The Formation and Dissociation of Silicon Carbide

In the formation of “amorphous” or micro‐crystalline silicon carbide, SiC, by interacting quartz and coke, carbon monoxide is evolved at a pressure of over an atmosphere at temperatures above 1,635° C. Experimental equipment and procedure are described. The reaction is inaugurated by the softening and melting of the quartz while in contact with the solid coke. Silicon vapor is produced which through a secondary reaction is consumed as SiC. The course of the reaction is described in detail. The pressure of silicon vapor above solid SiC and graphite is immeasurably small after equilibrium has been attained at 1,635° C. The boiling points of the reaction constituents at 1 mm., 10 mm., and 760 mm. are tabulated. Roland Brunner's research on SiC formation from SiO2 and C is critically reviewed. The supposition that siloxicon is a necessary intermediate product in the formation of SiC is unwarranted and contrary to experimental observations. The dissociation of SiC at various temperatures can be most accurately determined by measuring the equilibrium pressures of the gas phase above solid SiC and graphite, employing the spring balance method which is described in detail. The components of the gas phase were determined and from this analysis the partial pressures of silicon in contact with solid SiC and graphite are derived. The vapor pressure of liquid silicon was determined and its heat of vaporization calculated. On the basis of the experimentally determined heat of formation of solid SiC and the heat of vaporization of liquid silicon, the partial pressures for silicon vapor in contact with solid SiC and graphite are calculated and it is found that these calculated values are identical with those empirically found. The temperature of a commercial SiC furnace is selected not so much on the basis of minimum temperature required for the silicon carbide reaction to take place but rather a tern‐ perature characteristic for any particular furnace design which will bring about the carbide reaction at the best possible speed and efficiency. The reasons for the maintenance of a maximum temperature in the commercial silicon carbide furnace are discussed. In the re‐crystallization of carborundum it is suggested that an intermediate carbon‐oxygen compound is formed which is richer in C than CO. In the commercial carbide furnace the reaction sequence is more important than reaction equilibria. The presence of silicon in the gas phase of commercial furnaces is due to its formation as an intermediate product before combining with carbon to form the carbide. Localized condensation of larger masses of silicon is due to irregular operation of the furnace.