Investigation of the paramagnetic centres and electronic properties of silicon carbide nanomaterials

Ultrafine powders of silicon carbide (SIC) exhibit a substantial electric conductivity (sigma(dc) similar to 0.05 S cm(-1)) after annealing at temperatures between 1500 degrees C and 1800 degrees C. The origin of this phenomena is discussed with respect to structural modifications, sample composition and the delocalized paramagnetic centres involved in these materials. The structural evolution of SiC nanomaterials with annealing was monitored by Si-29 and C-13 Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). A consistent analysis of the electron paramagnetic resonance (EPR) spectra was performed and showed the coexistence of two paramagnetic centres (DI, DII) localized in the crystalline sites of the SiC polytypes. Delocalized DIII paramagnetic species were partially created by thermal conversions of DII defects with energies not exceeding 13 meV. The spin susceptibility, deduced from EPR measurements, was found to be marked by delocalized paramagnetic species above 150 K and by strongly correlated (DIII) unpaired spins around 30 K. An attempt is made to correlate the delocalized unpaired spins with the electric properties of these materials.