《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_Long b-Silicon Carbide Necklace-Like Whiskers Prepared by Carbothermal Reduction of Wood Flour/Silica/Phenolic Composite

J.Am. Ceran.Soc.9303499-3503(2010) C 2010 The American Ceramic Society urna Long B-Silicon Carbide Necklace-Like Whiskers Prepared by Carbothermal Reduction of Wood Flour /Silica/Phenolic Composite Zhong Li, Tie-Jun Shi, . and De-Xin Ta sChool of Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009,China 'School of Chemical Engineering, Anhui University of Science Technology, Huainan, Anhui 232001,China Long silicon carbide (SiC)microwhiskers with a necklace-like necklace-like Sic whiskers is also proposed based on the char morphology have been successfully synthesized by a carbothermal acterization results reduction process without using any catalyst. In the process, the wood flour/silica (SiO2)/phenolic composite was chosen as both silicon and carbon sources. The morphology and structure were II. Experimental Procedure vestigated by X-ray diffraction(XRD), Fourier transform in- rared spectroscopy(FT-IR), field-emission scanning electron mi- The synthesis of necklace-like B-Sic microwhiskers includes the croscopy(FESEM), and high-resolution transmission electron preparation of wood flour/SiO2/phenolic composite and subse- microscopy (HRTEM). Studies found that the as-synthesized nt carbothermal reduction of the direction with the length up to hundreds of micrometers. The described as follows. First, the Sio2 precursor solution wae whiskers were grown as single crystalline p-sic along the (111) d flour/SiO2/phe omposite preparation is briefl whiskers consisted of p-SiC strings with diameters of 1-2 um and pared from tetraethoxysilane/ethanol/H20(molar ratio=1: 4: 4) periodic B-Sic beads with diameters of 3-5 um. On the basis of Then 6 g of wood flour( fir flour, average size of 200 um,Moun- characterization results, a growth mechanism is proposed to clar ify the formation of necklace-like whiskers. precursor solution at 150C for 48 h in a self-made sealed infil- tration vessel, subsequently removed and dried in air at 130C overnight to prepare wood fiour /SiO composite(SiO, content of 48 wt%). Finally, 10 g of wood flour/SiO, composite im- L Introduction pregnated with 20 g of phenolic resin(solid content of 50%) for 12 h was dried and precured at 80C for 6 h, and further O ER the last decades, silicon carbide (SiC) has attracted ured at 130%-150oC for 4 h to obtain wood flour/SiO2/phenolic ch interest from both fundamental and practical view- composite points. It has been shown that Sic possesses unique physi The carbothermal reduction was carried out in a l promising candidate for reinforcing additives field-emission de- 500mm x 100mm x 80 mm(5g, 24 wt% SiO2, 26 wt%wood ties of Sic-based materials are determined by the architecture, middle of the furnace. Then, the furnace was heated up to riginal crystals .Many 1550.C at 2 C/min in a flowing ultra-high purity N2 atmosphere Dds, iai SiC whiskers with various shapes. at a rate of 3L/h and held there for 2 h. When the furnace wa and cooled to room temperature naturally, a large number of light spheres, 8, 19 have been reported, including laser ablation or green as-synthesized products were obtained c-discharge process, chemical vapor deposition, carbothermal The crystalline structures of as-synthesized products were an reduction, carbon(C) nanotube-converted reaction, and poly lyzed by XRD using CuKo radiation(Rigaku, D/MAX-rB meric precursor pyrolysis method. In terms of efficiency and Tokyo, Japan). The possible chemical composition was investi- economy, the carbothermal reduction process is considered to gated using Fourier tran orm infrared spectroscopy(FT-IR, be a promising method for the synthesis of SiC materials be- PerkinElmer, Spectrum 100, Waltham, MA)and FESEM (Sir- cause of the inexpensiveness and its simplicity. ion 200, FEl, Holland), equipped with an energy-dispersive X In this work, we report the necklace-like single crystallin ray spectrometer(EDS). The samples for FESEM were pre- B-SiC microwhiskers self-assembled through the carbothermal f Au for I min at 10 k reduction process without any templates and catalysts. In the FESEM, HRTEM (HRTEM, JEOL 2010, Japan) at an acce process, the wood flour/silica (SiO2)/phenolic composite was ration voltage of 200 