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Simple approach to fabricate SiC–SiO2 composite nanowires and their oxidation resistance
Materials Science and Engineering B 173 (2010) 117–121
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Simple approach to fabricate SiC–SiO2 composite nanowires and their oxidation resistance Wasana Khongwong ∗ , Katsumi Yoshida, Toyohiko Yano Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, Japan
a r t i c l e
i n f o
Article history: Received 16 June 2009 Received in revised form 21 January 2010 Accepted 22 January 2010 Keywords: SiC–SiO2 composite nanowire Oxidation resistance Electron microscope
a b s t r a c t A simple thermal evaporation technique without catalysts from an exterior has been developed to synthesize SiC–SiO2 composite nanowires. Silicon powder of micrometer size or coarser silicon powder was heated in a horizontal tube furnace up to 1350 ◦ C under CH4 gas flow. Large quantity of as-grown wool-like products was obtained on the silicon powder oxidized at 800 ◦ C in air for 1 h. Characterization by an X-ray diffractometer, a field-emission scanning electron microscope, a transmission electron microscope and an infrared spectroscope indicated that these products were SiC core/SiO2 shell composite nanowires. SiC core diameter was approximately 20–80 nm with SiO2 shell of about 10–20 nm in thickness and length up to 1–2 mm. Both of separate heating process, i.e., heating for oxidation of raw Si powder and nanowires synthesis reaction separately, and continuous heating process, i.e., multi-step continuous heating for oxidation and reaction, could produce SiC–SiO2 core/shell nanowires. Based on thermogravimetric analysis, it was suggested that the synthesized nanowires had better oxidation resistance than that of SiC nano-sized powder. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, many studies on the synthesis and characterization of SiC–SiO2 nanowires have reported since they were expected to be a superior semiconductor–insulator heterojunction in the radial direction [1,2]. Moreover, SiC–SiO2 nanowires may be applied as a good light emitting material, since they can emit stable and high-intensity blue-green [3] or violet-blue [4] light. Several techniques have been used to synthesize SiC–SiO2 nanowires including chemical vapor deposition [5–7], sol–gel [8], laser ablation [9] and arc-discharge technique [4]. However, most of these synthetic methods involved complex processes and additional catalysts were often required, which would increase the impurity, reduce the crystallinity and degrade the properties of nanowires [10,11]. Until now, there is no report, except for our previous works [12,13], about the synthesis and the effect of reaction conditions using Si nano- or micron-sized powder–CH4 gas system. In this work, we put forward a low-cost and simple route to prepare SiC–SiO2 composite nanowires without catalysts. A simple heating process was applied and cheap Si raw material was used as a precursor in this study. Particularly, using a continuous heating pro-
∗ Corresponding author. Tel.: +81 3 5734 3082; fax: +81 3 5734 2959. E-mail addresses: [email protected], [email protected] (W. Khongwong). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.01.042
cess which has inherent advantages including a simple production process, it is unnecessary to evacuate and flow Ar gas into the furnace at the beginning stage, and it could be shorten operation time as a result of continuous operation. Furthermore, oxidation resistance of the synthesized nanowires was compared with some SiC powders, since SiC nanowires are also candidates of reinforcements for various composites including engineering ceramics due to their superior mechanical properties [14,15]. Therefore, improved oxidation resistance is an issue to be examined. Schematic illustration of the growth process was proposed to explain the formation of SiC–SiO2 core/shell nanowires. 2. Experimental procedure Nanowires were synthesized in a horizontal mullite tube furnace, the same as previous studies [12,13]. Two types of silicon source, SP (silicon powder; average particle size ≈5 m, specific surface area obtained by calculation ≈0.52 m2 /g, dark gray, 99% nominal purity, Kojundo Chemical Laboratory Co., Ltd., Japan) and SG (silicon ingot; dark gray, 99% nominal purity, Hirano Seizaemon Co., Ltd., Japan) were used to generate Si vapor. In the case of using SG, the Si ingot was ground and then sieved with a No. 200 mesh sieve (74 m, specific surface area obtained by calculation ≈0.04 m2 /g) before used as a precursor (names as SGG). CH4 gas (purity: 99.99%) was used as carbon source. A small amount of silicon raw powder was put in the mullite boat which was then covered with an alumina fiber sheet (NextelTM Woven Fabric 610 Style,
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Fig. 1. Schematic diagrams of the specimen codes which relative to the heating process as given on the right-hand side. Upper half is specimens prepared from H1 heating pattern whereas lower half is specimens prepared from H2 heating pattern.
