Effects of excess carbon and vibrational properties in ultrafine SiC powders

Structural and vibrational properties are investigated in SiC nanopowders s ynthesized by a CO2 laser pyrolysis of (SiH4, C2H2) gaseous mixture and the rmally treated up to 1800 degrees C. The structural modifications of the Si C crystallites and the arrangement of the carbon in excess are monitored at different annealing stages. A critical behaviour is revealed in the powder annealed at T-a = 1500 degrees C through the features of the Raman spectra and the insulating-conductor transition. The significant electric conducti vity (sigma approximate to 0.05 S cm(-1)) which appears above T-a is discus sed with respect to the powder composition and the interface effects where the carbon in excess seems to play a key role.