kv were carried out to observe the mor. chosen for use as both C and SiO, sources. The conventional phology and the crystal lattice, respectively. techniques including field-emission scanning electron micros- copy(FESEM), X-ray diffraction(XRD), high-resolution trans- mission electron mic opy EM). and selected area II. Results and discussion electron diffraction(SAED) were used to investigate rphologies and crystal structures. The growth mechanism of the (1) Phase, Morphology, and Structural characterization The XRD result of the light-green products is displayed in T. Ohjcontributing editor ig. 1. As can be seen from the pattern, the diffraction peak an be indexed as the(111).(200), (220), and (311)reflections of cubic B-siC. the lattice parameter of the p-siC cubic cell cal- culated from the Xrd data is 0. 4353 nm, which is in good agreement with a=0.4359 nm(JCPDS card no. 29-1129. These Natural Science Fo on of China. under 017.50973024 sharp diffraction peaks indicate that the products are highly Author to whom correspondence should be addressed. e-mail: stjhfut@le rystalline. The additional diffraction peak is detected at 3499

Long b-Silicon Carbide Necklace-Like Whiskers Prepared by Carbothermal Reduction of Wood Flour/Silica/Phenolic Composite Zhong Li,z,y Tie-Jun Shi,w,z and De-Xin Tanz,y z School of Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, China y School of Chemical Engineering, Anhui University of Science & Technology, Huainan, Anhui 232001, China Long silicon carbide (SiC) microwhiskers with a necklace-like morphology have been successfully synthesized by a carbothermal reduction process without using any catalyst. In the process, the wood flour/silica (SiO2)/phenolic composite was chosen as both silicon and carbon sources. The morphology and structure were investigated by X-ray diffraction (XRD), Fourier transform in￾frared spectroscopy (FT-IR), field-emission scanning electron mi￾croscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM). Studies found that the as-synthesized whiskers were grown as single crystalline b-SiC along the (111) direction with the length up to hundreds of micrometers. The whiskers consisted of b-SiC strings with diameters of 1–2 lm and periodic b-SiC beads with diameters of 3–5 lm. On the basis of characterization results, a growth mechanism is proposed to clar￾ify the formation of necklace-like whiskers. I. Introduction OVER the last decades, silicon carbide (SiC) has attracted much interest from both fundamental and practical view￾points.1–4 It has been shown that SiC possesses unique physical, chemical, mechanical, and electronic properties, making it a very promising candidate for reinforcing additives, field-emission de￾vices, and electronic devices.5–7 Generally, the intrinsic proper￾ties of SiC-based materials are determined by the architecture, size, morphology, and crystallinity of original crystals.8,9 Many synthesis routes for growing SiC whiskers with various shapes, such as wires,10–12 rods,13,14 belts,15,16 tubes,17, springs, and spheres,18,19 have been reported, including laser ablation or arc-discharge process, chemical vapor deposition, carbothermal reduction, carbon (C) nanotube-converted reaction, and poly￾meric precursor pyrolysis method.20–25 In terms of efficiency and economy, the carbothermal reduction process is considered to be a promising method for the synthesis of SiC materials be￾cause of the inexpensiveness and its simplicity.26,27 In this work, we report the necklace-like single crystalline b-SiC microwhiskers self-assembled through the carbothermal reduction process without any templates and catalysts. In the process, the wood flour/silica (SiO2)/phenolic composite was chosen for use as both C and SiO2 sources. The conventional techniques including field-emission scanning electron micros￾copy (FESEM), X-ray diffraction (XRD), high-resolution trans￾mission electron microscopy (HRTEM), and selected area electron diffraction (SAED) were used to investigate the mo￾rphologies and crystal structures. The growth mechanism of the necklace-like SiC whiskers is also proposed based on the char￾acterization results. II. Experimental Procedure The synthesis of necklace-like b-SiC microwhiskers includes the preparation of wood flour/SiO2/phenolic composite and subse￾quent carbothermal reduction of the composite. Wood flour/SiO2/phenolic