Am Ceramo97177-820199 Stacking Faults in B-SiC Formed during Carbothermal Reduction of SiO2 Won-Seon Seo and Kunihito Koumoto' ry, School of Engineering, Nagoya University, Nagoya, 464-01, Japan g powder sources are listed in B-SiC formation, twice as as added to the I-milling nixed powders hydrogen order to con- formation. The for 3 h in air to owder, and SEM rostructure onditions: scan- at33.6and41.4 tio was calculated es a higher density is smaller sity can be obtair ult density. High-resolution d area diffraction patterns were done at Figures(b), 3(a)and (b), 6, and 9 reveal that spherical rs in the synthesized tion (XRD)intens stacking fault fo articles existed mainly morphological dif een(a) the bottom and (b) the (1) Synthesis of B-Sic The B-SiC powder in the present work was synthesized by fomead oniy at the bot carbon source and SiO2, SiO, or Si powder as a silicon source. Table I. P Starting Pur Particle T.. Mitchell-contributing editor materials ity size(um) S 99.9 Sio 99.99 1 Received March 1. 1995: approved January 5. 1996. Sio 2 99. 0.8 c Research No.05780370 ithe Japanese Carbon black 99.5 0.04-0.1 Science and Culture. Graphite 99.9 5 , Amencan Ceramic society 1777
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
uly 1996 Stacking Faults in B-SiC Formed during Carbothermal Reduction of siO, 1779 iin (b) Fig. 2. TEM micrographs and electron diffraction patterns of SiC particles synthesized at 1420.C for 30 min from SiO, and carbon black: (a)whisker, (b) spherical particle. Table Il. Stacking Fault Content and Reaction Products Formed in the stacked Layers of arious Carbon and Silicon Sources Contact method and CB CB+ SiC 1400°C.2h CB CB + Si 3,21 1420°C,10min (1)>(2) Sio SiO, t si+ CB+ SiC (1),(2 1420°C.3h Sio SiC t Sio, si(tr) CB CB+ SiC 5.06 1420°C,10min (1)>(2 SiO, SiO, ( CBCB + SiC 1)>(2) 1420C.3h SO2→sio(J)
Journal of the American Ceramic Society--Seo and Koumoto ol.79,No.7 2p Fig 3. SEM photographs of B-SiC powders formed in the stacked layers of various carbon and silicon sources: (a)carbon black layer of CB /SiO2 stacked layer; 1420C, 0. 1 h(b)CB layer of CB/SiO2 stacked layer; 1420C, 3 h(c)CB layer of CB/Si stacked layer: 1400.C, 2 h (d)Si layer of CB/Si stacked layer; 1400.C, 2 h signs, each reaction can occur thermodynamically. Specifically, the Sio gas pressure could be and Eq (4), indicates that they may occur competitively to Eq.(3), would be enhanced to he disprop ed somewhat higher oportionation reaction and Sio, so that si form Si particles would react with carbon particles to give rise to spheri es in the dβS according to the formation site(see Fig. 1)can be explained by onsidering the two reaction routes. The existence of different () Stacking Fault Formation morphologies within a reaction boat mixture implies that vari (A)Efect of Reaction Temperature: To investigate the ous reactions may have occurred competitively. Reactions (3) hange in stacking faults content with reaction temperature, and(4), route 2, probably occurred at the bottom of the boa the reaction time must be held constant, because fine silicon because spherical particles of B-Sic were observed, while carbide formed on carbon particles quickly coarsens to form eaction(2)occurred mainly in the upper part of the boat. This agglomerates, and the size of these agglomerates depends on ifference may owe to differences in the sio gas pressure in the reaction temperature and time. Figure 5 shows that the different parts of the boat. At the bottom of the boat, especially, (4) (2) O 8h △0.5h 12501300135014001450150015501600 120014001600180020002200 Temp(C) Fig 4.Standard Gibbs free energy vs temperature for various expressed as the relative X-ray intensity, k, for fault Fig. 5. rature dependence of the stack reactions
July 19 Stacking Faults in B-SiC Formed during Carbothermal Reduction of Sio, (b) Fig. 6. SEM photographs of B-SiC particles synthesized at(a)1300.C and( b)1550.C content of stacking faults decreased with increasing reaction B) Efect of Sio Gas: Generally, it is accepted that an temperature for B-Sic powders synthesized by firing the mixed increase in reaction time causes an increase in the grain size of powder at 1300% to 1420%C for 8 h and at 1420 to 1550c the formed B-SiC particles. Figure 7 shows stacking fault for 30 min content vs reaction time for B-SiC synthesized at 1420.C under Microstructural investigation was carried out with a view to an H, atmosphere. The mixed powder and the stacked powder explaining these results. Figure 6 shows SEM photographs of showed large differences in the manner in which stacking faults the cb layer of the CB SiO, stacked powder formed. Clearly, the content of stacking faults in the stacked bed formed at(a)1300.C and(b)1550C (30 min). Apparently, powder decreased with increasing reaction time, but in the the volume fractions of whiskers and the spherical particles mixed powder the value remained unchanged. Theoretically, the two synthesized powders were similar, but the average whisker diameters and the spherical particle sizes were quit te of Sio gas generation. Silicon monoxide gas in a stacked different. The coexistence of whiskers and spherical particles der is generated only in the contacting area of