《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_3SiC whsker1 Stacking faults in silicon carbide whiskers

CERAMICS INTERNATIONAL ELSEVIER Ceramics International 26(2000)7-12 Stacking faults in silicon carbide whiskers Heon Jin choi*. june-Gunn Lee Multifunctional Ceramics Research Center, Korea Institute of Science and Technology, PO Box 131 Cheongryang, Seoul 130-650, South Kore Received 15 September 1998; accepted 30 October 1998 Abstract k. Stacking faults in Sic whiskers grown by three different growth mechanisms; vapor-solid(VS), two-stage growth (TS)and por-liquid-solid (VLs)mechanism in the carbothermal reduction system were investigated by X-ray diffraction(XRD) and transmission electron microscopy (TEM). The content of stacking faults in Sic whiskers increased with decreasing the diameter of whiskers, i.e. the small diameter whiskers(2 um) grown by the VLS mechanism have little stacking faults. Heavy stacking faults of small dia meter whiskers were probably due to the high specific lateral surface area of small diameter whiskers. C 1999 Elsevier Science Ltd d Techna S.r. 1. All rights reserved Keywords: B. Whiskers; D. Silicon carbide; Stacking faults 1. ntroduction in the liquid droplets. The liquid droplets are formed by melting of metallic Sic whiskers are an effective material for the reinfor- ally added. The dissolved silicon and carbon atoms dif- cement of various composite materials, due mainly to fuse across the droplet, and precipitate at the growth eir superb mechanical properties [1-3]. At the present lane as sic whiskers is carbothermal reduction of silica [4, 5]. Sic affect various properties of whiskers [14-16]. The stack time, the preferred synthesis method for the Sic Stacking faults are usually found in Sic whiskers an whiskers are grown by the three different growth ing faults in the vSSCW and TSSCW have been studied mechanisms in carbothermal reduction; vapor-solid well in previous studies [6-9). However, the stacking (S)mechanism [6, 7, two-stage growth (TS) mechan- faults in the vlsSCW have been briefly referred in ism [8,9] and vapor-liquid-solid (VLs)mechanism [10- the literature [6, 11, 12, 17]. In this study, the VsSCw, 13,(hereafter, the grown whiskers are designated by TSSCW and VLsSCw were grown by carbotherm VSSCW, TSSCW and VLSSCW, respectively). The reduction. The stacking faults in these whiskers were VSSCW is grown by the direct accommodation of sili- characterized by X-ray diffraction (XRD)and trans con and carbon atoms to the growth plane from the mission electron microscopy(TEM)with the aim of silicon-and carbon-carrying vapors. The TSSCW is ascertain the important factor to the formation of grown in the raw materials containing metal impurities stacking faults. such as rice-hulls. The impurities form discrete liquid droplets on the growth plane. The droplets are quickly covered with vapor species because of their high 2. Experimental procedures accommodation coefficient and act as nucleation sites for the whisker growth. It results in axial growth of The system for the growing of SiC whiskers consisted whiskers (first stage), and, then in lateral thickening of substrate and boats in an alumina tube reactor. The (second stage). In the two-stage growth process, radial graphite boats containing carbon(carbon black, N220, dislocations are typically formed. The VLSSCW is Lucky Continental Co Ltd, Seoul, Korea )-silica(Aero- rown by dissolving silicon-and carbon-carrying vapors sil 200, Degussa Corp . NJ, USA)mixtures were placed at the left-hand end of the tube reactor at the beginning s Corresponding author. Fax: +82-2-958-5509 of each run. The substrate on which SiC whiskers grew E-mail address: hjchoi a kistmail kist re kr(H.J. Choi) was placed at the hot zone. Argon gas was introduced 0272-8842/99/$20.00@1999 Elsevier Science Ltd and Techna S.r.L. All rights reserved. PII:S0272-8842(99)00011-5

