Small Scale Synthesis Of Silicon Carbide

The raw materials used for synthesizing silicon carbide are mainly vein quartz or quartz sand with SiO2 as the main component and petroleum coke with C as the main component. Low grade silicon carbide can be made from anthracite with ground ash. The auxiliary materials are sawdust and salt.

Silicon carbide is available in black and green. When smelting green silicon carbide, it is required that the SiO2 content in siliceous raw materials should be as high as possible and the impurity content should be as low as possible. When producing black silicon carbide, SiO2 in siliceous raw materials can be slightly lower. The requirement for petroleum coke is that the fixed carbon content should be as high as possible, the ash content should be less than 1.2%, and the volatile content should be less than 12.0%. The particle size of petroleum coke is usually 2mm or less than 1.5mm. The sawdust is used to adjust the air permeability of the furnace charge, and the usually added amount is 3%~5% (volume). Common salt is only used for smelting green silicon carbide.

Silicon carbide is generated from siliceous raw materials and petroleum coke in a resistance furnace at 2000~2500 ℃ through the following reactions:

SiO2+3C→SiC+2CO↑-526.09Kj

CO is discharged through the charge. Adding salt can form chloride with impurities such as Fe, Al and volatilize. The sawdust makes the material form porous sintering body, which is convenient for CO gas discharge.

The formation of silicon carbide is characterized by that it does not pass through the liquid phase, and its process is as follows: from about 1700 ℃, the siliceous raw material changes from sand to melt, and then to steam (white smoke); SiO2 melt and steam drill into the pores of carbonaceous materials, penetrate into carbon particles, and generate Sic reaction; When the temperature rises to 1700~1900 ℃, b-SiC is generated; When the temperature further rises to 1900~2000 ℃, the fine b-SiC transforms into a-SiC, and the a-SiC grains gradually grow and compact; When the furnace temperature rises to about 2500 ℃, SiC begins to decompose into silicon steam and graphite.