Carbothermal Synthesis of Submicrometer B-SiC Powder Using Double Precursor Reaction Mixture Pridobivanje submikronskega B-SiC prahu s karbotermično sintezo reakcijske mešanice dvojnega prekurzorja Kevorkijan V.,1 Fakulteta za strojništvo Maribor The low temperature carbothermal reduction of colloidal silica has been applled for the synthesis of submicrometer B - SIC povvder. The intrmsic klnetics of the overall reaction was studied over the temperature range (1473(X-y)SiOIgl+ySiOj,;y - loss of SiO(?) caused by pumping (2) (X-y )SiO!„,+2.3Cl,1—>ZSiO+ZCO,„+ +(X-y-Z)SiO,sl+(2.3-2Z)C„, (3) Si02,s)+3.3C(s)—>ZSiC,sl+( 1 -X )Si02,sl+(3.3-X-2Z)Clsl+ +(X-y-Z)SiO • SiC) •• \ •/:( '() (4) Note that loss of Si() caused by pumping (y) is not deter-mined from the stoichiometry of the overall reaction (4). In order to determine the fraction of SiO,„, removed by pumping it is necessary to measure the conversion of SiO, , into SiC,,, |W") at special experimental conditions (see experim'ental procedure) when ali Si containing species remain in the system. Then y=0 and W'=Z/X. The aetual conversion of SiO,,,, into SiC,,, (W) (at y * 0) and W (when y=0) are than equal: Z'/X=Z/(X-y). Z >Z: Z" is the yield of SiC when y=0. According to this. the loss of SiO,,,, coused by pumping could be expressed as y=X(Z'-Z)/Z'. 1 dr. Varužan KEVORKIJAN. dipl. in/. kom. Fakullela za strojništvo Smetanova 17. (i2(K)<> Maribor 2. Experimental procedure The svnthesis of submicron B - SiC povvder by the carbothermal reduction of colloidal silica has been investigated in vac-uum ( = 1-5 Pa) for 0,5 h at temperatures betvveen 1125-1400°C. The double precursor mixture (Reaction mixture I) consists of SiO, particles (Cab - O - SiC, M5, Cabot) covered vvith carbon layer (one - to - one molar ratio) and fine carbon black particles (Carbon black, Monarch 1300, Cabot) doped vvith 0,6 vvt Vr of amorphous boron (Ventron, 00438 - 325 mesh). Fig. 1. An i n t i -mately mixed carbon black - colloidal silica precursor (one - to -one molar ratio) and carbon black particles doped vvith 0,6 vvt % of amorphous boron has been used as the comparative reaction mixture (Reaction mixture II). In order to analyse the kinetics of both steps of carbothermal reduction the vveight of the reaction mixture, the vveight of the specimens after carbothermal reduction and the vveight of the specimens after removal of the free carbon vvere determined, respectively. The loss of SiO,„, component caused by pumping vvas determined using the follovving procedure. The mixture of colloidal SiO, and phenolic resin (Viaphen, PR 881/60) vvas homogenised in acetone. After drying, the thick paste vvas pressed to form a solid composite. Samples vvere then heated in argon to 450° for 4 h. Pyrolysed SiO;/C specimen (one - to - one molar ratio) vvas placed into the crucible. SiO,/C core in the form of pallct vvas completely covered vvith carbon black povvder (doped vvith 0.6 wt% of boron). The crucible vvas than heated to temperatures be-tvveen 1150°C - 1400°C for 0,5 h in vacuum. After the heattreat-ment. the pallet vvas very carefully mechanically separated from the carbon black povvder. In order to remove free carbon, the protective povvder vvas heated in air at 750°C for 24 h. The residual povvder is pure SiC. Finallv. the vveigh of synthesised SiC mea-sured by accuracy ± 1%. Colloidal SiO, Phenolic resin T T Mixing in acetone T Evaporation with constant stirring T Drying T Pyrolysis in argon flow T Deagglomeration T Precursor for SiO(g) formation Carbon black Amorphous boron T T Mixing in acetone T Evaporation vvith constant stirring f Drying T Precursor for B-SiC L 1 r B Dry mixing T Reaction mrcture Figure 1: Flov. sheet of reaction mixture synthesis process Slika I: Shema priprave reakcijske mešanice 3. Kinetic studies 3.1. SiO, .. formation Assuming that the solid state reaction (2) is of zero order and its rate constantis defined