composite preparation is briefly described as follows. First, the SiO2 precursor solution was pre￾pared from tetraethoxysilane/ethanol/H2O (molar ratio 5 1:4:4). Then 6 g of wood flour (fir flour, average size of 200 mm, Moun￾tain Huangshan, Anhui, China) was infiltrated in 50 mL of SiO2 precursor solution at 1501C for 48 h in a self-made sealed infil￾tration vessel, subsequently removed and dried in air at 1301C overnight to prepare wood flour/SiO2 composite (SiO2 content of 48 wt%). Finally, 10 g of wood flour/SiO2 composite im￾pregnated with 20 g of phenolic resin (solid content of 50%) for 12 h was dried and precured at 801C for 6 h, and further cured at 1301–1501C for 4 h to obtain wood flour/SiO2/phenolic composite. The carbothermal reduction was carried out in a horizontally tubular furnace. The wood flour/SiO2/phenolic composite with 500mm 100mm 80 mm (5 g, 24 wt% SiO2, 26 wt% wood flour, and 50 wt% phenolic) in an alumina boat was sent to the middle of the furnace. Then, the furnace was heated up to 15501C at 21C/min in a flowing ultra-high purity N2 atmosphere at a rate of B3 L/h and held there for 2 h. When the furnace was cooled to room temperature naturally, a large number of light￾green as-synthesized products were obtained. The crystalline structures of as-synthesized products were an￾alyzed by XRD using CuKa radiation (Rigaku, D/MAX-rB, Tokyo, Japan). The possible chemical composition was investi￾gated using Fourier transform infrared spectroscopy (FT-IR, PerkinElmer, Spectrum 100, Waltham, MA) and FESEM (Sir￾ion 200, FEI, Holland), equipped with an energy-dispersive X￾ray spectrometer (EDS). The samples for FESEM were pre￾sputtered with a conducting layer of Au for 1 min at 10 kV. FESEM, HRTEM (HRTEM, JEOL 2010, Japan) at an accel￾eration voltage of 200 kV were carried out to observe the mor￾phology and the crystal lattice, respectively. III. Results and Discussion (1) Phase, Morphology, and Structural Characterization The XRD result of the light-green products is displayed in Fig. 1. As can be seen from the pattern, the diffraction peak can be indexed as the (111), (200), (220), and (311) reflections of cubic b-SiC. The lattice parameter of the b-SiC cubic cell cal￾culated from the XRD data is 0.4353 nm, which is in good agreement with a 5 0.4359 nm (JCPDS card no. 29-1129). These sharp diffraction peaks indicate that the products are highly crystalline. The additional diffraction peak is detected at T. Ohji—contributing editor This work was supported by the National Natural Science Foundation of China, under Grant No. 50773017, 50973024. w Author to whom correspondence should be addressed. e-mail: stjhfut@163.com Manuscript No. 27255. Received December 15, 2009; approved May 5, 2010. Journal J. Am. Ceram. Soc., 93 [10] 3499–3503 (2010) DOI: 10.1111/j.1551-2916.2010.03911.x r 2010 The American Ceramic Society 3499

3500 Journal of the American Ceramic Society--Li et al. Vol 93. No. 10 can be indexed based on B-SiC crystal, and along the [lll]zone defects exist within the whiskers because of the irregular depo- sition of silicon and C atom lavers.3 11) 1000 HRTEM lattice images of a single Sic bead and the string of SiC whisker (recorded from the encircled domains in Fig. 3(a)) are depicted in Figs. 4(c)and(d), respectively. The crystalline lattices show a periodic lattice structure and the spaces between wo adjacent lattice fringes are 0.25 nm for both the Sic bead and the core string, thus corresponding well to the [lll] planes acking of B-Sic.32 Stacking faults are also observed in the bead and string. The surface energy of the [lll] planes in the B-sic is 00) much smaller than those of the other crystal planes. Therefore B-SiC whisker can grow easily in the(lll) direction. To decrease the formation energy, stacking faults can be inserted easily in the [111] planes.33-35 (2) Growth Mechanism theta(degree) To study the growth mechanism of the necklace-like structured Fig1.X-ray diffraction pattern indicating that the samples is pure p-Sic microwhiskers, the products obtained under the reaction silicon carbide with stacking faults over 30, 60, 120 min were checked. Figure 5 shows the corre- sponding FESEM images Under the condition that the reaction planes in cubic i ts characterist c of stacking faults on the[Ill] temperature is 1550 C for 30 min, the strings obtained with