Sumitomo 3M Ltd., Japan). The whole set was carefully pushed into the middle of the furnace. Before heating, the tube furnace was evacuated for 1.30 h to a pressure below 1.33 Pa using a mechanical rotary pump, then the ultra high purity Ar gas (purity: 99.9995%) was released into the furnace at a flow rate of 0.6 dm3 /min to reduce the oxygen to a negligible level. The furnace was initially raised to 1200 ◦ C at a heating rate of 10 ◦ C/min and then continued to heat but at 5 ◦ C/min to a peak temperature (1350 ◦ C). At 1350 ◦ C, H2 gas (purity: 99.999%) at a flow rate of 20 sccm (1 sccm = 1.667 × 10−8 m3 /s) was fed for 2 min before flowing of CH4 gas at a flow rate of 10 sccm. The valve of CH4 gas was closed when the flowing time passed for 30 min. The reaction was kept at the target temperature for 1 h. Finally, the furnace was cooled to room temperature under an Ar atmosphere. Heating process as mentioned above was named as H1 (see Fig. 1). To enhance SiO vapor in the system; therefore, the oxidized SP at two different oxidation temperatures of 600 and 800 ◦ C for 1 h in air (named as SP/O6 and SP/O8, respectively) was substituted for nonoxidized SP as well. Oxidation process was conducted separately before nanowires production. To shorten time for the production, the oxidation process of SP or SGG powders was conducted continuously in the same tube furnace that was used for synthesis of nanowires at 800 ◦ C for 1 h in air atmosphere (named as SPO8 and SGGO8, respectively), and then the furnace continuously heated up to 1350 ◦ C under Ar atmosphere. This heating process was named as H2 or continuous heating process. Fig. 1 shows schematic diagrams of the specimen codes which relative to the heating process as given on the righthand side. By continuous heating process (H2), evacuation of the furnace before heating process could be omitted. Moreover, the H2 heating pattern can save an amount of Ar gas, which will be flowed into the furnace since Ar gas was fed into the furnace after the oxi-
dation process. These advantages coupled with shorter operation time lead to low-cost production. Several equipments were employed to characterize the asobtained products. The macroproducts were exhibited by a digital camera, crystallographic data was collected by an X-ray diffractometer (XRD, Cu K␣, PW 1700, Philips, The Netherlands), and morphology was observed by a scanning electron microscope (FESEM, field-emission type, S-4800, Hitachi, Japan). Further detailed structure information was collected by a transmission electron microscope (TEM, H-9000, Hitachi, Japan). Specimen preparation for TEM observation was described elsewhere [12]. An infrared spectroscope (FT-IR, Fourier transform type, FT/IR-460 Plus, JASCO, Japan) was applied to confirm chemical components of the products. Oxidation resistance of the samples was analyzed by TG-DTA (Model: Thermoflex TG 8110, Rigaku, Japan) up to 1000 ◦ C (heating rate: 10 ◦ C/min) and keep at 1000 ◦ C for 1 h in air. 3. Results and discussion The as-obtained products are given in Table 1. In the case of nonoxidized SP as a raw powder, only a small part of SP powder surface discolored from dark gray to bright gray; however, a larger quantity Table 1 Specimen codes and as-achieved products on Si surface. Specimen code (type of Si raw powder-heating process)
Product
SP-H1 SP/O6-H1 SP/O8-H1 SPO8-H2 SGGO8-H2
A small amount of soft bright gray product A small amount of white-blue wool-like product White-blue wool-like product White-blue and gray wool-like product A small amount of blue-white wool-like product
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Fig. 2. Photographs of products in mullite boat from SP-H1, SP/O6-H1 and SP/O8-H1.
of white-blue wool-like products were obtained on raw powder surface when they were oxidized previously (SP/O6-H1 and SP/O8H1). The amount of wool-like products was more if the SP powder was oxidized at the higher temperature. It was clearly observed that the yield of reaction products markedly increased when the oxidized SP was used as a raw powder, as shown in Fig. 2. These circumstances indicated that formation of the wool-like products increased with an increase of SiO, which should be generated by the reaction of SiO2 on the surface of Si powder and Si vapor. SPO8-H2 specimen was prepared to ascertain the effect of the continuous heating process. Wool-like products still obtained. The yield of as-grown products achieved from SPO8-H2 was almost the same as SP/O8-H1 (prepared from the separate heating process). This result suggested that the H2 process can produce a larger quantity of wool-like products compared with H1 process within the same total operation time. Furthermore, SGGO8-H2 was an example to prepare with the low-priced Si powder. Comparison between SPO8-H2 and SGGO8-H2 specimen, although the wool-like products from SGGO8-H2 was less than the wool-like products from SPO8-H2, this way is one alternative to synthesize wool-like products with low-cost production. More study to improve the yield of products should be continued in the future.
