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(1)

Reaction (1) takes place via gaseous intermediates, and in the presence of CO the first step is the production of SiO(g): SiO2(s) + CO(g) = SiO(g) + CO2(g)

(2)

In the presence of C(s), CO 2 may be consumed according to the Boudouard reaction:

CO2(g) + C(s) = 2CO(g)

(3)

When the gas phase is sufficiently enriched with respect to SiO(g) the formation of SiC(s) takes place: SiO(g) + 2C(s) = SiC(s) + CO(g)

(4)

Although a large number of investigations have been carried out during the last decades1,2,3A4 5,6,7,8,9, it still appears that there is a lack of fundamental understanding on the kinetics of reactions in the system Si-O-C. The present investigation focus on the reaction responsible for the formation of SiO(g): SiO2(s) + C(s) = SiO(g) + CO(g)

(5)

Reaction (5) is seen to be the sum of reaction (2) and (3). Furthermore, the formation of SiC, according to reaction (4), is also considered. EXPERIMENTAL AND RESULTS The experiments were performed in a thermobalance consisting of a vertical graphite tube furnace and an electronic balance. The system may be used with a high vacuum or controlled atmospheres at temperatures to about 2300'C. Temperature is measured and controlled by an automatic optical pyrometer. A peripheral gas support system provides atmospheres ranging from pure CO to pure Ar, as well as controlled mixtures of the two gas components. The experiments were performed by continuously monitoring the weight loss of a coarse grained mixture of silica and carbon contained in a graphite crucible at 1558°C and given gas compositions. Prior to the experiments the silica (natural quartz from Mount Rose, Canada. Purity>99.98%) was converted to cristobalite. The carbon reactant in question was a porous, fine435

Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society

grained graphite quality (CS49 from Union Carbide. Purity>99.5%). The grain size of both reactants were in the range 0.51-0.71mm. Crucible with screwed-on lid was made from graphite CS49, ID 23mm and IH 31rmm, the lid was perforated with six 4mm holes. The bottom and side wall of the crucible were virtually perforated with 0.5mm holes. This "open" crucible design provided easy access for reactant/product gases. The results from the first run are given in Fig. 1. The solid lines show the variation of rate of weight loss with partial pressure CO at constant total pressures. For each gas composition the rate of weight loss was recorded for approx. 20 minutes. .

30

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.

P(Iot.)-0.400 bar E2,5

*"

0.800 bar

o 2,0

1.066 bar

S1.5

1.333 bar

p1.0 1558"C 0,5

0,0

.

.

.

..

0,5 Partial

.

pressure

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i

.

1.0 COIber

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*

I

1.5

Fig. 1. Rate of weight loss of a cristobalite/graphite-charge (2.423 g silica and 0.726 g graphite) as function of partial pressure CO at 1558°C and total pressures 1.333, 1.066, 0.800 and 0.400 bar (1000, 800, 600, and 300 torr) respectively. P(tOt.)=PCO+PAr. Apart from the rounded maxima shown by three of the four curves, they all show decreasing rate with decreasing partial pressure of CO d