33 nanowires. (a) XRD pattern showing the orthorhombic Al18B4O33 and hexagonal BN mixture phase, and the EDS from an individual composite nanowires. (b) TEM images showing that all nanowires are uniformly coated by BN. (c) High-resolution TEM image of the coating wire with flat end, and the corresponding SAED pattern recorded along [-210] zone axis.
Al4B2O9 nanowires are converted to the Al18B4O33 nanowires coated by a uniform thin BN layer. The BN composition with a B/N ratio of ∼1 can be confirmed from the electron-energyloss spectrum (EELS) for individual coating nanowires, as also shown in Figure 2a. The sheathed BN layer has a uniform coating thickness ∼3-5 nm, independent of the diameter of the inner Al18B4O33 nanowires. Two typical morphologies of the BN-coated Al18B4O33 nanowires are shown in Figure 2, panels b and c. Over 90% of the nanowires were fully sheathed by BN tubular layers, which are composed of the cylindrical walls and the closed-ended tips. Although there are numerous voids and glassy boron oxides existing between the BN layers and inner nanowires, which results in an irregular surface of the nanowires, the BN coating exhibits a uniform and straight nanotube-like morphology. Other nanowires terminated at a flat and BN-free end, and the cylindrical BN layer closely attaches to the inner nanowires especially in the area far away from fibrous end. Combined with the SAED technique, the axis direction of the Al18B4O33 nanowires is along [001], and BN layer grows along the (120) planes of the inner nanowires. In conclusion, taking advantage of the boron oxide evaporation of Al4B2O9 nanowires at high temperature, uniform and complete coating of Al18B4O33 nanowires with BN was acheived. Although the growth mechanism needs to be further investigated, we believe that the method reported here opens up many opportunities for nanometallurgy in one-dimesional composite materials.
References and Notes (1) (a) Wei, G. C.; Becher, P. F. Am. Ceram. Soc. Bull. 1985, 64, 298. (b) Luthra, K. L. J. Am. Chem. Soc. 1988, 71, 1144. (2) (a) Wong, E. W.; Sheehan, P. E.; Lieber C. M. Science 1997, 277, 1971. (b) Kim, P.; Lieber, C. M. Science 1999, 286, 2148. (3) (a) Suganuma, K.; Fujita, T.; Suzuki, N.; Nihara, K. J. Mater. Sci. Lett. 1990, 9, 633. (b) Ray, S. P. J. Am. Ceram. Soc. 1992, 75, 2605. (4) (a)Kindl, B.; Liu, Y. L.; Nyberg, E.; Hansen, N. Composites Sci. Technol. 1992, 43, 85. (b) Salvo, L.; L’Esperance, G.; Suery, M.; Legoux, J. G. Mater. Sci. Eng. A 1994, 177, 173. (5) (a) Liaw, P. K.; Greggi, J. G. J. Mater. Sci. 1987, 22, 1613. (b) Wada, H.; Sakane, K.; Kitamura, T.; Hata, H.; Kambara, H.; J. Mater. Sci. Lett. 1991, 10, 1078. (6) Ning, X. G.; Pan, J.; Hu, Y.; Ye, H. Q. Mater. Lett. 1992, 13, 377. (7) Ding, D. Y.; Wang, D. Z.; Zhang, X. X.; Yao, C. K. Mater. Sci. Eng. A 2001, 308, 19. (8) (a) Tang, C. C.; Bando, Y.; Sato, T.; Kurashima, K. AdV. Mater. 2002, 14, 1046. (b) Tang C. C.; Bando, Y. Appl. Phys. Lett. 2003, 83, 659. (9) Pan, J.; Sasaki, G.; Yao, L. J.; Yoshida, M.; Fukunaga, H. Mater. Sci. Technol. 1999, 15, 1044. (10) (a) Readey, M. J.; J. Am. Ceram. Soc. 1992, 75, 352. (b) Narita, T.; Jizuka, T. J. Ceram. Soc. Jpn. 1998, 106, 402. (c) Wada, H.; Sakane K.; Kitamura, T.; Hata, H. J. Mater. Sci. 1996, 31, 537. (11) (a) Ma, R.; Bando, Y.; Sata, T. Appl. Phys. Lett. 2002, 81, 3467. (b) Ma, R.; Bando, Y.; Sata, T.; Tang C. C.; Xu, F. F.; J. Am. Chem. Soc. 2002, 124, 10668. (c) Cheng C.; Tang, C.; Ding, X. X.; Huang, X. T.; Huang, Z. X.; Qi, S. R.; Hu, L.; Li, Y. X. Chem. Phys. Lett. 2003, 373, 626. (12) Liu, Y. M.; Li, Q. Q.; Fan, S. S.; Chem. Phys. Lett. 2003, 375, 632. (13) Levin, E. M.; Robbins, C. R.; McMurdie, H. F. Phase Diagrams for Ceramists; American Ceramic Society: Columbus, OH, 1969.