Fig. 3. (a) Low-magnification FE-SEM image of wool-like product from SP/O8-H1. (b and c) FE-SEM images of wool-like product from SP/O8-H1 and SGGO8-H2 specimens, respectively.
XRD patterns of as-grown products both prepared from SP and SGG powder confirmed that crystalline phase of all the deposition products were -SiC, as same as the previous paper. No crystalline phase other than SiC was detected by XRD [12]. From our experimental results along with the previous researches [16,17], we believed that SiO vapor which might be formed from the reaction between SiO2 thin layer on Si powder and Si vapor was regarded as an essential factor to the growth of nanowires. Large amount of product could be obtained when oxidized Si powder was used as raw material, as mentioned above. FE-SEM observation indicated that all as-synthesized products consisted from randomly oriented wire-like products. As the examples, the typical FE-SEM images of nanowires prepared from oxidized silicon at 800 ◦ C but by different heating process (SP/O8-H1 and SGGO8-H2 specimens) were shown in Fig. 3. The low-magnification FE-SEM image of wool-like products from SP/O8-H1 specimen revealed that the white-blue wool-like prod-
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Fig. 4. TEM image of typical core-shell nanowire from SGGO8-H2 specimen. The lower left inset is the corresponding SAD pattern of SiC core and the upper right inset is high-magnification image of nanowire (the [1 1 1]-growth direction is indicated by an arrow).
ucts looked like a thread in a spool (see Fig. 3(a)). Both the wool-like products from SP/O8-H1 and SGGO8-H2 specimens (see Fig. 3(b) and (c), respectively) were composed of a large amount of straight, curved, tangled, randomly distributed nanowires. The length of the nanowires synthesized from oxidized SP at 800 ◦ C was too long to be measured under FE-SEM, and it is estimated to be 1–2 mm from the height of the product grown on SP raw powder surface. To obtain more details about the structure and crystallinity of synthesized nanowires, TEM and selected-area electron diffraction (SAD) methods were conducted and the results are shown in Fig. 4. A structure of as-grown nanowires was characterized with core/shell structure and the surface was very smooth. The diameter of core of nanowires was ranging from 20 to 80 nm, and it was wrapped with a uniform layer shell with a thickness of 10–20 nm. The SAD pattern showed that the crystalline SiC core had stacking faults and twins. It is well known that the SiC nanowires typically occupy high density of stacking faults and twin defects [18,19]. High-magnification image of SiC nanowire indicated that fringe of 0.25 nm-repeat corresponding to the d-spacing of the (1 1 1) plane.
Fig. 5. FT-IR spectrum of core-shell nanowires synthesized at 1350 ◦ C for 1 h (SP/O8H1 specimen).
Both suggested that the growth direction of the nanowire was [1 1 1] of -SiC. As described in the previous work [12], the elemental composition of core/shell nanowires was analyzed using an energy dispersive X-ray (EDX) spectroscope on a scanning TEM. The detected elements from both core and wrapping layer of a nanowire consisted from Si, C and O, but amount of O from the wrapping layer was significantly greater than that from core part. Fig. 5 is the FT-IR spectrum of the composite (core/shell) nanowires of the present study (SP/O8-H1). Two absorption bands from Si–O stretching vibration at around 1102 and 466 cm−1 , transversal optic (TO) mode of Si–C vibration at around 804 cm−1 and a shoulder which was marked by a small circle at around 860–950 cm−1 corresponding to the longitudinal optic (LO) mode of Si–C vibration were observed. The result was just in agreement with the previous reports [12,20,21]. Together with the XRD and TEM analyses, we believed that the outer shell was consisted from low crystallinity or amorphous SiO2 . In the growth process of the composite nanowires, the reaction is seemingly involved with SiO vapor phase. A schematic illustration of the growth process of SiC–SiO2 core/shell nanowire is shown in Fig. 6. As the temperature increases, SiO vapor is
Fig. 6. Schematic illustration of the growth process of SiC–SiO2 core/shell nanowire.