Stacking faults in silicon carbide whiskers Heon-Jin Choi *, June-Gunn Lee Multifunctional Ceramics Research Center, Korea Institute of Science and Technology, PO Box 131 Cheongryang, Seoul 130-650, South Korea Received 15 September 1998; accepted 30 October 1998 Abstract Stacking faults in SiC whiskers grown by three di€erent growth mechanisms; vapor±solid (VS), two-stage growth (TS) and vapor±liquid±solid (VLS) mechanism in the carbothermal reduction system were investigated by X-ray di€raction (XRD) and transmission electron microscopy (TEM). The content of stacking faults in SiC whiskers increased with decreasing the diameter of whiskers, i.e. the small diameter whiskers (2 mm) grown by the VLS mechanism have little stacking faults. Heavy stacking faults of small dia￾meter whiskers were probably due to the high speci®c lateral surface area of small diameter whiskers. # 1999 Elsevier Science Ltd and Techna S.r.l. All rights reserved. Keywords: B. Whiskers; D. Silicon carbide; Stacking faults 1. Introduction SiC whiskers are an e€ective material for the reinfor￾cement of various composite materials, due mainly to their superb mechanical properties [1±3]. At the present time, the preferred synthesis method for the SiC whiskers is carbothermal reduction of silica [4,5]. SiC whiskers are grown by the three di€erent growth mechanisms in carbothermal reduction; vapor±solid (VS) mechanism [6,7], two-stage growth (TS) mechan￾ism [8,9] and vapor±liquid±solid (VLS) mechanism [10± 13], (hereafter, the grown whiskers are designated by VSSCW, TSSCW and VLSSCW, respectively). The VSSCW is grown by the direct accommodation of sili￾con and carbon atoms to the growth plane from the silicon- and carbon-carrying vapors. The TSSCW is grown in the raw materials containing metal impurities such as rice-hulls. The impurities form discrete liquid droplets on the growth plane. The droplets are quickly covered with vapor species because of their high accommodation coecient and act as nucleation sites for the whisker growth. It results in axial growth of whiskers (®rst stage), and, then in lateral thickening (second stage). In the two-stage growth process, radial dislocations are typically formed. The VLSSCW is grown by dissolving silicon- and carbon-carrying vapors in the liquid droplets. The liquid droplets are formed by melting of metallic catalysts that are usually intention￾ally added. The dissolved silicon and carbon atoms dif￾fuse across the droplet, and precipitate at the growth plane as SiC. Stacking faults are usually found in SiC whiskers and a€ect various properties of whiskers [14±16]. The stack￾ing faults in the VSSCW and TSSCW have been studied well in previous studies [6±9]. However, the stacking faults in the VLSSCW have been brie¯y referred in the literature [6,11,12,17]. In this study, the VSSCW, TSSCW and VLSSCW were grown by carbothermal reduction. The stacking faults in these whiskers were characterized by X-ray di€raction (XRD) and trans￾mission electron microscopy (TEM) with the aim of ascertain the important factor to the formation of stacking faults. 2. Experimental procedures The system for the growing of SiC whiskers consisted of substrate and boats in an alumina tube reactor. The graphite boats containing carbon (carbon black, N220, Lucky Continental Co. Ltd., Seoul, Korea)-silica (Aero￾sil 200, Degussa Corp., NJ, USA) mixtures were placed at the left-hand end of the tube reactor at the beginning of each run. The substrate on which SiC whiskers grew was placed at the hot zone. Argon gas was introduced Ceramics International 26 (2000) 7±12 0272-8842/99/$20.00 #1999 Elsevier Science Ltd and Techna S.r.l. All rights reserved. PII: S0272-8842(99)00011-5 * Corresponding author. Fax: +82-2-958-5509. E-mail address: hjchoi@kistmail.kist.re.kr (H.-J. Choi)

