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