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as O2 diffusion barrier should be thicker than that of the nanosized SiC powder. TEM observation of the SiC powder of 30 nm in particle size indicated that thickness of SiO2 layer on the SiC powder was thinner (≈1–2 nm) than those of core-shell nanowires (≈10–20 nm). Together with good oxidation resistance indicated in this study, potentials of SiC–SiO2 core/shell nanowires as a good light emitting material and as reinforcements of composites should be improved particularly for the applications under high temperature oxidizing environments. 4. Conclusions
Fig. 7. TG profiles of three SiC samples during heating in air up to 1000 ◦ C (10 ◦ C/min) and kept for 1 h (initial weight 0.42 mg each).
generated by the reaction of SiO2 thin layer on the surface of Si and evaporated Si. Dense SiO smoke is deposited first near Si powder surface. Subsequently, when the furnace is heated up till close to the melting point of Si (1414 ◦ C), evaporation of Si accelerated. The main reaction of SiC nucleation might be SiO(v) + Si(v) + 3CH4 = 2SiC(s) + C(s) + H2 O(v) + 5H2 . The Gibbs free energy change (G) of this reaction at 1350 ◦ C is −512.673 kJ/mol [13]. The clusters of SiC nuclei assembled to form nanowire. The nanowires in a preferred orientation grow fast as more SiO vapor and CH4 gas co-exist in the system. These mechanisms were proposed as oxide-assisted growth for the nanowires growth directly from SiO powder–CH4 system by Lee et al. [17]. Subsequently, side surface of the synthesized SiC nanowires is gradually oxidized to form amorphous SiO2 outer shell by H2 O vapor, which is a byproduct of the formation reaction of SiC nanowires. The synthesized SiC nanowires from SP/O8-H1 was used as an example to study oxidation resistance using a TG analyzer and the result was compared with two kinds of commercial SiC powder. The TG profiles for the SiC nanowires (SiCNWs), SiC powder of an average particle size of 0.3 m and SiC powder of an average particle size of 30 nm with the same initial weight of 0.42 mg are shown in Fig. 7. The results showed that the SiC–SiO2 core/shell nanowires exhibited the lowest weight gain through the heating process. Furthermore, during soaking for 1 h at 1000 ◦ C, the weight gain rather suddenly increased. The SiC–SiO2 core/shell nanowires still exhibited the lowest weight gain than those of the 0.3 m SiC powder and the 30 nm SiC powder (0.29, 0.34 and 0.44 mg, respectively). These weight changes should be caused by passive oxidation of SiC. The SiO2 thin film on the SiC surface which occurred from passive oxidation of SiC can be an inhibitor in further oxidation as well. The SiC powder of 30 nm in diameter possessed very large surface area, resulted in easier to be oxidized. SiC–SiO2 nanowires had core diameter in the range from 20 to 80 nm and it also has very high surface area (estimated to be ≈22 m2 /g). Since SiC–SiO2 nanowires showed higher oxidation resistance than the nano-sized SiC powder, it was considered that the SiO2 layer of the nanowires acting
Different amounts of nanowires were synthesized from nonoxidized or oxidized Si powder as raw materials. In the case of a raw material of oxidized Si powder, larger quantity of SiC–SiO2 core/shell nanowires was synthesized. Diameter of core of the nanowires was 20–80 nm and 1–2 mm in length, and it was wrapped with a uniform layer of amorphous SiO2 with 10–20 nm in thickness. It was supposed that presence of SiO vapor is a key factor for the present nanowire formation. Moreover, it was demonstrated that the simple production process and using the low-cost raw material, such as the continuous heating pattern for reaction of coarse silicon powder, make the present approach attractive and outstanding. TG analysis suggested that the synthesized nanowires have higher oxidation resistance than that of the nano-sized SiC powder. References [1] Y. Zhang, K. Suenaga, C. Colliex, S. Iijima, Science 281 (1998) 973. [2] K.F. Cai, A.X. Zhang, J.L. Yin, H.F. Wang, X.H. Yung, Appl. Phys. A 91 (2008) 579. [3] D.H. Feng, T.Q. Jia, X.X. Li, Z.Z. Xu, J. Chen, S.Z. Deng, Z.S. Wu, N.S. Xu, Solid State Commun. 128 (2003) 295. [4] X.M. Liu, K.F. Yao, Nanotechnology 16 (2005) 2932. [5] X.T. Zhou, N. Wang, C.K.Au. Frederick, H.L. Lai, H.Y. Peng, I. Bello, C.S. Lee, S.T. Lee, Mater. Sci. Eng. A 286 (2000) 119. [6] Q.G. Fu, H.J. Li, X.H. Shi, K.Z. Li, J. Wei, Z.B. Hu, Mater. Chem. Phys. 100 (2006) 108. [7] D.Q. Zhang, A. Alkhateeb, H. Han, H. Mahmood, D.N. Mcllroy, M.G. Norton, Nano Lett. 3 (2003) 983. [8] C.H. Liang, G.W. Meng, L.D. Zhang, Y.C. Wu, Z. Cui, Chem. Phys. Lett. 329 (2000) 323. [9] W. Shi, Y. Zheng, H. Peng, N. Wang, C.S. Lee, S.T. Lee, J. Am. Ceram. Soc. 83 (2000) 3228. [10] K.Y. Cheong, Z. Lockman, J. Alloys Compd. 