October 2000 Morphology and Stacking Faults of B-sic Whisker Synthesied by Carbothermal Reduction schematic diagram of SiCy/CSia tetrahedron that is shown in Fig 12. The SiC,/CSia tetrahedron is composed of four ShC or C-Si bonds, directed toward the [111,[111],[1ll], and [111 direc- tions, and the angles among them are all 109.5. If two direc- [11 tions[11]and [1TIl--are chosen to be those of the two heads [11 of a Y-shaped whisker, the [001] direction would correspond to the growth direction of a leg. If the Y-shaped whisker is observed via TEM in the direction parallel to [100], two heads and one leg on the(100)plane(depicted by a dotted line in Fig 12(a)) show the same shape, as show two heads was measured to be 90 and the angles between a leg and two heads both were measured to be 135 (a) The right portion of a leg and the right head, and the left portion of a leg and the left head had the same stacking planes, as shown in Figs. 11(b)and(c), respectively. The leg had stacking faults that were inclined at an angle of 35 to the growth direction, similar to [1b that observed in type B whisker, whereas the two head portions [111 O Si or c had stacking faults that were perpendicular to the growth direction, similar to that observed in type a whisker. Thus, the stacking-faul density is considered to be higher in the leg than in the head ○ c or si portions. The stacking faults in the(111)and(111) planes of the leg were connected to each other, hence, the right and left portions of the leg must have grown simultaneously. The right and left heads also may have grown simultaneously, because they have (b) similar thickness and stacking-fault density The difference in the stacking-fault density and its insert of SiCa or CSi4 tetrahedra pictured with direction in a leg and the two head port ndicated that the order whisker, (b)schematic diagram of the of whisker growth can be assumed as follows. The rapid growth of a leg probably was followed by the growth of the two head rtions. Namely, the growth front probably moved from the leg to the heads, which was affected by the existence of a carbon source around the growth front, similar to that observed in the type b front becomes more distant from the enriched carbon source. suc whisker that was deflected at an angle of 125.. The growth of the that the growth rate becomes gradually lower, and, hence, stackin asymmetric Y-shaped whiskers with long heads and short heads, as faults rarely are formed. In response to the demand for less shown in Fig. 11(a), probably stacking-fault formation and slow growth speed, the growth rate of supply of the carbon source direction changes from[001]to[TTT], inclined at an angle of 1250 (111)to(111), similar to the case for type A whisker. The angle IV. Summar between the two faulted planes(111) and(Ill)was 70.5%at the bending part, and the angle between the featureless streaks B-SiC whisker was synthesized from silica(SiO2)and carbon perpendicular to the two stacking faults planes was 70.5 black(CB)powders via carbothermal reduction. The mechanism On the other hand, only the whisker that was deflected at an of whisker formation, the whisker morphology and growth direc- ngle of 70.5 was composed of type A whisker, as shown in Fig. tion, and the stacking-fault insertion were investigated, using some 10(b). The growth front of the whisker could not be distinguished model experiments and various sample-preparation methods. The from those. The stacking faults on the(111)and(111)planes were following conclusions can be drawn from the present study inserted into the whisker perpendicular to the growth directions The primary mechanism of whisker formation in the SiOx- 111] and [111 respectively ). The two stacking faults met at an carbon-hydrogen-gas system was solid-gas reaction between SiO angle of 109.5 at the inner region of the bent portion of the and CB, and their growth was strongly dependent on their whisker and formed a discontinuous boundary preparation conditions. These conditions included Sio generation A whisker that was branched at an angle of 109 also was and the transport and stacking manner of CB and SiO2 Wnmn上1g Growth planes and growth directions in the synthesized whisker closely related to the formation of type C whisker in the stacked are closely related to the surrounding growth conditions, such as der. The formation of additional 111) planes that are inclined the growth rate, the content of inserted stacking faults, and the supply at an angle of 109.5 on the type C whisker surface might hav of carbon and the Sio source. Three different stacking-fault types and acted as seed material for the branched whisker. As the growth morphologies in the synthesized Sic whisker were observed: time was prolonged to 3 h, the branched whisker grew, because it (1) Type A, where stacking faults that were perpendicular to was continuously supplied with Sio gas the growth direction were inserted in the whisker as a common leo gure 11 shows a Y-shaped whisker that is composed of one type. The whiskers had a wide distribution in their thickness and two heads. The leg grew in the [ool] direction 2) Type B, where the whiskers had a high density of stacking probably had a triangular cross section that consisted of (110) faults, because of the insertion at an inclination angle of 35, urfaces. The triangular(110)and(110) planes on the leg surface relative to the growth direction, and they had a finer radius than were connected by two heads, which were grown in the [1ll] and that of type a whisker [111 directions, respectively. The calculated angle between two (3) Type C, where the whiskers were synthesized from the eads grown in the [lll] and [lll] directions was 109.5, and stacked powder, because of the continuous supply of Sio gas. nose between the leg grown in the [oo1] direction and the two hey had a rough surface, similar to a sawtooth morphology, and heads were both 125.3. However, the angles observed via TEM the stacking faults existed in the three different (1ll) planes were I3s°and90°, instead of the real angles of I25.3°andl09.5 Whiskers that were deflected at angles of 125. and 70 and as shown in Figs. 11(b)and(c), respectively. To explain the Y-shaped whiskers were synthesized from mixed powder. Whi difference between the calculated and the measured angles, growth directions of a Y-shaped whisker were indicated infront becomes more distant from the enriched carbon source, such that the growth rate becomes gradually lower, and, hence, stacking faults rarely are formed. In response to the demand for less stacking-fault formation and slow growth speed, the growth direction changes from [001#] to[1#1#1#], inclined