J. An ceran.Soc,830]2584-9202000) urna Morphology and Stacking Faults of B-Silicon Carbide Whisker Synthesized by carbothermal Reduction Won-Seon Seo and Kunihito Koumoto* Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Jap MV Electron Microscopy Laboratory, Center for Integrated Research in Science and Engineering, Nagoya University Nagoya 464-8603, Japan The main formation reaction for whisker that has been losely related to the formation reactions and the reaction rate. synthesized from SiO2 and carbon black(CB)in a hydrogen- Thus, to be successful in forming specially shaped whiskers, the gas atmosphere was a solid-gas reaction between Sio and CB. insertion directions of the stacking faults and the whisker growth The synthesized whiskers were classified into three types, in rate each must be controlled terms of the morphology, growth direction, and stacking-fault In the present study, as the first step in our attempt to make bent planes:(i)type A, which has a relatively flat surface and the whiskers with different stacking-fault layers, we have investigated stacking-fault planes are perpendicular to the growth direc- several factors, using various sample-preparation methods. These tion;(ii)type B, which has a rough surface and the stacking Factors include the whisker formation reaction the whisker mor- fault planes are inclined at an angle of 35 to the growth phology and growth direction, and stacking-fault insertion of the direction; and (iii)type C, which has a rough sawtooth surface synthesized whisker. and the stacking faults exist concurrently in three (111 planes. The observed angles in the deflect branched whiskers were125°,70°,and109°. These m were composed of mixtures of type A and type B, type A only, Il. Experimental Procedures or parallel growth by two pairs of type A and type B whiskers. (1) Synthesis The whisker deflection was closely related to the difference in B-Sic powder was synthesized via carbothermal the growth speed of each type of whisker using carbon black(CB)powder and silica(SiO2)po L. Introduction theoretically needed was added to the sio, powder, and the SiLcoN ARBIDE (SiC) whiskers provide an effective means for polet ers were mixed via ball milling, using a-SiC balls in a The use of the naturally stacked powder bed of because of their good mechanical properties. The morphology and CB and Sio,(with CB positioned on the top), with a thickness of stacking faults of SiC whiskers are considered to be important, in 7mm, also was used as one of the methods for whisker synthesis regard to the mechanical properties of Sic whiskers themselves to provide a continuous supply of silicon monoxide (Sio) gas. The and whisker-reinforced composites. -4 To obtain good mechanical mixed powders(Fig. I(a), stacked powders(Fig. 1(b),and properties of the SiC-reinforced composite materials, most re- separated powder stacks( Fig. I(c)and(d)were annealed at a marchers are concerned about the interface between the matrix and mperature of 1420.C for 0. 1-3 h in a hydrogen atmosphere the whiskers, in addition to the homogeneous distribution of sic under vacuum( the latter condition is depicted in Fig. I(e)). The whiskers in the matrix 3,5 - However, only a few researchers have heating rate from room temperature to 1000C was 15C/min, and tried to improve the mechanical properties through a change in the that from 1000oC to 1420oC was 10 C/min. The flow rate of morphology of the Sic whiskers and control of the insertion hydrogen gas during heating was 0. 15 cm/s. The carbon layer in direction of stacking faults in Sic whiskers Moreover. because the stacked layers was separated physically from the SiO2 layer the grain growth and stacking-fault annihilation occur at high after the reaction run. The whisker that was formed in the cb layer temperatures in SiC, it is important that the final product that the during the reaction had sufficient handling strength to allow the Sic whisker morphology and the stacking-fault density be con- CB layer to be separated from the SiOz layer. The weight los trolled during the synthesis process. B-SiC whiskers generally the Sio, layer after the reaction was measured. The reactant of the(111) planes; hence, a stacking fault easily can be inserted into additional heating at 700C for 3 h in air, to eliminate excess the 111) planes that are perpendicular to the growth direc carbon xidation, and the weights of the synthesized Sic acking faults in B-SiC also are well-known to be powders were measured N. S. Jacobson--contributing editor Table I. Properties of Starting No. 189998. Received July 27, 1998; approved March 31, 2000 material n, Science and Culture(No. 1 1650857) 0.8 Carbon black 0.04-0.1 Institute of Ceramic Engineering and Technology, Seoul, Korea n Center,Korean tResidual after ignition