481 (2009) 345. [11] N. Wang, Y. Cai, R.Q. Zhang, Mater. Sci. Eng. R 60 (2000) 1. [12] W. Khongwong, M. Imai, K. Yoshida, T. Yano, J. Ceram. Soc. Jpn. 117 (2009) 194. [13] W. Khongwong, M. Imai, K. Yoshida, T. Yano, J. Ceram. Soc. Jpn. 117 (2009) 439. [14] L.K. Frevel, C.K. Saha, D.R. Petersen, J. Mater. Sci. 30 (1995) 3734. [15] L.H. Qian, Z.G. Wang, H. Toda, T. Kobayashi, Mater. Sci. Eng. A 357 (2003) 240. [16] H.J. Choi, J.G. Lee, J. Mater. Sci. 30 (1995) 1982. [17] Y. Yao, S.T. Lee, F.H. Li, Chem. Phys. Lett. 381 (2003) 628. [18] H.W. Shim, Y.F. Zhang, H.C. Huang, J. Appl. Phys. 104 (2008) 063511. [19] Z.J. Li, W.D. Gao, A. Meng, Z.D. Geng, L. Gao, J. Phys. Chem. C 113 (2009) 91. [20] G.W. Meng, L.D. Zhang, Y. Qin, C.M. Mo, F. Phillipp, Nanostruct. Mater. 12 (1999) 1003. [21] B.S. Li, R.B. Wu, Y. Pan, L.L. Wu, G. Yang, J.J. Chen, Q. Zhu, J. Alloys Compd. 462 (2008) 446.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Simple approach to fabricate SiC–SiO2 composite nanowires and their oxidation resistance Wasana Khongwong ∗ , Katsumi Yoshida, Toyohiko Yano Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, Japan
a r t i c l e
i n f o
Article history: Received 16 June 2009 Received in revised form 21 January 2010 Accepted 22 January 2010 Keywords: SiC–SiO2 composite nanowire Oxidation resistance Electron microscope
a b s t r a c t A simple thermal evaporation technique without catalysts from an exterior has been developed to synthesize SiC–SiO2 composite nanowires. Silicon powder of micrometer size or coarser silicon powder was heated in a horizontal tube furnace up to 1350 ◦ C under CH4 gas flow. Large quantity of as-grown wool-like products was obtained on the silicon powder oxidized at 800 ◦ C in air for 1 h. Characterization by an X-ray diffractometer, a field-emission scanning electron microscope, a transmission electron microscope and an infrared spectroscope indicated that these products were SiC core/SiO2 shell composite nanowires. SiC core diameter was approximately 20–80 nm with SiO2 shell of about 10–20 nm in thickness and length up to 1–2 mm. Both of separate heating process, i.e., heating for oxidation of raw Si powder and nanowires synthesis reaction separately, and continuous heating process, i.e., multi-step continuous heating for oxidation and reaction, could produce SiC–SiO2 core/shell nanowires. Based on thermogravimetric analysis, it was suggested that the synthesized nanowires had better oxidation resistance than that of SiC nano-sized powder. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, many studies on the synthesis and characterization of SiC–SiO2 nanowires have reported since they were expected to be a superior semiconductor–insulator heterojunction in the radial direction [1,2]. Moreover, SiC–SiO2 nanowires may be applied as a good light emitting material, since they can emit stable and high-intensity blue-green [3] or violet-blue [4] light. Several techniques have been used to synthesize SiC–SiO2 nanowires including chemical vapor deposition [5–7], sol–gel [8], laser ablation [9] and arc-discharge technique [4]. However, most of these synthetic methods involved complex processes and additional catalysts were often required, which would increase the impurity, reduce the crystallinity and degrade the properties of nanowires [10,11]. Until now, there is no report, except for our previous works [12,13], about the synthesis and the effect of reaction conditions using Si nano- or micron-sized powder–CH4 gas system. In this work, we put forward a low-cost and simple route to prepare SiC–SiO2 composite nanowires without catalysts. A simple heating process was applied and cheap Si raw material was used as a precursor in this study. Particularly, using a continuous heating pro-
∗ Corresponding author. Tel.: +81 3 5734 3082; fax: +81 3 5734 2959. E-mail addresses: [email protected], [email protected] (W. Khongwong). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.01.042
cess which has inherent advantages including a simple production process, it is unnecessary to evacuate and flow Ar gas into the furnace at the beginning stage, and it could be shorten operation time as a result of continuous operation. Furthermore, oxidation resistance of the synthesized nanowires was compared with some SiC powders, since SiC nanowires are also candidates of reinforcements for various composites including engineering ceramics due to their superior mechanical properties [14,15]. Therefore, improved oxidation resistance is an issue to be examined. Schematic illustration of the growth process was proposed to explain the formation of SiC–SiO2 core/shell nanowires. 