SiO, and carbon strongly indicates that reactions(2)and(4)must have occurre particles, while in mixed powders Sio gas is generated every simultaneously. In fact, the AG values for reactions(2)and where in the powder In a mixed powder, a reaction time long 4) were similar in this than 30 min thus has little effect on the formation of B-SiC, and stacking fault content probably did not change drastically with hence on stacking fault formation, because of the homogeneous a change in the reaction temperature. Since whiskers formed at generation of Sio gas. In a stacked layer, however, Sio gas 1550C were thicker than those formed at 1300C. however can be generated continuously for more than 30 min, so that the apparent decrease in stacking fault density with increasing thick whiskers with small aspect ratios can form, as shown reaction temperature should be attributed to a possible size effect on the XRD intensities, mainly because of the different Figure 8 shows that the stacking fault content increased with thicknesses of the formed whiskers increase in the heating rate from 1000"C up to 1420C. Figure 9 shows SEM photographs of B-Sic particles synthesized O mixed powder 1420°C,0.5h 1420°C 4 Reaction time(h) Heating rate C/min Fig. 7. Reaction time de ce of the stacking fault content for the powders synthesize to the reaction temperatur king fault content with the rate of heating up ture,1420°C
1782 Journalof the American Ceramic Society--Seo and Koumoto Vol. 79. No. 7 Fig 9. SEM photographs of p-SiC particles synthesized with heating rate of (a)0.5"C/min and(b)14" C/min. 1420C for 30 min with heating rates of (a)0.5 References (b)14 C/min. Since the sizes of the synthesized particles hardly Koumoto. M. Shimohigoshi, S. Takeda, and H and Thermoelectric Energy Comversion in Porous Si stacking fault density had little to do with a size Ceramic Socicty, Westerville, OH, 1989. change in the heating rate would affect the rate of G. Sasaki, K. Hiraga, M. Hirabayashi, K. Nihara, and T. Hirai, "Microstruc between the Sio gas and the carbon because of the ture around Indentation of Chemical Vapor Deposition Observed by Transmis- ates of Sio gas st hence stacking fault formation must Microscopy, "Yogyo Kyokaishi, 94, 779-83(1986 have been affected by the manner in which the Sic formed L. U. Ogbuji, T, E. Mitchell, and A. H. Heuer, "The B-a Transformation in The surface energy of the (1111 planes was much smaller olycrystalline SiC: Ill. The Thickening of a Plate, "J Am. Ceram Soc.,64, 91 than those of the other crystal planes. If atoms could be added continuously to the (111) planes, the crystal might easily grow S Seo, C.H. Pai. K. Kournoto, and H. Yanagida in the Phase Transformation of SiC J. Ceram Soc. Jpn, 100, 227-32(1992) in the [lll] direction, and stacking faults would form easily in 3. G. Lee and I.B. Cutler. "Formation of Silicon Carbide from Rice Hulls, the 111 planes in order to decrease the formation energy for Am. Ceran.Soc.Bm,.54[2]195-98(19 B-SiC. Moreover, under rapid reaction conditions, newly added oS.R. Nutt, "Microstructure and Growth Model for Rice- Hull-Derived SiC ous B-SiC Fabricated from Rice Hull Ash, J. pn,1017814-18(1993) IV. Conclusions d Factors affecting stacking fault formation during the syn- hesis of B-SiC powder by carbothermal reduction were inves- Doctoral thesis, pp 9-28. University of Tokyo, 1992. tigated. The fol ng conclusions can be drawn from the W.S. Seo, C. H. Pai, K. Koumoto, and H. Yanagi present study Development and Stacking Fault Annihilation in B-Sic The formed B-SiC usually exhibited two major morpholo gies, whiskers and spherical particles. The whiskers, with a Wssn4pm,9417(191) high density of stacking faults, formed perpendicular to the (7(10993) and Grain Growth in Porous Ceramics of B-SiC, ". Mater Res,8 cal particles, on the other hand, had a low density of stacking faults, and were formed by a solid-solid reaction. The soli bon; the solid-solid reaction took plac to as reaction occurred directly between Sio gas and solid car g: Thomas. w:. 6 103-11 (9 - ay Powder Profile Refinement Method Ceram Sot ell and H. M. Otte lectron Diffrac- ween solid Si and tion Patterns from Thin Platelets "Phys. Status Solidi, 12, 5366(1965 via a disproportionation reaction of gaseous Sio into Si M. Pickard and B. Derby, "TEM of Silicon Carbide Whisker O,.The average size of both whiskers and spherical Microstructures, "J Mater: Sci., 26, 6207-17(1991) particles increased with increasing reaction temperature and IL. Wang, H. Wada, and L F Allar esis and Characterization of sic time, although their stacking fault contents, measured by XRD, Whiskers ". Mater Res, 7[11 148-63(1 ppeared to decrease, possibly because of a size effect. Increas G.CWei"Beta SiC Powders Produced by Carbothermic Reduction Silica in a High-Temperature Rotary Furnace. "J. Am. Ceram. Soc., 62[7 ing the heating rate up to the reaction temperature greatly enhanced stacking fault formation. sV. D. Krstic, "Production of Fine, High-Purity Beta Silicon Carbide Pow Acknowledgment in Nagoya University for TEM Photo Mr, S. Arai of IMV E. Givargizov; Pp. 136-43 in Growth of Crystals; Vol. 11. Edited by A.A Chernov, translated by J E S. Bradley. Consultants Bu New Yorl