Morphology and Stacking Faults of b-Silicon Carbide Whisker Synthesized by Carbothermal Reduction Won-Seon Seo† and Kunihito Koumoto* Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464–8603, Japan Shigeo Aria 1MV Electron Microscopy Laboratory, Center for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464–8603, Japan The main formation reaction for whisker that has been synthesized from SiO2 and carbon black (CB) in a hydrogengas atmosphere was a solid–gas reaction between SiO and CB. The synthesized whiskers were classified into three types, in terms of the morphology, growth direction, and stacking-fault planes: (i) type A, which has a relatively flat surface and the stacking-fault planes are perpendicular to the growth direction; (ii) type B, which has a rough surface and the stackingfault planes are inclined at an angle of 35° to the growth direction; and (iii) type C, which has a rough sawtooth surface and the stacking faults exist concurrently in three different {111} planes. The observed angles in the deflected and branched whiskers were 125°, 70°, and 109°. These whiskers were composed of mixtures of type A and type B, type A only, or parallel growth by two pairs of type A and type B whiskers. The whisker deflection was closely related to the difference in the growth speed of each type of whisker. I. Introduction SILICON CARBIDE (SiC) whiskers provide an effective means for the reinforcement of metal and ceramic-matrix composites, because of their good mechanical properties. The morphology and stacking faults of SiC whiskers are considered to be important, in regard to the mechanical properties of SiC whiskers themselves and whisker-reinforced composites.1–4 To obtain good mechanical properties of the SiC-reinforced composite materials, most researchers are concerned about the interface between the matrix and the whiskers, in addition to the homogeneous distribution of SiC whiskers in the matrix.3,5–7 However, only a few researchers have tried to improve the mechanical properties through a change in the morphology of the SiC whiskers and control of the insertion direction of stacking faults in SiC whiskers.8 Moreover, because the grain growth and stacking-fault annihilation occur at high temperatures in SiC, it is important that the final product that the SiC whisker morphology and the stacking-fault density be controlled during the synthesis process.9,10 b-SiC whiskers generally grow in the [111] direction, because of the low surface energy of the {111} planes; hence, a stacking fault easily can be inserted into the {111} planes that are perpendicular to the growth direction.11–15 Stacking faults in b-SiC also are well-known to be closely related to the formation reactions and the reaction rate.11–13 Thus, to be successful in forming specially shaped whiskers, the insertion directions of the stacking faults and the whisker growth rate each must be controlled. In the present study, as the first step in our attempt to make bent whiskers with different stacking-fault layers, we have investigated several factors, using various sample-preparation methods. These factors include the whisker formation reaction, the whisker morphology and growth direction, and stacking-fault insertion of the synthesized whisker. II. Experimental Procedures (1) Synthesis b-SiC powder was synthesized via carbothermal reduction, using carbon black (CB) powder and silica (SiO2) powder. The properties of the starting powder are listed in Table I. To increase the efficiency of b-SiC formation, twice as much carbon powder as theoretically needed was added to the SiO2 powder, and the powders were mixed via ball milling, using a-SiC balls in a polyethylene jar. The use of the naturally stacked powder bed of CB and SiO2 (with CB positioned on the top), with a thickness of 7 mm, also was used as one of the methods for whisker synthesis, to provide a continuous supply of silicon monoxide (SiO) gas. The mixed powders (Fig. 1(a)), stacked powders (Fig. 1(b)), and separated powder stacks (Fig. 1(c) and (d)) were annealed at a temperature of 1420°C for 0.1–3 h in a hydrogen atmosphere and under vacuum (the latter condition is depicted in Fig. 1(e)). The heating rate from room temperature to 1000°C was 15°C/min, and that from 1000°C to 1420°C was 10°C/min. The flow rate of hydrogen gas during heating was 0.15 cm/s. The carbon layer in the stacked layers was separated physically from the SiO2 layer after the reaction run. The whisker that was formed in the CB layer during the reaction had sufficient handling strength to allow the CB layer to be separated from the SiO2 layer. The weight loss of the SiO2 layer after the reaction was measured. The reactant mixtures, which contained synthesized whiskers, were subjected to additional heating at 700°C for 3 h in air, to eliminate excess carbon via oxidation, and the weights of the synthesized SiC powders were measured. N. S. Jacobson—contributing editor Manuscript No. 189998. Received July 27, 1998; approved March 31, 2000. This work was supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science and Culture (No. 11650857). *Member, American Ceramic Society. † Currently at Advanced Materials Analysis and Evaluation Center, Korean Institute of Ceramic Engineering and Technology, Seoul, Korea. Table I. Properties of Starting Source Powders Starting material Purity (%) Particle size (mm) SiO2 99.9 0.8 Carbon black ,2† 0.04–0.1 † Residual after ignition. J. Am. Ceram. Soc., 83 [10] 2584–92 (2000) 2584 journal
