- Email: [email protected]
Microstructure characterization of SiC nanowires as reinforcements in composites
Materials Characterization 103 (2015) 37–41
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Microstructure characterization of SiC nanowires as reinforcements in composites Ronghua Dong a, Wenshu Yang a,⁎, Ping Wu b, Murid Hussain c, Ziyang Xiu a, Gaohui Wu a, Pingping Wang a a b c
Department of Material Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Northwest Institute of Nuclear Technology, Xi'an, 710024, China Department of Chemical Engineering, COMSATS Institute of Information Technology, M.A. Jinnah Building, Defence Road, Off Raiwind Road, Lahore 54000, Pakistan
a r t i c l e
i n f o
Article history: Received 10 November 2014 Received in revised form 13 March 2015 Accepted 15 March 2015 Available online 18 March 2015 Keywords: SiC nanowires Crystal structure Cross-section microstructure Hybrid structure
a b s t r a c t SiC nanowires have been rarely investigated or explored along their axial direction by transmission electron microscopy (TEM). Here we report the investigation of the cross-section microstructure of SiC nanowires by embedding them into Al matrix. Morphology of SiC nanowires was cylindrical with smooth surface or bamboo shape. Cubic (3C-SiC) and hexagonal structure (2H-SiC) phases were detected by X-ray diffraction (XRD) analysis. High density stacking faults were observed in both the cylindrical and bamboo shaped nanowires which were perpendicular to their axial direction. Selected area electron diffraction (SAED) patterns of the cylindrical and bamboo shaped SiC nanowires both in the perpendicular and parallel direction to the axial direction were equivalent in the structure. After calculation and remodeling, it has been found that the SAED patterns were composed of two sets of diffraction patterns, corresponding to 2H-SiC and 3C-SiC, respectively. Therefore, it could be concluded that the SiC nanowires are composed of a large number of small fragments that are formed by hybrid 3C-SiC and 2H-SiC structures. © 2015 Elsevier Inc. All rights reserved.
1. Introduction SiC nanowires have received a lot of attention due to their outstanding properties such as high mechanical strength [1,2], high thermal conductivity [3,4], and variable band gaps [5]. Zhang et al. [6] performed qualitative in-situ transmission electron microscopy (TEM) tension tests of SiC nanowires and found substantial plasticity at room temperature (e.g., SiC nanowires experience over 200% elongation before fracture). Yang et al. [7] reported that a small addition of SiC nanowires into a SiC whisker reinforced matrix would double the toughness of the nanocomposite. Despite these very interesting results, the comprehensive characterization of SiC nanowires has been rarely reported. The most common crystal structure of SiC is the cubic ones (3C or β) [8], the three hexagonals (2H, 4H and 6H) and some rhombohedral forms (9R, 15R and 21R). The cubic polytype is the most stable phase at low growth temperatures [9], despite the fact that the 6H-polytype is the most stable structure from the thermodynamic equilibrium point of view. Therefore, 3C-SiC and hexagonal SiC nanowires have been widely investigated in the vast majority of studies. The surface morphology of 3C-SiC and hexagonal SiC nanowires has been observed [10,11], while the morphology of the SiC nanowires has been turned to cylindrical or bamboo shape by altering the reaction temperature [12]. Moreover, high density of defects, which are usually in a periodic ⁎ Corresponding author at: P. O. 3023, Science Park, No. 2 Yikuang Street, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150080, China. E-mail addresses: [email protected], [email protected] (W. Yang).
http://dx.doi.org/10.1016/j.matchar.2015.03.013 1044-5803/© 2015 Elsevier Inc. All rights reserved.
fashion along the nanowire length due to its low stacking fault energy, is considered as one of the important reasons to form the zigzag appearance of the contoured surface of SiC nanowires [13]. Wang et al. [14] explained the growth mechanism of bamboo shaped 3C-SiC. The periodic twins in the SiC nanowires, which formed the zigzag appearance, were due to a minimum surface energy and strain energy. It is very interesting that indexing the XRD spectra to define the polytype of the SiC nanowires is not straightforward in many cases. Ryu et al. [15] and Li et al. [16] reported the XRD spectra of 3C-SiC nanowires and observed very weak diffraction peak at 34.2°, which was assigned to the [1011] plane of hexagonal SiC [10]. Zekentes et al. [9] reported that the diffraction peak at 34.2° might be due to the impurities of hexagonal SiC in 3C-SiC nanowires. However, Ryu et al. [15] proposed that the diffraction peak at 34.2° might be due to the high number of stacking faults (equivalent to hexagonal inclusions) in 3C-SiC nanowires. However, this interesting phenomenon has not been fully understood yet. Moreover, almost all the investigations [17,18] of microstructure of SiC nanowires are perpendicular to their axial direction. To the best of authors' knowledge, the characterization of the cross-section microstructure of SiC nanowires, which could be useful for the comprehensive understanding of the structure of SiC nanowires, has been rarely reported yet. It is very difficult to observe the cross-section microstructure of SiC nanowire in normal. However, it is well known that embedding the reinforcement in metal matrix is an effective way to observe their microstructure in detail [19,20]. Therefore, in the present work, in order to investigate the cross-section microstructure of SiC nanowires, the SiC nanowires were introduced into Al matrix composite. The comprehensive