H.J. Choi, J--G. Lee/ Ceramics International 26(2000)7-12 Table l Growth conditions and characteristics of Sic whiskers Designee Growth site Growth mechanism Reaction conditions Diameter (um) Relative intensity (336°/414) T(C) t(h)Sio generation rate(cmmi) VSSCW ubstrate 45059 None <0.2 2.83 ISSCW Rice-hulls Vapor-solid two-stage VLSSCW Substra Vapor-liquid-solid Fe(5um) 0.11 to maintain an inert atmosphere during the heat-up per lod. At the reaction temperature (1450oC), the process gas (hydrogen)was introduced at the rate of 191 ' min-I and the graphite boats were pushed forward into the hot zone at a predetermined rate of 0. 25 g batch min-.At this set-up, the sio generation rate from batch was fixed to 9 cm'min. The vsSCw was grown on the high purity graphite substrate(NP60, PF Grade, Toshiba (b) 04H (c) 40m g. 1. Representative microstructure of Sic whiskers: (a) VSSCw b) TSSCW and(c) VLSSCW Fig. 2.(a)B ge and(b) Sad pattern of the VSSCW

to maintain an inert atmosphere during the heat-up per￾iod. At the reaction temperature (1450C), the process gas (hydrogen) was introduced at the rate of 191 cm3 minÿ1 and the graphite boats were pushed forward into the hot zone at a predetermined rate of 0.25 g batch minÿ1 . At this set-up, the SiO generation rate from batch was ®xed to 9 cm3 minÿ1 . The VSSCW was grown on the high￾purity graphite substrate (NP60, PF Grade, Toshiba Table 1 Growth conditions and characteristics of SiC whiskers Designee Growth site Growth mechanism Reaction conditions Diameter (mm) Relative intensity (I33.6/I41.4) T (C) t (h) SiO generation rate (cm3 minÿ1 ) Catalyst VSSCW Substrate Vapor±solid 1450 5 9 None <0.2 2.83 TSSCW Rice-hulls Vapor±solid two-stage 1450 5 ± Impurities <0.6 1.01 VLSSCW Substrate Vapor±liquid±solid 1450 5 9 Fe (5m) 2±10 0.11 Fig. 1. Representative microstructure of SiC whiskers: (a) VSSCW, (b) TSSCW and (c) VLSSCW. Fig. 2. (a) BF image and (b) SAD pattern of the VSSCW. 8 H.-J. Choi, J.-G. Lee / Ceramics International 26 (2000) 7±12

H.J. Choi, J--G. Lee/ Ceramics International 26(2000)7-12 a 0.5m b) Fig 3. BF image and SAd patterns of the TSSCW; (a) BF image, (b) SAD pattern of the section A, and(c)Sad pattern of the section B Ceramics Co. Ltd, Tokyo, Japan) which was treated (CM30, Philips, Netherlands). The intensity ratios of with boiling hydrochloric acid (37%)for 4h. The the XRD peak of whiskers at 33. 6 and 41.4(20)(133.6/ VLSSCW was grown on the high-purity graphite sub- 141 40)were used to compare the stacking faults content strate with uniform coating of catalyst Fe powder with in the whiskers [18]. The whiskers for TEM were pre- a size of 5 um was used as catalyst. When control of the pared in two ways: (1) thin whiskers were dispersed in ze of catalyst was needed, Fe(OH)3 was precipitated isopropyl alcohol and a droplet of the suspension was on the substrate by using Fe(NO3).9H2O and NH4OH deposited onto a perforated carbon film that was sup- (Junsei Chemical Co., Tokyo, Japan). During reaction, ported on a 3 mm Cu grid; and()thick whiskers were the Fe(oH)3 precipitates converted to Fe. The TSSCW consolidated into a Si3 N4 matrix composites. The thin was grown by carbothermal reduction of rice-hulls in the foils for TEM were prepared by the standard proce- boat. Chemical analysis of the rice-hulls showed that dures of cutting, grinding, dimpling, and argon-ion they contained 2.6 wt% of various metal impurities such beam thinning followed by carbon coating as Fe, Ni, Mg, Mn, Na, Ca, and K. The whisker growth process was described in an earlier work [13] pon completing the reaction, the collected VSSCW, 3. Results and discussion SSCW and vlssCW were observed by scanning elec tron microscopy(SEM, S-4100, Hitachi, Japan). The Table I summarizes the growth conditions and char- stacking faults in the whiskers were characterized by acteristics of whiskers. Fig. I shows the microstructures XRD(RINT/DMAX-2000, Rigaku, Japan) and TEM of whiskers. Very thin vSSCW with the diameter less