October 2000 Morphology and Stacking Faults of B-sic Whisker Synthesied by Carbothermal Reduction 题圈 SiO, (s)+ 3C(s)= SiC(s)+ 2co(g) (1) 88 However. it is difficult to understand the formation mechanism of B-SiC completely via the carbothermal reduction, because the overall reaction implies several elementary reactions that occu (c) simultaneously, and because their reactions are dependent or the environmental conditions(such as the partial pressures of Sio and CO the reaction temperature, and the existence of H Vacuum impurities,). Although various growth mechanisms of B-SiC whisker have been suggested, such as vapor-liquid-solid (VLs) 闓盥虑 ● Carbon black birth-and-spread growth, vapor-vapor,two-dimensional Vs gas flow and two-stage mechanisms, depending on the use of catalysts, direction different types of starting sources, impurity contents, and growth conditions, the mechanism of whisker growth in the Sio2-carbon- hydrogen-gas system is not clearly understood yet. Fig. 1. Schematic diagram for various packing methods of starting From the model experiment, using various materials such as stacked powder, and (c) and(d) separated powder stacks, all in a ilicon, Sio, and Sio,, we already have reported that tw hydrogen-gas atmosphere, Fig. I(e)depicts mixed powder under vacuum) outes are possible for the formation of B-Sic in the SiOz-ca hydrogen-gas system. One route(route 1)is solid-gas re (reaction(2)), which occurs directly between Sio gas and solid Gold was evaporated onto the synthesized whiskers, and obser- 2) ations via scanning electron microscopy (SEM)(Model S-510 Sio(g)+ 2C(s)= SiC(s)+ Co(g)(route 1) hitachi, Tokyo, Japan)were conducted to examine the microstruc- The other route(route 2)is a solid-solid reaction between solid or ture. Powder samples for transmission electron microscopy(TEM) liquid silicon and carbon, which occurs via a disproportionation were dispersed ultrasonically in ethyl alcohol and transformed reaction of Sio gas into silicon and Sio onto a carbon microgrid that was affixed to copper grids. Conven- tional TEm and high-resolution transmission electron 2Sio(g)= Si(s)+ SiO2(s) (route 2) (HREM) images were acquired using different systems(Mode H-800, Hitachi and Model 2010, JEOL, Tokyo Si(s, 1)+C(s)= SiC(s) at were ated at an acceleration e of 200 kV. Morphology Previous results have observations and selected-area diffraction patterns of the whisker lat B-Sic that is formed via were performed, to investigate the growth direction of the whisker B-Sic that is formed via whisker morphology, whereas lid reaction produces a spherical Figure 2 shows SEM micrographs of B-SiC powders that were IlL. Results and Discussion formed from the mixed powder after reaction(Fig. 2(a)) and after the elimination of excess carbon(Fig. 2(b), as well as from the (I Formation Reaction of B-Silicon Carbide Whisker in the stacked powder(Fig. 2(c), as the starting powder was heated at a Silica-Carbon-Hydrogen System temperature of 1420oC for 0.5 h in a hydrogen-gas atmosphere It is well-known that the overall reaction for the formation of Spherical particles coexisted with fibrous whiskers in the synthe SiC via the carbothermal reduction of SiO, proceeds as follows sized B-sic powders. After the excess carbon was heated, the 2um m (b) Fig. 2. SEM micrographs of B-SiC powder synthesized from SiO2 and carbon black(CB) powders at 1420.C for 0.5 h via various sample-preparation methods(a)mixed powder, (b) after the elimination of excess carbon of the powder in Fig. 2(a), and (c)stacked powder
(2) Analysis Gold was evaporated onto the synthesized whiskers, and observations via scanning electron microscopy (SEM) (Model S-510, Hitachi, Tokyo, Japan) were conducted to examine the microstructure. Powder samples for transmission electron microscopy (TEM) were dispersed ultrasonically in ethyl alcohol and transformed onto a carbon microgrid that was affixed to copper grids. Conventional TEM and high-resolution transmission electron microscopy (HREM) images were acquired using different systems (Model H-800, Hitachi and Model 2010, JEOL, Tokyo, Japan) that were operated at an acceleration voltage of 200 kV. Morphology observations and selected-area diffraction patterns of the whisker were performed, to investigate the growth direction of the whisker and the insertion directions of the stacking faults. III. Results and Discussion (1) Formation Reaction of b-Silicon Carbide Whisker in the Silica–Carbon–Hydrogen System It is well-known that the overall reaction for the formation of SiC via the carbothermal reduction of SiO2 proceeds as follows: SiO2~s! 1 3C~s! º SiC~s! 1 2CO~ g! (1) However, it is difficult to understand the formation mechanism of b-SiC completely via the carbothermal reduction, because the overall reaction implies several elementary reactions that occur simultaneously12,16 and because their reactions are dependent on the environmental conditions (such as the partial pressures of SiO and CO,17,18 the reaction temperature,16 and the existence of impurities13,19). Although various growth mechanisms of b-SiC whisker have been suggested, such as vapor–liquid–solid (VLS),20 birth-and-spread growth,21 vapor–vapor,12 two-dimensional VS,11 and two-stage mechanisms,22 depending on the use of catalysts, different types of starting sources, impurity contents, and growth conditions, the mechanism of whisker growth in the SiO2–carbon– hydrogen-gas system is not clearly understood yet. From the model experiment, using various materials such as silicon, SiO, and SiO2, we already have reported that two main routes are possible for the formation of b-SiC in the SiO2–carbon– hydrogen-gas system.12 One route (route 1) is solid–gas reaction (reaction (2)), which occurs directly between SiO gas and solid carbon: SiO~ g! 1 2C~s! º SiC~s! 1 CO~ g! ~route 1! (2) The other route (route 2) is a solid–solid reaction between solid or liquid silicon and carbon, which occurs via a disproportionation reaction of SiO gas into silicon and SiO2: 2SiO~ g! º Si~s! 1 SiO2~s! ~route 2! (3) Si~s,l ! 