Morphology and Stacking Faults of B-sic Whisker Synthesied by Carbothermal Reduction 125 109 70 Fig 10. TEM micrographs of the three different whiskers deflected at an angle of (a)125,(b)70%, and(c)109 aligned with the electron beam parallel to the(110)zone axis. Insets in Figs. 10(a)and(b) show the corresponding electron diffraction patterns for each arrangement. for 0.5 A whisker commonly was observed in the 112,14,23 and the the (1ll, basal planes, the growth rate will be larger for thin whiskers than thick whiskers. If two whiskers have similar ranged fr m. The electron diffr thicknesses but different insertion directions of the (111) planes, hibited featureless are the growth rate may be larger for whisker that has tilted(Ill typical of a disordered layer structure, and they also were com- planes than whisker whose(11 1) planes are perpendicular to the posed of strong twin spots. Their electron diffraction streaks growth direction, because the area of the (111) plane, as an active always perpendicular to the(111) stacking-fault planes; 2, growing surface, is larger in the former than the latter. Although therefore, the stacking faults in the whiskers shown in Fig. 6 must type A and type B whiskers have similar stacking-fault densities be located on the basal planes and thicknesses, the growth rate of type B whisker has been Figure 7 shows a HREM image of type A whisker. Type a calculated to be more than three times' greater than that of type a whisker exhibited many stacking faults perpendicular to the whisker, because the( 1l) planes in type b whisker are tilted at an owth direction, and they were composed of many twin faults. angle of 35, relative to the growth direction. The diffusion The surface energy of the (11li planes in the B-sic is much distance of the solid carbon source also is an important factor, to smaller than those of the other crystal planes, therefore, it is control the reaction rate and stacking-fault content. We have generally accepted that B-SiC whisker can grow easily in the [lll] eported that increasing the reduction ability, using an active direction, to decrease the formation energy, and, hence, stacking carbon source, and adding a large carbon content both lead to the fata and Wang et al. demonstrated that 3C SiC polytype with aults can be inserted easily in the (1ll) planes Cheng et growth of long whiskers and an apparent increase in the stackin fault content, because of the continuous supply of carbon to the stacking fault has a lower energy than 3C Sic without stacking whisker tips that are facilitated by a decrease in the average faults. Thus, this phenomenon can explain the frequent occurrence distance between the carbon source particles, despite a similar of stacking faults in SiC whisker diffusion distance. 3 Thus, type B whisker(such as those shown in Figure 8 shows a HREM image of another type B whisker. Figs 6 and 8)is thinner and has larger areas of (111) planes than Stacking faults that were inclined to the growth direction were type A whisker; this observation would suggest faster growth of observed only in a branched whisker with a small thickness, and type b whisker than type A whisker they were composed primarily of twin faults. In the formation of he stacking-fault density should be dependent on the growth Sic whisker via reaction(2), the supersaturation of Sio gas Is rate and thickness of a whisker. In a thin whisker with a small closely related to the whisker thickness. A high flow rate of Sio cross-sectional area of (111) planes, many stacking faults can be as in small empty spaces among the starting particles leads to inserted, because of the time deficiency needed to realize their low supersaturation of Sio gas, and, hence, this condition becomes lowest-energy structure under rapid reaction conditions; however, favorable for thin whiskers to grow in a specific direction via in the thick whisker, only a small population of stacking faults can one-dimensional growth. On the other hand, a low flow rate of be inserted because of slow reactions that allow atoms to diffuse Sio gas in a relatively large space among the starting particles a long distance to form an equilibrium, defectless structure allows the growth of thick whiskers, because of the high super- In contrast to these two whisker types(types A and B), th saturation of Sio. The difference in the flow rate of sio gas must featureless streaks in the electron diffraction pattern of type C be one of the reasons for the difference in growth rate, if the carbon whisker show three different directions that are perpendicular to source supply is sufficient. The whisker growth rate is believed to be closely dependent on whisker thickness on direction of the (1l1) plane, and diffusion distance le solid carbon source. If one considers the supply of Sio the carbon source to be sufficient and also that Sic whisker is formed through layer-by layer growth of of the (ill)planefor 0.5 h. Type A whisker commonly was observed in the synthesized b-SiC powders,11,12,14,23 and the whisker thickness ranged from 20 nm to 0.2 mm. The electron diffraction patterns of all the whiskers clearly exhibited featureless streaks