Silicon carbon structure

Question 1

How does SiC exist?

Answer 1

~ in ~250 crystalline forms

~ structure is based on highly covalent sp3 hybrid tetrahedral coordination

~ all crystalline forms but one are α-SiC

Question 2

What are the α and β polymorphs?

Answer 2

~ β modification adopts a zinc blende crystal structure similar to diamond; forms below 1700C

~ all α modifications form above 1700C and have hexagonal crystal structures; metastable at room temp
~ α polymorphs are based on the pattern of stacking layers, ex: 2H, 4H, 6H, 8H, 15R (H: hexagonal, R: rhombohedral)

Question 3

What is the history of SiC?

Answer 3

~ SiC (moissanite) in nature is extremely rare, found in meteorites

~ first synthesized by EJ Acheson, who intended to form artificial diamonds
–> clay and powdered carbon were heated in an iron bowl via an electric arc to >1600
–> the hard crystals that formed upon cooling were originally thought to be a carbon-aluminum compound coined carborundum, still used as a name for abrasive and refractory grades of SiC
–> production method devised by Acheson has been largely unchanged
–> he founded carborundum corp

Question 4

What’s the first step to SiC synthesis?

Answer 4

1. electrodes are buried below a rxn mass of mixed silica (quartz sand) and carbon (petroleum coke) powder

Question 5

What is the second step to SiC synthesis?

Answer 5

2. heating to 1600-2000C for 36hours yields the net endothermic rxn: SiO2 + 3C = SiC + 2CO
   –> gaseous intermediate products facilitate reactant transport and interaction
   –> exiting CO2 is collected and transferred through pipelines to a power station and burned (20% energy savings)

Question 6

What is the third step to SiC synthesis?

Answer 6

3. concentric radial layers develop: graphite electrode at center, then α-SiC (coarse crystalline structure), then β-SiC metallurgical grade, and finally un-reacted material on the outside, which can be recycled
   –> metallurgical grade SiC is used as a batch additive for electric furnace production of cast iron

Question 7

What is the fourth step to SiC synthesis?

Answer 7

~ intensive milling in an attrition or jet mill is required to form sinter-grade α-SiC powders

~ vapor phase techniques produce high purity fine powders: SiCl4 + CH4 –> SiC + HCl

~ commercially produced SiC powder sustains an oxide coating; the oxide content of the powder increases with decreasing particle size