38
R. Dong et al. / Materials Characterization 103 (2015) 37–41
characterization of the microstructure of SiC nanowires was performed. It has been found that the morphology of SiC nanowires was composed of cylindrical with smooth surface and bamboo shapes. These two kinds of SiC nanowires have high density of stacking faults and nano-twins, leading towards the hybrid structure of cubic and hexagonal SiC, and correspond to the diffraction peak at 34.2°. Based on the microstructure observation results, the structure of SiC nanowires was remodeled. 2. Materials and methods SiC nanowires (3C-SiC, 99% in purity, Changsha Sinet Advanced Materials Co., Ltd. China) were dispersed firstly by ultrasonic in a gluing solution that was composed of water, ethanol and polyvinyl alcohol (PVA) in weight proportion of 10:10:1, and an unconsolidated SiC nanowire preform was obtained after drying. In the next step, the preform was put into a mold and further pressed to the set volume content (15 vol.%). The preheating temperatures for the preform and pressure infiltration dies were 500 and 770 °C, respectively. During the infiltration process, a pressure of 5 MPa was applied and maintained for 10 min, followed by the solidification of the composite in air. Before microstructure observation, 340 °C/2 h annealing treatment was performed to all the SiCnw/Al specimens. Morphology of SiC nanowires was observed by an FEI Sirion Quanta 200 scanning electron microscope (SEM). Microstructure observation of the SiCnw/6061Al composite was performed on JEM-2010F high resolution transmission electron microscopy (HRTEM). XRD analysis was carried out by using a Rigaku D/max-rB diffractometer. The specimens were subjected to Cu-Kα radiation (0.15418 nm) with a scanning speed set at 2°/min, while 2θ scans were performed between 25 and 90°. 3. Results and discussion Fig. 1 shows the morphology of SiC nanowires observed by SEM analysis. The distribution range of length and diameter of SiC nanowires was from 10 to 50 μm (Fig. 1a) and 100 to 500 nm (Fig. 1b), respectively. Furthermore, two kinds of SiC phases were observed in the raw nanowires, which showed cylindrical with smooth surface (pointed out by green arrow in Fig. 1b) and bamboo shape (shown by yellow arrows in Fig. 1b), respectively. This phenomenon has been widely observed in SiC nanowires [11]. XRD analysis result of SiC nanowires is shown in Fig. 2, and diffraction peaks at 33.7°, 35.7°, 41.4° 60.0°, 71.8° and 75.6° were found. Compared to the standard powder XRD spectra (JCPDS cards), it is clear that the peaks at 35.7°, 41.4° 60.0°, 71.8° and 75.6° could be well matched with the diffraction peaks of (111), (200), (220), (311) and (222) planes
Fig. 2. XRD analysis results of SiC nanowires.
of 3C-SiC [15]. However, the peak at 33.7° could be related to the hexagonal SiC. Usually, diffraction peaks of hexagonal SiC could be found in 3C-SiC nanowires [21]. It might be intuitively thought that the raw SiC nanowires were composed of 2H-SiC nanowires and 3C-SiC while the 2H-SiC could be considered as the impurity in 3C-SiC nanowires. However, Zekentes and Rogdakis proposed that the multiple XRD peaks could be a textured structure of the nanowires themselves or the existence of single-crystalline nanowires with different orientations [9]. Unfortunately, no detailed information and structure of the SiC nanowire have been provided. Our present investigation indicated that both the 3C-SiC and 2H-SiC could be found in single SiC nanowires, and the 2HSiC was formed due to the high density stacking faults in 3C-SiC nanowires. It will be explained with detail in the following discussion. Microstructure of SiCnw/Al composite was also observed by TEM analysis, as shown in Fig. 3. It is shown that SiC nanowires were uniformly distributed in Al matrix and no significant interfacial products were observed, implying that the preparation process of the composites caused slight damage to the SiC nanowires. The TEM microstructures of cylindrical and bamboo shaped SiC nanowires in the composite, which were perpendicular to their axial direction, are shown in Fig. 4a and b, respectively. It is obvious that high density stacking faults were found in both the cylindrical (Fig. 4a) and bamboo shaped (Fig. 4b) nanowires. SAED patterns of cylindrical (Fig. 4a) and bamboo shaped (Fig. 4b) nanowires are shown in Fig. 4c and d, respectively. Regardless of the morphology, 3C-SiC usually consists of three types of structures: pure face-centered cubic (3C) structure, 3C structure with inclined stacking faults, and high density
Fig. 1. Representative morphology of SiC nanowires used in the present work. (a) Morphology; (b) Magnification of nanowires. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
R. Dong et al. / Materials Characterization 103 (2015) 37–41
39
Fig. 3. Microstructure of SiCnw/Al composite. (a) Cylindrical SiC nanowires in the composite; (b) Bamboo shaped SiC nanowires in the composite.