Ceramics Co. Ltd., Tokyo, Japan) which was treated with boiling hydrochloric acid (37%) for 4 h. The VLSSCW was grown on the high-purity graphite sub￾strate with uniform coating of catalyst. Fe powder with a size of 5 mm was used as catalyst. When control of the size of catalyst was needed, Fe(OH)3 was precipitated on the substrate by using Fe(NO3).9H2O and NH4OH (Junsei Chemical Co., Tokyo, Japan). During reaction, the Fe(OH)3 precipitates converted to Fe. The TSSCW was grown by carbothermal reduction of rice-hulls in the boat. Chemical analysis of the rice-hulls showed that they contained 2.6 wt% of various metal impurities such as Fe, Ni, Mg, Mn, Na, Ca, and K. The whisker growth process was described in an earlier work [13]. Upon completing the reaction, the collected VSSCW, TSSCW and VLSSCW were observed by scanning elec￾tron microscopy (SEM, S-4100, Hitachi, Japan). The stacking faults in the whiskers were characterized by XRD (RINT/DMAX-2000, Rigaku, Japan) and TEM (CM30, Philips, Netherlands). The intensity ratios of the XRD peak of whiskers at 33.6 and 41.4 (2) (I33.6/ I41.4) were used to compare the stacking faults content in the whiskers [18]. The whiskers for TEM were pre￾pared in two ways: (1) thin whiskers were dispersed in isopropyl alcohol and a droplet of the suspension was deposited onto a perforated carbon ®lm that was sup￾ported on a 3 mm Cu grid; and (2) thick whiskers were consolidated into a Si3N4 matrix composites. The thin foils for TEM were prepared by the standard proce￾dures of cutting, grinding, dimpling, and argon-ion￾beam thinning followed by carbon coating. 3. Results and discussion Table 1 summarizes the growth conditions and char￾acteristics of whiskers. Fig. 1 shows the microstructures of whiskers. Very thin VSSCW with the diameter less Fig. 3. BF image and SAD patterns of the TSSCW; (a) BF image, (b) SAD pattern of the section A, and (c) SAD pattern of the section B. H.-J. Choi, J.-G. Lee / Ceramics International 26 (2000) 7±12 9

H.J. Choi, J--G. Lee/ Ceramics International 26(2000)7-12 5 Hm Fig 4. BF images [(a) and(c), and Sad patterns [(b)and (d) of the VlssCW. than 0. 2 um were grown on the substrate [Fig. 1(a showed heavy stacking faults along the entire whisker Sic whiskers grown in the rice-hulls batch were length, which was evidenced by the contrast bands in observed in a wide variation of the diameters within the the bF image and streaks in the Sad pattern. Fig range of less that 0.6 um [Fig. 1(b)]. It may be due to the shows the BF image and SAD patterns of Sic whiskers gradual decrease of the Sio generation rate with time in grown in the rice-hulls batch. Some whiskers showed the rice-hulls batch. Generally, the diameter of Sic alternating section without stacking faults [ marked A in whiskers is decreased with increasing Sio generation Fig. 3(a)] and with heavy stacking faults [marked B in rate [10). The VLSSCW that was grown by using the Fe Fig. 3(a). The Sad patterns of sections A and b also powder was thick with the diameters of 2-8 um showed, respectively, no stacking faults and heavy [Fig. I(c)]. Liquid droplets at the end of the whiskers. stacking faults [Fig. 3(b)and(c)]. It was inferred that which are typical for the VLS mechanism [6, 10-13, 17]. these whiskers are the vSSCw [6]. The other whiskers were found in the VLSSCW. All