di- 20=33.7°,whi whiskers. --Moreover, the stronger (Ill) ameters of about I um are shown in Fig. 5(a). It is worth noting diffraction peak indicates that the [lll] is the dominant growth that their surface is not smooth, which provides an energy direction of the sic whiskers avored site for the initial nucleation of B-sic and adds the The FT-IR spectrum of the products is shown in Fig. 2. A growth of the beads by absorbing gas-phase C atoms and silicor strong absorption band at 790 cm is ascribed to the C-si toms. After reacting for 60 min some strings are decorate stretching vibration mode in the SiC crystalline phase,30 which is with complete and incomplete beads, as indicated with an arrow consistent with XRD result FESEM examination was conducted to investigate the mor- Fig. 5(c). Previous studies demonstrated that the si whisker phology characteristics of as-synthesized SiC products. Figure 3 with periodic instability were grown via an extension of the va- shows a set of the images at different magnificat The lo magnification FESEM image shown in Fig 3(a) displays that the alysts The whiskers obtained always carried a droplet most evident characteristic of the products is the unique neck (metal catalyst) at the growth end, which is a typical feature lace-like morphology; the amount of the necklace-like structures for whiskers grown by the VLS mechanism. In our pres ork, no catalyst was used in the growth process and no add amount and purity quotient of the products. As indicated by tional metal droplet was detected on the of the whiskers FESEM images, most of the necklace-like whiskers are straight, The chemical compositions of starting material of wood flour hundreds of micrometers in length. a closer examination(Figs. the Sic strings on the tip of whiskers, and the beads were ex 3(b),(c), and(d) of the whiskers indicates that the every strin mined using EDS analyses, as shown in Fig. 6. In all the eDs vith 1-2 um in diameter is regularly decorated with numerous pectra, Au element coming from the conducting layer of Au in equal-sized beads. Uniform beads have 3-5 um diameter the FESEm characterization was detected. The EDs material The morphological details of the necklace-like SiC whiskers of wood flour(Fig. 6(a)) revealed that the starting material re highlighted in TEM images(Fig. 4). Figure 4(a) shows a typical TEM image of individual necklace-like whisker with lement hydrogen cannot be identified). EDS analysis of strings beads indicated that the synthesized whiskers were com- structure. The corresponding SAED pattern shown in Fig. 4(b) is close to 1: I(Figs. 6(b)and(c)). It is observed that no signal of 34 the impurity( Ca)could be detected on the whiskers Based on the above-mentioned experimental results, we pro- 32 osed a possible growth mechanism for the development of necklace-like microwhiskers, which is schematically illustrated in ig.7. 28 Several investigators have studied the reaction kinetics of the 26 carbothermal reduction of Sio, and C to SiC38-I SiO2(s)+3C(s)-SiC(s)+2CO(g (1) The generally accepted mechanism for the overall reaction ( s a mul ep process that involves the formation of volatile Sio gas and its subsequent reduction to Sic Sio2(s)+C(s)- Sio(g)+co(g) SiO2(s)+Co(g)- Sio(g)+CO2(g) Wavenumbers(cm CO2(g)+C(s)→2COg) thesized p-silicon carbide t Infrared spectroscopy spectrum of the syn- Fig. 2. Fourier tra Sio(g)+2C(s)- SiC(s)+Co(g)

2y 5 33.71, which is characteristic of stacking faults on the [111] planes in cubic SiC whiskers.28,29 Moreover, the stronger (111) diffraction peak indicates that the [111] is the dominant growth direction of the SiC whiskers. The FT-IR spectrum of the products is shown in Fig. 2. A strong absorption band at 790 cm1 is ascribed to the C–Si stretching vibration mode in the SiC crystalline phase,30 which is consistent with XRD result. FESEM examination was conducted to investigate the mor￾phology characteristics of as-synthesized SiC products. Figure 3 shows a set of the images at different magnifications. The low￾magnification FESEM image shown in Fig. 3(a) displays that the most evident characteristic of the products is the unique neck￾lace-like morphology; the amount of the necklace-like structures in the sample exceeds 95%, and it indicates the relatively high amount and purity quotient of the products. As indicated by FESEM