1 C~s! º SiC~s! (4) Previous results have suggested that b-SiC that is formed via solid–gas reactions produces a whisker morphology, whereas b-SiC that is formed via solid–solid reaction produces a spherical shape.12 Figure 2 shows SEM micrographs of b-SiC powders that were formed from the mixed powder after reaction (Fig. 2(a)) and after the elimination of excess carbon (Fig. 2(b)), as well as from the stacked powder (Fig. 2(c)), as the starting powder was heated at a temperature of 1420°C for 0.5 h in a hydrogen-gas atmosphere. Spherical particles coexisted with fibrous whiskers in the synthesized b-SiC powders. After the excess carbon was heated, the Fig. 1. Schematic diagram for various packing methods of starting powders used in the synthesis of b-SiC powder ((a) mixed powder, (b) stacked powder, and (c) and (d) separated powder stacks, all in a hydrogen-gas atmosphere; Fig. 1(e) depicts mixed powder under vacuum). Fig. 2. SEM micrographs of b-SiC powder synthesized from SiO2 and carbon black (CB) powders at 1420°C for 0.5 h via various sample-preparation methods ((a) mixed powder, (b) after the elimination of excess carbon of the powder in Fig. 2(a), and (c) stacked powder). October 2000 Morphology and Stacking Faults of b-SiC Whisker Synthesized by Carbothermal Reduction 2585
86 Journal of the American Ceramic Socieny-Seo et al. Vol. 83. No. 10 spherical particles that existed primarily in lumps were observed more clearly, whereas the whiskers seem to ●c‖lso have formed primarily in the empty spaces among the starting osio‖c particles, which offered a path for the reaction gases(SiO). The morphology differences in the synthesized B-Sic powders can be associated with the reaction routes, according to the formation site A reducing reaction in a gas volume, just as that in reaction (3), robably occurs under high SiO-gas pressure, whereas a constant- volume reaction in the gas volume, such as that in reactions (2)and (4), probably has no relation to the gas pressure. Thus, the high 0.2 SiO-gas pressure, just as the agglomerates begin to lump together, creates possibly convenient conditions for reaction (3) to occur, whereas the low SiO-gas pressure, in empty space, creates easy onditions for reaction(2) to occur. In the CB/SiOz stacked- powder bed (where CB is on the top), Sic formed only within the CB layer, whereas the Sio, layer decreased continuously in Flow rate(cm/sec) content as the reaction time increased The whisker content and Fig. 4. Ratio of the change of SiC formation content to the weight loss of whisker thickness of the synthesized powder also are greater and SiO2, relative to the hydrogen-gas flow rate, during the synthesis of SiC at thicker, respectively, in the stacked powder than in the mixed 1420C for 0.5 h wder, as shown in Fig. 2(c). The Sio gas is generated in a region that is physically separated from the carbon; hence, the reaction occurs at a whisker front rather than at a location whose volume that mass transport via a gas phase might be included in the Sic varies. Thus, a sequential deposition of Sio gas at the same formation reaction. Because there is no contact between the CB location probably is possible, and these locations can grow in the empty spaces without being inhibited by nearby whiskers. How- via the reduction of Sio, by hydrogen gas. The transport process as is consumed competitively by the surrounding carbon. More- When the flow of Sio gas from the Sio, stack toward the CB stack over, whisker growth in the mixed powder can stop by impingin on other particles; hence, smaller whiskers are formed. was in the same direction as the flow of hydrogen gas(Fig. I(c)), the sic formation content did not change. relative to the flow rate Figure 3 shows the formation content of SiC, relative to the On the other hand, when the direction of sio gas flow was counter increasing carbon content in the CB/SiO, stacked powder during the synthesis of Sic at a temperature of 1420.C for 3 h. The content of sic decreased as the flow rate increased. because of the formation content of Sic increases linearly as the weight of carbo decrease in the transport rate of sio gas. The formed whiskers had increases. The contact area between the CB and Sio stacked layers does not change as the carbon content in the reaction boat increases; however, the contact area has only a minor effect on increases in the sic formation content. The increase of sic formation content probably is closely related to the increase in capture content of Sio gas as the thickness of the carbon layer To clarify the SiC formation mechanism, some model experi- ments were conducted, and these experiments are described as follows. The separated SiO, and CB powder stacks were placed on the same reaction boat(which was made of alumina(Al,O3)),as shown in Figs. I(c)and(d). The stacks were separated distance of >l cm, and their setting order along the flow directio of hydrogen gas was the Sio, powder stack, followed by the CB powder stack(Fig. I(c); an alternate setting order also was used (see Fig. I(d)). Figure 4 shows the ratio of SiC formation content to the weight loss of SiO,, relative to changes in the flow rate of hydrogen gas at a temperature of 1420.C for 0.5 h In both cases, Sic formed only within the CB powder stacks, which indicated 1420°C,3h 50nm C weight(gr Fig. 5. TEM micrograph of B-SiC powder synthesized from a mixture of Fig 3. Change of SiC formation content, relativ added, in the stacked powder during the synthesis to tce amoo c carbon SiO2 and CB at 1420oC for 0.5 h under vacuum. The corresponding electron diffraction pattern is given in the inset