whiskers in this study showed Moire patterns [marked C in Fig 3(a)], which were identified as cubic 3C SiC(B-phase) by XRD. The was the evidence of the presence of dislocations and intensity ratio of the XRD peak at 33.6 and 41.4, 133.6/ stacking faults in the whiskers [14]. Therefore, it 1414, of the vSSCw, tssCw and vlssCw were 2.8 believed that the vsscw and tsscw were simulta- 0l and 0.1l, respectively (Table 1). The intensity ratio neously grown in the rice-hulls. Fig. 4 shows the BF increased with contents of stacking faults [7, 17]. The images and SAD patterns of VLSSCW. The VLSSCW RD results indicated that there are heavy stacking with a diameter of about 2 um showed no contrast band faults in the VSSCW and little stacking faults in the in the BF image[Fig 4(a)] and streaks in the Sad pat Ⅴ LSSCV tern [Fig. 4(b)]. Other VLSSCW with a diameter of The bright field(BF) image [Fig. 2(a)] and selected about 7 um showed contrast bands in the narrow region area diffraction(SAD) pattern[Fig. 2(b)] of the VSSCw indicated by the arrow in Fig. 4(c)that indicates the

than 0.2 mm were grown on the substrate [Fig. 1 (a)]. SiC whiskers grown in the rice-hulls batch were observed in a wide variation of the diameters within the range of less that 0.6 mm [Fig. 1(b)]. It may be due to the gradual decrease of the SiO generation rate with time in the rice-hulls batch. Generally, the diameter of SiC whiskers is decreased with increasing SiO generation rate [10]. The VLSSCW that was grown by using the Fe powder was thick with the diameters of 2±8 mm [Fig. 1(c)]. Liquid droplets at the end of the whiskers, which are typical for the VLS mechanism [6,10±13,17], were found in the VLSSCW. All whiskers in this study were identi®ed as cubic 3C SiC (-phase) by XRD. The intensity ratio of the XRD peak at 33.6 and 41.4, I33.6/ I41.4, of the VSSCW, TSSCW and VLSSCW were 2.83, 1.01 and 0.11, respectively (Table 1). The intensity ratio increased with contents of stacking faults [7,17]. The XRD results indicated that there are heavy stacking faults in the VSSCW and little stacking faults in the VLSSCW. The bright ®eld (BF) image [Fig. 2(a)] and selected area di€raction (SAD) pattern [Fig. 2(b)] of the VSSCW showed heavy stacking faults along the entire whisker length, which was evidenced by the contrast bands in the BF image and streaks in the SAD pattern. Fig. 3 shows the BF image and SAD patterns of SiC whiskers grown in the rice-hulls batch. Some whiskers showed alternating section without stacking faults [marked A in Fig. 3(a)] and with heavy stacking faults [marked B in Fig. 3(a)]. The SAD patterns of sections A and B also showed, respectively, no stacking faults and heavy stacking faults [Fig. 3(b) and (c)]. It was inferred that these whiskers are the VSSCW [6]. The other whiskers showed Moire patterns [marked C in Fig. 3 (a)], which was the evidence of the presence of dislocations and stacking faults in the whiskers [14]. Therefore, it is believed that the VSSCW and TSSCW were simulta￾neously grown in the rice-hulls. Fig. 4 shows the BF images and SAD patterns of VLSSCW. The VLSSCW with a diameter of about 2 mm showed no contrast band in the BF image [Fig. 4(a)] and streaks in the SAD pat￾tern [Fig. 4(b)]. Other VLSSCW with a diameter of about 7 mm showed contrast bands in the narrow region indicated by the arrow in Fig. 4(c) that indicates the Fig. 4. BF images [(a) and (c)], and SAD patterns [(b) and (d)] of the VLSSCW. 10 H.-J. Choi, J.-G. Lee / Ceramics International 26 (2000) 7±12