2. Experimental procedure Nanowires were synthesized in a horizontal mullite tube furnace, the same as previous studies [12,13]. Two types of silicon source, SP (silicon powder; average particle size ≈5 m, specific surface area obtained by calculation ≈0.52 m2 /g, dark gray, 99% nominal purity, Kojundo Chemical Laboratory Co., Ltd., Japan) and SG (silicon ingot; dark gray, 99% nominal purity, Hirano Seizaemon Co., Ltd., Japan) were used to generate Si vapor. In the case of using SG, the Si ingot was ground and then sieved with a No. 200 mesh sieve (74 m, specific surface area obtained by calculation ≈0.04 m2 /g) before used as a precursor (names as SGG). CH4 gas (purity: 99.99%) was used as carbon source. A small amount of silicon raw powder was put in the mullite boat which was then covered with an alumina fiber sheet (NextelTM Woven Fabric 610 Style,
118
W. Khongwong et al. / Materials Science and Engineering B 173 (2010) 117–121
Fig. 1. Schematic diagrams of the specimen codes which relative to the heating process as given on the right-hand side. Upper half is specimens prepared from H1 heating pattern whereas lower half is specimens prepared from H2 heating pattern.
Sumitomo 3M Ltd., Japan). The whole set was carefully pushed into the middle of the furnace. Before heating, the tube furnace was evacuated for 1.30 h to a pressure below 1.33 Pa using a mechanical rotary pump, then the ultra high purity Ar gas (purity: 99.9995%) was released into the furnace at a flow rate of 0.6 dm3 /min to reduce the oxygen to a negligible level. The furnace was initially raised to 1200 ◦ C at a heating rate of 10 ◦ C/min and then continued to heat but at 5 ◦ C/min to a peak temperature (1350 ◦ C). At 1350 ◦ C, H2 gas (purity: 99.999%) at a flow rate of 20 sccm (1 sccm = 1.667 × 10−8 m3 /s) was fed for 2 min before flowing of CH4 gas at a flow rate of 10 sccm. The valve of CH4 gas was closed when the flowing time passed for 30 min. The reaction was kept at the target temperature for 1 h. Finally, the furnace was cooled to room temperature under an Ar atmosphere. Heating process as mentioned above was named as H1 (see Fig. 1). To enhance SiO vapor in the system; therefore, the oxidized SP at two different oxidation temperatures of 600 and 800 ◦ C for 1 h in air (named as SP/O6 and SP/O8, respectively) was substituted for nonoxidized SP as well. Oxidation process was conducted separately before nanowires production. To shorten time for the production, the oxidation process of SP or SGG powders was conducted continuously in the same tube furnace that was used for synthesis of nanowires at 800 ◦ C for 1 h in air atmosphere (named as SPO8 and SGGO8, respectively), and then the furnace continuously heated up to 1350 ◦ C under Ar atmosphere. This heating process was named as H2 or continuous heating process. Fig. 1 shows schematic diagrams of the specimen codes which relative to the heating process as given on the righthand side. By continuous heating process (H2), evacuation of the furnace before heating process could be omitted. Moreover, the H2 heating pattern can save an amount of Ar gas, which will be flowed into the furnace since Ar gas was fed into the furnace after the oxi-
dation process. These advantages coupled with shorter operation time lead to low-cost production. Several equipments were employed to characterize the asobtained products. The macroproducts were exhibited by a digital camera, crystallographic data was collected by an X-ray diffractometer (XRD, Cu K␣, PW 1700, Philips, The Netherlands), and morphology was observed by a scanning electron microscope (FESEM, field-emission type, S-4800, Hitachi, Japan). Further detailed structure information was collected by a transmission electron microscope (TEM, H-9000, Hitachi, Japan). Specimen preparation for TEM observation was described elsewhere [12]. An infrared spectroscope (FT-IR, Fourier transform type, FT/IR-460 Plus, JASCO, Japan) was applied to confirm chemical components of the products. Oxidation resistance of the samples was analyzed by TG-DTA (Model: Thermoflex TG 8110, Rigaku, Japan) up to 1000 ◦ C (heating rate: 10 ◦ C/min) and keep at 1000 ◦ C for 1 h in air. 3. Results and discussion The as-obtained products are given in Table 1. In the case of nonoxidized SP as a raw powder, only a small part of SP powder surface discolored from dark gray to bright gray; however, a larger quantity Table 1 Specimen codes and as-achieved products on Si surface. Specimen code (type of Si raw powder-heating process)
Product
SP-H1 SP/O6-H1 SP/O8-H1 SPO8-H2 SGGO8-H2
A small amount of soft bright gray product A small amount of white-blue wool-like product White-blue wool-like product White-blue and gray wool-like product A small amount of blue-white wool-like product
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Fig. 2. Photographs of products in mullite boat from SP-H1, SP/O6-H1 and SP/O8-H1.