Fig. 4. The microstructure characterization of SiC nanowires perpendicular to their axial direction. (a) Cylindrical SiC nanowires; (b) Bamboo shaped SiC nanowires; (c) and (d) Selected h i area electron diffraction patterns of cylindrical (a) and bamboo shaped (b) nanowires, respectively. (e) The superposition of the simulated diffraction patterns of 2H-SiC along 2110 zone axes (blue spots) and 3C-SiC along [011] zone axes (red spots). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
stacking faults perpendicular to the nanowire axis. In the present work, the SAED patterns of the cylindrical and bamboo shaped SiC nanowires perpendicular to the axial direction were equivalent in the structure, and their SAED patterns of nanowires were composed of two sets of spots, and the streaked diffraction spots were formed due to the high density of planar defects. Similar results have also been found by Pozuelo et al. [22] and Kang et al. [23]. We calculated the interplanar spacing of (0002) plane in 2H-SiC and (111) plane in 3C-SiC, which are the close-packed planes in their crystal structures. It has been found that the interplanar spacing of (0002) plane in 2H-SiC (2.518 Å) was very close to that of (111) plane in 3C-SiC (2.517 Å). Therefore, based on Zekentes and Rogdakis suggestion [9], it is hypothesized that the SiC nanowires were composited of the 3C-SiC and 2H-SiC, and h i then the simulated diffraction patterns of 2H-SiC along 2110 zone
axes (blue spots in Fig. 4e) and 3C-SiC along [011] zone axes (red spots in Fig. 4e) were superimposed. The superimposed patterns were agreed very well with the experimental SAED patterns (Fig. 4c and d), indicating the feasibility of our hypothesis that SiC nanowires were comprised of a composite of 3C-SiC and 2H-SiC. In order to further confirm our hypothesis, the cross-section microstructure of the cylindrical (Fig. 5a) and bamboo shaped (Fig. 5b) SiC nanowires, which has been rarely reported in literatures, was observed by the TEM analysis. The cross-section morphology of cylindrical (Fig. 5a) and bamboo shaped (Fig. 5b) SiC was quasi-circular with fuzzy hexagonal shape and standard hexagonal shape, respectively. Generally, the SiC nanowires with hexagonal shape are corresponding to the hexagonal structure [11]. However, our present work indicated that the morphology of SiC nanowires was not mandatory related to its crystal structure. Furthermore, SAED patterns of the cylindrical
40
R. Dong et al. / Materials Characterization 103 (2015) 37–41
Fig. 5. The cross-section microstructure characterization of SiC nanowires. (a) Cylindrical type SiC nanowires; (c) Bamboo shaped SiC nanowires; (b) and (d) Selected area electron diffraction patterns of cylindrical (a) and bamboo shaped (c) nanowires, respectively. (e) The superposition of the simulated diffraction patterns of 2H-SiC along [0001] zone axes (blue spots) and 3C-SiC along [111] zone axes (red spots). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(Fig. 5a) and bamboo shaped (Fig. 5b) nanowires are shown in Fig. 5c and d, respectively. The SAED patterns of the cylindrical and bamboo shaped SiC nanowires parallel to the axial direction were also equivalent in the structure. The central spots were very bright (Fig. 5c and d), while the first hexagonal spots were relatively dark. However, the second hexagonal spots were anomalous bright. Based on our hypothesis that SiC nanowires were composited of the 3C-SiC and 2H-SiC, we calculated the interplanar spacing of 1210 plane in 2H-SiC and
202 plane in 3C-SiC. It has been found that the interplanar spacing of 1210 plane in 2H-SiC (1.540 Å) was very close to that of 202 plane in 3C-SiC (1.541 Å). Eventually, we plotted the diffraction patterns of 2H-SiC along [0001] zone axes (blue spots in Fig. 5e) and 3C-SiC along [111] zone axes (red spots in Fig. 5e). The superimposed patterns of 2HSiC and 3C-SiC were well correlated with the experiment SAED patterns (Fig. 5b and d).
Fig. 6. The high density stacking faults in SiC nanowires. (a) HRTEM micrograph of SiC nanowires with high density stacking faults; (b) The structure schematic diagram of SiC nanowires along axial direction. The red spots were 3C-SiC with ABCABC stacking sequence and the blue spots were 2H-SiC with ABAB stacking sequence, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
R. Dong et al. / Materials Characterization 103 (2015) 37–41
Therefore, the cylindrical and bamboo shaped SiC nanowires have the same structure since no difference in their diffraction patterns was observed. Since the remodeled results (Figs. 4e and 5e) agreed very well with the SAED (Figs. 4c and d, 5b and d) results both in the parallel and perpendicular direction to the axial direction of SiC nanowires, it could be concluded that the SAED patterns were composed of two sets of diffraction patterns, corresponding to 2H-SiC and 3C-SiC, respectively. Therefore, the nominally 3C-SiC nanowires in the present work were the hybrid structure of cubic and hexagonal. The HRTEM image of the representative SiC nanowires in the present work is shown in Fig. 6a. Based on the microstructure observation and analysis result, the structure of SiC nanowires along axial direction was remodeled and its schematic diagram is shown in Fig. 6b. The red spots corresponded to 3C-SiC with ABCABC stacking sequence, while the blue spots were 2H-SiC with ABAB stacking sequence, respectively. Due to the low stacking faults energy of SiC, formation of Shockley partial dislocations, which lead to change the stacking sequence, could easily be occurred in the SiC nanowires. Therefore, high density stacking faults and nanotwins could be generated during the preparation of the cylindrical and bamboo shaped SiC nanowires. Therefore, the SiC nanowires contain a large number of small fragments that are formed by hybrid 3C-SiC and 2H-SiC structures. It is usually considered that the defective structures, such as stacking faults and twins, would be the weakest place in the materials. However, Cheng et al. [24] reported recently that the highly defective structures might not be the weakest type of structures in SiC nanowires. The failure of SiC nanowires firstly occurred in the 3C structures with inclined stacking faults rather than in the highly defective structures. Therefore, it is suggested that the hybrid structure of SiC nanowires also has outstanding mechanical properties. Moreover, the hybrid structure of SiC nanowires might be more suitable to be used as reinforcement since their highly defective structures could better promote their interlocking with metal matrix. 4. Conclusions In the present work, the comprehensive characterization of the microstructure of the SiC nanowires was performed. SiC nanowires were embedded in the Al matrix to investigate their cross-section microstructure. It has been found that the morphology of the SiC nanowires was consisted of cylindrical type with smooth surface and bamboo shape. Diffraction peaks of cubic structure (3C-SiC) phase and hexagonal structure (2H-SiC) phase were detected by XRD analysis. High density stacking faults were found in both the cylindrical and bamboo shaped nanowires perpendicular to their axial direction, and the SAED patterns of the cylindrical and bamboo shaped SiC nanowires perpendicular to the axial direction were equivalent in the structure. The cross-section microstructure of the cylindrical and bamboo shaped SiC nanowires was further observed by TEM in the inclined composite. The SAED patterns of the cylindrical and bamboo shaped SiC nanowires parallel to the axial direction were also equivalent in the structure, while the second hexagonal spots were anomalous bright. After calculation and remodeling, it has been found that the SAED patterns were composed of two sets of diffraction patterns, corresponding to 2H-SiC and 3C-SiC, respectively. Therefore, it could be concluded that the SiC nanowires consisted of a
41
large number of small fragments that are formed by hybrid 3C-SiC and 2H-SiC structures.