H.J. Choi, J--G. Lee/ Ceramics International 26(2000)7-12 existence of stacking faults. However, no streak (1103. Therefore, with regard to energetic considera- appeared in the Sad pattern [Fig. 4(d)], which may tion, formation of stacking faults during the growth of result from the low density of stacking faults. It was Sic whiskers is favorable due to the contribution of the confirmed that the consolidation process for the pre- stacking faults themselves and (1ll) facets at the lateral paration of TEM sample did not induce the annihilation surface to the formation energy of Sic whiskers. The of stacking faults in SiC whiskers. The observations of specific lateral surface area of whiskers becomes rela- stacking faults by using TEM support the results of tively large when the whisker diameter is small. In this XRD analysis. Heavy stacking faults were found in the case, the contribution of surface energy to the formation vSSCW that show the high intensity ratio of 2.83. energy of Sic whisker becomes large. It may enhance However, little stacking faults were found in the the formation of stacking faults to reduce the contribu- VlSSCW that show the low intensity ratio of 0.11 tion of lateral surface energy. Wang et al. reported that The results shown in Table I and TEM observations the more stacking faults were found in the relatively indicate that the contents of stacking faults relate to the small diameter VSSCW(0. 2 um)[6] diameter of whiskers, i.e. heavy stacking faults are It was documented that the growth of Sic whiskers found with decreasing diameters. According to the axial through the liquid phase (i.e. the VLS mechanism) is a next nearest-neighbor Ising(ANNNI) model [6, 19], Sic more near-equilibrium process than the growth of Sic whiskers with stacking faults have a lower energy than through the vapor phase (i.e. the VS or TS mechanism) Sic whiskers without stacking faults. Furthermore, for- [17], and that the near-equilibrium process may suppress mation of stacking faults promotes (111) facets, which the formation of stacking faults during the growth of ve muc ch lower surface energy than (211) or(110 SiC whiskers. Fig. 5 shows the BF image and SAd pat- facets, at the lateral surface and reduce the total surface terns of the small diameter VlSSCW(<l um) that was energy of whiskers [6]. The lateral surfaces of a whisker grown by using the Fe(OH)3 precipitates. As shown in without stacking faults are usually parallel to(211 or Fig. 5(a), the liquid droplets at the end of whisker were found in the vlsscW hows the contrast band (a) originating from the stacking faults which was also evi- denced by SAd pattern [Fig. 5(b)]. The result indicates that heavy stacking faults can also be formed in the VLSSCW when the diameter is small and, consequently the specific lateral surface area of whiskers becomes Stacking faults in Sic whiskers grown by three dif- ferent growth mechanisms in the carbothermal reduc- tion system were investigated. The content of stacking faults, characterized by XRD and TEM, increased with a decrease in the diameter of whisker. The increasing specific lateral surface area of whiskers with decreasing diameter may enhance the formation of stacking faults References []GC. Wei, P.F. 了P Development of SiC-whisker-reinforced oc.Bull.64(1985)298-304. 2 P D. Shalek, J.J. Petrovic, G.F. Hurley, F D. Gac, Hot pressed Sic whisker/Si3 N4 matrix composites, Am. Ceram Soc. Bull. 6 (1986)351-356 3R. Lundberg, L. Kahlman, R. Pompe, R. Carlsson, R. 4 P.A. Janeway, ART: reinforcing tomorrow technology, Ceram d.(1992)42-44 Fig. 5.(a) BF image and (b) SAD pattern of the small diameter [5]J M. Schoenung, The economics of silicon carbide whisker fabri- cation, Ceram. Eng. Sci. Proc. 12(1991)1943-1951