spherical particles that existed primarily in the agglomerated lumps were observed more clearly, whereas the whiskers seem to have formed primarily in the empty spaces among the starting particles, which offered a path for the reaction gases (SiO). The morphology differences in the synthesized b-SiC powders can be associated with the reaction routes, according to the formation site. A reducing reaction in a gas volume, just as that in reaction (3), probably occurs under high SiO-gas pressure, whereas a constantvolume reaction in the gas volume, such as that in reactions (2) and (4), probably has no relation to the gas pressure. Thus, the high SiO-gas pressure, just as the agglomerates begin to lump together, creates possibly convenient conditions for reaction (3) to occur, whereas the low SiO-gas pressure, in empty space, creates easy conditions for reaction (2) to occur. In the CB/SiO2 stackedpowder bed (where CB is on the top), SiC formed only within the CB layer, whereas the SiO2 layer decreased continuously in content as the reaction time increased. The whisker content and whisker thickness of the synthesized powder also are greater and thicker, respectively, in the stacked powder than in the mixed powder, as shown in Fig. 2(c). The SiO gas is generated in a region that is physically separated from the carbon; hence, the reaction occurs at a whisker front rather than at a location whose volume varies. Thus, a sequential deposition of SiO gas at the same location probably is possible, and these locations can grow in the empty spaces without being inhibited by nearby whiskers. However, the reaction in the mixed powder is volumetric and the SiO gas is consumed competitively by the surrounding carbon. Moreover, whisker growth in the mixed powder can stop by impinging on other particles; hence, smaller whiskers are formed. Figure 3 shows the formation content of SiC, relative to the increasing carbon content in the CB/SiO2 stacked powder during the synthesis of SiC at a temperature of 1420°C for 3 h. The formation content of SiC increases linearly as the weight of carbon increases. The contact area between the CB and SiO2 stacked layers does not change as the carbon content in the reaction boat increases; however, the contact area has only a minor effect on increases in the SiC formation content. The increase of SiC formation content probably is closely related to the increase in capture content of SiO gas as the thickness of the carbon layer increases. To clarify the SiC formation mechanism, some model experiments were conducted, and these experiments are described as follows. The separated SiO2 and CB powder stacks were placed on the same reaction boat (which was made of alumina (Al2O3)), as shown in Figs. 1(c) and (d). The stacks were separated by a distance of .1 cm, and their setting order along the flow direction of hydrogen gas was the SiO2 powder stack, followed by the CB powder stack (Fig. 1(c)); an alternate setting order also was used (see Fig. 1(d)). Figure 4 shows the ratio of SiC formation content to the weight loss of SiO2, relative to changes in the flow rate of hydrogen gas at a temperature of 1420°C for 0.5 h. In both cases, SiC formed only within the CB powder stacks, which indicated that mass transport via a gas phase might be included in the SiC formation reaction. Because there is no contact between the CB and SiO2 stacks, only one possible gas species—SiO—is generated via the reduction of SiO2 by hydrogen gas. The transport process of SiO gas to form SiC was related closely to the formation of SiC. When the flow of SiO gas from the SiO2 stack toward the CB stack was in the same direction as the flow of hydrogen gas (Fig. 1(c)), the SiC formation content did not change, relative to the flow rate. On the other hand, when the direction of SiO gas flow was counter to that of the flow of hydrogen gas (Fig. 1(d)), the formation content of SiC decreased as the flow rate increased, because of the decrease in the transport rate of SiO gas. The formed whiskers had Fig. 5. TEM micrograph of b-SiC powder synthesized from a mixture of SiO2 and CB at 1420°C for 0.5 h under vacuum. The corresponding electron diffraction pattern is given in the inset. Fig. 3. Change of SiC formation content, relative to the amount of carbon added, in the stacked powder during the synthesis of SiC at 1420°C for 3 h. Fig. 4. Ratio of the change of SiC formation content to the weight loss of SiO2, relative to the hydrogen-gas flow rate, during the synthesis of SiC at 1420°C for 0.5 h. 2586 Journal of the American Ceramic Society—Seo et al. Vol. 83, No. 10
October 2000 Morphology and Stacking Faults of p-siC Whisker Synthesized 35 90 109 Fig. 6. TEM micrographs of the three different types of whiskers(a)type A, (b) type B, and(c)type C aligned with the electron beam parallel to the(110) axis). The corresponding electron diffraction patterns are shown as insets in each figure 125 Fig. 7. HREM image of type A whisker Inset shows the corresponding electron diffraction pat te ame morphology as SiC whisker that was synthesized from stacked powder. Here, if carbon monoxide (co)gas is ponsible for the sic formation reaction, the Sic formation ontent might change with the flow rate, because of the gaseous reaction of SiO and co, when the flow of sio gas from the Sio tack to the cb stack is in the same direction as the flow of hydrogen gas. In addition, if CO is used as a reaction gas, SiO, and Sic powders should be formed through the following reactions in Sio(g)+ 3Co(g)- SiC(s)+ 2Co(g) 3SiO(g)+Co(g)- SiC(s)+ 2SiO,(s) However, SiO, and Sic powders were not observed, even near the Fig 8. pe B whisker. Top figure shows a TEM image, reaction boat. Thus, we can conclude that Sic whisker formation IREM image (inset in the HREM image shows reaction occurs directly between the Sio gas and solid carbon. the co diffraction pattern)