References [1] S. Perisanu, V. Gouttenoire, P. Vincent, A. Ayari, M. Choueib, M. Bechelany, D. Cornu, S. Purcell, Mechanical properties of SiC nanowires determined by scanning electron and field emission microscopies, Phys. Rev. B 77 (2008) 165434. [2] T.Y. Kim, S.S. Han, H.M. Lee, Nanomechanical behavior of beta-SiC nanowire in tension: molecular dynamics simulations, Mater. Trans. 45 (2004) 1442–1449. [3] N. Papanikolaou, Lattice thermal conductivity of SiC nanowires, J. Phys. Condens. Matter 20 (2008) 135201. [4] K.M. Lee, T.Y. Choi, S.K. Lee, D. Poulikakos, Focused ion beam-assisted manipulation of single and double beta-SiC nanowires and their thermal conductivity measurements by the four-point-probe 3-omega method, Nanotechnology 21 (2010) 125301. [5] W.L.C.M. Lieber, Nanoelectronics from the bottom up, Nat. Mater. 6 (2007) 841–850. [6] Y. Zhang, X. Han, K. Zheng, Z. Zhang, X. Zhang, J. Fu, Y. Ji, Y. Hao, X. Guo, Z.L. Wang, Direct observation of super-Plasticity of beta-SiC nanowires at low temperature, Adv. Funct. Mater. 17 (2007) 3435–3440. [7] W. Yang, H. Araki, C.C. Tang, S. Thaveethavorn, A. Kohyama, H. Suzuki, T. Noda, Single-crystal SiC nanowires with a thin carbon coating for stronger and tougher ceramic composites, Adv. Mater. 17 (2005) 1519–1523. [8] L.P. Xin, Q. Shi, J.J. Chen, W.H. Tang, N.Y. Wang, Y. Liu, Y.X. Lin, Morphological evolution of one-dimensional SiC nanomaterials controlled by sol–gel carbothermal reduction, Mater. Charact. 65 (2012) 55–61. [9] K. Zekentes, K. Rogdakis, SiC nanowires: material and devices, J. Phys. D. Appl. Phys. 44 (2011) 133001. [10] G. Wei, W. Qin, G. Wang, J. Sun, J. Lin, R. Kim, D. Zhang, K. Zheng, The synthesis and ultraviolet photoluminescence of 6H-SiC nanowires by microwave method, J. Phys. D. Appl. Phys. 41 (2008) 235102. [11] R. Wu, B. Li, M. Gao, J. Chen, Q. Zhu, Y. Pan, Tuning the morphologies of SiC nanowires via the control of growth temperature, and their photoluminescence properties, Nanotechnology 19 (2008) 335602. [12] Y.B. Li, S.S. Xie, W.Y. Zhou, L.J. Ci, Y. Bando, Cone-shaped hexagonal 6H-SiC nanorods, Chem. Phys. Lett. 356 (2002) 325–330. [13] Y.J. Zhang, J. Zhu, Synthesis and characterization of several one-dimensional nanomaterials, Micron 33 (2002) 523–534. [14] D.H. Wang, D. Xu, Q. Wang, Y.J. Hao, G.Q. Jin, X.Y. Guo, K.N. Tu, Periodically twinned SiC nanowires, Nanotechnology 19 (2008) 215602. [15] Y. Ryu, Y. Tak, K. Yong, Direct growth of core–shell SiC–SiO(2) nanowires and field emission characteristics, Nanotechnology 16 (2005) S370–S374. [16] F. Li, G. Wen, A novel method for massive fabrication of β-SiC nanowires, J. Mater. Sci. 42 (2007) 4125–4130. [17] F. Fabbri, F. Rossi, P. Lagonegro, M. Negri, J.S. Ponraj, M. Bosi, G. Attolini, G. Salviati, 3C-SiC nanowires luminescent enhancement by coating with a conformal oxides layer, J. Phys. D. Appl. Phys. 47 (2014) 394006. [18] Y. Liu, J. Men, W. Feng, L. Cheng, L. Zhang, Catalyst-free growth of SiC nanowires in a porous graphite substrate by low pressure chemical vapor infiltration, Ceram. Int. 40 (2014) 11889–11897. [19] Q. Guo, D.L. Sun, L.T. Jiang, G.H. Wu, G.Q. Chen, Residual microstructure and damage geometry associated with high speed impact crater in Al2O3 and TiB2 particles reinforced 2024 Al composite, Mater. Charact. 66 (2012) 9–15. [20] J. Hu, G. Wu, Q. Zhang, P. Kang, Y. Liu, Microstructure of multilayer interface in an Al matrix composite reinforced by TiNi fiber, Micron 64 (2014) 57–65. [21] F.M. Gao, W.Y. Yang, H.T. Wang, Y. Fan, Z.P. Xie, L.A. An, Controlled Al-doped singlecrystalline 6H-SiC nanowires, Cryst. Growth Des. 8 (2008) 1461–1464. [22] M. Pozuelo, W.H. Kao, J.-M. Yang, High-resolution TEM characterization of SiC nanowires as reinforcements in a nanocrystalline Mg-matrix, Mater. Charact. 77 (2013) 81–88. [23] P. Kang, B. Zhang, G. Wu, H. Gou, G. Chen, L. Jiang, S. Mula, Synthesis of β-SiC nanowires by ball milled nanoparticles of silicon and carbon, J. Alloys Compd. 604 (2014) 304–308. [24] G. Cheng, T.H. Chang, Q. Qin, H. Huang, Y. Zhu, Mechanical properties of silicon carbide nanowires: effect of size-dependent defect density, Nano Lett. 14 (2014) 754–758.