as: k=dX/dt=k„exp(-Ea/RT). The aeti-vation energy, Ea, for Reaction 2 is determined by the slope of ln k against l/T, Fig. 2-3. The experimental values for both types of reaction mixtures are almost the same: = 300 ± 30 KJ/mol vvhich is in good agreement vvith literature data:: = 377 KJ/mol. boron rring 1100 1200 1300 1400 TEMPERATURE fCl Figure 2: The conversion of SiO, into SiO.,., at different temperatures after the same holding tirne - 0,5 h Slika 2: Konverzija SiO, v SiO(E1 pri različnih temperaturah. Čas sinteze - 0,5 h 3.2. SiClu formation Using vveight loss data, the conversion of SiO,„, into SiC,„, (W) defined as: W=Z/(l-Y+y) is calculated and plotted in Fig. 4. On the basis of a shrinking core model, the rate constant, k, is:k = 1 - (1 -W)"i = k„ exp(- Ea/RT) if the chemical reaction is the rate-liming step or: k = 1-3(1 - WČ! + 2 (1 - W) = k„ exp (- Ea/RT) if the diffusion through SiC layer is rate-controlling. The aetivation energies for both scenarios are determined from the slope of the plots Ink against l/T. The values obtained by the assumption that the chemical reaction is the rate-controlling step. are 391 KJ/mol (for the reaction mixture I) and 220 KJ/mol (for C* -5 -6 m 0.35 < x < 0.9 t=1800 s 335 KJ CURVE 1 7 7.5 1/T [°Kx10*| Figure 3: The aetivation energy for reaction 2 (Curve I -SiO, particles cover with carbon laver: Curve 2-SiO. particles in contact vvith carbon black particles). Holding time 0,5 h Slika 3: Aktivacijska energija za reakcijo 2 (Krivulja l-SiO, delci prevlečeni s slojem ogljika: Krivulja 2-SiO, delci v stiku z delci ogljikovih saj l. Čas sinteze - 0.5 h the reaction mixture II), Fig. 5. Hovvever. the values of the rate constant calculated for a diffusion controlled process can be also fitted well by linear plots. Fig. 6. The aetivation energies calculated on this way are much higher: 577 KJ/mol for the reaction mixture I and 483 KJ/mol for the reaction mixture II. 4. Conclusions The transformation of carbon black particles into SiC is modelled by a shrinking core model. On the basis of a detailed investigation of the reaction kinetics at different temperatures. it is postulated that the chemical reaction is the rate - controlling step at lower temperatures (T<1473 K ) vvhile diffusion of carbon through SiC ,, layer being rate controlling at T > 1673 K. Hovvever, over the temperature range of special interest (1473 < T < 1673 K) in the lovv temperature, high yield svnthesis of sub-micron 6 - SiC povvder, it seems, that both rate steps proceed by almost the same rate masking on this way each other as the real rate limiting step. For the overall carbothermal reduction, col-lected experimental data strongly suggest that the SiO(i genera-tion is the rate controlling step. 1100 1200 1300 1400 TEMPERATURE |*C] Figure 4: The conversion of SiOlal into SiC Slika 4: Konverzija SiO v SiC . o CURVE 1 0.20 < W < 0.60 CURVE 2 0.20 < W < 0.90 -1 o\ V, CURVE 1 \ \° -2 -3 CURVE 2 \o\ o\\ -4 6 6.5 7 7^5 1/T |°Kx10'] 1/T |°Kx10'] Figure 5: The activation energy for Reaction (3) based on the assump-tion that the chemical reaction is the rate - controlling step Slika 5: Aktivacijska energija za Reakcijo (3) ob predpostavki, da kemijska reakcija določa hitrost karbotermične sinteze 5. References V. M. Kevorkijan. M. Komac. D. Kolar. Low - temperature synthesis of sinterable SiC povvders by carbothermic reduction of colloidal SiO:. ./. Mat. S c i.. 27. 1992. 10, 2705 Figure 6: The activation energy for Reaction (3) based on the assumption that the diffusion is the rate - controlling step Slika 6: Aktivacijska energija za Reakcijo (3) ob predpostavki, da kemijska reakcija določa hitrost karbotermične sinteze J. J. Biernacki. Formation of Silicon Monoxide and Application to the Grovvth of Vapor - Liquid - Solid Silicion Carbide Whiskers: Doctoral Dissertation. Fenn College of Engineering, Cleveland State University, Cleveland. OH. 1987