the same morphology as SiC whisker that was synthesized from the stacked powder. Here, if carbon monoxide (CO) gas is responsible for the SiC formation reaction, the SiC formation content might change with the flow rate, because of the gaseous reaction of SiO and CO, when the flow of SiO gas from the SiO2 stack to the CB stack is in the same direction as the flow of hydrogen gas. In addition, if CO is used as a reaction gas, SiO2 and SiC powders should be formed through the following reactions in the reactor:18 SiO~ g! 1 3CO~ g! 3 SiC~s! 1 2CO2~ g! (5) 3SiO~ g! 1 CO~ g! 3 SiC~s! 1 2SiO2~s! (6) However, SiO2 and SiC powders were not observed, even near the reaction boat. Thus, we can conclude that SiC whisker formation reaction occurs directly between the SiO gas and solid carbon. Fig. 6. TEM micrographs of the three different types of whiskers ((a) type A, (b) type B, and (c) type C aligned with the electron beam parallel to the ^110& zone axis). The corresponding electron diffraction patterns are shown as insets in each figure. Fig. 7. HREM image of type A whisker. Inset shows the corresponding electron diffraction pattern. Fig. 8. Micrographs of type B whisker. Top figure shows a TEM image, and the bottom shows an HREM image (inset in the HREM image shows the corresponding electron diffraction pattern). October 2000 Morphology and Stacking Faults of b-SiC Whisker Synthesized by Carbothermal Reduction 2587
2588 Journal of the American Ceramic Sociery--Seo et al VoL. 83. No. 10 o985 20 (b) 405 紧 Fig 9.(a) TEM image and(b)and(c) HREM images of type C whisker synthesized from the stacked powder at 1420.C for 0. I h. Insets in Figs. 9(b)and (c) schematically depict the lattice configuration of each arrangement Also, the synthesis of sic powder under vacuum was attempted,(2) Whisker-Growth Direction and Insertion Direction of sing a mixture of CB and Sio as shown in Fig. l(e). The Stacking Faults Sic content synthesized under m was of the sic content that was synthesized in hydrogen-gas atmosphere. The Figure 6 shows TEM micrographs and electron diffraction synthesized powders composed primarily of angular particles patterns of the three different whiskers, each aligned with the with various facets, whereas whiskers rarely were observed, as electron beam parallel to(1 10)zone axis. The type A whisker(Fig shown in Fig. 5. Their synthesis reaction probably is caused by 6(a) has a relatively flat surface and the stacking-fault planes are direct reaction of the CB and SiO,(solid-solid reaction), indepen- perpendicular to the growth direction. The type B whisker(Fig dent of the gas component. the main whisker formatio 6(b)) has a rough surface and the stacking-fault planes are inclined mechanism in the SiO2-carbon-hydrogen-gas system was solid- 35, relative to the growth direction. The type C whisker(Fig. 6(c) gas reaction between Sio and CB, such as that described in has a rough sawtooth surface and the stacking faults exist concur. reaction(2), and their grow dependent closely on their rently in three different (111) planes. Type a and type B whiskers preparation conditions, such as Sio generation and the stacking were synthesized from the mixed powder, and type C whisker was manner of CB synthesized from the stacked powder at a temperature of 1420C
Also, the synthesis of SiC powder under vacuum was attempted, using a mixture of CB and SiO2 powder, as shown in Fig. 1(e). The SiC content synthesized under vacuum was ;1⁄20 of the SiC content that was synthesized in hydrogen-gas atmosphere. The synthesized powders were composed primarily of angular particles with various facets, whereas whiskers rarely were observed, as shown in Fig. 5. Their synthesis reaction probably is caused by direct reaction of the CB and SiO2 (solid–solid reaction), independent of the gas component.16 Thus, the main whisker formation mechanism in the SiO2–carbon–hydrogen-gas system was solid– gas reaction between SiO and CB, such as that described in reaction (2), and their growth was dependent closely on their preparation conditions, such as SiO generation and the stacking manner of CB. (2) Whisker-Growth Direction and Insertion Direction of Stacking Faults Figure 6 shows TEM micrographs and electron diffraction patterns of the three different whiskers, each aligned with the electron beam parallel to ^110& zone axis. The type A whisker (Fig. 6(a)) has a relatively flat surface and the stacking-fault planes are perpendicular to the growth direction. The type B whisker (Fig. 6(b)) has a rough surface and the stacking-fault planes are inclined 35°, relative to the growth direction. The type C whisker (Fig. 6(c)) has a rough sawtooth surface and the stacking faults exist concurrently in three different {111} planes. Type A and type B whiskers were synthesized from the mixed powder, and type C whisker was synthesized from the stacked powder at a temperature of 1420°C Fig. 9. (a) TEM image and (b) and (c) HREM images of type C whisker synthesized from the stacked powder at 1420°C for 0.1 h. Insets in Figs. 9(b) and (c) schematically depict the lattice configuration of each arrangement. 2588 Journal of the American Ceramic Society—Seo et al. Vol. 83, No. 10