Contents lists available at ScienceDirect
Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Microstructure characterization of SiC nanowires as reinforcements in composites Ronghua Dong a, Wenshu Yang a,⁎, Ping Wu b, Murid Hussain c, Ziyang Xiu a, Gaohui Wu a, Pingping Wang a a b c
Department of Material Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Northwest Institute of Nuclear Technology, Xi'an, 710024, China Department of Chemical Engineering, COMSATS Institute of Information Technology, M.A. Jinnah Building, Defence Road, Off Raiwind Road, Lahore 54000, Pakistan
a r t i c l e
i n f o
Article history: Received 10 November 2014 Received in revised form 13 March 2015 Accepted 15 March 2015 Available online 18 March 2015 Keywords: SiC nanowires Crystal structure Cross-section microstructure Hybrid structure
a b s t r a c t SiC nanowires have been rarely investigated or explored along their axial direction by transmission electron microscopy (TEM). Here we report the investigation of the cross-section microstructure of SiC nanowires by embedding them into Al matrix. Morphology of SiC nanowires was cylindrical with smooth surface or bamboo shape. Cubic (3C-SiC) and hexagonal structure (2H-SiC) phases were detected by X-ray diffraction (XRD) analysis. High density stacking faults were observed in both the cylindrical and bamboo shaped nanowires which were perpendicular to their axial direction. Selected area electron diffraction (SAED) patterns of the cylindrical and bamboo shaped SiC nanowires both in the perpendicular and parallel direction to the axial direction were equivalent in the structure. After calculation and remodeling, it has been found that the SAED patterns were composed of two sets of diffraction patterns, corresponding to 2H-SiC and 3C-SiC, respectively. Therefore, it could be concluded that the SiC nanowires are composed of a large number of small fragments that are formed by hybrid 3C-SiC and 2H-SiC structures. © 2015 Elsevier Inc. All rights reserved.
1. Introduction SiC nanowires have received a lot of attention due to their outstanding properties such as high mechanical strength [1,2], high thermal conductivity [3,4], and variable band gaps [5]. Zhang et al. [6] performed qualitative in-situ transmission electron microscopy (TEM) tension tests of SiC nanowires and found substantial plasticity at room temperature (e.g., SiC nanowires experience over 200% elongation before fracture). Yang et al. [7] reported that a small addition of SiC nanowires into a SiC whisker reinforced matrix would double the toughness of the nanocomposite. Despite these very interesting results, the comprehensive characterization of SiC nanowires has been rarely reported. The most common crystal structure of SiC is the cubic ones (3C or β) [8], the three hexagonals (2H, 4H and 6H) and some rhombohedral forms (9R, 15R and 21R). The cubic polytype is the most stable phase at low growth temperatures [9], despite the fact that the 6H-polytype is the most stable structure from the thermodynamic equilibrium point of view. Therefore, 3C-SiC and hexagonal SiC nanowires have been widely investigated in the vast majority of studies. The surface morphology of 3C-SiC and hexagonal SiC nanowires has been observed [10,11], while the morphology of the SiC nanowires has been turned to cylindrical or bamboo shape by altering the reaction temperature [12]. Moreover, high density of defects, which are usually in a periodic ⁎ Corresponding author at: P. O. 3023, Science Park, No. 2 Yikuang Street, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150080, China. E-mail addresses: [email protected], [email protected] (W. Yang).
http://dx.doi.org/10.1016/j.matchar.2015.03.013 1044-5803/© 2015 Elsevier Inc. All rights reserved.
fashion along the nanowire length due to its low stacking fault energy, is considered as one of the important reasons to form the zigzag appearance of the contoured surface of SiC nanowires [13]. Wang et al. [14] explained the growth mechanism of bamboo shaped 3C-SiC. The periodic twins in the SiC nanowires, which formed the zigzag appearance, were due to a minimum surface energy and strain energy. It is very interesting that indexing the XRD spectra to define the polytype of the SiC nanowires is not straightforward in many cases. Ryu et al. [15] and Li et al. [16] reported the XRD spectra of 3C-SiC nanowires and observed very weak diffraction peak at 34.2°, which was assigned to the [1011] plane of hexagonal SiC [10]. Zekentes et al. [9] reported that the diffraction peak at 34.2° might be due to the impurities of hexagonal SiC in 3C-SiC nanowires. However, Ryu et al. [15] proposed that the diffraction peak at 34.2° might be due to the high number of stacking faults (equivalent to hexagonal inclusions) in 3C-SiC nanowires. However, this interesting phenomenon has not been fully understood yet. Moreover, almost all the investigations [17,18] of microstructure of SiC nanowires are perpendicular to their axial direction. To the best of authors' knowledge, the characterization of the cross-section microstructure of SiC nanowires, which could be useful for the comprehensive understanding of the structure of SiC nanowires, has been rarely reported yet. It is very difficult to observe the cross-section microstructure of SiC nanowire in normal. However, it is well known that embedding the reinforcement in metal matrix is an effective way to observe their microstructure in detail [19,20]. Therefore, in the present work, in order to investigate the cross-section microstructure of SiC nanowires, the SiC nanowires were introduced into Al matrix composite. The comprehensive