images, most of the necklace-like whiskers are straight, hundreds of micrometers in length. A closer examination (Figs. 3(b), (c), and (d)) of the whiskers indicates that the every string with 1–2 mm in diameter is regularly decorated with numerous equal-sized beads. Uniform beads have 3–5 mm diameters. The morphological details of the necklace-like SiC whiskers are highlighted in TEM images (Fig. 4). Figure 4(a) shows a typical TEM image of individual necklace-like whisker with three beads, owning a very clean surface and regularly periodic structure. The corresponding SAED pattern shown in Fig. 4(b) can be indexed based on b-SiC crystal, and along the [111] zone axis shows bright spots, as well as streaks, indicating that some defects exist within the whiskers because of the irregular depo￾sition of silicon and C atom layers.31 HRTEM lattice images of a single SiC bead and the string of SiC whisker (recorded from the encircled domains in Fig. 3(a)) are depicted in Figs. 4(c) and (d), respectively. The crystalline lattices show a periodic lattice structure and the spaces between two adjacent lattice fringes are 0.25 nm for both the SiC bead and the core string, thus corresponding well to the [111] planes of b-SiC.32 Stacking faults are also observed in the bead and string. The surface energy of the [111] planes in the b-SiC is much smaller than those of the other crystal planes. Therefore, b-SiC whisker can grow easily in the (111) direction. To decrease the formation energy, stacking faults can be inserted easily in the [111] planes.33–35 (2) Growth Mechanism To study the growth mechanism of the necklace-like structured b-SiC microwhiskers, the products obtained under the reaction over 30, 60, 120 min were checked. Figure 5 shows the corre￾sponding FESEM images. Under the condition that the reaction temperature is 15501C for 30 min, the strings obtained with di￾ameters of about 1 mm are shown in Fig. 5(a). It is worth noting that their surface is not smooth, which provides an energy￾favored site for the initial nucleation of b-SiC and adds the growth of the beads by absorbing gas-phase C atoms and silicon atoms.15 After reacting for 60 min, some strings are decorated with complete and incomplete beads, as indicated with an arrow in Fig. 5(b). After 120 min, necklace-like whiskers are formed (Fig. 5(c)). Previous studies demonstrated that the Si whiskers with periodic instability were grown via an extension of the va￾por–liquid–solid (VLS) mechanism in the presence of metal cat￾alysts.36,37 The whiskers obtained always carried a droplet (metal catalyst) at the growth end, which is a typical feature for whiskers grown by the VLS mechanism. In our present work, no catalyst was used in the growth process and no addi￾tional metal droplet was detected on the tips of the whiskers. The chemical compositions of starting material of wood flour, the SiC strings on the tip of whiskers, and the beads were ex￾amined using EDS analyses, as shown in Fig. 6. In all the EDS spectra, Au element coming from the conducting layer of Au in the FESEM characterization was detected. The EDS spectrum of wood flour (Fig. 6(a)) revealed that the starting material mainly contained C and O with 0.26 at.% impurity, Ca (light element hydrogen cannot be identified). EDS analysis of strings and beads indicated that the synthesized whiskers were com￾posed of Si and C elements and the corresponding atomic ratio is close to 1:1 (Figs. 6(b) and (c)). It is observed that no signal of the impurity (Ca) could be detected on the whiskers. Based on the above-mentioned experimental results, we pro￾posed a possible growth mechanism for the development of necklace-like microwhiskers, which is schematically illustrated in Fig. 7. Several investigators have studied the reaction kinetics of the carbothermal reduction of SiO2 and C to SiC38–41: SiO2ðsÞ þ 3CðsÞ ! SiCðsÞ þ 2COðgÞ (1) The generally accepted mechanism for the overall reaction (1) is a multiple-step process that involves the formation of volatile SiO gas and its subsequent reduction to SiC SiO2ðsÞ þ CðsÞ ! SiOðgÞ þ COðgÞ (2) SiO2ðsÞ þ COðgÞ ! SiOðgÞ þ CO2ðgÞ (3) CO2ðgÞ þ CðsÞ ! 2COðgÞ (4) SiOðgÞ þ 2CðsÞ ! SiCðsÞ þ COðgÞ (5) Fig. 1. X-ray diffraction pattern indicating that the samples is pure b-silicon carbide with stacking faults. Fig. 2. Fourier transform infrared spectroscopy spectrum of the syn￾thesized b-silicon carbide whiskers. 3500 Journal of the American Ceramic Society—Li et al. Vol. 93, No. 10