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)plane
for 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 composed 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 supersaturation 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 stackingfault 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
Journal of the American Ceramic Sociery- VoL. 83. No. 10 (109.5 90° (109.5 0253)"1 1135 90 111 69G0N 12589 Fig. 11. TEM micrographs of a Y-shaped whisker synthesized from powder mixtures at 1420C for 0.5 h based on the right portion of the leg and the right nification view and(b)tilted to a( 110)direction( toward the (111)direction); Fig. 1I(c) shows the left portion of the nd the left head (toward the(11l)direction). Insets in Figs. I l()and(c)show the corresponding on diffraction patterns each set of stacking-fault layers ( Fig. 6(c)). The angles between the axis. Whiskers that were deflected at angles of 125and 70, and stacking-fault planes in the center and those in small portions on the right side of the whisker or those on the left side of the whisker heated at a temperature of 1420.C for 0.5 h, as shown in Figs. vere similar(-109o). To explain this result, HREM observation of type C whisker that was synthesized from CB/SiO2 stacked an angle of 109 also was formed from the stacked powder that powder at a temperature of 1420C for 0 I h was conducted, as was heated at 1420 C for 3 h(see Fig. 10(c). Whisker that was shown in Fig. 9. B-SiC belongs to the face-centered cubic(fcc) deflected at an angle of 125 was composed of two parts: type a ystem and has four equivalent (1ll) planes. If the primary whisker as a common type, as shown on the left side of Fig. 10(a), growth plane of type C whisker is(111)and the whisker is and type B whisker, which had a stacking fault that was inclined observed with the electron beam aligned parallel to the(110) at an angle of 35. to the growth direction, as shown on the right direction, as shown in Fig. 9, only the(lll)and (111) planes can side. Their thickness rarely changed, relative to the whisker type be observed, because the planes that are perpendicular to the( 110) For flat whiskers, however, the radius of type B whisker normally direction are inclined at an angle of 109.5 relative to each other. was smaller than that of type A whisker, as mentioned in section On the other hand. because the whiskers that were formed via 1(2). Thus, the whisker probably does not interlink different types carbothermal reduction had many twin faults, the twin (lily plane of whiskers but is grown as one whisker via the growth of different of the original(lID) plane could exist in the whisker. +, Thus, stacking planes. The growth front of the whisker that was deflected (111)and(111) planes that are perpendicular to the(110) at an angle of 125. probably starts from the end side of a type b direction appear in type C whisker, and the angle between them whisker(see the top side of Fig. 10(a)). Near the enriched carbon also is70.5°(1800-109.5°. The regions labeled“A"and"B"in Fig 9 clearly have a twin relation with each other, in the direction source, which is an agglomerated lump of carbon, the type B perpendicular to the growth direction of the whisker whisker has grown rapidly by stacking the(1l 1) planes inclined at an angle of 35, relative to the growth direction. The growth planes and growth directions are related closely to the surrounding growth (3) Whisker Branching conditions, such as differences in the growth rate, the content of Figures 10 and 1 l show TEM micrographs of the four deflected the inserted stacking faults, and the supply of carbon and sio whiskers, aligned with the electron beam paral the(110)zone source. Under a constant supersaturation of Sio gas, the growth
each set of stacking-fault layers (Fig. 6(c)). The angles between the stacking-fault planes in the center and those in small portions on the right side of the whisker or those on the left side of the whisker were similar (;109°). To explain this result, HREM observation of type C whisker that was synthesized from CB/SiO2 stacked powder at a temperature of 1420°C for 0.1 h was conducted, as shown in Fig. 9. b-SiC belongs to the face-centered cubic (fcc) system and has four equivalent {111} planes. If the primary growth plane of type C whisker is (111) and the whisker is observed with the electron beam aligned parallel to the ^110& direction, as shown in Fig. 9, only the (111) and (111#) planes can be observed, because the planes that are perpendicular to the ^110& direction are inclined at an angle of 109.5°, relative to each other. On the other hand, because the whiskers that were formed via carbothermal reduction had many twin faults, the twin (111)9 plane of the original (111) plane could exist in the whisker.13,14,21 Thus, (111)9 and (1#11#)9 planes that are perpendicular to the ^110& direction appear in type C whisker, and the angle between them also is 70.5° (180° 2 109.5°). The regions labeled “A” and “B” in Fig. 9 clearly have a twin relation with each other, in the direction perpendicular to the growth direction of the whisker. (3) Whisker Branching Figures 10 and 11 show TEM micrographs of the four deflected whiskers, aligned with the electron beam parallel to the ^110& zone axis. Whiskers that were deflected at angles of 125° and 70°, and Y-shaped whiskers, were formed from the mixed powder that was heated at a temperature of 1420°C for 0.5 h, as shown in Figs. 10(a), 