38
R. Dong et al. / Materials Characterization 103 (2015) 37–41
characterization of the microstructure of SiC nanowires was performed. It has been found that the morphology of SiC nanowires was composed of cylindrical with smooth surface and bamboo shapes. These two kinds of SiC nanowires have high density of stacking faults and nano-twins, leading towards the hybrid structure of cubic and hexagonal SiC, and correspond to the diffraction peak at 34.2°. Based on the microstructure observation results, the structure of SiC nanowires was remodeled. 2. Materials and methods SiC nanowires (3C-SiC, 99% in purity, Changsha Sinet Advanced Materials Co., Ltd. China) were dispersed firstly by ultrasonic in a gluing solution that was composed of water, ethanol and polyvinyl alcohol (PVA) in weight proportion of 10:10:1, and an unconsolidated SiC nanowire preform was obtained after drying. In the next step, the preform was put into a mold and further pressed to the set volume content (15 vol.%). The preheating temperatures for the preform and pressure infiltration dies were 500 and 770 °C, respectively. During the infiltration process, a pressure of 5 MPa was applied and maintained for 10 min, followed by the solidification of the composite in air. Before microstructure observation, 340 °C/2 h annealing treatment was performed to all the SiCnw/Al specimens. Morphology of SiC nanowires was observed by an FEI Sirion Quanta 200 scanning electron microscope (SEM). Microstructure observation of the SiCnw/6061Al composite was performed on JEM-2010F high resolution transmission electron microscopy (HRTEM). XRD analysis was carried out by using a Rigaku D/max-rB diffractometer. The specimens were subjected to Cu-Kα radiation (0.15418 nm) with a scanning speed set at 2°/min, while 2θ scans were performed between 25 and 90°. 3. Results and discussion Fig. 1 shows the morphology of SiC nanowires observed by SEM analysis. The distribution range of length and diameter of SiC nanowires was from 10 to 50 μm (Fig. 1a) and 100 to 500 nm (Fig. 1b), respectively. Furthermore, two kinds of SiC phases were observed in the raw nanowires, which showed cylindrical with smooth surface (pointed out by green arrow in Fig. 1b) and bamboo shape (shown by yellow arrows in Fig. 1b), respectively. This phenomenon has been widely observed in SiC nanowires [11]. XRD analysis result of SiC nanowires is shown in Fig. 2, and diffraction peaks at 33.7°, 35.7°, 41.4° 60.0°, 71.8° and 75.6° were found. Compared to the standard powder XRD spectra (JCPDS cards), it is clear that the peaks at 35.7°, 41.4° 60.0°, 71.8° and 75.6° could be well matched with the diffraction peaks of (111), (200), (220), (311) and (222) planes
Fig. 2. XRD analysis results of SiC nanowires.
of 3C-SiC [15]. However, the peak at 33.7° could be related to the hexagonal SiC. Usually, diffraction peaks of hexagonal SiC could be found in 3C-SiC nanowires [21]. It might be intuitively thought that the raw SiC nanowires were composed of 2H-SiC nanowires and 3C-SiC while the 2H-SiC could be considered as the impurity in 3C-SiC nanowires. However, Zekentes and Rogdakis proposed that the multiple XRD peaks could be a textured structure of the nanowires themselves or the existence of single-crystalline nanowires with different orientations [9]. Unfortunately, no detailed information and structure of the SiC nanowire have been provided. Our present investigation indicated that both the 3C-SiC and 2H-SiC could be found in single SiC nanowires, and the 2HSiC was formed due to the high density stacking faults in 3C-SiC nanowires. It will be explained with detail in the following discussion. Microstructure of SiCnw/Al composite was also observed by TEM analysis, as shown in Fig. 3. It is shown that SiC nanowires were uniformly distributed in Al matrix and no significant interfacial products were observed, implying that the preparation process of the composites caused slight damage to the SiC nanowires. The TEM microstructures of cylindrical and bamboo shaped SiC nanowires in the composite, which were perpendicular to their axial direction, are shown in Fig. 4a and b, respectively. It is obvious that high density stacking faults were found in both the cylindrical (Fig. 4a) and bamboo shaped (Fig. 4b) nanowires. SAED patterns of cylindrical (Fig. 4a) and bamboo shaped (Fig. 4b) nanowires are shown in Fig. 4c and d, respectively. Regardless of the morphology, 3C-SiC usually consists of three types of structures: pure face-centered cubic (3C) structure, 3C structure with inclined stacking faults, and high density
Fig. 1. Representative morphology of SiC nanowires used in the present work. (a) Morphology; (b) Magnification of nanowires. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
R. Dong et al. / Materials Characterization 103 (2015) 37–41
39
Fig. 3. Microstructure of SiCnw/Al composite. (a) Cylindrical SiC nanowires in the composite; (b) Bamboo shaped SiC nanowires in the composite.