at an angle of 125°, relative to each other, and the growth plane also changes from (111#) to (111), similar to the case for type A whisker. The angle between the two faulted planes—(111) and (111#)—was 70.5° at the bending part, and the angle between the featureless streaks perpendicular to the two stacking faults planes was 70.5°. On the other hand, only the whisker that was deflected at an angle of 70.5° was composed of type A whisker, as shown in Fig. 10(b). The growth front of the whisker could not be distinguished from those. The stacking faults on the (111) and (111#) planes were inserted into the whisker perpendicular to the growth directions ([1#1#1#] and [111#], respectively). The two stacking faults met at an angle of 109.5° at the inner region of the bent portion of the whisker and formed a discontinuous boundary. A whisker that was branched at an angle of 109° also was formed in the stacked powder, as shown in Fig. 10(c), which is closely related to the formation of type C whisker in the stacked powder. The formation of additional {111} planes that are inclined at an angle of 109.5° on the type C whisker surface might have acted as seed material for the branched whisker. As the growth time was prolonged to 3 h, the branched whisker grew, because it was continuously supplied with SiO gas. Figure 11 shows a Y-shaped whisker that is composed of one leg and two heads. The leg grew in the [001] direction and it probably had a triangular cross section that consisted of {110} surfaces. The triangular (1#10) and (11#0) planes on the leg surface were connected by two heads, which were grown in the [1#11] and [11#1] directions, respectively. The calculated angle between two heads grown in the [1#11] and [11#1] directions was 109.5°, and those between the leg grown in the [001] direction and the two heads were both 125.3°. However, the angles observed via TEM were 135° and 90°, instead of the real angles of 125.3° and 109.5°, as shown in Figs. 11(b) and (c), respectively. To explain the difference between the calculated and the measured angles, the growth directions of a Y-shaped whisker were indicated in the schematic diagram of SiC4/CSi4 tetrahedron that is shown in Fig. 12. The SiC4/CSi4 tetrahedron is composed of four SiOC or COSi bonds, directed toward the [11#1], [1#11], [111], and [1#1#1] direc￾tions, and the angles among them are all 109.5°. If two direc￾tions—[1#11] and [11#1]—are chosen to be those of the two heads of a Y-shaped whisker, the [001] direction would correspond to the growth direction of a leg. If the Y-shaped whisker is observed via TEM in the direction parallel to [1#00], two heads and one leg on the (100) plane (depicted by a dotted line in Fig. 12(a)) show the same shape, as shown in Fig. 12(b). Thus, the angle between the two heads was measured to be 90° and the angles between a leg and two heads both were measured to be 135°. The right portion of a leg and the right head, and the left portion of a leg and the left head had the same stacking planes, as shown in Figs. 11(b) and (c), respectively. The leg had stacking faults that were inclined at an angle of 35° to the growth direction, similar to that observed in type B whisker, whereas the two head portions had stacking faults that were perpendicular to the growth direction, similar to that observed in type A whisker. Thus, the stacking-fault density is considered to be higher in the leg than in the head portions. The stacking faults in the (1#11) and (11#1) planes of the leg were connected to each other; hence, the right and left portions of the leg must have grown simultaneously. The right and left heads also may have grown simultaneously, because they have similar thickness and stacking-fault density. The difference in the stacking-fault density and its insert direction in a leg and the two head portions indicated that the order of whisker growth can be assumed as follows. The rapid growth of a leg probably was followed by the growth of the two head portions. Namely, the growth front probably moved from the leg to the heads, which was affected by the existence of a carbon source around the growth front, similar to that observed in the type B whisker that was deflected at an angle of 125°. The growth of the asymmetric Y-shaped whiskers with long heads and short heads, as shown in Fig. 11(a), probably was caused by the difference in the rate of supply of the carbon source. IV. Summary b-SiC whisker was synthesized from silica (SiO2) and carbon black (CB) powders via carbothermal reduction. The mechanism of whisker formation, the whisker morphology and growth direc￾tion, and the stacking-fault insertion were investigated, using some model experiments and various sample-preparation methods. The following conclusions can be drawn from the present study. The primary mechanism of whisker formation in the SiO2– carbon–hydrogen-gas system was solid–gas reaction between SiO and CB, and their growth was strongly dependent on their preparation conditions. These conditions included SiO generation and the transport and stacking manner of CB and SiO2. Growth planes and growth directions in the synthesized whisker are closely related to the surrounding growth conditions, such as the growth rate, the content of inserted stacking faults, and the supply of carbon and the SiO source. Three different stacking-fault types and morphologies in the synthesized SiC whisker were observed: (1) Type A, where stacking faults that were perpendicular to the growth direction were inserted in the whisker as a common type. The whiskers had a wide distribution in their thickness. (2) Type B, where the whiskers had a high density of stacking faults, because of the insertion at an inclination angle of 35°, relative to the growth direction, and they had a finer radius than that of type A whisker. (3) Type C, where the whiskers were synthesized from the stacked powder, because of the continuous supply of SiO gas. They had a rough surface, similar to a sawtooth morphology, and the stacking faults existed in the three different {111} planes. Whiskers that were deflected at angles of 125° and 70° and Y-shaped whiskers were synthesized from mixed powder. Whis￾kers that branched at an angle of 109.5° were formed on the surface of type C whisker by continuously offering SiO gas. The Fig. 12. (a) Schematic diagram of SiC4 or CSi4 tetrahedra pictured with directions and angles of Y-shaped whisker; (b) schematic diagram of the observed image in the case of aligning with the electron beam parallel to the ^100& zone axis. October 2000 Morphology and Stacking Faults of b-SiC Whisker Synthesized by Carbothermal Reduction 2591