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 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 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 stacked￾powder 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. How￾ever, the reaction in the mixed powder is volumetric and the SiO gas is consumed competitively by the surrounding carbon. More￾over, 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 experi￾ments 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