10(b), and 11, respectively. A whisker that was branched at an angle of 109° also was formed from the stacked powder that was heated at 1420°C for 3 h (see Fig. 10(c)). Whisker that was deflected at an angle of 125° was composed of two parts: type A whisker as a common type, as shown on the left side of Fig. 10(a), and type B whisker, which had a stacking fault that was inclined at an angle of 35° to the growth direction, as shown on the right side. Their thickness rarely changed, relative to the whisker type. For flat whiskers, however, the radius of type B whisker normally was smaller than that of type A whisker, as mentioned in section III(2). Thus, the whisker probably does not interlink different types of whiskers but is grown as one whisker via the growth of different stacking planes. The growth front of the whisker that was deflected at an angle of 125° probably starts from the end side of a type B whisker (see the top side of Fig. 10(a)). Near the enriched carbon source, which is an agglomerated lump of carbon, the type B whisker has grown rapidly by stacking the (111#) planes inclined at an angle of 35°, relative to the growth direction. The growth planes and growth directions are related closely to the surrounding growth conditions, such as differences in the growth rate, the content of the inserted stacking faults, and the supply of carbon and SiO source. Under a constant supersaturation of SiO gas, the growth Fig. 11. TEM micrographs of a Y-shaped whisker synthesized from powder mixtures at 1420°C for 0.5 h based on the right portion of the leg and the right head ((a) low-magnification view and (b) tilted to a ^110& direction (toward the ^111& direction)); Fig. 11(c) shows the left portion of the leg and the left head (toward the ^111& direction). Insets in Figs. 11(b) and (c) show the corresponding electron diffraction patterns. 2590 Journal of the American Ceramic Society—Seo et al. Vol. 83, No. 10
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 in
front 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] directions, and the angles among them are all 109.5°. If two directions—[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 direction, 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. Whiskers 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
592 ican Ceramic societ Seo et ai whiskers that were deflected at angles of 125 and 70. were w.S Seo, C.H. Pai, K. Koumoto, and H. Yanagida, "Microstructure Develop- omposed of type A and type B whiskers, and two types of type a ment and stacking Fault Annihilation in p-Sic Powder Compact, "JCeramSoc. whiskers, respectively, depending on the di e in the growth rate loW.-S. Seo and K. Koumoto, "Kinetics and Mechanism of Stacking ault whisker had one leg and two heads, and their angles between the leg 164-0m153 Grain Growth in Porous Ceramics of B-sic,"JMater Res,8, and the stacking-fault content of each type of whisker. The Y-shaped and the heads were both 125.3 the angle between the two heads was L. Wang, H. Wada, and L. F. Allard hesis and Characterization of Sic 109.5. The growth of Y-shaped whisker was accomplished by a 12W.-S. Seo and K. Koumoto, "Stacking Faults in B-SiC Formed during Carbo- parallel growth of two pairs of type A and type B whiskers thermal Reduction of SiO2, ". An. Ceram Soc., 79[7] 1777-82(1996 W-S. Seo. K. Koumoto, and S. Arai. "Effects SiC Particles in the System SiOx-C-H2, J. Am. Ceram Soc, 81[5] 1255-61(1998). S M. Pickard and B. Derby,"TEM Study of Silicon Carbide Whisker Micro- G. Sasaki, K. Hiraga, M. Hirabayashi, K. Nihara, and T. Hirai, "Microstructure sV. V Pujar and J D Cawley, "Effect of Stacking Faults on the X-ray Diffraction Around Indentation of CVD-SiC Observed by Transmission Electron Microscopy, Yokyo Kyokaish, 94, 779-83(1986) V. D Krstic,"Production of Fine, High-Purity Beta Silicon Carbide Powders, Y.C. Zhou and F. Xia, "Effect of Processing Temperature on the Morphology of J Am Ceram Soc., 75 [1170-74(1992 G. C. Wei"Beta SiC Powders Produced by Carbothermic Reduction of Silica in P. F. Becher, C -H. Hsueh, P. Angelini, and T. N. Tiegs, ""Toughening Behavi a High-Temperature Rotary Furnace,J. Am. Ceran. Soc., 66[7 C-111-C-113 hisker-Reinforced Ceramic Matrix Composites, "J. A Ceram Soc., 71 I 1050-61(1988) M. Saito, S. Nagashima, and A. Kato, "Crystal Growth of Sic whisker from the J J. Comer, "A Study of Contrast Bands in B-siC Whiskers, Mater. Res. Bull. io(gHCO System,J. Mater: Sci. Lett, 11, 373-76(1992) 4,279-88(1969) L-G. Lee and 1. B. Cutler. "Formation of Silicon Carbide from Rice Hulls. " Ar SM. Singh, R. M. Dickerson, F. A Olmstead, and J. 1. Eldridge,"SiC (SCS-6)Fiber Ceram. Soc. Bul, 54[2]195-98(1975) Rm1ma2图 Curzio. M. K.F.mdrM、是 id growth model f13 by Single-Fiber push-Out Tests "5. Am. Ceram Soc. 81 (4)965-78(1998). Whiskers, " J. A Ceram Soc., 71 [3]149-56(1988)- H. Wang, Y. Berta, and G, S. Fischman, "Microstructure of Silicon S V Nair and Y L Wang,"Toughening Behavior of a Two-Dimensional Woven Whisker Synthesized by Carbothermal Reduction of Silicon Nitride,J.Am. Ceram. . Damage and R-Curve Behavior .J. Am Soc,755]1080-84(1992) sW.-S. Seo and K. Koumoto, "Stacking Fault and Growth Direction of B-Si Vapor-Grown B-S Whisker Synthesized by Carbothermal Reduction, Key Eng. Mater, 159[1601 2C. Cheng, R. J. Needs, and V. Heine, "Inter-layer Interactions and the Origin of 95-100(1999) SiC Polytypes, " J. Phys. C: Solid State Phys., 21, 1049(1988)
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