Fig. 4. The microstructure characterization of SiC nanowires perpendicular to their axial direction. (a) Cylindrical SiC nanowires; (b) Bamboo shaped SiC nanowires; (c) and (d) Selected h i area electron diffraction patterns of cylindrical (a) and bamboo shaped (b) nanowires, respectively. (e) The superposition of the simulated diffraction patterns of 2H-SiC along 2110 zone axes (blue spots) and 3C-SiC along [011] zone axes (red spots). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
stacking faults perpendicular to the nanowire axis. In the present work, the SAED patterns of the cylindrical and bamboo shaped SiC nanowires perpendicular to the axial direction were equivalent in the structure, and their SAED patterns of nanowires were composed of two sets of spots, and the streaked diffraction spots were formed due to the high density of planar defects. Similar results have also been found by Pozuelo et al. [22] and Kang et al. [23]. We calculated the interplanar spacing of (0002) plane in 2H-SiC and (111) plane in 3C-SiC, which are the close-packed planes in their crystal structures. It has been found that the interplanar spacing of (0002) plane in 2H-SiC (2.518 Å) was very close to that of (111) plane in 3C-SiC (2.517 Å). Therefore, based on Zekentes and Rogdakis suggestion [9], it is hypothesized that the SiC nanowires were composited of the 3C-SiC and 2H-SiC, and h i then the simulated diffraction patterns of 2H-SiC along 2110 zone
axes (blue spots in Fig. 4e) and 3C-SiC along [011] zone axes (red spots in Fig. 4e) were superimposed. The superimposed patterns were agreed very well with the experimental SAED patterns (Fig. 4c and d), indicating the feasibility of our hypothesis that SiC nanowires were comprised of a composite of 3C-SiC and 2H-SiC. In order to further confirm our hypothesis, the cross-section microstructure of the cylindrical (Fig. 5a) and bamboo shaped (Fig. 5b) SiC nanowires, which has been rarely reported in literatures, was observed by the TEM analysis. The cross-section morphology of cylindrical (Fig. 5a) and bamboo shaped (Fig. 5b) SiC was quasi-circular with fuzzy hexagonal shape and standard hexagonal shape, respectively. Generally, the SiC nanowires with hexagonal shape are corresponding to the hexagonal structure [11]. However, our present work indicated that the morphology of SiC nanowires was not mandatory related to its crystal structure. Furthermore, SAED patterns of the cylindrical
40
R. Dong et al. / Materials Characterization 103 (2015) 37–41
Fig. 5. The cross-section microstructure characterization of SiC nanowires. (a) Cylindrical type SiC nanowires; (c) Bamboo shaped SiC nanowires; (b) and (d) Selected area electron diffraction patterns of cylindrical (a) and bamboo shaped (c) nanowires, respectively. (e) The superposition of the simulated diffraction patterns of 2H-SiC along [0001] zone axes (blue spots) and 3C-SiC along [111] zone axes (red spots). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(Fig. 5a) and bamboo shaped (Fig. 5b) nanowires are shown in Fig. 5c and d, respectively. The SAED patterns of the cylindrical and bamboo shaped SiC nanowires parallel to the axial direction were also equivalent in the structure. The central spots were very bright (Fig. 5c and d), while the first hexagonal spots were relatively dark. However, the second hexagonal spots were anomalous bright. Based on our hypothesis that SiC nanowires were composited of the 3C-SiC and 2H-SiC, we calculated the interplanar spacing of 1210 plane in 2H-SiC and
202 plane in 3C-SiC. It has been found that the interplanar spacing of 1210 plane in 2H-SiC (1.540 Å) was very close to that of 202 plane in 3C-SiC (1.541 Å). Eventually, we plotted the diffraction patterns of 2H-SiC along [0001] zone axes (blue spots in Fig. 5e) and 3C-SiC along [111] zone axes (red spots in Fig. 5e). The superimposed patterns of 2HSiC and 3C-SiC were well correlated with the experiment SAED patterns (Fig. 5b and d).
Fig. 6. The high density stacking faults in SiC nanowires. (a) HRTEM micrograph of SiC nanowires with high density stacking faults; (b) The structure schematic diagram of SiC nanowires along axial direction. The red spots were 3C-SiC with ABCABC stacking sequence and the blue spots were 2H-SiC with ABAB stacking sequence, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
R. Dong et al. / Materials Characterization 103 (2015) 37–41
Therefore, the cylindrical and bamboo shaped SiC nanowires have the same structure since no difference in their diffraction patterns was observed. Since the remodeled results (Figs. 4e and 5e) agreed very well with the SAED (Figs. 4c and d, 5b and d) results both in the parallel and perpendicular direction to the axial direction of SiC nanowires, it could be concluded that the SAED patterns were composed of two sets of diffraction patterns, corresponding to 2H-SiC and 3C-SiC, respectively. Therefore, the nominally 3C-SiC nanowires in the present work were the hybrid structure of cubic and hexagonal. The HRTEM image of the representative SiC nanowires in the present work is shown in Fig. 6a. Based on the microstructure observation and analysis result, the structure of SiC nanowires along axial direction was remodeled and its schematic diagram is shown in Fig. 6b. The red spots corresponded to 3C-SiC with ABCABC stacking sequence, while the blue spots were 2H-SiC with ABAB stacking sequence, respectively. Due to the low stacking faults energy of SiC, formation of Shockley partial dislocations, which lead to change the stacking sequence, could easily be occurred in the SiC nanowires. Therefore, high density stacking faults and nanotwins could be generated during the preparation of the cylindrical and bamboo shaped SiC nanowires. Therefore, the SiC nanowires contain a large number of small fragments that are formed by hybrid 3C-SiC and 2H-SiC structures. It is usually considered that the defective structures, such as stacking faults and twins, would be the weakest place in the materials. However, Cheng et al. [24] reported recently that the highly defective structures might not be the weakest type of structures in SiC nanowires. The failure of SiC nanowires firstly occurred in the 3C structures with inclined stacking faults rather than in the highly defective structures. Therefore, it is suggested that the hybrid structure of SiC nanowires also has outstanding mechanical properties. Moreover, the hybrid structure of SiC nanowires might be more suitable to be used as reinforcement since their highly defective structures could better promote their interlocking with metal matrix. 4. Conclusions In the present work, the comprehensive characterization of the microstructure of the SiC nanowires was performed. SiC nanowires were embedded in the Al matrix to investigate their cross-section microstructure. It has been found that the morphology of the SiC nanowires was consisted of cylindrical type with smooth surface and bamboo shape. Diffraction peaks of cubic structure (3C-SiC) phase and hexagonal structure (2H-SiC) phase were detected by XRD analysis. High density stacking faults were found in both the cylindrical and bamboo shaped nanowires perpendicular to their axial direction, and the SAED patterns of the cylindrical and bamboo shaped SiC nanowires perpendicular to the axial direction were equivalent in the structure. The cross-section microstructure of the cylindrical and bamboo shaped SiC nanowires was further observed by TEM in the inclined composite. The SAED patterns of the cylindrical and bamboo shaped SiC nanowires parallel to the axial direction were also equivalent in the structure, while the second hexagonal spots were anomalous bright. After calculation and remodeling, it has been found that the SAED patterns were composed of two sets of diffraction patterns, corresponding to 2H-SiC and 3C-SiC, respectively. Therefore, it could be concluded that the SiC nanowires consisted of a
41
large number of small fragments that are formed by hybrid 3C-SiC and 2H-SiC structures.