that are typical of a disordered layer structure, and they also were com￾posed of strong twin spots. Their electron diffraction streaks are always perpendicular to the {111} stacking-fault planes;11,12,14,23 therefore, the stacking faults in the whiskers shown in Fig. 6 must be located on the basal planes. Figure 7 shows a HREM image of type A whisker. Type A whisker exhibited many stacking faults perpendicular to the growth direction, and they were composed of many twin faults. The surface energy of the {111} planes in the b-SiC is much smaller than those of the other crystal planes; therefore, it is generally accepted that b-SiC whisker can grow easily in the [111] direction, to decrease the formation energy, and, hence, stacking faults can be inserted easily in the {111} planes.11–14 Cheng et al.24 and Wang et al.11 demonstrated that 3C SiC polytype with a stacking fault has a lower energy than 3C SiC without stacking faults. Thus, this phenomenon can explain the frequent occurrence of stacking faults in SiC whisker. Figure 8 shows a HREM image of another type B whisker. Stacking faults that were inclined to the growth direction were observed only in a branched whisker with a small thickness, and they were composed primarily of twin faults. In the formation of SiC whisker via reaction (2), the supersaturation of SiO gas is closely related to the whisker thickness. A high flow rate of SiO gas in small empty spaces among the starting particles leads to a low supersaturation of SiO gas, and, hence, this condition becomes favorable for thin whiskers to grow in a specific direction via one-dimensional growth.11 On the other hand, a low flow rate of SiO gas in a relatively large space among the starting particles allows the growth of thick whiskers, because of the high super￾saturation of SiO. The difference in the flow rate of SiO gas must be one of the reasons for the difference in growth rate, if the carbon source supply is sufficient. The whisker growth rate is believed to be closely dependent on whisker thickness, insertion direction of the (111) plane, and diffusion distance from the solid carbon source. If one considers the supply of SiO gas and the carbon source to be sufficient and also that SiC whisker is formed through layer-by layer growth of the {111} basal planes, the growth rate will be larger for thin whiskers than thick whiskers. If two whiskers have similar thicknesses but different insertion directions of the {111} planes, the growth rate may be larger for whisker that has tilted (111) planes than whisker whose (111) planes are perpendicular to the growth direction, because the area of the (111) plane, as an active growing surface, is larger in the former than the latter. Although type A and type B whiskers have similar stacking-fault densities and thicknesses, the growth rate of type B whisker has been calculated to be more than three times‡ greater than that of type A whisker, because the (111) planes in type B whisker are tilted at an angle of 35°, relative to the growth direction. The diffusion distance of the solid carbon source also is an important factor, to control the reaction rate and stacking-fault content. We have reported that increasing the reduction ability, using an active carbon source, and adding a large carbon content both lead to the growth of long whiskers and an apparent increase in the stacking￾fault content, because of the continuous supply of carbon to the whisker tips that are facilitated by a decrease in the average distance between the carbon source particles, despite a similar diffusion distance.13 Thus, type B whisker (such as those shown in Figs. 6 and 8) is thinner and has larger areas of (111) planes than type A whisker; this observation would suggest faster growth of type B whisker than type A whisker. The stacking-fault density should be dependent on the growth rate and thickness of a whisker. In a thin whisker with a small cross-sectional area of (111) planes, many stacking faults can be inserted, because of the time deficiency needed to realize their lowest-energy structure under rapid reaction conditions; however, in the thick whisker, only a small population of stacking faults can be inserted, because of slow reactions that allow atoms to diffuse a long distance to form an equilibrium, defectless structure. In contrast to these two whisker types (types A and B), the featureless streaks in the electron diffraction pattern of type C whisker show three different directions that are perpendicular to ‡ The area of (111) planes in type B whisker is three times as large as that in type A whisker, based on the following estimations: Dtype B/Dtype A 5 1/(sin 35.3°) 5 =3 and Stype B/Stype A 5 (=3) 2 5 3 (where D is the whisker diameter and S is the area of the (111) plane). Fig. 10. TEM micrographs of the three different whiskers deflected at an angle of (a) 125°, (b) 70°, and (c) 109° aligned with the electron beam parallel to the ^110& zone axis. Insets in Figs. 10(a) and (b) show the corresponding electron diffraction patterns for each arrangement. October 2000 Morphology and Stacking Faults of b-SiC Whisker Synthesized by Carbothermal Reduction 2589