of white-blue wool-like products were obtained on raw powder surface when they were oxidized previously (SP/O6-H1 and SP/O8H1). The amount of wool-like products was more if the SP powder was oxidized at the higher temperature. It was clearly observed that the yield of reaction products markedly increased when the oxidized SP was used as a raw powder, as shown in Fig. 2. These circumstances indicated that formation of the wool-like products increased with an increase of SiO, which should be generated by the reaction of SiO2 on the surface of Si powder and Si vapor. SPO8-H2 specimen was prepared to ascertain the effect of the continuous heating process. Wool-like products still obtained. The yield of as-grown products achieved from SPO8-H2 was almost the same as SP/O8-H1 (prepared from the separate heating process). This result suggested that the H2 process can produce a larger quantity of wool-like products compared with H1 process within the same total operation time. Furthermore, SGGO8-H2 was an example to prepare with the low-priced Si powder. Comparison between SPO8-H2 and SGGO8-H2 specimen, although the wool-like products from SGGO8-H2 was less than the wool-like products from SPO8-H2, this way is one alternative to synthesize wool-like products with low-cost production. More study to improve the yield of products should be continued in the future.
Fig. 3. (a) Low-magnification FE-SEM image of wool-like product from SP/O8-H1. (b and c) FE-SEM images of wool-like product from SP/O8-H1 and SGGO8-H2 specimens, respectively.
XRD patterns of as-grown products both prepared from SP and SGG powder confirmed that crystalline phase of all the deposition products were -SiC, as same as the previous paper. No crystalline phase other than SiC was detected by XRD [12]. From our experimental results along with the previous researches [16,17], we believed that SiO vapor which might be formed from the reaction between SiO2 thin layer on Si powder and Si vapor was regarded as an essential factor to the growth of nanowires. Large amount of product could be obtained when oxidized Si powder was used as raw material, as mentioned above. FE-SEM observation indicated that all as-synthesized products consisted from randomly oriented wire-like products. As the examples, the typical FE-SEM images of nanowires prepared from oxidized silicon at 800 ◦ C but by different heating process (SP/O8-H1 and SGGO8-H2 specimens) were shown in Fig. 3. The low-magnification FE-SEM image of wool-like products from SP/O8-H1 specimen revealed that the white-blue wool-like prod-
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Fig. 4. TEM image of typical core-shell nanowire from SGGO8-H2 specimen. The lower left inset is the corresponding SAD pattern of SiC core and the upper right inset is high-magnification image of nanowire (the [1 1 1]-growth direction is indicated by an arrow).
ucts looked like a thread in a spool (see Fig. 3(a)). Both the wool-like products from SP/O8-H1 and SGGO8-H2 specimens (see Fig. 3(b) and (c), respectively) were composed of a large amount of straight, curved, tangled, randomly distributed nanowires. The length of the nanowires synthesized from oxidized SP at 800 ◦ C was too long to be measured under FE-SEM, and it is estimated to be 1–2 mm from the height of the product grown on SP raw powder surface. To obtain more details about the structure and crystallinity of synthesized nanowires, TEM and selected-area electron diffraction (SAD) methods were conducted and the results are shown in Fig. 4. A structure of as-grown nanowires was characterized with core/shell structure and the surface was very smooth. The diameter of core of nanowires was ranging from 20 to 80 nm, and it was wrapped with a uniform layer shell with a thickness of 10–20 nm. The SAD pattern showed that the crystalline SiC core had stacking faults and twins. It is well known that the SiC nanowires typically occupy high density of stacking faults and twin defects [18,19]. High-magnification image of SiC nanowire indicated that fringe of 0.25 nm-repeat corresponding to the d-spacing of the (1 1 1) plane.