References [1] S. Perisanu, V. Gouttenoire, P. Vincent, A. Ayari, M. Choueib, M. Bechelany, D. Cornu, S. Purcell, Mechanical properties of SiC nanowires determined by scanning electron and field emission microscopies, Phys. Rev. B 77 (2008) 165434. [2] T.Y. Kim, S.S. Han, H.M. Lee, Nanomechanical behavior of beta-SiC nanowire in tension: molecular dynamics simulations, Mater. Trans. 45 (2004) 1442–1449. [3] N. Papanikolaou, Lattice thermal conductivity of SiC nanowires, J. Phys. Condens. Matter 20 (2008) 135201. [4] K.M. Lee, T.Y. Choi, S.K. Lee, D. Poulikakos, Focused ion beam-assisted manipulation of single and double beta-SiC nanowires and their thermal conductivity measurements by the four-point-probe 3-omega method, Nanotechnology 21 (2010) 125301. [5] W.L.C.M. Lieber, Nanoelectronics from the bottom up, Nat. Mater. 6 (2007) 841–850. [6] Y. Zhang, X. Han, K. Zheng, Z. Zhang, X. Zhang, J. Fu, Y. Ji, Y. Hao, X. Guo, Z.L. Wang, Direct observation of super-Plasticity of beta-SiC nanowires at low temperature, Adv. Funct. Mater. 17 (2007) 3435–3440. [7] W. Yang, H. Araki, C.C. Tang, S. Thaveethavorn, A. Kohyama, H. Suzuki, T. Noda, Single-crystal SiC nanowires with a thin carbon coating for stronger and tougher ceramic composites, Adv. Mater. 17 (2005) 1519–1523. [8] L.P. Xin, Q. Shi, J.J. Chen, W.H. Tang, N.Y. Wang, Y. Liu, Y.X. Lin, Morphological evolution of one-dimensional SiC nanomaterials controlled by sol–gel carbothermal reduction, Mater. Charact. 65 (2012) 55–61. [9] K. Zekentes, K. Rogdakis, SiC nanowires: material and devices, J. Phys. D. Appl. Phys. 44 (2011) 133001. [10] G. Wei, W. Qin, G. Wang, J. Sun, J. Lin, R. Kim, D. Zhang, K. Zheng, The synthesis and ultraviolet photoluminescence of 6H-SiC nanowires by microwave method, J. Phys. D. Appl. Phys. 41 (2008) 235102. [11] R. Wu, B. Li, M. Gao, J. Chen, Q. Zhu, Y. Pan, Tuning the morphologies of SiC nanowires via the control of growth temperature, and their photoluminescence properties, Nanotechnology 19 (2008) 335602. [12] Y.B. Li, S.S. Xie, W.Y. Zhou, L.J. Ci, Y. Bando, Cone-shaped hexagonal 6H-SiC nanorods, Chem. Phys. Lett. 356 (2002) 325–330. [13] Y.J. Zhang, J. Zhu, Synthesis and characterization of several one-dimensional nanomaterials, Micron 33 (2002) 523–534. [14] D.H. Wang, D. Xu, Q. Wang, Y.J. Hao, G.Q. Jin, X.Y. Guo, K.N. Tu, Periodically twinned SiC nanowires, Nanotechnology 19 (2008) 215602. [15] Y. Ryu, Y. Tak, K. Yong, Direct growth of core–shell SiC–SiO(2) nanowires and field emission characteristics, Nanotechnology 16 (2005) S370–S374. [16] F. Li, G. Wen, A novel method for massive fabrication of β-SiC nanowires, J. Mater. Sci. 42 (2007) 4125–4130. [17] F. Fabbri, F. Rossi, P. Lagonegro, M. Negri, J.S. Ponraj, M. Bosi, G. Attolini, G. Salviati, 3C-SiC nanowires luminescent enhancement by coating with a conformal oxides layer, J. Phys. D. Appl. Phys. 47 (2014) 394006. [18] Y. Liu, J. Men, W. Feng, L. Cheng, L. Zhang, Catalyst-free growth of SiC nanowires in a porous graphite substrate by low pressure chemical vapor infiltration, Ceram. Int. 40 (2014) 11889–11897. [19] Q. Guo, D.L. Sun, L.T. Jiang, G.H. Wu, G.Q. Chen, Residual microstructure and damage geometry associated with high speed impact crater in Al2O3 and TiB2 particles reinforced 2024 Al composite, Mater. Charact. 66 (2012) 9–15. [20] J. Hu, G. Wu, Q. Zhang, P. Kang, Y. Liu, Microstructure of multilayer interface in an Al matrix composite reinforced by TiNi fiber, Micron 64 (2014) 57–65. [21] F.M. Gao, W.Y. Yang, H.T. Wang, Y. Fan, Z.P. Xie, L.A. An, Controlled Al-doped singlecrystalline 6H-SiC nanowires, Cryst. Growth Des. 8 (2008) 1461–1464. [22] M. Pozuelo, W.H. Kao, J.-M. Yang, High-resolution TEM characterization of SiC nanowires as reinforcements in a nanocrystalline Mg-matrix, Mater. Charact. 77 (2013) 81–88. [23] P. Kang, B. Zhang, G. Wu, H. Gou, G. Chen, L. Jiang, S. Mula, Synthesis of β-SiC nanowires by ball milled nanoparticles of silicon and carbon, J. Alloys Compd. 604 (2014) 304–308. [24] G. Cheng, T.H. Chang, Q. Qin, H. Huang, Y. Zhu, Mechanical properties of silicon carbide nanowires: effect of size-dependent defect density, Nano Lett. 14 (2014) 754–758.