existence of stacking faults. However, no streak appeared in the SAD pattern [Fig. 4(d)], which may result from the low density of stacking faults. It was con®rmed that the consolidation process for the pre￾paration of TEM sample did not induce the annihilation of stacking faults in SiC whiskers. The observations of stacking faults by using TEM support the results of XRD analysis. Heavy stacking faults were found in the VSSCW that show the high intensity ratio of 2.83. However, little stacking faults were found in the VLSSCW that show the low intensity ratio of 0.11. The results shown in Table 1 and TEM observations indicate that the contents of stacking faults relate to the diameter of whiskers, i.e. heavy stacking faults are found with decreasing diameters. According to the axial next nearest-neighbor Ising (ANNNI) model [6,19], SiC whiskers with stacking faults have a lower energy than SiC whiskers without stacking faults. Furthermore, for￾mation of stacking faults promotes {111} facets, which have much lower surface energy than {211} or {110} facets, at the lateral surface and reduce the total surface energy of whiskers [6]. The lateral surfaces of a whisker without stacking faults are usually parallel to {211} or {110}. Therefore, with regard to energetic considera￾tion, formation of stacking faults during the growth of SiC whiskers is favorable due to the contribution of the stacking faults themselves and {111} facets at the lateral surface to the formation energy of SiC whiskers. The speci®c lateral surface area of whiskers becomes rela￾tively large when the whisker diameter is small. In this case, the contribution of surface energy to the formation energy of SiC whisker becomes large. It may enhance the formation of stacking faults to reduce the contribu￾tion of lateral surface energy. Wang et al. reported that the more stacking faults were found in the relatively small diameter VSSCW (0.2 mm) [6]. It was documented that the growth of SiC whiskers through the liquid phase (i.e. the VLS mechanism) is a more near-equilibrium process than the growth of SiC through the vapor phase (i.e. the VS or TS mechanism) [17], and that the near-equilibrium process may suppress the formation of stacking faults during the growth of SiC whiskers. Fig. 5 shows the BF image and SAD pat￾terns of the small diameter VLSSCW (<1 mm) that was grown by using the Fe(OH)3 precipitates. As shown in Fig. 5(a), the liquid droplets at the end of whisker were found in the VLSSCW. It shows the contrast bands originating from the stacking faults which was also evi￾denced by SAD pattern [Fig. 5(b)]. The result indicates that heavy stacking faults can also be formed in the VLSSCW when the diameter is small and, consequently, the speci®c lateral surface area of whiskers becomes large. 4. Summary Stacking faults in SiC whiskers grown by three dif￾ferent growth mechanisms in the carbothermal reduc￾tion system were investigated. The content of stacking faults, characterized by XRD and TEM, increased with a decrease in the diameter of whisker. The increasing speci®c lateral surface area of whiskers with decreasing diameter may enhance the formation of stacking faults in SiC whiskers. References [1] G.C. Wei, P.F. Becher, Development of SiC-whisker-reinforced ceramics, Am. Ceram. Soc. Bull. 64 (1985) 298±304. [2] P.D. Shalek, J.J. Petrovic, G.F. Hurley, F.D. Gac, Hot pressed SiC whisker/Si3N4 matrix composites, Am. Ceram. Soc. Bull. 65 (1986) 351±356. [3] R. Lundberg, L. Kahlman, R. Pompe, R. Carlsson, R. Warren, SiC-whisker-reinforced Si3N4 composites, Am. Ceram. Soc. Bull. 66 (1987) 330±333. [4] P.A. Janeway, ART: reinforcing tomorrow technology, Ceram Ind. (1992) 42±44. [5] J.M. Schoenung, The economics of silicon carbide whisker fabri￾cation, Ceram. Eng. Sci. Proc. 12 (1991) 1943±1951. Fig. 5. (a) BF image and (b) SAD pattern of the small diameter VLSSCW. H.-J. Choi, J.-G. Lee / Ceramics International 26 (2000) 7±12 11

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