October 2010 Long B-SiC Necklace-like Whiskers 350l 10 um 20m 2 um Fig 3. (a) Field electron microscopy (FESEM) image B-silicon carbide whiskers.(b),(c), and (d) Higher magnifications FESEM images of B-silicon carbide whiskers. These clearly indicate the necklace-like morphology in which every whisker is self-assembled revealing numerous qual-sized beads that regularly decorate the uniform string Sio(g)+3CO( C(s)+2CO2(g) The Sio gas obtained from reactions(2)and ()reacts with C hrough reactions (5)and(6) In our experiment, at the reaction temperature, the pyrolysis of wood flour/SiO,/phenolic composite yields a large quantity of C atoms and silicon atoms. Initially, the freshly formed C atoms and silicon atoms, which are not stable because of high energy react and create SiC nuclei. Then, the Sic strings along the [lll direction, which has lower energy than those of others in B-Sic, grow fast by absorbing gas-phase C atoms and silicon atoms. To (c (d) long with the consecutive growth of Sic. with the extension of he reaction time. more C atoms and silicon atoms are fed into the gas phase due to a continuous provision from the pyrolysis of wood flour/SiO2/phenolic composite SiC strings are circum- ented by gas-phase C atoms and silicon atoms, which nucleate around the defects on the surface of strings in the nucleation process, as the energy for nucleating SiC around the defects is far lower than on the other places. With an increasing supply of C atoms and silicon atoms diffusing to the nucleation regimes because of the lower growth energy, the spherical SiC beads are gradually created by an epitaxial growth process. The epitaxial orientation relationship is preserved to reduce the lattice mis- match energy. Further asymmetrical growth of the beads forms the necklace-like SiC microwhisker. But the exact growth mech- ism of the periodicity of the beads formation and their almost perfect spherical surface is still unclear, and further work is needed for a full understanding Necklace-like single crystalline B-sic microwhiskers have been Fig 4.(a)Transmission electron microscopy image of the successfully synthesized by a simple carbothermal reduction isker. (b) Corresponding selected area electron diffrac process. Characterizations of the as-svntnesiz zed products have tion pattern. (c) and(d) High-resolution transmission electron micros been carried out and the possible growth mechanism has been copy images recorded from the parts marked in(a), respectively. proposed for the necklace-like structure formation. These micro-

SiOðgÞ þ 3COðgÞ ! SiCðsÞ þ 2CO2ðgÞ (6) The SiO gas obtained from reactions (2) and (3) reacts with C or CO to yield SiC through reactions (5) and (6). In our experiment, at the reaction temperature, the pyrolysis of wood flour/SiO2/phenolic composite yields a large quantity of C atoms and silicon atoms. Initially, the freshly formed C atoms and silicon atoms, which are not stable because of high energy, react and create SiC nuclei. Then, the SiC strings along the [111] direction, which has lower energy than those of others in b-SiC, grow fast by absorbing gas-phase C atoms and silicon atoms. To decrease the formation energy, numerous stacking faults are in￾serted in the {111} planes.31,34 The longer strings can be formed along with the consecutive growth of SiC. With the extension of the reaction time, more C atoms and silicon atoms are fed into the gas phase due to a continuous provision from the pyrolysis of wood flour/SiO2/phenolic composite. SiC strings are circum￾vented by gas-phase C atoms and silicon atoms, which nucleate around the defects on the surface of strings in the nucleation process, as the energy for nucleating SiC around the defects is far lower than on the other places. With an increasing supply of C atoms and silicon atoms diffusing to the nucleation regimes, because of the lower growth energy, the spherical SiC beads are gradually created by an epitaxial growth process. The epitaxial orientation relationship is preserved to reduce the lattice mis￾match energy. Further asymmetrical growth of the beads forms the necklace-like SiC microwhisker. But the exact growth mech￾anism of the periodicity of the beads formation and their almost perfect spherical