1960 年 68 巻 777 号 p. 210-221
This paper presents the results of the investigations on the effect of vanadium oxide on the oxidation of pure silicon oxide powder, since the former has proved to be most influential material for such action (J. Ceram. Assoc., Japan, 67  189 (1959)).
V2O5 melts at 660°C and in order to investigate the wettability of the molten oxide against silicon carbide the change of contact angle by temperature as well as the penetrability of the molten liquid into the powder were measured.
Through the measurement of the change of weight and by X-ray analysis of mixed powder heated in argon atmosphere the temperature dependency of the chemical reaction and its details were studied. From the results obtained the rate of reaction at 950° and 1150°C in oxygen stream was measured. Furthmore, through the measurement of the penetrability V2O5 into thedry-pressed silica powder, and of the pyrometric cone equivalent temperature of the mixture the author has tried to approach to the mechanism of chemical reaction.
The results are summarised as follows:
1) Large wettability of SiC with the melt from V2O5 in air as well as in argon atmosphere was confirmed. At the temperatures higher than the melting point the contact angle decreased rapidly approaching soon to practical zero value. No bubble at the interface was observed. Also, the melt denetrated rapidly into the pressed mass of the fine powder of SiC. Especially, the rate of penetration of the melt into the pressed silica powder was to be very high between the temperatures of 950° and 1150°C.
2) V2O5 reacts with SiC in three steps in the atmosphere of argon, namely at 640°-670°C, 750-920°C and 1020°-1180°C-. It was confirmed that V2O5 was reduced to V2O4 in the second step, and to V2O3 in the third step. The temperature of the completion of the third step changed by the nature of V2O5, but not by the kind of SiC used. On the other hand, as V2O3 was oxidized to V2O5 through V2O4 at the temperatures as low as 500°C, and it may be concluded that the coexistence of O2 is to accelerate the rate of oxidation of SiC by V2O5 which naturaly will be influenced strongly by temperature.
3) When V2O5+SiC mixture was heated to 900°C in argon and then O2 was introduced oxidation proceed very slowly at the begining, fast gradually, and again slowed down with time. This may be interpreted by assuming the fact that when SiO2, the reaction product of the oxidation of SiC, accumulate over ten per cent the migration of V+5 became difficult owing to the solidification of the melt. In this stage V2O4 is formed at the interface.
4) When the same experiment was carried out at a higher temperature, 1150°C, two types of the reaction were observed, namely
a) When all V2O5 was reduced to V2O3 the reaction rate was possible to be expressed by a second order equation, i.e. very high at the initial stage falling gradually with time.
b) When most part was reduced to V2O4 the reaction represented by the equation.
where Δr is the amount of the oxidized SiC measured by the thickness from the surface, t is the time, and k, c are constants.
Taking a serious view of the roll played by SiO2 at the interface between the oxide film and the surface of SiC crystal the author discussed the mechanism of the oxidation of SiC with the mobility of V+5 in the system SiO2-V2O5 and the cone equivalent temperature.