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 obser￾vations via scanning electron microscopy (SEM) (Model S-510, Hitachi, Tokyo, Japan) were conducted to examine the microstruc￾ture. 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. Conven￾tional 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 synthe￾sized 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