surface is still unclear, and further work is needed for a full understanding. IV. Conclusions Necklace-like single crystalline b-SiC microwhiskers have been successfully synthesized by a simple carbothermal reduction process. Characterizations of the as-synthesized products have been carried out and the possible growth mechanism has been proposed for the necklace-like structure formation. These micro￾Fig. 3. (a) Field emission scanning electron microscopy (FESEM) image b-silicon carbide whiskers. (b), (c), and (d) Higher magnifications FESEM images of b-silicon carbide whiskers. These clearly indicate the necklace-like morphology in which every whisker is self-assembled revealing numerous equal-sized beads that regularly decorate the uniform string. Fig. 4. (a) Transmission electron microscopy image of the necklace-like silicon carbide whisker. (b) Corresponding selected area electron diffrac￾tion pattern. (c) and (d) High-resolution transmission electron micros￾copy images recorded from the parts marked in (a), respectively. October 2010 Long b-SiC Necklace-like Whiskers 3501

Journal of the American Ceramic Sociery--Li et al. Vol 93. No. 10 Fig. 5. Field emission scanning electron microscopy images of the products obtained at different reaction time:(a)30 min, (b)60 min, and(c)120 min (b) (c) 5638 5143 CK 8 157 345678910 Energy(kev) Energy (kev) Fig. 6. Energy-dispersive X-ray spectrometer spectrum of (a) the starting material of wood flour, (b)the silicon carbide strings on the tip of whisk and(c) the beads. Necklace-like Obtained SiC nucle SiC whisker SiC whisker Fig. 7. Schematic illustration of a necklace-like silicon carbide whisker growth process. whiskers with the length up to hundreds of micrometers are com- References posed of uniform B-SiC oriented in the(111)direction and stack ing faults perpendicular to the [lll plane. The special B Wong. S. 1. L ee. and s. xie. "Oriented silico n Carbide: wang ires. morphology Sic whiskers are expected to build novel electronic devices. The periodic structure in whiskers could be used as -H. Dai, E. w. Wong. Y. Z. Lu, S. Fan, and C. M. Lieber " Synthesis and benchmarks for precisely positioning them in device fabrication The necklace-like structure is considered to enhance the interfacial G. w. Ho. A.S. w. Wong. D. J. Kang and M. E. Welland. "Three-Dimen- adhesion between the matrix and the whiskers, which can improve oftechnology. 15. 996-9(2004). u,J. Zheng. M. Wang. ynthesis and Characterization of the mechanical properties of Sic-reinforced composite materials. Z ca arbide Whiskers. "Carl 92941(2001

whiskers with the length up to hundreds of micrometers are com￾posed of uniform b-SiC oriented in the (111) direction and stack￾ing faults perpendicular to the [111] plane. The special morphology SiC whiskers are expected to build novel electronic devices. The periodic structure in whiskers could be used as benchmarks for precisely positioning them in device fabrication. The necklace-like structure is considered to enhance the interfacial adhesion between the matrix and the whiskers, which can improve the mechanical properties of SiC-reinforced composite materials. References 1 Z. Pan, H. L. Lai, F. C. K. Au, X. Duan, W. Zhou, W. Shi, N. Wang, C. S. Lee, N. B. Wong, S. T. Lee, and S. Xie, ‘‘Oriented Silicon Carbide Nanowires: Syn￾thesis and Field Emission Properties,’’ Adv. Mater., 12, 1186–9 (2000). 2 H. Dai, E. W. Wong, Y. Z. Lu, S. Fan, and C. M. Lieber, ‘‘Synthesis and Characterization of Carbide Nanorods,’’ Nature, 375, 769–72 (1995). 3 G. W. Ho, A. S. W. Wong, D. J. Kang, and M. E. Welland, ‘‘Three-Dimen￾sional Crystalline SiC Nanowire Flowers,’’ Nanotechnology, 15, 996–9 (2004). 4 Z. Ryu, J. Zheng, M. Wang, and B. Zhang, ‘‘Synthesis and Characterization of Silicon Carbide Whiskers,’’ Carbon, 39, 1929–41 (2001). Fig. 5. Field emission scanning electron microscopy images of the products obtained at different reaction time: (a) 30 min, (b) 60 min, and (c) 120 min. Fig. 6. Energy-dispersive X-ray spectrometer spectrum of (a) the starting material of wood flour, (b) the silicon carbide strings on the tip of whiskers, and (c) the beads. Fig. 7. Schematic illustration of a necklace-like silicon carbide whisker growth process. 3502 Journal of the American Ceramic Society—Li et al. Vol. 93, No. 10

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