778 Journal of the American Ceramic Sociery-Seo and Koumoto Vol. 79. No. 7 1 (b) SEM photographs of B-SiC powder synthesized at(a)the bottom and(b)the upper part of a reaction boat containing SiO, and C powders 300C for 8 h under H, atmosphere mixture of whiskers and sperical particles formed in layer showed a morphological I re of spherical particles part. The existence of different hologies in powder synthesized by carbothermal reduction seem CB/SiO, bed. On the other hand, the p-SiC formed in the Si elated to the different growth mechanisms. It is obvious in layer consisted only of spherical particles with an average grain Fig. 2 that the whiskers had many stacking faults with planes size nearly equal to that of the starting Si particles. It can be perpendicular to the growth axis, whereas the spherical parti assumed that solid carbon had diffused into the layer and that cles exhibited few recognizable stacking faults spherical B-siC particles formed by a solid-solid reaction. The simultaneous formation of spherical particles and whiskers in stals also showed remarkable differences, as evident from the CB layer might have resulted from a reaction between the Fig. 2. The eD pattern from the whisker clearly exhibits fea surface oxides of the Si particles and the CB tuneless streaks typical of a disordered layer structure, while the In the Sio and CB stacked powder bed, B-SiC also formed concurrently in both the Sio layer and the CB layer the ed streaks always are perpendicular to the stacking fault Table In). X-ray diffraction analysis of the stacked CBSio lanes (1111, 4- the main growth direction of the whiskers can layer indicated that solid Sio decomposed into Sio, and Si judged as parallel to the [lll] direction during the reaction. The same reactions that occurred concur (2) Reaction Mechanism recur ed in g he stacked cesi and cB/sio layers. The p-sic he coexistence of different morphologies and their stacking fault densities in the B-SiC particles indicate reaction between carbon and Si, just as in the case of the p-Sic or more than two reaction routes were involved in th formation reaction. To concretely investigate the reaction formed on Si layer of the stacked CB/Si layer, and those formed some model experiments were conducted as follows in the CB layer seem to have originated from the reaction between carbon and SiO,, as in the case of the B-SiC formed on Three pairs of the stacked powder beds of CB/Si, CB/SiO CB layer of the stacked CB/SiO, layer and CB/SiO, were heated at 1420.C for 0. 1 and 3 h under an H, Consequently, the formation mechanism of B-SiC particles atmosphere. Table II shows the phases formed and the stacking by carbothermal reduction can be summarized as follows. Two fault contents(expressed as 133.6 /a14)in the p-SiC. Figure 3 routes may exist for SiC formation: route 1, a solid-gas reaction shows the sample morphologies In the stacked powder bed of CB/SiOz, SiC formed only within the CB layer, whereas the solid carbon: and route 2, a solid-solid reaction between solid SiO, layer decreased continuously in content as the reactie Si and carbon(see Eq. 4), prior to which a disproportionation time increased, without forming SiC. The B-SiC particles reaction of gaseous Sio into Si and Sio2 takes place(Eq(3) formed in the CB layer sted of a mixture of spherical particles and whiskers, as shown in Figs. 3 (a)and(b). Tha The B-SiC formed by the solid-gas reaction showed a whisker same morphology formed from the mixture of Sio, and CB morphology with a high density of stacking faults; that formed by the solid-solid reaction showed spherical particles with a (see Fig. 1(b). Silicon monoxide(Sio)gas was generated by the reaction between SiO, and CB at the interface of two low density of stacking faults, as mentioned previously stacked layers and was transported immediately to the Route 1: SiO(g)+ 2C(5)= SiC(s)+ Co(g) of the CB layer to form SiC. Accordingly, the two dif morphologies of the formed SiC particles seem closely Route 2: 2Sio(g)= Si(s)+ SiO2(s) to the reaction between Sio gas and carbon particles Si(s)+C(s)= SiC(s) In contrast, Sic was formed concurrently in both the Si layer and the CB layer in the Si/CB powder bed. As obvious from The standard Gibbs free energy change, AG, of reactions igs. 3(c)and(d ), however, the SiC particles formed in the tw (2)to(4), calculated from the JANAF tables, are shown in layers had different morphologies. The B-SiC formed in the CB Fig. 4. Since the AG values of all the reactions carry negativ