Fig. 5. FT-IR spectrum of core-shell nanowires synthesized at 1350 ◦ C for 1 h (SP/O8H1 specimen).
Both suggested that the growth direction of the nanowire was [1 1 1] of -SiC. As described in the previous work [12], the elemental composition of core/shell nanowires was analyzed using an energy dispersive X-ray (EDX) spectroscope on a scanning TEM. The detected elements from both core and wrapping layer of a nanowire consisted from Si, C and O, but amount of O from the wrapping layer was significantly greater than that from core part. Fig. 5 is the FT-IR spectrum of the composite (core/shell) nanowires of the present study (SP/O8-H1). Two absorption bands from Si–O stretching vibration at around 1102 and 466 cm−1 , transversal optic (TO) mode of Si–C vibration at around 804 cm−1 and a shoulder which was marked by a small circle at around 860–950 cm−1 corresponding to the longitudinal optic (LO) mode of Si–C vibration were observed. The result was just in agreement with the previous reports [12,20,21]. Together with the XRD and TEM analyses, we believed that the outer shell was consisted from low crystallinity or amorphous SiO2 . In the growth process of the composite nanowires, the reaction is seemingly involved with SiO vapor phase. A schematic illustration of the growth process of SiC–SiO2 core/shell nanowire is shown in Fig. 6. As the temperature increases, SiO vapor is
Fig. 6. Schematic illustration of the growth process of SiC–SiO2 core/shell nanowire.
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as O2 diffusion barrier should be thicker than that of the nanosized SiC powder. TEM observation of the SiC powder of 30 nm in particle size indicated that thickness of SiO2 layer on the SiC powder was thinner (≈1–2 nm) than those of core-shell nanowires (≈10–20 nm). Together with good oxidation resistance indicated in this study, potentials of SiC–SiO2 core/shell nanowires as a good light emitting material and as reinforcements of composites should be improved particularly for the applications under high temperature oxidizing environments. 4. Conclusions
Fig. 7. TG profiles of three SiC samples during heating in air up to 1000 ◦ C (10 ◦ C/min) and kept for 1 h (initial weight 0.42 mg each).
generated by the reaction of SiO2 thin layer on the surface of Si and evaporated Si. Dense SiO smoke is deposited first near Si powder surface. Subsequently, when the furnace is heated up till close to the melting point of Si (1414 ◦ C), evaporation of Si accelerated. The main reaction of SiC nucleation might be SiO(v) + Si(v) + 3CH4 = 2SiC(s) + C(s) + H2 O(v) + 5H2 . The Gibbs free energy change (G) of this reaction at 1350 ◦ C is −512.673 kJ/mol [13]. The clusters of SiC nuclei assembled to form nanowire. The nanowires in a preferred orientation grow fast as more SiO vapor and CH4 gas co-exist in the system. These mechanisms were proposed as oxide-assisted growth for the nanowires growth directly from SiO powder–CH4 system by Lee et al. [17]. Subsequently, side surface of the synthesized SiC nanowires is gradually oxidized to form amorphous SiO2 outer shell by H2 O vapor, which is a byproduct of the formation reaction of SiC nanowires. The synthesized SiC nanowires from SP/O8-H1 was used as an example to study oxidation resistance using a TG analyzer and the result was compared with two kinds of commercial SiC powder. The TG profiles for the SiC nanowires (SiCNWs), SiC powder of an average particle size of 0.3 m and SiC powder of an average particle size of 30 nm with the same initial weight of 0.42 mg are shown in Fig. 7. The results showed that the SiC–SiO2 core/shell nanowires exhibited the lowest weight gain through the heating process. Furthermore, during soaking for 1 h at 1000 ◦ C, the weight gain rather suddenly increased. The SiC–SiO2 core/shell nanowires still exhibited the lowest weight gain than those of the 0.3 m SiC powder and the 30 nm SiC powder (0.29, 0.34 and 0.44 mg, respectively). These weight changes should be caused by passive oxidation of SiC. The SiO2 thin film on the SiC surface which occurred from passive oxidation of SiC can be an inhibitor in further oxidation as well. The SiC powder of 30 nm in diameter possessed very large surface area, resulted in easier to be oxidized. SiC–SiO2 nanowires had core diameter in the range from 20 to 80 nm and it also has very high surface area (estimated to be ≈22 m2 /g). Since SiC–SiO2 nanowires showed higher oxidation resistance than the nano-sized SiC powder, it was considered that the SiO2 layer of the nanowires acting
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